Camille Escudé

est destinée au dépôt

Pourquoi les États-Unis rompent le consensus arctique

à considérer les détroits entre les îles de l'archipel arctique comme des détroits internationaux où demeure la liberté de transit permanente de ses navires, ce que réfute Ottawa (3). L'intérêt étatsunien en Arctique est également fortement lié aux ressources naturelles, depuis la découverte en 1968 du champ pétrolier de Prudhoe Bay -et sa mise en exploitation en 1977 -dont les réserves en font le plus gros gisement du pays.

Après la chute de l'URSS, l'Alaska a perdu de son importance stratégique pour Washington. La présidence Obama (2009Obama ( -2017) ) rompra avec la réticence traditionnelle envers la coopération régionale. L'engagement dans la coopération s'est même accru pendant la présidence américaine du Conseil de l'Arctique, entre 2015 et 2017, les États-Unis utilisant la région comme point d'ancrage de leur politique climatique. Cet intérêt est cependant uctuant et, alors que Washington avait un représentant spécial pour les questions arctiques pendant la présidence du Conseil, ce poste n'existe plus : l'amiral Robert Papp, qui l'occupait, est devenu en 2017 lobbyiste dans une entreprise de construction navale.

La priorité donnée aux enjeux de sécurité Il est frappant de voir que les diplomates états-uniens rencontrés sont les seuls à a rmer sans aucun complexe l'importance de l'aspect sécuritaire de leurs intérêts en Arctique. Selon eux, il y a deux axes pour la politique états-unienne en Arctique. Le premier, que l'on pourrait quali er de « politiquement correct », est l'a rmation o cielle de l'Arctique comme zone de coopération, de recherche scienti que, de mesure des e ets du changement climatique, de lieu de vie des populations autochtones. Le deuxième aspect, sans aucun doute le plus important pour Washington, consiste à considérer l'Arctique comme une zone stratégique et militaire, dont les enjeux sont discutés en dehors des instances multilatérales.

Comme partout en Arctique, la tension entre perspectives de développement et de conservation divise aux États-Unis. Sous l'administration Obama, l'Arctique avait été déclarée, avec l'interdiction de l'exploitation pétrolière, zone de conservation. La nouvelle administration se calque plutôt sur la politique norvégienne d'exploitation des ressources, en insistant moins sur l'environnement et plus sur la sécurité et le développement durable -notion élastique consistant à vanter l'équilibre de la balance entre le développement économique et la protection de l'environnement. (2) L'Alaska est acheté à la Russie en 1867 pour 7,2 millions de dollars.

(

  Il faut souligner que les États-Unis sont le seul État côtier de l'Arctique à ne pas avoir rati é la Convention des Nations Unies sur le droit de la mer, rédigée à Montego Bay en 1982, qui permet de régler les di érends dans le cadre du droit international. Barack Obama avait fait un e ort en ce sens, mais l'administration Trump se gardera d'entrer dans l'accord. Par conséquent, les États-Unis ne peuvent pas revendiquer de zone économique exclusive étendue comme le font le Danemark, le Canada et la Russie [lire l'article de F. Lasserre, p. 46]. Acteur en retrait de la gouvernance arctique naissante, les États-Unis ne se réinvestissent que timidement. Washington s'est montré réticent à valider des accords juridiquement contraignants au sein du Conseil de l'Arctique, accords qui changeraient la législation nationale américaine ou créeraient des institutions supranationales. La stratégie arctique publiée sous la présidence de George W. Bush, en janvier 2009, était déjà très claire sur ce sujet : le Conseil doit rester un forum de discussions. Malgré le scepticisme de l'administration Trump vis-à-vis de la coopération internationale

  ) Comme pour la route maritime du Nord qui longe les côtes russes, le statut des eaux du passage du Nord-Ouest est sujet à désaccords. La position canadienne est d'inclure le passage dans ses eaux intérieures, donc de le soumettre à son entière souveraineté. Les États-Unis défendent

  en général et de la coopération climatique en particulier, l'engagement états-unien dans la coopération régionale arctique a été réa rmé par le secrétaire d'État Rex Tillerson lors de la réunion ministérielle de Fairbanks (Alaska) en mai 2017, qui a clôt la présidence états-unienne du Conseil. L'administration Trump, trouble-fête de la coopération arctique L'année 2019 a cependant marqué un tournant dans l'implication états-unienne dans la coopération arctique, qui, de timide, est devenue agressive. Pour la première fois depuis la création du Conseil de l'Arctique, les huit ministres des A aires étrangères de la région ont échoué à s'entendre sur une déclaration commune à l'issue de la 11e réunion ministérielle du Conseil de l'Arctique, tenue en mai 2019 à l'issue de la présidence nlandaise. Les États-Unis ont en e et refusé de signer un document faisant mention du terme « changement climatique ». Lors de la précédente réunion en 2017, Rex Tillerson -limogé le 13 mars 2018 -avait pourtant été conciliant, même si deux mois plus tard les États-Unis quittaient l'accord de Paris. Cette fois, l'administration Trump s'est montrée intransigeante. La veille de la réunion, le secrétaire d'État Mike Pompeo avait donné le ton, fustigeant dans un discours extrêmement virulent la présence chinoise dans la région ainsi que les positions canadienne et russe concernant les passages maritimes. Mike Pompeo a soupçonné publiquement la Chine d'utiliser une présence civile pour renforcer sa présence militaire à travers le déploiement de sous-marins. Selon Washington, la Chine aurait investi 90 milliards de dollars dans la région entre 2012 et 2017, principalement dans des infrastructures, notamment dans le cadre de son gigantesque projet des « routes de la soie polaires ». L'attitude peu consensuelle de la présidence états-unienne a trouvé une nouvelle illustration dans la proposition de Donald Trump d'acheter le Groenland [lire p. 64], au mois d'août 2019. Bien plus qu'une simple plaisanterie, il s'agit pour les États-Unis de réa rmer leur présence en Arctique vis-àvis de la Russie et de la Chine. Face à la perception de cette double menace, et après avoir laissé longtemps l'Arctique au dernier plan de ses priorités, la diplomatie américaine multiplie donc les coups d'éclat. Parallèlement, ont été publiés plusieurs documents o ciels consacrés à l'Arctique, dont la stratégie du ministère de la Défense et celle des Garde-côtes, qui pointent également du doigt la Russie et la Chine comme principaux dangers pour la stabilité de la région. Un an plus tôt, « Trident Juncture » rassemblait en Norvège 50 000 soldats de l'OTAN, faisant de cet exercice le plus important depuis la guerre froide. Depuis peu, l'administration Trump se place ainsi à contre-courant du discours consensuel d'implication plus ou moins active dans la coopération régionale, et rend son futur incertain. Cette expression vient de l'auteur Rob Huebert, « United States Arctic Policy : The Reluctant Arctic Power », School of Public Policy Brie ng Papers, vol. 2 no 2, University of Calgary, mai 2009 [NdlR].

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Pourquoi les États-Unis rompent le

//www.areion24.news/2020consensus arctique https://www.areion24.news/

À propos de l'auteur

Camille Escudé-Joffres

Professeur agrégée de géographie, Sciences Po -CERI.

   0


LA DIPLOMATIE ARCTIQUE EN 2021 CAMILLE ESCUDÉ JOFFRES PROFESSEURE AGRÉGÉE DE GÉOGRAPHIE, DOCTEURE ASSOCIÉE AU CERI, SCIENCES PO

Au sens large, la diplomatie concerne la branche de la science politique qui s'intéresse aux relations internationales. Analyser la diplomatie en Arctique reviendrait alors à observer à la fois la construction de liens interpersonnels et l'organisation d'événements facilitant la coopération dans cette région du monde (Landriault, 2020). C'est un fait largement accepté que les États ne sont plus les seuls acteurs des relations internationales, et l'Arctique ne fait pas exception. Si l'année 2021 a été riche en événements diplomatiques traditionnels, comme le retour de la présidence russe au Conseil de l'Arctique, 2021 a marqué également la reprise en présentiel de conférences comme Arctic Circle, plateformes pour ouvrir la diplomatie arctique au-delà des limites de la région et des acteurs traditionnels.

La résilience de la coopération arctique

Le Conseil de l'Arctique, dont la Russie occupe la présidence tournante depuis 2021, a célébré le 19 septembre 2021 ses 25 ans d'existence. Par sa promotion de la coopération internationale entre tous les acteurs qui ont un intérêt dans l'Arctique, le Conseil a progressivement pris une place centrale dans la diplomatie de l'Arctique. À l'occasion de ce 25ème anniversaire, les États arctiques et les participants permanents ont réaffirmé leur engagement à maintenir l'Arctique comme une région de paix, de stabilité et de coopération, lors d'une table ronde organisée par la présidence russe à Moscou le 17 septembre.

À l'issue de la réunion ministérielle de Rovaniemi en mai 2019 clôturant la présidence finlandaise, et pour la première fois depuis la création du Conseil de l'Arctique, les ministres des Affaires étrangères avaient échoué à s'entendre sur une déclaration commune. Les États-Unis avaient alors refusé d'approuver un document faisant mention de la formule « changements climatiques » et avaient dénoncé les ambitions économiques chinoises et militaires russes dans la région (Pic, 2019).

La réunion ministérielle du 20 mai 2021 semble ainsi montrer la résilience de la coopération arctique, dans un contexte de nouvelle administration états-unienne. En effet, non seulement une déclaration contenant une dizaine de fois la mention « changements climatiques » a été signée (Arctic Council, 2021a), mais de plus, à l'occasion du 25ème anniversaire du Conseil, les ministres ont adopté le tout premier plan stratégique du Conseil. Ce plan couvre la période 2021-2030 et développe la vision stratégique du Conseil pour améliorer le développement durable, la protection de l'environnement et la gouvernance de l'Arctique (Arctic Council, 2021 b).

La Russie comme meneuse de la diplomatie Arctique en 2021 ? Lors du passage de témoin de la réunion ministérielle de 2021, la présidence islandaise fortement impactée par pandémie de Covid-19 a été unanimement saluée avec l'adoption de nombreux rapports scientifiques, notamment celui de 2021 sur le changement climatique dans l'Arctique. Le ministre des Affaires étrangères russe, Sergueï Lavrov, a réitéré la volonté de la Russie de maintenir les efforts engagés au cours de ces deux dernières années afin de poursuivre une politique pragmatique en Arctique où les Etats arctiques sont parvenus jusqu'à présent à préserver le dialogue et la coopération.

Se plaçant dans la continuité des présidences islandaise et finlandaise, la Russie met en avant quatre priorités pour les deux ans où elle présidera la Conseil : 1. la qualité de vie des communautés arctiques ; 2. la protection contre les effets du changement climatique ; 3.la coopération socio-économique entre les régions arctiques (afin de développer la construction d'infrastructures) ; 4. garantir la stabilité du Conseil de l'Arctique et agir en faveur de son développement [START_REF] Canova | Communication conjointe sur un engagement renforcé de l'UE en faveur d'une région arctique pacifique, durable et prospère[END_REF].

Enfin, Sergueï Lavrov a exprimé le souhait de voir la bonne coopération entre les Etats arctiques être étendue à la coopération militaire. Si les questions de sécurité sont exclues du mandat du Conseil de l'Arctique, cela pourrait être une ouverture pour la reprise du dialogue annuel entre les chefs des forces armées des États arctiques, suspendu depuis 2014.

Il est indiqué clairement que la présidence russe au Conseil de l'Arctique est à situer dans le cadre de la stratégie arctique russe 2020-2035 qui vise à développer l'Arctique à grande échelle. On peut cependant souligner la contradiction entre l'agenda russe au Conseil de l'Arctique tourné vers le développement durable et la lutte contre les effets du changement climatique et sa politique intérieure de développement de ses régions arctiques (exploitation minière et d'hydrocarbures au coeur du développement de la zone arctique russe, développement de la route maritime du Nord) [START_REF] Laruelle | La politique arctique de la Russie : Une stratégie de puissance et ses limites[END_REF].

La diplomatie arctique à géométrie variable, entre ouverture et fermeture L'une des priorités de la présidence islandaise du Conseil de l'Arctique était de renforcer le Conseil de l'Arctique en tant qu'institution de coopération, notamment par une meilleure collaboration avec les Observateurs. Pourtant, face au manque de consensus, aucun nouvel Observateur (État, ONG ou organisation internationale) n'a été admis lors de cette réunion. Rappelons qu'il y a déjà 38 Observateurs (13 États non Arctiques, 13 organisations internationales et 12 ONG) et que cette année l'Estonie, la République Tchèque et l'Irlande ont officiellement déposé leur candidature, en plus de la Turquie (dont la candidature a été déposée en 2015), la Mongolie et la Grèce qui sont toujours en attente de validation (Quinn, 2021).

Compte tenu du fait que de plus en plus de parties intéressées souhaitent rejoindre le Conseil, certains experts arguent depuis quelques années déjà [START_REF] Knecht | New Observers Queuing Up: Why the Arctic Council should expand -and expel[END_REF] qu'il serait temps que le Conseil de l'Arctique reconsidère cette pratique et appliquent plus rigoureusement les règles de procédures et critères tels que modifiés à la réunion ministérielle de Kiruna en 2013 pour ne pas décrédibiliser ce statut. Par ailleurs, les Observateurs réclament d'avoir une participation plus active aux débats.

En parallèle ou peut-être en réaction à des formats diplomatiques traditionnels perçus comme trop fermés par certains, l'année 2021 a marqué une reprise des conférences arctiques. Plateforme privilégiée des acteurs non-traditionnels des relations internationales (entreprises, ONG, think tanks, populations autochtones, État fédérés etc.), celles-ci ont pu reprendre au moins au format dit hybride.

La conférence « Arctic Frontiers qui a lieu d'ordinaire tous les ans depuis 2007 à Tromsø en Norvège s'était tenue pour la première fois en ligne. En 2021, le programme a mis à l'honneur de hautes personnalités politiques, notamment de l'Union européennes (UE) et de la Russie. Non seulement l'Ambassadeur de l'UE pour l'Arctique Michael Mann a fait une rapide intervention mais aussi, beaucoup plus inhabituel, le Haut Représentant de l'UE pour les Affaires extérieures et de sécurité, Josep Borell. Ce dernier a véhiculé très clairement le message devenu de plus en plus récurrent des représentants de l'UE selon lequel l'Union européenne est dans l'Arctique. Quatre représentants de l'UE sont intervenus à « Arctic Frontiers ».

Alors que l'Union européenne s'est plutôt distinguée en Arctique par ses maladresses diplomatiques qui lui valent toujours le refus d'un siège d'observateur au sein du Conseil de l'Arctique [START_REF] Canova | Communication conjointe sur un engagement renforcé de l'UE en faveur d'une région arctique pacifique, durable et prospère[END_REF], la nomination du nouvel envoyé spécial Michael Mann depuis avril 2020 marque une relance de la politique européenne en Arctique à la ligne encore peu claire. La nouvelle stratégie arctique européenne annoncée depuis novembre 2019 a été publiée le 13 octobre 2021 (EEAS, 2021). L'UE est confrontée à une situation paradoxale : elle prétend être dans l'Arctique [START_REF] Mann | Exclusive interview with Michael Mann, EU's Ambassador at large for the Arctic/Special envoy for Arctic matters[END_REF] tout en étant considérée comme un acteur extérieur parfois considéré avec suspicion par les autres acteurs arctiques pour ses efforts d'ingérence dans les affaires arctiques (Raspotnik, 2018).

La diplomatie dans les instances multilatérales : des avancées lentes mais sûres en faveur de la protection de l'environnement arctique

L'année 2021 a marqué des avancées certaines en faveur de la protection de l'environnement arctique, autant de victoires pour des travaux diplomatiques commencés souvent il y a des années.

Les travaux à l'OMI sur les mesures de réduction des impacts sur l'environnement arctique du transport maritime international durent depuis 11 ans. Le projet de directives vise à aider les administrations des États-Parties à la Convention MARPOL, dont les côtes sont bordées par les eaux arctiques, à mettre en oeuvre des mesures au niveau national pour réduire les risques liés à l'utilisation et au transport de fioul lourd par les navires dans les eaux arctiques. Le projet de directives vise également à fournir aux exploitants de navires qui prévoient d'effectuer des voyages dans l'Arctique des recommandations sur les mesures à prendre pour réduire les risques de déversement de fioul lourd dans les eaux arctiques.

En juin 2021, le comité MEPC 76 de l'organisation maritime internationale (OMI) a définitivement approuvé le moratoire sur le fioul lourd en Arctique (IMO, 2021). L'interdiction porte sur l'utilisation et le transport de fioul lourd en vue de leur utilisation comme combustible et prendra effet le 1er juillet 2024. Il existe toutefois des dérogations et exemptions. Les navires qui assurent la sécurité des navires ou qui participent à des opérations de recherche et de sauvetage, ainsi que les navires qui se consacrent à la préparation et à la lutte contre les déversements d'hydrocarbures, seront exemptés. Les navires qui répondent à certaines normes de construction en matière de protection des réservoirs de carburant devront se conformer à partir du 1er juillet 2029. Un État Partie à la convention MARPOL dont le littoral borde les eaux arctiques peut temporairement renoncer aux exigences pour les navires battant son pavillon lorsqu'ils sont exploités dans les eaux relevant de la souveraineté ou de la juridiction de cet Etat jusqu'au 1er juillet 2029.

Deuxième victoire pour les défenseurs de l'environnement, le moratoire sur la pratique de la pêche commerciale en Arctique ratifié en 2018 a pris effet. Après dix années de négociations initiées notamment par George W. Bush en 2008, le moratoire interdisant la pêche commerciale dans l'Océan Arctique central a été ratifié par une dizaine de pays incluant le Canada, la Chine, le Danemark avec le Groenland et les îles Féroé), l'Union européenne, l'Islande, le Japon, la Norvège, la Russie, la Corée du Sud et les États-Unis [START_REF] Schreiber | A long-awaited Central Arctic Ocean commercial fishing ban takes effect[END_REF]. Valable pendant 16 ans, cet accord ne concerne pas la pêche de subsistance. Premier accord ratifié par des pays arctiques et non arctiques, ce moratoire est un exemple encourageant de coopération arctique en faveur de la préservation de l'écosystème polaire. La fonte des glaces en Arctique conduit à l'ouverture de passages maritimes permettant notamment l'exploitation devenant progressivement libre de glace sur une longue période. Anticipant le risque d'exploitation massive de ces eaux internationales, ce moratoire se place en faveur de la recherche scientifique permettant de mieux connaître l'écosystème marin. Créant un centre de recherche scientifique, cet accord souhaite notamment impulser la recherche sur la pêche durable, encore minoritaire en Arctique comme dans le reste du monde.

Il est intéressant de noter que ce moratoire prévoit d'inclure l'expertise et la participation des peuples autochtones arctiques, marquant une fois de plus l'ouverture des pratiques diplomatiques au-delà des cercles traditionnels. Cette intégration des connaissances autochtones au sein d'un accord international est inédite et démontre une véritable volonté de coopération sur les questions industrielles et de diplomatie arctique. Malgré des négociations mises en danger par les différentes crises diplomatiques entre la Russie et l'Union Européenne, cet accord marque un véritable tournant en termes de coopération inter-étatique permettant la protection d'une région fragile avant même l'industrialisation de celle-ci. Dans cette région polaire où le réchauffement climatique se fait ressentir trois fois plus que dans le reste du monde et où la fonte des glaces est majoritairement appréhendée comme source de nouvelles opportunités économiques (Deshayes, 2021), ce moratoire représente une véritable avancée diplomatique et environnementale.

Conclusion

L'année 2020, en Arctique comme ailleurs, avait été surtout dominée par la gestion de la COVID-19. L'élection de Joe Biden et la nouvelle présidence russe du Conseil de l'Arctique ont infléchi l'année 2021 vers une coopération arctique qui semble s'être relevée de quelques années difficileson se souvient de l'échec de la réunion ministérielle de Rovaniemi en 2019. Plus que jamais, la coopération arctique s'est montrée résiliente, à la fois en résultat des efforts diplomatiques au sein des formats traditionnels comme le Conseil de l'Arctique avec l'adoption d'un plan stratégique, et dans des instances qui font la part belle aux acteurs non arctiques et non traditionnels, comme « Arctic Frontiers ». À un autre niveau enfin, des efforts diplomatiques entretenus depuis des années ont porté leurs fruits en faveur de la protection environnementale, malgré un résultat parfois mitigé aux dires de certaines ONG en raison des freins d'États arctiques comme la Russie et le Canada justement : l'interdiction du fioul lourd et le moratoire contre la pêche dans l'Arctique central en sont deux exemples frappants. Il reste un an et demi à Moscou pour s'affirmer comme meneur de la diplomatie arctique avant de passer le relais. Quoi qu'il en soit, l'année 2021 a montré une fois de plus que l'Arctique est une région où la diplomatie et la coopération à géométrie variable sont bien assurées.

Références

  Arctic Council. 2021a. The Arctic Council celebrates 25 years of Arctic cooperation. Arctic Council. Disponible au : https://arctic-council.org/news/arctic-council-celebrates-25-years-of-arctic-cooperation/ Arctic Council. 2021b. Reykavik Strategic Plan 2021-2023. Disponible au https://oaarchive.arctic-L'ANNÉE ARCTIQUE 2021 OBSERVATOIRE DE LA POLITIQUE ET LA SÉCURITÉ DE L'ARCTIQUE (OPSA) POUR PLUS D'INFORMATIONS : CIRRICQ.ORG/OPSA


J Khatua

S Bhattacharya

A M Strydom

A Zorko

J S Lord

A Ozarowski

E Kermarrec

P Khuntia

Magnetic properties and spin dynamics in a spin-orbit driven J eff = 1/2 triangular lattice antiferromagnet

Frustration induced strong quantum fluctuations accompanied by spin-orbit coupling and crystal electric field can give rise to rich and diverse magnetic phenomena associated with unconventional low-energy excitations in rare-earth based quantum magnets. Herein, we present crystal structure, magnetic susceptibility, specific heat, muon spin relaxation (µSR), and electron spin resonance (ESR) studies on the polycrystalline samples of Ba6Yb2Ti4O17 in which Yb 3+ ions constitute a perfect triangular lattice in ab-plane without detectable anti-site disorder between atomic sites. The Curie-Weiss fit of low-temperature magnetic susceptibility data suggest the spin-orbit entangled J eff = 1/2 degrees of freedom of Yb 3+ spin with weak antiferromagnetic exchange interactions in the Kramers doublet ground state. The zero-field specific heat data reveal the presence of longrange magnetic order at TN = 77 mK which is suppressed in a magnetic field µ0H ≥ 1 T. The broad maximum in specific heat is attributed to the Schottky anomaly implying the Zeeman splitting of the Kramers doublet ground state. The ESR measurements suggest the presence of anisotropic exchange interaction between the moments of Yb 3+ spins and the well separated Kramers doublet state. µSR experiments reveal a fluctuating state of Yb 3+ spins in the temperature range 0.1 K ≤ T ≤ 10 K owing to depopulation of crystal electric field levels, which suggests that the Kramers doublets are well separated consistent with thermodynamic and ESR results. In addition to the intraplane nearest-neighbor superexchange interaction, the interplane exchange interaction and anisotropy are expected to stabilize long-range ordered state in this triangular lattice antiferromagnet.

I. INTRODUCTION

Quantum materials wherein the interplay between competing degrees of freedom, frustration-induced strong quantum fluctuations, and quantum entanglement is prominent have drawn considerable attention due to their connection to the emergence of exotic quantum phenomena with potential to address some of the recurring themes in quantum condensed matter [1][2][3][4][5][6]. One noteworthy example is the quantum spin liquid (QSL), in which strongly entangled electron spins do not exhibit long-range magnetic order down to absolute zero temperature despite strong spin correlations [7,8]. Originally, the QSL state was proposed by Anderson for a system composed of S = 1/2 spins on a two-dimensional triangular lattice antiferromagnet [9]. Its materialization has been proposed in a few triangular lattice antiferromagnets with next-nearest interaction, and anisotropic exchange interaction [10][11][12][13][14][15]. Furthermore, the quantum nature of this entangled * pkhuntia@iitm.ac.in state offers an outstanding track to study gauge theories incorporating fractionalized excitations, as well as its relevance in the field of quantum computing [16][17][18]. Despite the fact that several QSL candidate materials show analogous behavior in various observables [19][20][21][22][23], understanding the microscopic spin Hamiltonian to identify fractionalized excitations remains a significant challenge because of the presence of extra exchange couplings, intrinsic disorder, and unavoidable defects in real materials [12,17,[24][25][26]. In this context, rare-earth-based materials featuring J eff = 1/2 quantum spins decorated on geometrically frustrated lattices offer a promising avenue for the experimental realization of exotic quantum phenomena [11,23,[27][28][29][30][31]. Moreover, it has been suggested that anisotropic magnetic interactions between J eff = 1/2 moments in the Kramers crystal-field ground state of these magnetic materials play an important role in the materialization of QSL state, in contrast to isotropic interactions between pure S = 1/2 moments on the triangular lattice Heisenberg model, which tend to promote a 120 • long-range magnetic ordered state [32]. For example, the rare-earth triangular antiferromagnets YbMgGaO 4 [33][34][35] and chalcogenides NaYbCh 2 (Ch = O, S, Se ) [36][37][38][39][40][41][42][43] with Yb 3+ ions with J eff = 1/2 moment have attracted considerable interest due to the emergence of a QSL ground state brought about by the presence of dominant easy-plane anisotropic magnetic interactions and spin frustration [44,45]. In contrast to QSL state with dominant easy-plane anisotropic magnetic interactions, the spin-liquid candidates on a perfect triangular lattice with dominant easy-axis anisotropy are very rare [46] and have been found only very recently in NdTa 7 O 19 which appears to be the first realization of spin-liquid ground state with Isinglike magnetic correlations driven by strong easy-axis anisotropic magnetic interaction [11,[START_REF] Ulaga | Finite-temperature properties of the easy-axis heisenberg model on frustrated lattices[END_REF]. Furthermore, the impact of dipolar magnetic interactions to the observed spin excitation continuum, widely considered as the most robust evidence of spin fractionalization in insulators, remains a dynamic and actively researched domain in quantum condensed matter [START_REF] Yao | [END_REF]49]. In addition to the anisotropic exchange interactions, there have been proposals for a dominant dipolar interaction-driven spin-liquid state in certain triangular lattice antiferromagnets, such as Yb(BaBO 3 ) 3 [50][51][52] and ABaYb(BO 3 ) 2 (where A = Na, K) [53,54]. In addition to their dynamic ground states, the phase transitions observed in triangular lattice antiferromagnets are of significant interest for gaining insights into intriguing quantum phenomena such as Berezinskii-Kosterlitz-Thouless (BKT) physics [55,56]. For example, the observed quasi long-range magnetic ordered state in the two-dimensional triangular lattice material TmMgGaO 4 may represent the manifestation of the BKT phase [57][58][59]. Recently, it has been suggested that the dipolar spin liquid candidate KBaGd(BO 3 ) 2 is a promising candidate to host a BKT phase and an unconventional quantum critical point [START_REF] Xiang | Dipolar spin liquid ending with quantum critical point in a gd-based triangular magnet[END_REF]. Furthermore, the rare-earth-based magnets with weakly coupled magnetic moments, approaching the paramagnetic limit, have shown potential for lowering system temperatures through adiabatic demagnetization cooling, as observed in triangular lattice antiferromagnets such as KBaGd(BO 3 ) 2 [START_REF] Jesche | [END_REF] and KBaYb(BO 3 ) 2 [62]. Interestingly, in such rare-earth quantum magnets, anisotropic magnetic interaction induced by spin-orbit coupling, crystal-electric field of localized 4f electrons and lattice symmetry offer a viable platform to host unconventional phases including spiral spin liquid [63,64]. In a similar vein, Kitaev spin liquid state induced by bond-dependent anisotropic magnetic interactions [65][66][67], and multipolar orders [68] in spin-orbit entangled J eff = 1/2 quantum magnets remains largely unexplored. The current effort is devoted towards the design, discovery and investigation of twoand three dimensional spin-orbit entangled 4f based frustrated magnets potential to host myriads of exotic quantum phases that may aid for the establishment of new theoretical paradigms in quantum condensed matter. In order to realize rich variety of spin-orbit driven quantum phenomena as a result of competing exchange interaction, crystal electric field and anisotropic interaction, it is therefore essential in exploring disorder free rare-earth based quantum magnets. Herein, we report the crystal structure and magnetic properties of a unexplored triangular lattice antiferromagnet Ba 6 Yb 2 Ti 4 O 17 , (henceforth, BYTO) which crystallizes in the hexagonal space group P 6 3 /mmc where the Yb 3+ ions constitute a perfect triangular lattice in the ab-plane. The Rietveld analysis of x-ray diffraction data shows the absence of anti-site disorder between the crystallographic sites. Magnetic susceptibility data suggest that Yb 3+ spins acquire J eff = 1/2 degrees of freedom and the presence of a weak antiferromagnetic exchange interaction between rareearth moments. The zero-field specific heat data show an anomaly possibly due to the long-range magnetic order at T N = 77 mK which disappears in a magnetic field µ 0 H ≥ 1 T. µSR experiments suggest the lack of long-range magnetic order down to 80 mK, while spin fluctuations are observed up to 10 K. The anisotropic magnetic exchange interaction between J eff = 1/2 moments of Yb 3+ spins was suggested from electron spin resonance measurements. The combination of magnetic anisotropy, intraplane, and interplane exchange interactions are expected to stabilize a long-range magnetic ordered state in this antiferromagnet.

II. EXPERIMENTAL DETAILS

Polycrystalline samples of BYTO were prepared by a conventional solid state method. Prior to use, we preheated a stoichiometric amount of BaCO 3 (Alfa Aesar, 99.997 %), and Yb 2 O 3 (Alfa Aesar, 99.998 %) at 200 • C and 800 • C, respectively to prevent moisture contamination. The stoichiometric mixtures were pelletized and sintered at 1400 • C for 72 hours with intermittent grindings. The phase purity of the final product was confirmed by the x-ray diffraction at 300 K by employing Rigaku smartLAB x-ray diffractometer with Cu Kα radiation (λ = 1.54 Å). Magnetization measurements were carried out using a Quantum Design, SQUID (MPMS) in the temperature range 2 K ≤ T ≤ 350 K in several magnetic fields. Specific heat measurements were performed using a Quantum Design, physical properties measurement system (PPMS) in the temperature range 2 K ≤ T ≤ 270 K and in magnetic fields up to 7 T. In addition, specific heat measurements were carried out in the temperature range 0.049 K ≤ T ≤ 4 K in 0 T, 1 T, 3 T in a dilution refrigerator using a Dynacool PPMS instrument from Quantum Design, San Diego, USA. Thermal conductivity measurements were conducted in the temperature range 2 K ≤ T ≤ 300 K in magnetic fields up to 7 T by two probe method also using Quantum Design, PPMS. µSR experiments in zero field and in longitudinal magnetic fields were performed on the MuSR spectrometer at the ISIS pulsed neutron and muon source at Rutherford Appleton Laboratory, UK. The powder sample (∼ 1 gm) was mixed with a small amount of GE-varnish and fixed to a silver plate. A 25 µm-thick silver foil was placed on top of the sample to maximize the number of implanted muons in the sample and to reduce the thermal radiation. A dilution fridge was used for the temperature range 0.08 K to 4 K, and subsequently an helium flow cryostat to reach temperatures from 1.4 K to 300 K. Electron spin resonance (ESR) measurements were performed at the National High Magnetic Field Laboratory, Tallahassee, USA on a polycrystalline sample using a custom-made transmission-type ESR spectrometer with homodyne detection. The measurements were conducted in the Faraday configuration at the irradiation frequency of 212 GHz. The magnetic field was swept between 2 and 10 T using a superconducting magnet and the tempera-ture was varied between 5 and 200 K using a continuousflow He cryostat. A standard field-modulation technique was used with the modulation field of about 2 mT.

III. RESULTS

A. Rietveld refinement and crystal structure

To examine the phase purity and crystal structure, we carried out the Rietveld refinement of room temperature x-ray diffraction data using GSAS software [69]. The XRD results suggest that the polycrystalline BYTO samples contain a tiny fraction of unreacted magnetic Yb 2 O 3 impurity, which has a minimal influence on the overall magnetic properties of the material under examination. A similar situation with the unavoidable Yb 2 O 3 secondary phase in polycrystalline samples has been observed in a few other Yb-based magnets [70][71][72]. To accurately determine the percentages of the main and secondary phase, a two-phase Rietveld refinement was performed. The crystallographic parameters of isostructural compound Ba 6 Y 2 Ti 4 O 17 were incorporated as a reference to perform the Rietveld refinement for the main phase [73]. I. Figure 1 (a) depicts the refined crystal structure of BYTO drawn using VESTA software [74]. The nearest neighbor magnetic Yb 3+ ion (Yb-Yb ≈ 5.907 Å) constitutes two dimensional triangular layers stacked along the c-axis (see Figure 1 (b)) similar to the extensively studied triangular lattice antiferromagnet YbMgGaO 4 with Yb-Yb distance of approximately 3.4 Å [33]. As shown in Figure 1 (b), one unit cell of BYTO is composed of four triangular layers of Yb 3+ ions where the first and last two layers from the bottom are separated by the inter-planar distance of 7.26 Å while the middle two layers are separated by the inter-planar distance 7.45 Å. In BYTO, the Yb 3+ ion occupies a single crystallographic site (4e) of the hexagonal lattice and forms a distorted YbO 6 tetrahedron (see Figure 1 (a)) with local O 2-ions. Due to such an octahedral environment, Yb 3+ ions are exposed to a strong crystal electric field, which splits the degenerate multiplet of J = 7/2 into four Kramers doublet states of Yb 3+ ions with spin-orbit entangled J eff = 1/2 moment. In BYTO, the intra-plane YbO 6 octahedra are not interconnected through a common O 2-ions, as observed in other triangular lattice antiferromagnets such as YbMgGaO 4 or NaYbX 2 (X = O, S, Se). Instead, they are linked by TiO 4 tetrahedra. On the other hand, the inter-plane YbO 6 octahedra are connected through Ti 2 O 9 dimers, as illustrated in Figure 1 (a). The larger unit cell of BYTO compared to YbMgGaO 4 is the primary reason for the formation of isolated YbO 6 octahedra, which could potentially reduce the strength of intraplane magnetic exchange interactions between the Yb 3+ moments due to the increased bond length for the f -p-dp-f (Yb-O-Ti-O-Yb) hybridization. The presence of weak exchange interactions, resulting from similar superexchange pathways, has been observed in several triangular lattice antiferromagnets, including NaBaYb(BO 3 ) 2 [53], Ba 3 Yb(BO 3 ) 3 [52,54], and Ba 3 YbB 9 O 18 [29] (as shown in Table II). However, this scenario differs somewhat for 3d transition ion-based magnets, as the electrons of 3d ions are not highly localized in real space, unlike the 4f ions. For instance, relatively strong exchange interaction between Co 2+ ions is observed in the 6H-hexagonal spin lattice of Ba 3 CoSb 2 O 9 , despite the absence of a common oxygen mediator for intra-plane superexchange interactions between the J eff = 1/2 moment of Co 2+ ions [75,76]. From a structural perspective, it is worth noting that BYTO and Ba 3 CoSb 2 O 9 share several similarities, aside from differences in their stacking layers [75]. They have similar exchange paths, nearest-neighbor distances of 5.90 Å, and interlayer separations of 7.2 Å. However, the magnetic properties are anticipated to be distinct due to the localized nature and distinct anisotropy of Yb 3+ ions in BYTO.

θ CW (K) ( low T ) µ eff (µB) ( low T ) T N (K) (Ref.) YbMgGaO4 (R 3m) Yb-O-Yb -4 2.8 - [33, 77] NaYbO2 (R 3m) Yb-O-Yb -5.6 2.84 - [41] NaBaYb(BO3)2 (R 3m) Yb-O-B-O-Yb -0.069 2.23 0.41 [53] K3Yb(VO4)2 (P 63m) Yb-O-V-O-Yb -1 2.41 - [78] NdTa7O19 (P 6c2) Nd-O-Ta-O-Nd -0.46 1.9 - [11] Ba6Yb2Ti4O17 (P 63/mmc) Yb-O-Ti-O-Yb -0.49 2.5 0.

Present

B. Magnetic susceptibility

In Figure 2 (a), we show the temperature dependence of magnetic susceptibility (χ(T )) in two magnetic fields in the temperature range 2 K ≤ T ≤ 350 K. The magnetic susceptibility data do not show any signature of long-range magnetic order at least above 2 K in BYTO. The absence of any marked difference between zerofield cooled and field-cooled magnetic susceptibility data (see inset of Figure 2 (a)) suggests Yb 3+ spins are not frozen at least down to 2 K. In order to calculate the value of effective magnetic moment (µ eff ) and Curie-Weiss temperature (θ CW ), the inverse χ(T ) data (see Figure 2 (b)) were fitted by the Curie-Weiss (CW) law χ = C/(T -θ CW ). Here, the Curie constant (C ) is associated with the effective moment through the formula µ eff = √ 8Cµ B , and θ CW signifies the energy scale of the magnetic exchange interactions between Yb 3+ moments. The high-temperature CW fit in the temperature range 150 K ≤ T ≤ 350 K yields θ CW = -102 K and µ eff = 4.57 µ B . The large negative Curie-Weiss temperature indicates the presence of elevated energy levels of crystal electric field excitations. The obtained µ eff = 4.57 µ B is comparable to the effective moment of free Yb 3+ ions (µ free eff = 4.54 µ B ). As previously mentioned, the Yb 3+ moments are located in an octahedral environment, and as a result, a strong crystal electric field naturally causes the eight-fold degenerate J = 7/2 multiplet to split into four Kramers doublet states. The preliminary indication of the presence of the groundstate Kramers doublet state can be observed in the temperature-dependent inverse susceptibility data. For instance, in BYTO, one can observe a deviation in the measured susceptibility data from the high-temperature Curie-Weiss fit (indicated by the red line) as depicted in Figure 2 (b), where this deviation becomes noticeable below 100 K. It is important to note that such deviations in transition metal-based magnets typically reflects the development of magnetic correlations between electronic spins, but in rare-earth magnets, it is typically due to presence of crystal electric field excitations.

χ ( 1 0 -1 c m 3 / m o l ) T ( K ) 0 . 1 T 1 T ( b ) χ (1 0 -1 c m 3 / m o l ) T ( K ) Z F C F C m 0 H = 0 . 0 1 T ( c ) M ( m B / f . u . ) m 0 H ( T ) 2 K 5 K 1 0 K -θ C W ( K ) T m a x ( K ) θ C W = -0 . 4 9 K 1 / χ ( m o l / c m 3 ) T ( K ) m 0 H = 1 T
To gain insights into the Kramers doublet ground state and the nature of magnetic interactions, it is necessary to perform a Curie-Weiss fit at low temperatures sufficiently below the first excited crystal electric field level. It has been brought out in the literature that the Curie-Weiss temperature for rare-earth-based magnets is strongly dependent on the temperature range for the Curie-Weiss fitting, owing to the influence of excited crystal electric field levels [11,72].

To estimate the nature of dominant magnetic interactions between Yb 3+ moments in the ground-state Kramers doublet of BYTO, we performed fittings of the lowtemperature inverse χ(T ) data in various temperature ranges. In this analysis, the lower temperature limit was set at 4 K, while the upper temperature limit was systematically varied in 0.5 K increments up to 18 K (see inset of Figure 2 (b)) following a procedure as described in ref. [11]. The estimated temperature independent fit parameter gives µ eff = 2.51 µ B , which is much smaller than µ eff = 4.54 µ B resulting from the Hund's rule for a free Yb 3+ (J = 7/2) spin. This indicates the formation of Kramers doublet ground state at low-temperatures and θ CW = -0.49 K ± 0.02 K which suggests the presence of a weak antiferromagnetic interaction between J eff = 1/2 moments of Yb 3+ ions at low-temperature. Based on the effective magnetic moment (µ eff ) value of 2.51, the powder averaged Landé g factor is found to be 2.89. In the mean-field approximation, the Curie-Weiss temperature (θ CW ) can be expressed as θ CW = (-zJS(S + 1))/(3k B ), where J represents the exchange interaction between the J eff = 1/2 moment of Yb 3+ ions in the ab-plane, and z denotes the number of nearest neighbors. In the case of BYTO, where S = J eff = 1/2 and z = 6, the obtained value J/k B is 0.32 K [58]. The solid lines in Figure 2 (c) correspond to the fitting of the Brillouin function, yielding an average value of g = 2.76, which is close to that estimated from the Curie-Weiss fit of low-temperature susceptibility data.

C. Specific heat

In order to unveil the magnetic ground state influenced by spin frustration and the correlation between Yb 3+ moments within the Kramers doublet ground state with J eff = 1/2 moments in BYTO, it is crucial to conduct experiments at mK temperatures owing to the weak exchange coupling between Yb 3+ moments. Hence, we performed specific heat measurements in a broad range of temperatures and magnetic fields. Figure 3 (a) depicts the total specific heat data (C p ) measured in a zero-magnetic field down to 49 mK. Below T ≤ 3 K, the specific heat data in zero magnetic field reveal two anomalies, one at 2.22 K and another at 77 mK, as depicted in the inset of

T ( K ) 0 T L a t t i c e C p ( J / ( m o l . K ) ) T ( K ) 0 T L a t t i c e C p -C L a t t i c e ( J / ( m o l . K ) ) T ( K ) C m a g + C n C n C m a g C m a g ( J / ( m o l . K ) ) T ( K ) 0 T 1 T 3 T 5 T 7 T ( a ) ( b ) ( c ) ( d ) ∆/k B ( K ) m 0 H ( T ) ∆/ k B ∆S ( J / m o l . K ) T ( K ) 0 T 1 T 3 T 5 T 7 T F 2 4 A R l n 2 FIG. 3. (a)
The temperature dependence of total specific heat (Cp) of BYTO down to 49 mK in zero magnetic field. The solid line shows the Debye-Einstein model fit (see text) which represents the phonon specific heat. The inset depicts the appearance of two anomalies on decreasing the temperature below 3 K. (b) The temperature dependence of specific heat due to the sum of magnetic and nuclear contributions (circle) while the filled triangle represents the intrinsic magnetic specific heat obtained after subtracting nuclear contributions (solid line). The inset shows the temperature dependence of entropy change up to 20 K in several magnetic fields. The dotted horizontal pink line is the expected entropy for J eff = 1/2 moment. (c) The temperature dependence of the magnetic specific heat in several magnetic fields where the solid line represents two-level Schottky fit as described in the text. (d) The evolution of a field induced gap as a function of applied magnetic field where the solid line represents a linear fit.

. magnetic ordering due to the presence of unavoidable minor impurity phase of Yb 2 O 3 [70][71][72]. On the other hand the anomaly observed at 77 mK is attributed to the longrange magnetic order of Yb 3+ ions decorated on the triangular lattice.

In BYTO, the total specific heat can be represented as the combination of three components: the magneticspecific heat (C mag. (T )) originating from magnetic Yb 3+ ions, the lattice-specific heat (C lat. (T )) attributed to the phonons, and the nuclear-specific heat resulting from the nuclear spins of Yb [79]. In order to extract the entropy release resulting from the antiferromagnetic phase transition and the magnetic-specific heat, it is necessary to subtract the lattice and nuclear contributions to the total specific heat data. For this purpose, first, the lattice contribution was subtracted following the Debye-Einstein model of lattice specific heat with one Debye term and three Einstein terms i.e., C lat.

(T ) = C D [9k B T θ D 3 θ D /T 0 x 4 e x (e x -1) 2 dx] + 3 i=1 C Ei [3R θi T 2 exp( θ E i T ) (exp( θ E i T )-1) 2
], where θ D is the Debye temperature, θ i are Einstein temperatures, and R and k B are the molar gas constant and Boltzmann constant, respectively. The good fit, as shown in Figure 3 (a), suggests that the Debye-Einstein model reproduces well the lattice specific heat in this material [START_REF] Kittel | Introduction to solid state physics[END_REF]. The best fit in the temperature range 30 K ≤ T ≤ 160 K (see Figure 3 (a)) yielded the values θ D = 179 K, θ E1 = 260 K, θ E2 =506 K, and θ E1 = 1500 K [START_REF]Specific heats at low temperatures[END_REF]. During the fitting procedure, the relative weight of acoustic and optical modes of vibration i.e., C D and C Ei were assigned at a fixed ratio of C D : C E1 : C E1 : C E2 : C E3 = 1 : 2 : 3 : 8.5, which is consistent closely with the ratio of heavy atoms (Ba, Yb, Ti) and light atoms (O) in BYTO [29,79]. The olive circle in Figure 3 (b) depicts the zero-field specific heat data obtained after subtracting the lattice contribution. The anomaly at T N = 77 mK indicates that BYTO undergoes an antiferromagnetic phase transition at low-temperature similar to the triangular lattice NaBaYb(BO 3 ) 2 [53]. The weak upturn in specific heat on decreasing temperature (T ≤ 77 mK) can be attributed to a nuclear Schottky specific heat owing to the Yb nuclear spins akin to that observed in other Yb based magnets [53,[START_REF] Sala | [END_REF]. To account for this contribution, we further subtracted the specific heat due to nuclear contribution following C n ∝ 1/T 2 that is shown by the solid line in Figure 3 (b) [39]. After subtracting the nuclear Schottky and lattice contributions, the resulting magnetic-specific heat data (represented by triangles) is displayed in Figure 3 (b) as a function of temperature in zero-magnetic field. Below 1 K, there is a noticeable increase in magnetic-specific heat, followed by a sharp anomaly at 77 mK. This strongly suggests the presence of antiferromagnetic long-range order in BYTO. The occurrence of this anomaly at very low temperatures is in agreement with the presence of a weak exchange interaction between Yb 3+ moments. The inset in Figure 3 (b) shows the temperaturedependent magnetic entropy change, denoted as ∆S(T ), which is calculated as the integral of C mag (T )/T dT in the several magnetic fields. In a zero magnetic field, below the lowest measured temperature, the magnetic specific heat was obtained using linear interpolation down to zero temperature. Notably, it is observed that the entropy tends to attain a plateau at a value of 1.20 J/mol•K. This value corresponds to approximately 20% of the expected entropy value of Rln 2 (equal to 5.76 J/mol•K) for Yb 3+ spins with J eff = 1/2 moments. The missing 80 % entropy can be due to the presence of shortrange spin correlations [52,53,83]. The overestimation of the lattice contribution to the missing entropy can be ignored as the experimental temperature range is sufficient to adequately consider the impact of lattice contributions. Furthermore, below the transition temperature T N , the change in entropy amounts to approximately 5% of R ln 2. This observation implies the existence of substantial magnetic entropy below T N , likely attributable to significant spin fluctuations with a small ordered moment [84]. Furthermore, the spin fluctuation associated with short-range spin correlations above the transition temperature is also evident from the µSR experiments as discussed in the next section. A common scenario of missing 80 % entropy is also observed in recently reported triangular lattice antiferromagnet Ba 3 Yb(BO 3 ) 3 [52]. Nonetheless, the significant entropy retained at temperatures approaching absolute zero may arise from strong quantum fluctuations similar to the one observed in the long-range ordered triangular lattice antiferromag- net KBaGd(BO 3 ) 2 [START_REF] Xiang | Dipolar spin liquid ending with quantum critical point in a gd-based triangular magnet[END_REF]85]. Magnetic entropy tends to reach 5.76 J/molK in high magnetic fields, indicating the presence of J eff = 1/2 moments. In order to investigate the field effect on the antiferromagnetically ordered ground state of BYTO, specific heat measurements were performed in different magnetic fields. After subtracting the lattice contributions, the obtained magnetic specific heat is shown in Figure 3 (c) for different magnetic fields. It is observed that the anomaly at 77 mK disappears when a magnetic field of µ 0 H = 1 T is applied. This is a common feature observed in certain rare-earth magnets, including the compound under investigation, where strong magnetic field suppresses the exchange interactions. In such a scenario, the external magnetic field induces the Zeeman splitting of the lowest Kramers doublet state. Consequently, a Schottky-like broad peak emerges in the specific heat data as depicted in Figure 3 (c). Furthermore, it is worth noting that as the strength of the applied magnetic field increases, the broad peak shifts towards higher temperatures similar to that observed in several other rare-earth magnets [28,53]. To deduce the gap induced by the Zeeman splitting of the lowest Kramers doublet due to the applied magnetic field, we employed the simplest two-level Schottky specific heat model [START_REF]Specific heats at low temperatures[END_REF] i.e.,

C sch = f R ∆ k B T 2 exp(∆/k B T ) (1 + exp(∆/k B T )) 2 , ( 1
)
where ∆ is the Zeeman splitting of the ground state Kramers doublet of Yb 3+ ion, k B is the Boltzmann constant, R is the universal gas constant and f measures the fraction of Yb 3+ spins which contributes to the splitting of the ground state doublet. In Figure 3 (c), the solid line illustrates that two-level Schottky fit effectively describes the magnetic-specific heat data acquired in several magnetic fields. Notably, the estimated Zeeman gap (∆) is observed to exhibit a linear variation with the external magnetic field, as depicted in Figure 3 (d). From a linear fit the value of the Landé g factor, estimated to be g = 2.66 ± 0.02, which is close to that determined from the magnetization data. Moreover, the Schottky fit yields a value of approximately f ∼ 0.9 for non-zero magnetic fields. This indicates that nearly all of the Yb 3+ spins are involved in contributing to the Schottky specific heat in magnetic fields.

D. Thermal conductivity

In correlated quantum materials, thermal conductivity is a very sensitive probe to identify low-energy excitations which can carry heat or scatter heat carriers at low-temperature. Since the specific heat data are dominated by nuclear Schottky and lattice contributions, thermal conductivity is highly advantageous to probe the nature of ground state excitations, whether it is gapped or gapless. Figure 4 depicts the temperature dependence thermal conductivity (κ) of a polycrystalline sample of BYTO. With decreasing temperature a welldefined broad peak appears around 20 K, which can be understood as the so-called phonon peak [86][87][88]. The appearance of such a peak is expected for typical insulating behavior of good quality polycrystalline samples. As depicted in inset (a) of Figure 4, there is no effect of magnetic field on thermal conductivity, which implies that up to 2 K thermal conductivity is entirely dominated by phonons. Given that the interaction energy scale is approximately 0.30 K, magnetic excitations may potentially contribute to thermal conductivity at sub-Kelvin temperatures. For some insulating materials, thermal conductivity at low-temperature can be described as κ/T = a + b T α-1 , where the first term represents the contribution of itinerant low-energy magnetic excitations and the second term is due to the contribution of phonons where the value of α typically lies in the range 2 ≤ α ≤ 3 due to scattering of phonons from sample boundaries [89,90]. As the measured temperature range is not sufficient to observe the contribution from magnetic excitations, the low temperature κ/T data were fitted by κ/T = b T α-1 as shown in inset (b) of Figure 4. The obtained value for α was found to be 1.87 ± 0.05, which is below the expected value [91,92]. For triangular lattice antiferromagnets NaYbSe 2 and YbMgGaO 4 , a similar values of α is also observed at lower temperature (T << θ CW ) [91,92].

E. Muon spin relaxation

In order to gain microscopic insights into the ground state spin dynamics, we performed highly sensitive muon spin relaxation measurements down to the base temperature of 80 mK. Figure 5 depicts the zero field (ZF) and longitudinal field (LF) data, showing its evolution at several temperatures for BYTO. As shown in Figure 5 (a), the ZF-µSR spectra exhibit no indications of either an oscillating component or a "1/3" tail that reflects the lack of internal static magnetic fields down to 0.11 K. Nevertheless, the transition to magnetic ordering may not always be defined by oscillations in the zero field muon spin asymmetry, instead it can manifest as modifications in the asymmetry line shape [93]. Neither this is observed in BYTO as all the ZF spectra are well fitted with a stretched exponential function, A(t) = exp[-(λt) β ] + B, with a moderate β ≃ 0.8(1), and a background constant term B. The stretching parameter β is found to be temperature independent and thus likely accounts for multiple relaxation rates due to a non-unique muon stopping site in the crystal structure. Upon cooling, the spin relaxation rate λ reaches a plateau at a value of 0.58(3) µs -1 below 10 K and down to 0.125 K. We noticed a

E S R s i g n a l ( a r b . u n i t s )

B ( T ) d a t a s i m . 1 s i m . 2 g x = 2 . 3 8 g x = 2 . 6 5
g y = 2 . 8 0 g y = 2 . 6 5

g z = 3 . 7 8 g z = 2 . 9 2 ∆B x = 1 . 0 T ∆B x = 2 . 0 T ∆B y = 0 . 9 T ∆B y = 2 . 0 T ∆B z = 2 . 5 T ∆B z = 3 . 8 T ( a ) 4 0 K 3 0 K 2 0 K 1 5 K 1 0 K 5 K 2 1 2 G H z E S R s i g n a l ( a r b . u n i t s ) B ( T ) 2 0 0 K 1 0 0 K 9 0 K 8 0 K 6 5 K 5 0 K ( c ) g f a c t o r T ( K ) ( d )
E S R l i n e w i d t h ( T ) slight increase of the muon spin relaxation rate at 100 mK, perhaps indicative of a crossover towards a more correlated regime, as pointed out by the anomaly found at 77 mK in specific heat. The slowing down of spin fluctuations observed between 300 and 30 K occurs in a typical temperature range of crystal electric field levels of Yb 3+ [94], to which the muon is sensitive through the Orbach process, with a muon spin relaxation rate modelled by [11,[START_REF] Dalmas De Réotier | HFI/NQI 2004[END_REF]

T ( K )
λ = Ce -∆/k B T + 1 λ 0 -1 (2)
with λ 0 = 0.58(3) µs -1 the relaxation rate when T → 0, C an amplitude parameter and ∆ = 418(20) K the gap to the first excited crystal field level. The large value of ∆ ensures that only the Kramers ground-state doublet is occupied at low temperature and is well separated from the excited states and confirms the validity of the J eff = 1/2 picture in BYTO.

In addition, a series of LF measurements were performed at 80 mK (Figure 5 [34], but in agreement with the lower energy scale of the interaction in BYTO.

F. Electron spin resonance

To study the nature of dominant magnetic interaction between Yb 3+ spins in BYTO, we performed ESR mea-surement on polycrystalline samples of BYTO. The ESR spectra contain a broad main component centered around B 0 = 5.8 T, and a secondary narrow component centered at 7.54 T (Figure 6 (a)). At 5 K the intensity of the secondary component amounts to only 0.4% of the main one and is therefore attributed to a minor impurity phase. The main ESR line is composed of several pronounced features due to the polycrystalline nature of our sample. A simulation of the powder-averaged spectrum works well with Lorentzian line shape and three independent g-factor components and line widths (simulation 1 in Figure 6 (b)), which is compatible with the distortion of the YbO 6 octahedra away from cubic symmetry. At 5 K, we find the g-factor eigenvalues g x = 2.38, g y = 2.80, and g z = 3.78, and the corresponding line widths ∆B x = 1.0 T, ∆B y = 0.9 T, and ∆B z = 2.5 T. The fit is considerably worse if only two independent g factors and line widths are considered (simulation 2 in Figure 6 (b)). The latter model assuming axial symmetry around the crystallographic c axis would apply for strongly exchange coupled Yb 3+ moments on the unit triangle [START_REF] Gatteschi | EPR of Exchange Coupled Systems[END_REF]. The fact that the ESR spectrum does not obey the uniaxial symmetry of the underlying spin lattice limits the intra-trimer exchange interactions to J ≪ µ B ∆gB 0 /k B ≃ 6 K, where µ B and k B are the Bohr magneton and the Boltzman constant, respectively, and ∆g = g z -g x = 1.4 denotes the spread of the g factor at 5 K. The detected ESR linewidth is extremely broad and cannot be accounted for by simple dipolar interactions between Yb 3+ moments, which would result in ESR line width of the order µ 0 µ/(4πr 3 ) = 11 mT, where µ 0 is the vacuum permeability, µ = 2.51µ B is the Yb 3+ magnetic moment in the ground state and r = 5.91 Å is the nearest-neighbor distance. Therefore, much larger magnetic anisotropy resulting from exchange interactions must also be present. With increasing temperature the intensity of the ESR signal decreases profoundly (Figure 6 (a)), following a Curie-like dependence, which is in agreement with small exchange interactions. The fit of the ESR spectrum becomes unreliable above 100 K and the spectrum is completely lost at 200 K. We find that the ESR line widths (Figure 6 (d)) and g factors (Figure 6 (c)), however, only slightly change with temperature, in agreement with the observation that the ground-state Kramers doublet is well separated from excited Kramers doublets. Furthermore, phonon-related ESR broadening mechanisms which are all characterized by profound temperature dependence [START_REF] Abragam | Electron paramagnetic resonance of transition ions[END_REF] must be negligible in this temperature range.

IV. DISCUSSION

In rare-earth magnets, the electronic state of the rareearth ion is mostly governed by the crystalline electric field at the rare-earth site and the number of electrons in the 4f shell. Depending on such electronic state, crystalline anisotropy and favorable exchange path, rareearth magnets can host distinct ground state properties [31]. In rare-earth magnets with odd number of 4f electrons, the crystal electric field usually splits the magnetic ground state of a free rare-earth ion into Kramers doublets, where 4f ions acquire pseudospin-J eff = 1/2 moments in the ground-state doublet that is protected by time-reversal symmetry [43]. In the present antiferromagnet BYTO (Yb 3+ , 4f 13 ), crystalline electric field generated by nearby O 2-ions can split the 2J + 1 = 8 degenerate ground state ( 2 F 7/2 ) into four Kramers doublets. The presence of these low-energy Kramers doublet states in BYTO has been indicated already through magnetic susceptibility measurements. At low-temperatures, the magnetic susceptibility data follow a Curie-Weiss law with effective moment µ eff = 2.51 µ B that is much less than the effective moment of free Yb 3+ ions, which suggests the formation of Kramers doublet state with spin-orbit entangled J eff = 1/2 moment of Yb 3+ ions. The estimated negative θ CW = -0.49 K from low-temperature susceptibility data indicates the presence of antiferromagnetic interaction though the strength of interaction is quite small which is typical of 4f magnets [29]. The zero-field specific heat data clearly shows an anomaly at T N = 77 mK, which is tentatively assigned to antiferromagnetic long-range magnetic order in BYTO. Nonetheless, the anomaly occurring at T N vanishes when subjected to a magnetic field with µ 0 H ≥ 1 T, and instead, a broad maximum emerges at higher temperatures. This scenario is a common occurrence in rare-earth magnets, where the influence of Zeeman splitting resulting from an external magnetic field surpasses the weak exchange interactions between rare-earth moments, leading the system towards a field-polarized state [53,[START_REF] Xiang | Dipolar spin liquid ending with quantum critical point in a gd-based triangular magnet[END_REF]. As the magnetic field strength increases, the observed broad peak in specific heat for µ 0 H ≥ 1 T broadens and shifts towards higher temperatures. This reflects the Zeeman splitting of the lowest Kramers doublet state of BYTO, a phenomenon commonly observed in rare-earth magnets [28,72]. The obtained 20 % entropy in zero-magnetic field is less than the expected entropy (Rln2) for J eff = 1/2, suggesting there could be two origins of missing entropy; one is spin fluctuations in the ordered state and another is the presence of short-range spin correlation above the transition temperature due to moderate spin frustration defined by f = |θ CW |/T N ≈ 6 [53,83]. In literature there are several reports on Yb-based triangular lattice antiferromagnets where the exchange interaction between rare-earth moments is found to be the sum of dipolar and superexchange interactions [START_REF] Xiang | Dipolar spin liquid ending with quantum critical point in a gd-based triangular magnet[END_REF]. The intraplane Yb-Yb distance in BYTO is almost double that of the intra-planar distance in YbMgGaO4, which suggests the presence of weak magnetic dipole-dipole interaction approximately 0.017 K as estimated using E dip ≈ µ 0 g 2 avg µ 2 B /4πa 3 where g avg is the powder average Landé g factor and a is the nearest-neighbor Yb-Yb distance in BYTO. The obtained dipolar interaction is only 5 % of nearest-neighbor exchange interaction as estimated from the Curie-Weiss temperature following mean-field approximation. This suggests the presence of dominant superexchange interaction between the J eff = 1/2 moments of Yb 3+ ions. Our ESR results also indicate apart from dipolar interaction, the presence of finite exchange interactions in BYTO, which however is anisotropic. In BYTO, YbO 6 octahedra are isolated instead of corner and side sharing regular octahedra as observed in YbMgGaO 4 . Thus, in BYTO, the nearest neighbor intra-plane superexchange interaction can only be possible via the f -p -d -p -f (Yb-O-Ti-O-Yb) virtual path, which is likely one of the reasons of weak antiferromagnetic interaction strength in BYTO. Whereas, in YbMgGaO 4 , the nearest neighbor superexchange interaction is directly mediated by the oxygen ion via the f -p -f (Yb-O-Yb) virtual electron hopping processes, as a result in YbMgGaO 4 , antiferromagnetic interaction is a bit stronger (≈ 2 K) [START_REF] Onoda | [END_REF]. From crystallographic parameters, it is observed that the interplanar distance in BYTO is approximately 7.26 Å and is slightly larger than the nearest-neighbor distance (∼ 5.92 Å) between rareearth moments. Furthermore, in the triangular planes, the bond distance of second nearest-neighbor of Yb 3+ ion is roughly 10.23 Å that is larger than the interplanar separation to make significant exchange coupling in BYTO. Therefore, it is most likely that in addition to intraplane nearest-neighbor exchange interactions, the presence of non-negligible interplane exchange interactions through the Ti 2 O 9 dimer lead to long-range magnetic order in this triangular lattice antiferromagnet. An antiferromagnetic phase transition due to interlayer interaction is also observed in triangular lattice antiferromagnet Ba 3 CoSb 2 O 9 (Co 2+ ; J eff = 1/2), which has similar crystallographic symmetry, intralayer, interlayer exchange paths, and interlayer distance with that of BYTO [76,99]. µSR experiments reveal a fluctuating state of Yb 3+ moments in the temperature range 0.1 K ≤ T ≤ 10 K and do not show any signature of long-range magnetic order down to 80 mK. The broadening of ESR spectrum at 5 K indicates the presence of anisotropy in the exchange interaction between the Yb 3+ spins, otherwise one would observe narrow ESR spectrum for isotropic exchange interaction [100]. We also found large g-factor anisotropy, which is compatible with the distortion of the YbO 6 octahedra from cubic symmetry. The temperature independent g factor in the broad temperature range implies the well separated lowest Kramers doublet state that is consistent with µSR results. The estimated g value from ESR spectrum at 5 K leads to powder-average value g = (g 2

x + g 2 y + g 2 z )/3 = 3.04, which is close to that obtained from magnetization data. Theoretically, it is predicted that apart from interlayer exchange interaction, the nearest-neighbor exchange interaction with either easy-plane or easy-axis anisotropy can lead to longrange ordered state in rare-earth based triangular lattice antiferromagnet [31]. The role of anisotropic exchange interaction in stabilizing long-range order state is not yet clear from the present study and will be a subject of detailed future investigations.

V. CONCLUSION

We presented the synthesis, crystal structure, thermodynamic, muon spin relaxation, and electron spin resonance results of a novel triangular lattice antiferromagnet Ba 6 Yb 2 Ti 4 O 17 , which crystallizes in the highly symmetric crystal structure with space group P 6 3 /mmc without any detectable anti-site disorder between atomic sites. The present compound constitutes close to structurally perfect two dimensional triangular layers of Yb 3+ ion perpendicular to the crystallographic c-axis. The magnetic susceptibility and electron spin resonance measurements suggest the presence of weak antiferromagnetic interaction between J eff = 1/2 moment of Yb 3+ ions. The anomaly observed at T N = 77 mK in zero-field specific heat data is attributed to the presence of long-range magnetic order which vanishes in magnetic field µ 0 H ≥ 1 T. Furthermore, the specific heat data suggest the presence of a field induced gap due to Zeeman splitting of the Kramers doublet ground state in weak magnetic field. µSR experiments reveal a fluctuating regime in 0.1 K ≤ T ≤ 10 K ascribed to the depopulation of crystal electrical field levels, in agreement with a well seprated Kramers doublet, and the enhancement of muon spin relaxation rate below 100 mK might be associated with a more correlated regime of Yb 3+ moments that is consistent with the presence of an anomaly in specific heat. Our ESR results further confirm that the lowest Kramers doublet is well separated from the excited doublet state and point to the presence of anisotropic exchange interaction between Yb 3+ spins in the present antiferromagnet. Despite the moderate spin frustration, a combination of exchange anisotropy and interlayer magnetic interaction are most likely to stabilize long-range magnetically ordered state in Ba 6 Yb 2 Ti 4 O 17 . Future experiments may shed detailed insights into the microscopic Hamiltonian, precise nature of anisotropy and spin correlations in this class of spinorbit driven J eff = 1/2 triangular lattice antiferromagnet.

FIG. 1 .

 1 FIG. 1. (a) Schematics of the unit cell of Ba6Yb2Ti4O17. The oxygen atoms coordinate with Yb 3+ ions in an octahedral arrangement (blue). Two top and bottom layers of YbO6 octahedra are connected through Ti2O9 double octahedra while two middle layers of YbO6 connected through TiO4 tetrahedra. (b) Two-dimensional triangular layers of Yb 3+ ions arranged in the unit cells. (c) The Rietveld refinement of powder x-ray diffraction data recorded at room temperature. The experimentally observed points (scattered orange points), calculated Rietveld refinement profile (solid navy line), Bragg reflection positions of BYTO (olive vertical bars), Bragg reflection position of Yb2O3 (pink bars) and the difference between observed and calculated intensity (solid blue line) are shown..

Figure 1 (

 1 c) depicts the Rietveld refinement pattern of the x-ray diffraction data indicating that our polycrystalline samples consist of 98.5 % BYTO and 1.5 % Yb 2 O 3 phases. The Rietveld refinement results indicate the compound BYTO crystallizes in the 12H hexagonal structure with space group P 6 3 /mmc without inter-site mixing between constituents ions. The obtained lattice parameters are a = b = 5.907 Å, c = 29.426 Å, α = β = 90 • and γ = 120 • . The estimated fractional atomic coordinates and R factors are summarized in table

FIG. 2 .

 2 FIG. 2. (a)The temperature dependence of magnetic susceptibility χ(T ) in two different magnetic fields. The inset shows the temperature dependence of zero-field cooled (ZFC) and field-cooled (FC) magnetic susceptibility measured in µ0H = 100 Oe. (b) The temperature dependence of inverse magnetic susceptibility (1/χ(T )). The red and orange lines are the Curie-Weiss fit to the high-temperature and low-temperature inverse magnetic susceptibility data, respectively. The inset shows the estimated Curie-Weiss temperature obtained by varying upper limit of temperature range while the constant value of Curie-Weiss temperature at low-temperature is shown by dotted orange line. (c) Magnetization as a function of external magnetic field at several temperatures. The solid lines are the Brillouin function fit for paramagnetic Yb 3+ spins with J eff = 1/2..

Figure 2 (

 2 c) displays the magnetization curve at various temperatures. As corroborated by the specific heat data discussed below, BYTO behaves like a paramagnet above 1 K, therefore one can extract the powder-averaged Landé g-factor following the relation M /M s = B 1/2 (y), where B J (y) = [ 2J+1 2J coth y(2J+1) 2J -1 2J coth y 2J ] represents the Brillouin function. Here, M is the measured magnetization, M s (= gJµ B ) is the saturation magnetization, and y = gµ B Jµ 0 H/k B T , with µ B denoting the Bohr magneton and g representing the Lande g-factor.

  Figure 3 (a). The 2.22 K anomaly arises from the long-range

FIG. 4 .

 4 FIG.4. The temperature dependence of thermal conductivity divided by temperature (κ/T ) of BYTO in zero magnetic field. The inset (a) shows κ/T versus temperature in different magnetic fields up to 7 T. The inset (b) displays κ/T versus temperature in a zero-field where the red line represents the fitted phenomenological model described in the text.

FIG. 5 .

 5 FIG. 5. (a) Evolution in temperature of the zero-field asymmetry as a function of time for BYTO. Lines are fits with a stretched exponential. (b) Evolution of the asymmetry under applied longitudinal fields (circles from ZF to 300 Oe) and comparison with the behaviour of the static Kubo-Toyabe function for corresponding magnetic fields (lines, in Oe), highlighting the dynamical behaviour of Yb 3+ moments at 0.08 K. (c) Temperature dependence of the muon spin relaxation rate λ extracted from fits to the data shown in (a).

FIG. 6 .

 6 FIG. 6. (a) The temperature evolution of the ESR spectra of BYTO. (b) Fit of the ESR spectrum at 5 K (points) with a powder-averaged Lorentzian line shape with three independent g factors (sim. 1) and two independent g factors (sim. 2), the latter assuming uniaxial symmetry of the crystal lattice. (c) The temperature dependence of the three g factor eigenvalues and (d) ESR line widths for sim 1.

TABLE I

 I

		. Refined structural parameters based on x-ray
	diffraction data at 300 K. (space group: P 63/mmc, α = β
	= 90.0 0 , γ = 120.0 0 ), a = b = 5.907 Å, c = 29.426 Å and χ 2
	= 4.22, Rwp = 5.88, Rp = 3.39, and Rexp = 2.86)
	Atom Wyckoff position	x	y	z	Occ.
	Yb	4e	0	0	0.126	1
	Ba1	2a	0	0	0	1
	Ba2	24l	0.666 0.333 0.089	1
	Ba3	24l	0.333 0.666 0.182	1
	Ba4	2b	0	0	0.25	1
	Ti1	12k	0.666 0.333 -0.055 1
	Ti2	12k	0.666 0.333 0.203	1
	O1	12k	0.666 0.333 -0.006 1
	O2	12k	-0.347 -0.173 0.077	1
	O3	12k	0.346 0.173 -0.170 1
	O4	12j	0.573 0.014 0.25	1

TABLE II .

 II Some promising rare-earth based frustrated triangular lattice antiferromagnets and their ground-state mag-

	netic properties.
		.
	Materials	Exchange
	(symmetry)	path

ACKNOWLEDGMENTS P.K. acknowledges funding by the Science and Engineering Research Board and Department of Science and Technology, India through research grants. A.Z. acknowledges the financial support of the Slovenian Research and Innovation Agency through Program No. P1-0125 and Projects No. J1-50008 and No. BI-US/22-24-065. E.K. acknowledges financial support from the labex PALM for the QuantumPyroMan project (ANR-10-LABX-0039-PALM). A.M.S. thanks the URC/FRC of UJ and the SANational Research Foundation (93549) for financial support. The National High Magnetic Field Laboratoryis supported by National Science Foundation throughNSF/DMR-1644779 and the State of Florida.


K Takahashi
email: kohki.takahashi.e5@tohoku.ac.jp

A Badel

A Zampa

T Okada

S Awaji

G Nishijima

T Uto

H Takewa

S Hanai

S Ioka

J Inagaki

A Takahashi

T Zampa

S Okada

Awaji

Performance Test of 20-Stacked Two-Tapes-Bundled REBCO

Keywords: Axial strain, electromagnetic stress, high field superconducting magnet, hoop strain, REBCO coil

I. INTRODUCTION

25 T CRYOGEN-FREE superconducting magnet (25T-CSM) developed at the High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research, Tohoku University [START_REF] Awaji | First performance test of a 25 T cryogen-free superconducting magnet[END_REF] has been successfully operated more than six years; it has been subjected to various kinds of experiments as a user magnet. The 25T-CSM consists of CuNb/Nb3Sn and NbTi outsert coils and Bi2Sr2Ca2Cu3Oy (Bi2223) insert pancake coils. Now, an upgrade plan to a 30T CSM by replacing the Bi2223 insert coils with REBa2Cu3Oy (REBCO, RE: rare earth and yttrium) coil was underway [START_REF] Awaji | Robust REBCO insert coil for upgrade of 25 T cryogenfree superconducting magnet[END_REF], since REBCO tapes have high in-field critical current density as well as great mechanical properties [START_REF] Barth | Electro-mechanical properties of REBCO coated conductors from various industrial manufactures at 77 K, self-field and 4.2 K, 19 T[END_REF], [START_REF] Fujita | Characteristics of REBCO coated conductors for 25 T cryogen-free superconducting magnet[END_REF]. Meanwhile, a new 33T-CSM project was launched at the HFLSM in 2022 [START_REF] Awaji | 33T cryogen-free superconducting magnet project at HFLSM[END_REF]. The 33T-CSM will consist of newly-designed 14 T outsert coils wound from both CuNb/Nb3Sn and NbTi Rutherford cables, and a 19 T REBCO insert coil. In any case, REBCO coil technology plays an important role in the success of the magnet development projects.

Coil technologies using REBCO tapes can be broadly classified into two types: layer-winding and pancake-winding methods. In layer-wound REBCO coils, insert coils for GHzclass NMR magnets have been developed and put into practical use [START_REF] Wikus | Commercial gigahertz-class NMR magnets[END_REF], [START_REF] Yanagisawa | Review of recent developments in ultra-high field (UHF) NMR magnets in the Asia region[END_REF]. As for pancake coils, NHMFL reached 32 T with an all-superconducting magnet in 2017 [START_REF]32 Tesla all-superconducting magnet[END_REF], and IEECAS successfully generated 32.35 T as well in 2019 [START_REF] Liu | Word record 32.35 tesla direct-current magnetic field generated with an all-superconducting magnet[END_REF]. In terms of winding technology of REBCO pancake coil, no-insulation technique has attracted much attention from the viewpoint of quench protection. Since Hahn et al. demonstrated the NI coils [START_REF] Hahn | HTS pancake coils without turn-to-turn insulation[END_REF], a partial-insulation coil [START_REF] Choi | Partial insulation of GdBCO single pancake coils for protection-free HTS power applications[END_REF], a metal-as-insulation coil [START_REF] Lécrevisse | A (RE)BCO pancake winding with metalasinsulation[END_REF] and a pancake coil using an electrically conductive epoxy resin [START_REF] Miyazaki | Over-current test of REBCO pancake coils impregnated with electrically conductive epoxy resin under conductioncooled conditions[END_REF] have been proposed. On the other hand, our group has suggested a robust coil concept, in which two REBCO tapes are co-wound and fluorine-coated polyimide tape is used as an inter-bundle insulation as described later. To date, we have investigated mechanical properties of the robust coils with subscale double pancake REBCO coils and full-scale four-stacked REBCO coils and have shown the structure has an advantage with respect to the electromagnetic stress [START_REF] Takahashi | Electromagnetic characteristics study of two-ply REBCO tapes pancake coils[END_REF], [START_REF] Takahashi | Mechanical properties of four-stacked two tape bundled REBCO pancake coils[END_REF]. Furthermore, two-tape bundled coil have been shown to be robust against a local degradation in the tape and thermal runaway [START_REF] Abe | REBCO coil with robust behavior against local defects wound using two-tape bundle[END_REF], [START_REF] Badel | Detection and protection against quench/local thermal runaway for a 30 T cryogen-free magnet[END_REF]. In this study, to further validate our coil design for the 30T-CSM, we investigate electromagnetic and current-voltage properties of 20 stacked REBCO pancake coils with the robust coil concept under large electromagnetic stress. The20-stacked coils were designed and fabricated with the expectation that they will actually be installed into the 25T-CSM by adding required number of coils for generating the target magnetic field after this test.

A TABLE I

II.

EXPERIMENTAL

The 20-stacked coils consist of 10 double pancake (DP) coils which are stacks of single pancake (SP) coils electrically connected at their inner copper electrode with a silver plate. Adjacent DP coils are connected at the outermost winding as descried later. Each SP coil was wound by the PLD processed EuBCO tape with artificial pinning centers (Fujikura FESCSCH04 (40)) [START_REF] Fujita | Mechanical properties of BaHfO3-doped EuBCO coated conductors fabricated by hot-wall PLD on IBAD template[END_REF], [START_REF]Flux-pinningpropertiesofBaHfO3-dopedEuBCO-coated conductors fabricated by hot-wall PLD[END_REF]. The specifications of the EuBCO tape are listed in Table I. In all the coils, two EuBCO tapes are co-wound in a face-to-back configuration with the superconductor (SC) layer inside and at the start of winding, the SC layer side of both the inner tape and the outer tape (with the end shifted) are soldered to the inner electrode. The coils were wound with a winding tension of 5.5 kgf (90 MPa) for each EuBCO tape. A fluorine-coated polyimide tape is used as the inter-bundle insulation. Thin FRP plates were glued onto the edges of each coil with epoxy resin, which is one of the features of our robust coilconcept.Aluminumsheets,dividedradiallyinto12sections, are sandwiched in each DP coils for conduction cooling as in the case of the four-stacked coils [START_REF] Takahashi | Mechanical properties of four-stacked two tape bundled REBCO pancake coils[END_REF]. For all SP coils, the critical current, Ic and n-value were evaluated in liquid nitrogen and were 121-174 A and 22-27, respectively. The coil specifications are listed in Table II. The SP coils are arranged so that its Ic is higher toward the top and bottom edges than at the mid plane, considering magnetic field angle dependence of Ic of the EuBCO tape.

To measure hoop and axial strains, two sets of strain gauges were attached to the outer surface of the winding of SP#10 that is 10th coil from the top as shown in Fig. 1. One set consists of two strain gauges (one for hoop strain, SG160h and the other for axial strain, SG160a) attached orthogonally, with another set on the opposite side circumferentially (SG340h and SG340a). The numbers of 160 and 340 represent the azimuth angle of the coil viewed from top as the direction of the electrodes is 90°. A data logger UCAM-60B (Kyowa Electronic Instruments Co., Ltd.) was used for the strain measurements.

Hoop stress tests of the coils were performed using a 14 T large bore superconducting magnet at National Institute for Materials Science. The coils were immersed in liquid helium bath and an operating current was supplied by a DC current source (up to 300 A) under a background field of 14 T.

III. RESULTS AND DISCUSSION

The operating current-voltage (I-V) properties of the coils are shown in Fig. 2. Fig. 2(a) shows the I-V properties of SP and DP coils, and Fig. 2(b) shows to the voltage across the overall coils. Coils are numbered from the top, with DP#1 consisting of SP#1 and SP#2,DP#2 of SP#3 and SP#4, etc. A balance voltage, which is the voltage difference between the upper and lower 10 SP coils, is also monitored as shown in Fig. 2(c). An inductive voltage of approximately 0.025 V for each SP coil and 0.05 V for each DP coil are observed at a ramping rate of 0.18 A/s, respectively in Fig. 2(a). The 20-stacked coils have totally nine outer electrical joints between adjacent DP coils. At the joint, an additional EuBCO tape with the SC layer outside is soldered to the SC layer side of the inner tape and the outer tape which is longer than the inner one. The SC layer of both tapes in the bundle are soldered to the SC layer of the additional tape in a face-to-face configuration. Then, a silver plate (96 mm long, 8 mm wide, and 1 mm thick) is soldered to make a current path with covering the SC outside sections of the upper and lower DP coils. The resistance of each joint was measured in liquid nitrogen and ranged from 0.09 to 0.25 µΩ. There was a concern about damage due to electromagnetic forces on the joint and the resulting increased resistance. Fig. 2(b) shows that no dissipative voltage appeared without the joint resistance, which was totally less than a few µΩ from inner joints as well as outer ones. No resistive or anomalous voltages, although relatively noisy, were observed also in the balance voltage. The spike noises appearing in the coil voltages are probably due to slight wire movement in the coil. Those reduced drastically in ramping down from 300 A. No noises were observed in measurements up to 300 A at zero background field. These I-V properties indicate that there is no serious problem in the windings and connections of the coils. The magnetic field measured by a Hall probe at the center of the coils is plotted against the operating current in Fig. 3(a). The magnetic field error, which is a difference between the measured data and the fit line is shown in Fig. 3(b). A clear hysteresis for the operating current is observed, which is well-known behavior as an effect of the screening current in superconducting tapes [START_REF] Yanagisawa | Operation of a 400 MHz NMR magnet using a (RE:Rare Earth)Ba2Cu3O7-x high temperature superconducting coil: Towards an ultra-compact super-high field NMR spectrometer operated beyond 1 GHz[END_REF]. In addition to the overall hysteresis, relaxations of the magnetic field as indicated by vertical arrows are observed every 50 A where a plateau of the operating current was made. This behavior is due to the effect of coupling current flowing between the two bundled tapes [START_REF] Yokoyama | AC losses and induced fields in HTS coil wound using two-ply coated conductors[END_REF], [START_REF] Kajikawa | AC loss measurements in an HTS coil wound using two-ply bundle conductor[END_REF]. The axial magnetic field generated by the coupling current is in the same direction of the magnetic field of the coils. Therefore, when the current sweep is stopped, the coupling current decays with relatively small time constant, and the total axial field decreases during ramping up and increases during ramping down the operating current. 4 shows hoop strains obtained from strain gauges on the SP#10. Although the data is noisy, the strains show almost the same trend, and approximately 0.23% at 298 A. The stress/strain distribution in the coil calculated by the BJR relation and FEM analysis are shown in Fig. 5, where B, J and R are a magnetic flux density, current density and coil radius, respectively. The measured hoop strain is in good agreement with the FEM analysis taking into account the thin FRP plates glued on the edges of the coil [START_REF]Validation of robust REBCO insert concept for 30T+high field magnets on a 20-pancake quasi full-size prototype[END_REF]. According to the FEM calculation, the maximum hoop stress is approximately 330 MPa, which is smaller than that estimated by the BJR relation. This result is consistent with those we have reported previously [START_REF] Takahashi | Electromagnetic characteristics study of two-ply REBCO tapes pancake coils[END_REF], [START_REF] Takahashi | Mechanical properties of four-stacked two tape bundled REBCO pancake coils[END_REF], suggesting one of the advantages of the robust concept.

Fig. 6 shows axial strains measured on the SP#10 coil in the axial midplane of the 20-stacked coils. The axial strain at 298 A, averaged over the two gauges, is approximately -0.15%, being compressive strain. Assuming simple compressive deformation, i.e. the coil is compressed uniformly, the axial stress is estimated to be 195 MPa from the strain with a Young's modulus of 130 GPa for the EuBCO tape. This stress value is larger than the compressive stress estimated in a comparable-sized magnet. This discrepancy in stress estimations indicates that the axial deformation is not simple, especially at the outermost winding of the coil, where bending of the tape may be also taken into account for the strain analysis.

IV. CONCLUSION

We investigated the electromagnetic and current-voltage properties of 20-stacked REBCO pancake coils with the robust coil concept under a background field of 14 T. The coil could be operated with no degradation under an electromagnetic stress up to 330 MPa estimated by the FEM analysis considering the robust coil concept. The obtained strain at the outer surface of the coil (SP#10) at the midplane of the 20stacked coils, is approximately 0.23%, which is in good agreement to the FEM analysis. It suggests that the coil structure with the robust concept can reduce the maximum stress due to the optimization of stress distribution in the coil. Although the operating current was limited up to 300 A in this case due to the power supply, total magnetic field of 25 T could be generated, and higher magnetic fields are expected to be generated because no serious problems were observed in the current-voltage behavior. This is a benefit of the robust coil concept, and we plan to perform further experiments with this coil, including under a conduction cooling condition.

Fig. 1 .

 1 Fig. 1. Photograph of 20-stacked coils and schematic illustration of strain gauge arrangement. Strain gauges of hoop strain (SG160h and SG340h) are attachedtothecenterofthetapewidth,andforaxialstrain(SG160aandSG340a) are slightly offset to the lower side. Active length of the gauge is 0.2 mm.

Fig. 2 .

 2 Fig. 2. I-V properties of (a) SP and DP coils in 20-stacked coils, and of (b) the overall coils. Coils are numbered from top to bottom. (c) Balance voltage of the 20-stacked coils as a function of the operating current.

Fig. 3 .

 3 Fig. 3. (a) Generated magnetic field measured by a Hall probe at the center of 20-stacked coils under a background field of 14 T. (b) Difference between the measured value and the fitted line as a function of the operating current.

Fig. 4 .

 4 Fig. 4. Hoop strains of SP#10 measured by strain gauges as a function of the operating current

Fig.

  Fig.4shows hoop strains obtained from strain gauges on the SP#10. Although the data is noisy, the strains show almost the same trend, and approximately 0.23% at 298 A. The stress/strain distribution in the coil calculated by the BJR relation and FEM analysis are shown in Fig.5, where B, J and R are a magnetic flux density, current density and coil radius, respectively. The measured hoop strain is in good agreement with the FEM analysis taking into account the thin FRP plates glued on the edges of the coil[START_REF]Validation of robust REBCO insert concept for 30T+high field magnets on a 20-pancake quasi full-size prototype[END_REF].

Fig. 5 .

 5 Fig. 5. Stress distribution in SP#10 at an operating current 298 A, calculated by the BJR relation and the FEM analysis considering the robust coil concept. Open circle with an error bar represents averaged measured strain.

Fig. 6 .

 6 Fig. 6. Axial strains of SP#10 measured by strain gauges as a function of the operating current.

ACKNOWLEDGMENT

The authors would like to thank Dr. S. Nimori of Cryogenic Station, National Institute for Materials Science and K. Fushimi of High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University for their technical assistance with the experiments.

This work was supported by JSPS KAKENHI under Grant 18H05248.