Guillaume Cabanac
email: guillaume.cabanac@univ-tlse3.fr

What is the primordial reference for . . . ? -Redux

Keywords: Primordial reference chasing, Publish or Perish, Eugene Garfield

Eugene Garfield's quest of the primordial reference for the familiar and ubiquitous phrase 'Publish or Perish' led him to a 1942 monograph (The Scientist, 1996, 10(12)). This quest is resumed two decades later here. Text mining applied to a sample of the mainstream and academic literature ever published, as well as crowdsourcing, yielded earlier references dating from 1934 and 1927. This search experiment suggests that 'primordial reference chasing' in full-text corpora remains an open problem for the community intersecting bibliometrics and information retrieval. Addressing it has the potential to rejuvenate Garfield's work on historio-bibliography to improve our understanding of the genesis and diffusion of ideas, concepts, and associated metaphors.

Introduction

Academics are expected and encouraged to cite all their influences, but they rarely do so (M. H. [START_REF] Macroberts | Quantitative measures of communication in science: A study of the formal level[END_REF][START_REF] Harzing | The Publish or Perish book: Your guide to effective and responsible citation analysis[END_REF]. Sometimes one chooses to select the most relevant references to his/her work; sometimes the primordial reference for a phrase or idea is nowhere to be found. Hedging by surrounding an expression with quotes is one way of dealing with this issue. These quotes act as a signalling device for the reader: the quoted phrase was coined elsewhere and it would be misleading to think it is genuine here.

Eugene Garfield had long demonstrated his enduring interest for primordial concepts and historio-bibliography [START_REF] Garfield | Primordial concepts, citation indexing, and historio-bibliography[END_REF] when he embarked on the quest of the primordial reference for the phrase 'Publish or Perish'. . .

Garfield's take on the primordial reference problem in 1996

An academic asked Garfield if he knew of the original reference for the familiar phrase 'Publish or Perish.' In a witty commentary published in The Scientist, [START_REF] Garfield | What is the primordial reference for the phrase 'Publish or Perish'?[END_REF] recounts his attempts to chase this ubiquitous expression. He had recourse to various strategies involving a variety of information sources. He did a literature search, searched dictionaries (both in print and online, he notes), and several databases. A Stanford librarian even joined forces in vain to post his query on Internet newsgroups and electronic bulletin boards.

Crowdsourcing appeared to be more effective: word of mouth spread through Garfield's invisible college and a Yale librarian and editor of the Oxford Dictionary of American Legal Quotations dug up an occurrence of 'Publish or Perish' in a 1951 letter (Molarino, McLuhan, & Toye, 1987, p. 226). Later on, he found an earlier occurrence (Wilson, 1942, p. 197). Noting that Wilson was a sociologist, Garfield reached to Robert K. Merton, who had taught Wilson at Harvard University. Merton suggested that 'Publish or Perish' was an expression in the air in pre-war academe, but no earlier account of it was provided.

Contemporary authors of Garfield's chase endeavour (e.g., Tenopir, 1995, p. 575) were still quoting 'Publish or Perish' with no reference to its primordial source. Note that all of these authors (i.e., of the 1951 letter, 1942 book, and 1995 paper) cared to word the expression surrounded by quotes, as if to warn readers that they were not coining it and that they had failed to find the primordial reference for it.

In closing his commentary, Garfield (1996, p. 11) tossed a bottle into the ocean: "Perhaps one of The Scientist's readers can identify an earlier source for this common phrase." This is the challenge I have taken up two decades later using modern computing and networking capabilities.

Opportunities of text mining on scholarly materials

Nowadays various online services can help one track primordial references. For instance, Google Books digitised, OCRised, and tokenised the 8 million books fuelling the Ngram Viewer search interface. This amounts to 6% of all books ever published as of 2012 [START_REF] Michel | Quantitative analysis of culture using millions of digitized books[END_REF][START_REF] Lin | Syntactic annotations for the Google Books Ngram corpus[END_REF], that is a corpus including but not limited to scholarly books. Figure 1 suggests that the phrase 'Publish or Perish' appeared in print in this corpus around the late 1940s, which is in line with the findings reported in [START_REF] Garfield | What is the primordial reference for the phrase 'Publish or Perish'?[END_REF]. This expression gained popularity in the 1960s, peaked and plateaued in the 1970s, only to slowly decrease during the next decades.

Search engines focused on scholarly materials offer an other option to chase expressions appearing in the full-text of academic papers, whereas leading citation indexes are restricted to title, abstract, and keywords fields. Google Scholar, introduced a decade ago [START_REF] Giles | Science in the web age: Start your engines[END_REF], is now an established and widely used academic search engine [START_REF] Jacso | As we my search -Comparison of major features of the Web of Science, Scopus, and Google Scholar citation-based and citation-enhanced databases[END_REF][START_REF] Harzing | Google Scholar, Scopus and the Web of Science: A longitudinal and cross-disciplinary comparison[END_REF]. Unfortunately, searching for 'Publish or Perish' and sorting results by date of publication retrieves papers added in the last year only, which has close to no use for primordial reference chasing.

The JSTOR digital library developed Data for Research 1 [START_REF] Burns | JSTOR -Data for Research[END_REF] which now indexes the full-text of 9 million papers dated between 1545 and 2014. The recent studies by [START_REF] Mccain | Assessing obliteration by incorporation in a full-text database: JS-TOR, Economics, and the concept of "bounded rationality[END_REF][START_REF] Mccain | Mining full-text journal articles to assess obliteration by incorporation: Herbert A. Simon's concepts of bounded rationality and satisficing in economics, management, and psychology[END_REF] relied on this service to track down obliterations by incorporation (see OBI in Merton, 1988, p. 622), namely expressions such as "bounded reality" or "Nash equilibrium" appearing in the body of research papers with no (more) associated references in the bibliography.

Data for Research allowed me to further Garfield's quest of the primordial reference for 'Publish or Perish.' The oldest occurrence in this corpus appears in an obituary by Bowman (1934, p. 180), who recounted that the late Professor Davis urged the Association of American Geographers to 'Publish or Perish,' back in a 1904 meeting. The longitudinal data shown in Fig. 2 correlate earlier observations made on mainstream books (Fig. 1) for the rise in popularity of this expression in the 1960s. Then, it seems that 'Publish or Perish' stuck in academics' minds and texts. The sudden surge in popularity just before 2010 might reflect the release and massive uptake of the Publish or Perish software2 [START_REF] Harzing | The Publish or Perish book: Your guide to effective and responsible citation analysis[END_REF] that has been developed since 2006. In addition, the subject group breakdown suggests that the expression appears the most in articles classified in the Social Sciences.

Reflecting on Garfield's recourse to crowdsourcing, I turned to Wikipedia and, to my utmost surprise, found there an even older reference to a 1927 journal article of sociology added3 on August 18, 2016 with a note about the source: 'via Google Books' hyperlinked to a 5-line snippet including the famous phrase.4 I requested a copy from a university library in Strasbourg and updated the incomplete Wikipedia reference, adding the page range for (Case, 1927(Case, -1928) that states on page 325: . . . the system of promotion used in our universities amounts to the warning, "Publish or perish!" In the second place, publication in general . . . It is striking how this ninety-year-old critique sounds like many contemporary rants overheard in academe nowadays. Plus ça change, plus c'est la même chose. Plagiarising Garfield, I dare to hope that perhaps one the readers of Scientometrics can identify an earlier source for this common phrase.

Towards search methods for primordial reference chasing

These anecdotal accounts of Garfield's chase for a primordial reference back in 1996, and my own two decades later, suggest a challenging research issue for the researchers in- volved at the crossroads between bibliometrics and information retrieval. Tackling the 'primordial reference chasing' problem would contribute to tighten up the ties between these two communities of information scientists that have somewhat drifted apart from each other (see White & McCain, 1998, p. 345). Initially launched to foster re-connections, the Bibliometric-enhanced Information Retrieval workshop series seek to address such challenging issues requiring competence in both domains [START_REF] Mayr | Report on the 5th International Workshop on Bibliometric-enhanced Information Retrieval (BIR 2017)[END_REF]).

Garfield's pioneering work on historio-bibliography [START_REF] Garfield | Primordial concepts, citation indexing, and historio-bibliography[END_REF] led to the development of the HistCite TM software [START_REF] Garfield | Algorithmic citation-linked historiographymapping the literature of science[END_REF] allowing one to reconstitute the genealogy of papers chained by citations. With literature-based discovery as a framework [START_REF] Bruza | Literature-based discovery[END_REF], maybe time is ripe to rejuvenate and extend this legacy by moving from the citation level to the full-text level.

The following sections provide examples of variations in the primordial reference chasing problem that one might need to address.

Primordial reference for a phrase

Seeking the primordial reference for a phrase in a corpus of academic materials can be as simple as an exact string search. Take for instance the 'invisible college' [START_REF] Price | Collaboration in an invisible college[END_REF] and the 'Matthew Effect' [START_REF] Merton | The Matthew Effect in Science: The reward and communication systems of science are considered[END_REF]. 5 Trouble arises when we are looking for a phrase used as a metaphor: it is ubiquitous but, in a certain context, it was used for the first time to describe a phenomenon or a concept. Let us consider here the use of 'gatekeepers' by [START_REF] Crane | The gatekeepers of science: Some factors affecting the selection of articles for scientific journals[END_REF] and the 'Sleeping Beauties' of [START_REF] Van Raan | Sleeping Beauties in science[END_REF]. Other phrases such as 'salami slicing' might even be more tricky to track in the literature -here the search engine should not return papers from the Meat Science journal if we are considering bibliometrics as the search context.

A challenging extension of this search task consists in retrieving the derivatives of a given phrase. Figure 1 shows the weak uptake of the ironic derivative 'Publish and Perish' whose eponymous primordial reference appeared in an insightful journal article [START_REF] Hurt | Publish and perish[END_REF] according to JSTOR's Data for Research. Another derivative recently coined by Ye and Bornmann (forthcoming) is 'Smart Girls,' in contrast to van Raan's 'Sleeping Beauties' to qualify instant versus delayed recognition of research works.

Primordial reference for an eponym Stigler (1980, p. 148) concisely summarised two empirical observations about eponyms: "First, names are not given to scientific discoveries by historians of science or even by individual scientists, but by the community of practicing scientists (most of whom have no special historical expertise). Second, names are rarely given, and never generally accepted unless the namer (or accepter of the name) is remote in time or place (or both) from the scientist being honored."

Here a search task might be to: find the primordial reference coining a given eponym. For instance, who started calling a particular disease 'Alzeihmer's,' and in which paper? The interested reader is referred to [START_REF] Cabanac | Extracting and quantifying eponyms in full-text articles[END_REF](Cabanac, , p. 1637) ) for a chase of 'Hirsch's index' [START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF], an interesting case of an eponym coined and popularised without spatiotemporal distance. Derivative seeking is also an open problem: take the Northern and Eastern blotting techniques named after the eponym 'Southern blotting' in molecular biology [START_REF] Thomas | The development of eponymy: A case study of the Southern blot[END_REF]).

Stigler's Law of Eponymy states that "No scientific discovery is named after its original discoverer" (Stigler, 1980, p. 147), but there is an earlier declaration of this principle in (Kennedy, 1972, p. 67). A true challenge here lies in the designing of a method to identify links between an established eponym and the same discovery made earlier in order to revise questionable or misleading attributions and inform the historians of science.

Primordial reference for a concept and re-discoveries

A third even more challenging search task consists in finding the references that could have influenced a given discovery or concept, provided that its author had known about a specific, earlier, reference. This relates to re-discoveries, like in the case of [START_REF] Edwards | System to rank scientists was pedalled by Jeffreys [Correspondence[END_REF] sending a letter to the editor of Nature to stress that Sir Harold Jeffreys had told him, decades ago, that he conceived an indicator to measure his cycling prowess -this indicator being computed the same way as the h-index [START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF].

By text mining the digitised literature, an effective algorithm could perhaps trace the genesis of the Open Access movement [START_REF] Tennant | The academic, economic and societal impacts of Open Access: An evidence-based review [version 3; referees: 4 approved, 1 approved with reservations[END_REF] back to Merton (1942, p. 122):

"The institutional conception of science as part of the public domain is linked with the imperative for communication of findings. Secrecy is the antithesis of this norm; full an open communication its enactment. The pressure for diffusion of results is reenforced by the institutional goal of 'advancing the boundaries of knowledge' and by the incentive of recognition which is, of course, contingent upon publication."

In this passage about the norm of 'Communism' in the Ethos of Science, let us note the colloquialism 'advancing the boundaries of knowledge' and the quotes surrounding it, as if the author wished to refer to some source he couldn't establish. . . (1959, p. 461) coined the concept of 'Unified Index to Science' that he defined as "a single inter-disciplinary index to all documents, primarily periodical literature in all fields of science." Today and according to recent estimates, this corpus would include the 2.5 million articles published each year in about 30.000 peer-reviewed journals (Ware & Mabe, 2015, p. 27) that are still active among the 300.000 journals ever recorded (Kaplan, Killough, & Thomas, 2012, p. 152). Conference proceedings, books, and grey literature also contribute to the mass of scholarly documents produced by the 7.8 million scientists worldwide (Soete, Schneegans, Eröcal, Angathevar, & Rasiah, 2015, p. 32). Some online services seem to have been inspired by this vision of a 'Unified Index to Science,' providing search capabilities to scholars searching the literature and seeking primordial references like 'Publish or Perish. ' Khabsa and Giles (2014, p. 3) estimated that Google Scholar was indexing the full-text of about 100 million scholarly documents. And yet, one may wonder how many primordial references are still not digitized, not indexed, and not yet accessible online. . .

Concluding remarks

Garfield

Fig. 1

 1 Fig.1Result of the Ngram Viewer for query ["publish or perish", "publish and perish"] on the English corpus of Google Books comprising 8 million books (see http://bit.ly/pop-ngram).

Fig. 2

 2 Fig. 2 Results of Data for Research for query ["publish or perish"] on the JSTOR DFR corpus of 9 million academic papers (see http://bit.ly/pop-jstor).

https://harzing.com/resources/publish-or-perish

https://en.wikipedia.org/w/index.php?title=Publish_or_perish&diff=prev&oldid=735106196

https://books.google.co.uk/books?id=qBAZAAAAIAAJ&q=%22publish+or+perish%22

Disclaimer: the references provided here are not necessarily primordial references.


Charles Paillard
email: charles.paillard@centralesupelec.fr

Engin Torun

Ludger Wirtz

Jorge Íñiguez

Laurent Bellaiche

Photo-induced phase transitions in ferroelectrics

Ferroic materials naturally exhibit a rich number of functionalities, which often arise from thermally, chemically or mechanically induced symmetry breakings or phase transitions. Based on Density Functional Calculations, we demonstrate here that light can drive phase transitions as well in ferroelectric materials such as the perovskite oxides lead titanate and barium titanate. Phonon analysis and total energy calculations reveal that the polarization tends to vanish under illumination, to favor the emergence of non-polar phases, potentially antiferroelectric, and exhibiting a tilt of the oxygen octahedra. Strategies to tailor photo-induced phases based on phonon instabilities in the electronic ground state are also discussed.

Ferroelectric materials possess a spontaneous electric polarization that is switchable by electric fields. Active the Supplemental Material [31] and related discussion).

research in bandgap engineering [1] and unconventional photovoltaic mechanisms [START_REF] Sturman | The Photovoltaic and 349 Photorefractive Effects in Noncentrosymmetric Materi-350 als[END_REF][START_REF] Tan | [END_REF][4] have prompted perovskite oxide ferroelectrics as potential light absorbers to convert light into electric [5], mechanical [6][7][8] or chemical energy [9], or for optical memory writing [10] and non-destructive reading [11]. Numerous works focused on the photo-induced deformation of the unit cell without apparent symmetry breaking [8]. However, a recent experimental work hinted at possible structural phase transitions occurring in ferroelectric BaTiO 3 nanowires under illumination [12]. Using first-principles calculations, we show that photo-induced phase transitions are indeed possible in ferroelectrics such as barium and lead titanates. In both cases, transitions to non-polar phases are predicted. Furthermore, material engineering strategies based on competing lattice instabilities are suggested to tailor the critical concentration of photo-induced carriers at which the transition occurs, thus opening the way to a rational design of materials exhibiting specific properties under illumination. ABO 3 perovskite oxides are versatile materials exhibiting a wide range of properties (magnetism, polar distortions, etc.). Their high symmetry reference structure (see Fig. 1) is cubic with the B cation enclosed in an octahedral cage of oxygens at the center, and the A cation at the edges of the cube. Simple atomic displacement patterns -e.g. as the octahedra tilts or electric dipoles sketched in Fig. 1 -make it possible to obtain various symmetry breakings and control the materials properties. For convenience, here we denote the most relevant of such distortions using a generalized and simplified Glazer notation [13] αβγ/u α v β w γ , which is introduced in the caption of Fig. 1.

Among perovskite oxides, barium titanate (BTO) and 47 lead titanate (PTO) are prototypical ferroelectric mate-48 rials. They exhibit a number of phases [14,15] associated 49 with lattice instabilities of the reference cubic phase [16].

50

The transitions between phases are commonly driven by 51 temperature [14,15], pressure [17][18][19] or static electric 52 fields [20]. Here, we demonstrate that light can also act

69

We performed Density Functional Theory (DFT) cal-70 culations using the Abinit package software [23], with 71 the Projector Augmented Wave (PAW) method [24,25].

72

We employ a 2 × 2 × 2 supercell (with respect to the 5-73 atom pseudo-cubic perovskite cell) containing 40 atoms.

74

The k-point mesh used was a Γ-centered 8 × 8 × 8 grid; 75 the plane-wave cut-off was 35 Ha. A Fermi-Dirac distri-76 bution with 0.1 eV smearing was used to populate the 77 electronic states. The electronic density was considered 78 converged when the difference in the calculated forces be-79 tween two Self-Consistent Field (SCF) steps was smaller 80 than 10 -8 Ha.Bohr -1 . The structural relaxation was 000/000 000/00w + 00+/000 00-/000 000/u + u + 0 - p o la r d is p la c e m e n ts T il ts FIG. 1. Sketch of a 2×2×2 supercell in the paraelectric cubic reference structure 000/000. A and B cations are in black and blue, oxygens in red. The symmetry of a phase is denoted by (i) its oxygen octahedra tilt pattern αβγ, where α, β and γ are either 0 (no tilt), "+" (in-phase tilt) or "-" (antiphase tilt) and represent whether the oxygen octahedra tilts around the first, second and third pseudocubic axis, respectively. Similarly, we denote the electric dipole pattern using a Glazer-like notation u α v β w γ , in which u, v, w represent the magnitude of the dipole inside the perovskite 5-atom cell, and α , β and γ describes whether dipole vectors [uvw] are aligned parallel ("+") or antiparallel ("-") when moving by one perovskite 5-atom cell along the first, second and third pseudocubic direction. Typical polar (000/00w + ), antipolar (000/u + u + 0 -), in-phase (00c + /000) and antiphase tilt (00c -/000) displacement patterns are depicted using arrows.

stopped when all the forces on the atoms were smaller Field (SCF) iteration, the following system of equations

95 n h = i≤Nv k,σ w k [1 -f (ε ik,σ , µ h )], (1)
n e = i>Nv k,σ w k f (ε ik,σ , µ e ), (2)
is solved for µ e and µ h using a bisection algorithm.

96

In the above equation, w k is the weight of k-point k,

Γ X R M Γ R X M Γ 0 10 20 hω (THz) Γ X R M Γ R X M Γ 0 10 20 hω (THz) Γ X R M Γ R X M Γ 0 10 20 hω (THz) Γ X R M Γ R X M Γ 0 10 20 hω (THz)
BTO n ph (e/f.u.) The polar Γ dominant instability in BTO becomes stabilized (i.e. positive and real frequency) for n ph ≥ 0.10 e/f.u., as shown in (c) and (d). In PTO, the polar instability at Γ is replaced by a non-polar tilted instability at M and R.

Γ X R M Γ R X M Γ -

Material [31]

). We note that such a critical concentration would match recent observations of a monoclinic phase in BTO nanowires under illumination [12,33].

PTO exhibits a single transition from the non-polar cubic 000/000 phase to the ferroelectric tetragonal 000/00w + structure below 765 K [15]. The phonon dis- The ferroelectric 000/00w + phase, which is the ground state in dark conditions, is the black dashed baseline, while the cubic phase is plotted as green squares. We also plot a rhombohedral polar ---/u + u + u + structure (navy circles) and its non-polar counterpart, a ---/000 (navy diamonds). We consider as well an orthorhombic phase (violet triangles) with the --0/u -ū-0 -antipolar displacement pattern [34]. A 00c -/000 non-polar tetragonal phase is also calculated (gold triangles). According to

persion

212

The large concentration of 0.125 e/f.u. needed to

E -E ---/u + u + u + (meV/f.u.) 000/000 ---/000 --0/u -ū-0 - 000/0 + 0 + w + 00 -/000 (a) (b) (c) FIG. 3. (a)
Energy of different phases in BTO with respect to the 000/u + u + u + structure (dashed dark line), which is the ground state in the dark. The tetragonal 000/00w + and then the cubic 000 phase become more energetically favorable. (b) Energy of different phases in PTO with respect to the 000/00w + structure (dashed dark line), which is the ground state in the dark, showing that non-polar antiferrodistortive phases become more stable at large concentration of photo-excited carriers. (c) Energy of different phases in PSTO with respect to the rhombohedral ---/u + u + u + structure (dashed dark line), which is the lowest energy phase considered when no carriers are excited.

231

From this work, it also becomes apparent that the fer-232 roelectric instability can be hampered by the photogen-233 eration of free carriers. Obviously, free carriers screen the 234 long-range Coulombic interaction responsible for the po-235 lar order [37,[START_REF] Wang | Tsym-457 bal[END_REF], thus seemingly disfavoring polar phases 236 with respect to non-polar ones. This is consistent with 237 calculations [37,[START_REF] Wang | Tsym-457 bal[END_REF] and experiments [42] performed by directions [38]. From the evidence presented here, as well 262 as in other works involving doping and photoexcited cal-263 culations [37,38], electrostatic screening by photoexcited 264 carriers appears as a universal mechanism for "dipole-265 dipole driven" ferroelectric materials such as BTO [39]. [43], and that screening effects play a central role.

279

There are, however, potential limitations of this work, 280 since DFT is known to underestimate the quasi-particle 281 bandgap. Furthermore, excitonic effects are described 282 within a mean-field approximation using constrained 283 DFT to mimic excited states occupations. First, we per-284 formed G 0 W 0 [44] calculations with the Yambo [START_REF] Marini | Yambo: An ab initio tool for excited state calculations[END_REF] pack-285 age -using DFT input from Quantum Espresso [START_REF] Giannozzi | QUANTUM ESPRESSO: a modular 467 and open-source software project for quantum simula-468 tions of materials[END_REF][START_REF] Giannozzi | Advanced capabilities for materials 471 modelling with Quantum ESPRESSO[END_REF] shown that ST holes were unlikely in PTO [49,50], but 317 could potentially be stabilized in cubic BTO [49]. In 318 order to check the appearance of polaronic states, we 319 ran calculations in BaTiO3 under photoexcitation with 320 n ph = 0.075 e/f.u. and n ph = 0.150 e/f.u (with no sym-321 metry). No trace of polaronic states was found.

322

In conclusion, we have shown that light absorption is

53 as a knob

 53 photo-induced phase transitions in ferroelectrics.

82 than 5 × 10 - 7

 5107 Ha.Bohr -1 and all components of the 83 stress tensor smaller than 0.1 MPa. BTO was described 84 using the PBESol [26] exchange-correlation functional, 85 while the Local Density Approximation [27, 28] was em-86 ployed to treat PTO. Photo-excited thermalized carriers 87 are mimicked using Fermi-Dirac distribution with two 88 quasi-Fermi levels µ e and µ h , as commonly used to de-89 scribe photovoltaic effects in, for instance, p -n junc-90 tions [29]. As a result, during our DFT calculations, the 91 density is self-consistently converged under the constraint 92 of having n e = n ph (and n h = n ph ) electrons (holes) in 93 the CB (VB). In other words, at each Self-Consistent 94

97f

  photoexcited carriers n ph . In the dark (n ph = 0 e/f.u., 119

15 FIG. 2 .

 152 FIG. 2. (a-d) Phonon band structure in paraelectric cubic BTO and (e-h) PTO for different concentrations of photoexcited carrier pairs n ph = 0.00 e/f.u. (a & e), n ph = 0.05 e/f.u. (b & f), n ph = 0.10 e/f.u. (c & g) and n ph = 0.15 e/f.u. (d & h).The polar Γ dominant instability in BTO becomes stabilized (i.e. positive and real frequency) for n ph ≥ 0.10 e/f.u., as shown in (c) and (d). In PTO, the polar instability at Γ is replaced by a non-polar tilted instability at M and R.

  of the cubic phase for n ph = 0 e/f.u. (Fig. (2.e)) exhibits one main polar instability at Γ, but also unstable modes at the R and M points associated with octahedra tilting 00-and 00+, respectively. The strength of the polar instability at Γ decreases with increasing the number of photo-excited carriers (see Figs. (2.e-h)) as in BTO. However, it never totally disappears, indicating that ferroelectricity in PTO is more robust against photocarrier generation. In contrast, the strengths of the tilting instabilities at R and M are slightly enhanced, and they become dominant for n ph ≥ 0.1 (Figs. (2.g-h)), suggesting a transition from a polar to a non-polar structure with oxygen tilts under illumination. Hence, we compute the energies of several prototypical phases upon illumination, as shown in Fig. (3.b).

Fig. ( 3

 3 Fig. (3.b), the ferroelectric 000/00w + phase is the most

(Pb 1 / 2

 12 material such as SrTiO 3 , and form the solid solution 218 Sr 1/2 )TiO 3 (PSTO) to favor the tilt instability 219 with respect to the polar one. As a proof of concept, 220 several phases of PSTO with rocksalt order were cal-221 culated in Fig. (3.c). In particular, a polar rhombohe-222 dral ---/u + u + u + phase (black dashed line) is the 223 ground state in the dark. The 000/00w + (purple tri-224 angles) and the high-symmetry cubic 000/000 phase (in 225 green squares) have increasing energy with n ph . On the 226 contrary, the antipolar tilted phase --0/u -ū-0 -(light 227 blue triangles) becomes the most stable phase above 228 n ph = 0.05 e/f.u., thus supporting the proposed design 229 strategy to reduce the critical concentration for the polar-230 to-antipolar/antiferrodistortive transition.

  photoexcitation, and (ii) the most affected IFCs corre-254

266

  In addition, the volume of most phases decreases with 267 increasing concentration of excited carriers in both BTO 268 and PTO, except that of the cubic 000/000 phase, which 269 increases instead (see Supplementary Material [31]). In-270 terestingly, the decrease of volume of the polar phases 271 is consistent with the disappearance of the polar order, 272 and is reminiscent of the well-known [17-19] effect of hy-273 drostatic compression; yet, the volume increase of the 274 cubic phase under illumination should in principle favor 275 the polar instabilities. Hence, our results suggest that the 276 behavior of the polar distortion upon illumination cannot 277 be understood solely in terms of a photo-induced volume 278 change

286-

  in order to accurately describe the conduction and va-287 lence bands (see Supplementary Material [31]). The cor-288 rected band structure is overall not altered, except for 289 an almost rigid upshift of the conduction bands. This 290 shift slightly depends on the studied phase (with differ-291 ences smaller than 100 meV between phases). The shift is 292 smaller for non-polar phases, therefore facilitating further 293 the light-induced transitions towards such latter states. 294 For BTO, we checked the robustness of our conclu-295 sions by performing calculations of zone center phonons 296 of the cubic phase with hybrid DFT. We compare calcu-297 lations performed with the semi-local PBEsol functional 298 and the hybrid HSEsol [48] functional. Both functionals 299 give similar lattice constants and similar frequency for 300 the imaginary frequency of the soft ferroelectric mode, 301 even though the band gaps are quite different (1.8 eV for 302 PBEsol and 3.4 eV, close to the GW value, for HSEsol).

303

  Despite the very different band gaps, the phonon frequen-304 cies as a function of the excited charge density evolve sim-305 ilarly: we observe a stabilization of the soft mode at an 306 excited charge density of 0.09 electron per formula unit 307 (see Supplementary Material [31]). Hence, although our 308 DFT-based description may slightly alter the critical con-309 centrations at which transitions occur, the present qual-310 itative behavior should be correct, especially given our 311 results in Fig. 2 for the dynamical lattice instabilities of 312 the paraelectric cubic state. 313 At last, let us discuss the possibility of having photo-314 induced phase transitions to (small self-trapped, or ST) 315 polaronic states. Recent (doping) calculations have 316

323a

  viable route to switch on/off specific lattice instabili-324 ties in complex materials such as ferroelectric perovskite 325 oxides. Via first-principles calculations we have demon-326 strated the microscopic mechanism leading to a light-327 controlled stabilization of the cubic phase in BTO but 328 not in PTO. The calculations are a promising tool to en-329 gineer and optimize such light-triggered transformations, 330 suggesting strategies for their practical realization in suit-331 ably chosen materials. The results presented in this work 332 could offer a route to design light-controlled memory, by 333 accessing light-induced metastable states (see Fig. 4 in

thank the ARO grant W911NF-support from the 337 National Research Fund, Luxembourg through project 338 INTER/ANR/13/20/NANOTMD (E. T. and L. W.) 339 and the intermobility program (Grant No. 15/9890527 grant for use of computing resources.

433

This order of magnitude matches the region where we 434 observed flattening of the energy landscape in Fig. 3a 435 (around the 000/u + u + u + to 000/00w + transition) which 436 could potentially harbor a monoclinic phase as discussed 437 in the Supplementary Material [31].