Creative The Crucial Things in Science often Happen Quite Unexpectedly—Das Entscheidende in der Wissenschaft Geschieht oft Ganz Unerwartet (K. Alex Müller)

Reinhard K. Kremer 1,* , Annette Bussmann-Holder 1, Hugo Keller 2 and Robin Haunschild 1 1 Institute for State Research, Heisenbergstrasse 1, 70569 Stuttgart, ; [email protected] (A.B.-H.); [email protected] (R.H.) 2 Physik Institut der Universität Zürich, Winterthurerstr. 190, 8057 Zürich, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +49-711-689-1688

 Received: 10 June 2020; Accepted: 28 June 2020; Published: 1 July 2020 

Abstract: We analyzed the publication output of one of the 1987 awardees, K. Alex Müller, using bibliometric methods. The time-dependent number of publications and citations and the network with respect to the coauthors and their affiliations was studied. Specifically, the citation history of the Nobel Prize awarded 1986 article on “Possible high-temperature in the Ba-La-Cu-O system” has been evaluated in terms of the overall number of articles on superconductivity and the corresponding citations of other most frequently referenced articles. Thereby, a publication with “delayed recognition” was identified.

1. Introduction Rarely in the history of has a discovery had such a prompt and enduring impact as the observation of high-temperature superconductivity (high-Tc) in the Ba-La-Cu-O system by J. Georg Bednorz and K. Alex Müller in 1986 [1]. The unexpected disclosure of superconductivity at temperatures about twice as high as had been known before [2,3] triggered immediate (sometimes hectic and often very competitive and controversial) research activity targeted at the synthesis of compounds with even higher Tc’s and the still contentious theoretical explanation of high-Tc superconductivity. The bursting reaction in the search for new and deeper insight into already known high-Tc materials is reflected in the instantaneous rise of the number of publications indexed by the Web of Science (WoS) [4,5] before and after 1986 (see Figure1). Figure1 shows that after 1986 about 30% of the publications related to superconductivity covered high-Tc research (see [6–8]). Since 1986, the number of publications dealing with superconductivity essentially remains constant at a high level of about 6500 per year, whereas the number of high-Tc papers decreased gradually after a peak in 1991 with approximately 3100 publications and currently levels off at about 1400 publications per year. A closer inspection of Figure1 also reveals a couple of historically very interesting aspects: After the discovery of superconductivity in mercury by H. Kamerling Onnes in 1911 [9] there was an extended period lasting until the late 1950s with a continuous but moderate increase of the publication output with about 10 to 50 superconductivity publications per year. Note the plunge in the number of publications in the late years of the Second World War. Certainly triggered by the development of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity [10] but very likely also by the first placing of a satellite into Earth’s orbit by the Soviet Union in October 1957, the number of publications rose steeply in the late 1950s and early 1960s [11]. This event, perceived in the Western world as the ‘Sputnik crisis’, initiated massive funding increase into science, especially in the USA [12]. As a consequence, the number of scientists working in the field and the publication

Condens. Matter 2020, 5, 43; doi:10.3390/condmat5030043 www.mdpi.com/journal/condensedmatter Condens. Matter 2018, 3, x FOR PEER REVIEW 2 of 9 steeply in the late 1950s and early 1960s [11]. This event, perceived in the Western world as the ‘Sputnik crisis’,Condens. Matterinitiated2020, 5 ,massive 43 funding increase into science, especially in the USA 2[12]. of 9 As a consequence, the number of scientists working in the field and the publication output increased steeply afteroutput 1957 increased [13,14]. steeply The afterdata 1957 also [13 ,show14]. The that data after also show 1970 that the after publication 1970 the publication output outputstalled and flattened, stalledbecause and no flattened, new discoveries because no newwere discoveries made, and were increased made, and dramatically increased dramatically after 1987. after 1987.

Figure 1. Number of publications indexed by the Web of Science (WoS) either with the keywords Figure 1. Number of publications indexed by the Web of Science (WoS) either with the keywords ‘superconduct*’ (grey) or ‘high-Tc’ (red). [4]. ‘superconduct*’ (grey) or ‘high-Tc ‘(red). [4]. 2. The Publication Output of Original Research Publications 2. The PublicationK. Alex Output Müller began of Original his scientific Research career with Publications his PhD thesis entitled ‘Paramagnetische Resonanz 3+ von Fe in SrTiO3 Einkristallen’ submitted to the ETH Zürich in 1958. In his thesis he summarized K. Alexhis microwaveMüller began resonance his experiments scientific oncareer spin S =with5/2 ions his substituted PhD thesis for Ti entitled in SrTiO3 ,‘Paramagnetische a compound Resonanz whichvon Fe fascinated3+ in SrTiO him during3 Einkristallen’ all his future submitted scientific activity. to the The ETH thesis Zürich was published in 1958. in the In Helvetica his thesis he summarizedPhysica his Actamicrowave in 1958 [15 resonance]. The results experiments of his thesis were on summarizedspin S=5/2 inions a short substituted single-authored for Ti Physical in SrTiO3, a compoundReview which Letter fascinated (PRL) published him during in 1959, all already his future mentioning scientific his new activity. affiliation The at the thesis Battelle was Memorial published in Institute in Geneva [16]. This PRL referenced a publication, apparently K. Alex Müller’s very first the Helveticaone (possiblyPhysica publishedActa in 1958 more [15]. or less The simultaneously results of his with thesis his PhDwere thesis), summarized in the proceedings in a short of single- authored Physicalthe 7th Colloquium Review Letter Ampère (PRL) held in published 1958 in Geneva in 1959, (Résonance already paramagn mentioningétique duhis Cr new3+ dans affiliation des at the Battellemonocristaux Memorial deInstitute SrTiO3)[ in17 Geneva]. [16]. This PRL referenced a publication, apparently K. Alex Müller’s very Figurefirst one2a shows (possibly the number published of papers more published or less by simultaneously K. Alex Müller per yearwith indexed his PhD by thethesis), WoS in the as source items beginning from his first Physical Review Letter, which appeared in 1959. Figure2b is a proceedings of the 7th Colloquium Ampère held in 1958 in Geneva (Résonance paramagnétique du ‘vintage’ diagram, showing the number of citations for papers published in the particular year. 3+ Cr dans des monocristauxUnfortunately, the de ‘Archives SrTiO3 des) [17]. Sciences éditées par la Société de Physique et d’Histoire Naturelle Figurede 2(a) Gen èshowsve’ are notthe indexed number by theof papers WoS and published a reliable bibliometric by K. Alex analysis Müller of this per publication year indexed cannot by the WoS as sourcebe carried items out beginning [18]. from his first Physical Review Letter, which appeared in 1959. Figure 2b is a ‘vintage’In diagram, total, K. Alex showing Müller publishedthe number 311 papersof citations in the for 61 years papers of his published scientific activityin the particular until 2019. year. Unfortunately,Until 1986, hethe authored ‘Archives between des three Sciences and eight éditées publications par perla Société year (see Figurede Physique2a). Interestingly, et d’Histoire this number did not increase significantly over the following years, except in the first two immediate Naturelle de Genève’ are not indexed by the WoS and a reliable bibliometric analysis of this years after the seminal paper on the discovery of high-Tc superconductivity in 1986 [1] when he publicationauthored cannot or be co-authored carried out up [18]. to 30 papers in 1987 and 22 in 1988 (i.e., about one-sixth of Müller’s In total,publications K. Alex appearedMüller published in these two 311 years). papers It is worthin the mentioning 61 years of that his the scientific numberof activity publications until 2019. Until 1986,dealing he authored with SrTiO between3 (50) is almostthree and as high eight as those public coveringations cuprates per year (52). (see Including Figure 112a). additional Interestingly, this numberpapers did whichnot increase treated titanates significantly in general over (mostly the BaTiOfollowing3), the years, number except of publications in the first on titanates two immediate (61) even exceeds that on cuprates. years after the seminal paper on the discovery of high-Tc superconductivity in 1986 [1] when he authored or co-authored up to 30 papers in 1987 and 22 in 1988 (i.e., about one-sixth of Müller’s publications appeared in these two years). It is worth mentioning that the number of publications dealing with SrTiO3 (50) is almost as high as those covering cuprates (52). Including 11 additional papers which treated titanates in general (mostly BaTiO3), the number of publications on titanates (61) even exceeds that on cuprates. Figure 2b displays a ‘vintage’ diagram showing the number of citations to papers published in a particular year. Besides the citation spikes following immediately after the discovery of high-Tc superconductivity, two more years stand out, namely the years 1979 and 1996. The high citation

Condens. Matter 2018, 3, x FOR PEER REVIEW 3 of 9 Condens. Matter 2020, 5, 43 3 of 9 number in 1979 is due to one particular paper, which also exhibited an unusual citation history (see details below).Figure In 21996,b displays the ahigh ‘vintage’ citation diagram number showing is theowed number to two of citations publications to papers on published La1-xCa inxMnO3+y, namely ana particular article in year. Nature Besides together the citation with spikes Zhao, following Conder, immediately and Keller after on the giant discovery oxygen of high-isotopeTc shifts (622 citations)superconductivity, and a PRL two published more years standjointly out, with namely Shengelaya, the years 1979 Zhao, and 1996. and The Keller high citation providing number electron paramagneticin 1979 resonance is due to one (EPR) particular evidence paper, for which Jahn-Teller also exhibited an unusual formation citation (215 history citations) (see details (see Table below). In 1996, the high citation number is owed to two publications on La1-xCaxMnO3+y, namely an 2 and Ref.article [19] infor Nature the detailed together withreferences). Zhao, Conder, and Keller on giant oxygen isotope shifts (622 citations) K. Alexand aMüller’s PRL published publication jointly withoutput Shengelaya, compares Zhao, well, and for Keller example, providing with electron that paramagneticof V. L. Ginzburg, another Nobelresonance prize (EPR) winner evidence for for Jahn-Teller“Pioneering polaron Contri formationbutions (215 to citations) the Theory (see Table of 2Superconductors and Ref. [19] for and Superfluids",the detailed whose references). scientific activity covered a similar period of time as of K. Alex Müller’s [20].

Figure 2.Figure (a) Number 2. (a) Number of papers of papers published published by by K. K. Alex Alex Müller per per year year indexed indexed by the by WoS the as WoS source as source items beginning from his first Physical Review Letter, which appeared in 1959. (b) ‘Vintage’ diagram items beginning from his first Physical Review Letter, which appeared in 1959. (b) ‘Vintage’ diagram showing the number of citations for papers published in the particular year. showing the number of citations for papers published in the particular year. K. Alex Müller’s publication output compares well, for example, with that of V. L. Ginzburg, 3. Editorialanother Activities Nobel prize winner for “Pioneering Contributions to the Theory of Superconductors and Superfluids”, whose scientific activity covered a similar period of time as of K. Alex Müller’s [20]. Over his long-lasting scientific activity, K. Alex Müller edited and co-edited numerous collections3. Editorialof lectures, Activities conference proceedings, and series volumes, to which he usually contributed an article or Overa chapter. his long-lasting Table 1 scientific provides activity, a compilation K. Alex Müller of edited Müller’s and co-edited editorial numerous activity. collections Clearly, in the 1970s andof 1980s lectures, these conference books proceedings, focused around and series phase volumes, transition-related to which he usually phenomena. contributed anLater, article after the or a chapter. Table1 provides a compilation of Müller’s editorial activity. Clearly, in the 1970s and discovery1980s of high- theseT booksc superconductivity, focused around phase these transition-related were in his fo phenomena.cus. Noteworthy Later, after are the the discovery two conference of proceedingshigh- ofTc superconductivity,international symposia these were on in hisphase focus. separation Noteworthy problems are the two in conference cuprate proceedings superconductors held in Ericeof international and Cottbus symposia in 1992 on phase and separation 1993, respectively. problems in cuprate These superconductorsmeetings and heldthe incontributions Erice summarizedand Cottbusin the inproceedings 1992 and 1993, reflect respectively. Müller’s These early meetings interest and thein essential contributions inhomogeneity summarized in thein high-Tc superconductors.proceedings reflect Müller’s early interest in essential inhomogeneity in high-Tc superconductors.

Table 1. Compilation of K. Alex Müller’s editorial activities.

Müller, K.A.; Rigamonti, A. Local Properties at Phase Transitions. Proprietà Locali alle Transizioni di Fase, Societa Italiana di Fisica Bologna, 1976. Müller, K.A.; Thomas, H. Structural Phase Transitions I. Springer-Verlag, Berlin, Heidelberg, New York, 1981. Müller, K.A.; Thomas, H. Structural Phase Transitions II. Springer-Verlag, Berlin, Heidelberg, New York, 1991.

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Bednorz, J.G.; Müller, K.A. Earlier and Recent Aspects of Superconductivity, Lectures from the International School, Erice, Trapani, Sicily, 4–16 July, 1989. Springer Verlag, 1st Edition 1990, 2nd EditionCondens. 1991. Matter 2020, 5, 43 4 of 9 Müller, K.A.; Benedek, G. Proceedings of the Workshop on Phase Separation in Cuprate Superconductors. Erice,Table Italy, 1. CompilationWorld Scientific of K. Alex Publishing, Müller’s editorial 1993. activities. Müller, K.A.; Rigamonti, A. Local Properties at Phase Transitions. Proprietà Locali alle Transizioni di Fase, Societa Sigmund,Italiana E.; di Fisica Müller, Bologna, K.A. 1976. Phase Separation in Cuprate Superconductors. Proceedings of the secondMüller, international K.A.; Thomas, workshop H. StructuralPhase on ‘Phase Transitions Separation I. Springer-Verlag, in Cuprate Berlin, Heidelberg, Superconductors’, New York, 1981. 4–10 September,Müller, K.A.; 1993. Thomas, Cottbus, H. Structural Germany, Phase Transitions Springer-Verlag, II. Springer-Verlag, 1994. Berlin, Heidelberg, New York, 1991. Bednorz, J.G.; Müller, K.A. Earlier and Recent Aspects of Superconductivity, Lectures from the International School, Müller,Erice, K.A.; Trapani, Bussmann-Holder, Sicily, 4–16 July, 1989. Springer A. Superconductivity Verlag, 1st Edition 1990, in 2nd Complex Edition 1991. Systems, Structure and BondingMüller, 114, K.A.; Springer-Verlag, Benedek, G. Proceedings 2005. of the Workshop on Phase Separation in Cuprate Superconductors. Erice, Italy, World Scientific Publishing, 1993. Müller,Sigmund, K.A.; E.; Kool, Müller, T.W. K.A. Phase Properties Separation of in Perovs Cuprate Superconductors.kites and Other Proceedings Oxides. of World the second Scientific, international 2010. workshop on ‘Phase Separation in Cuprate Superconductors’, 4–10 September, 1993. Cottbus, Germany, Müller,Springer-Verlag, K.A. Letters 1994. Section in the Journal of Superconductivity and Novel Magnetism, publishedMüller, K.A.;by Springer Bussmann-Holder, Nature. A. Superconductivity in Complex Systems, Structure and Bonding 114, Springer-Verlag, 2005. Müller, K.A.; Kool, T.W. Properties of Perovskites and Other Oxides. World Scientific, 2010. 4. TheMüller, Citation K.A. Letters Record Section—Overall in the Journal of Superconductivity and Novel Magnetism, published by Springer Nature. In his long scientific career, K. Alex Müller’s publications were cited about 25,000 times. By far 4. The Citation Record—Overall the highest citation count of all of his publications was acquired by the Zeitschrift für Physik article of 1986In on his ‘Possible long scientific High- career,Tc Superconductivity K. Alex Müller’s publications in the BA-LA-CU-O were cited about System’ 25,000 times.(see Figure By far the3). Until Februaryhighest citation2020, it count was ofcited all of more his publications than an exceptio was acquirednal 10,000 by the Zeitschrifttimes. Note für that Physik we article did not of 1986 find any publicationon ‘Possible on High- superconductivityTc Superconductivity that inwas the BA-LA-CU-Ocited more System’frequently. (see FigureThe citation3). Until Februarycount for this 2020, it was cited more than an exceptional 10,000 times. Note that we did not find any publication publication peaks two years after its appearance at a value of about 1250 citations per year. on superconductivity that was cited more frequently. The citation count for this publication peaks Subsequently, the citations decrease to pass through a broad minimum around the year 2005, two years after its appearance at a value of about 1250 citations per year. Subsequently, the citations whereupondecrease to they pass start through to grow a broad again minimum until today. around Such the a year second 2005, increase whereupon is remarkable they start to and grow must be attributedagain until to the today. discovery Such a secondof new increase classes isof remarkablehigh-Tc superconductors, and must be attributed e.g., MgB to the2 or discoverythe iron-based superconductors.of new classes of Forty high- Tofc superconductors,K. Alex Müller’s e.g., 311 MgB scientific2 or the papers iron-based acquired superconductors. more than Forty100 citations. of TheK. 10 Alex most Müller’s frequently311 scientific cited papers papers are acquired compiled more in than Table 100 2. citations. The 10 most frequently cited papers are compiled in Table2.

FigureFigure 3. 3.TimeTime evolution evolution of thethe numbernumber of of citations citations to K.to AlexK. Alex Müller’s Müller’s papers. papers. Theblack The black solid linesolid line givesgives the the number number of of all all citations perper year,year, the the red red solid solid line line the the citations citations per yearper ofyear the of article the article in in ZeitschriftZeitschrift für für Physik Physik in in 1986 1986 [[1],1], andand the the blue blue solid solid line line the the diff differenceerence between between the first the and first the and latter. the latter.

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Table 2. The 10 most cited publications authored and co-authored by K. Alex Müller (search date February 2020), ordered by their citation count.

1. Bednorz, J.G.; Müller, K.A. Possible High-Tc Superconductivity in the BA-LA-CU-O System. Z. Phys. B Condens. Matter 1986, 64, 189–193. Doi: 10.1007/BF01303701. times cited: 10210 2. Müller, K.A.; Burkard, H. SrTiO3—Intrinsic Quantum Para-Electric Below 4-K. Phys. Rev. B 1979, 19, 3593–3602. Doi: 10.1103/PhysRevB.19.3593. times cited: 1196 3. Müller, K.A.; Takashige, M.; Bednorz, J.G. Flux Trapping and Superconductive Glass State in La2CuO4-y:Ba. Phys. Rev. Lett. 1987, 58, 1143–1146. Doi: 10.1103/PhysRevLett.58.1143. times cited: 1094 4. Zhao, G.M.; Conder, K.; Keller, H.; Müller, K.A. Giant Oxygen Isotope Shift in the Magnetoresistive La1-xCaxMnO3+y. Nature 1996, 381, 676–678. Doi: 10.1038/381676a0. times cited: 622 5. Deutscher, G.; Müller, K.A. Origin of Superconductive Glassy State and Extrinsic Critical Currents in High-Tc Oxides. Phys. Rev. Lett. 1987, 59, 1745–1747. Doi: 10.1103/PhysRevLett.59.1745. times cited: 584 6. Blazey, K.W.; Müller, K.A.; Bednorz, J.G.; Berlinger, W.; Amoretti, G.; Buluggiu, E.; Vera, A.; Matacotta, F.C. Low-Field Microwave-Absorption in the Superconducting Copper Oxides. Phys. Rev. B 1987, 36, 7241–7243. DOI: 10.1103/PhysRevB.36.7241. times cited: 327 7. Bednorz, J.G.; Müller, K.A. Perovskite-Type Oxides - The New Approach to High-Tc Superconductivity. Rev. Modern Phys. 1988, 60, 585–600. Doi: 10.1103/RevModPhys.60.585. times cited: 306 8. Zhao, G.M.; Hunt, M.B.; Keller, H.; Müller, K.A. Evidence for Polaronic Supercarriers in the Copper Oxide Superconductors La2-xSrxCuO4. Nature 1997, 385, 236–239. Doi: 10.1038/385236a0. times cited: 273 9. Müller, K.A.; Berlinger, W.; Waldner, F. Characteristic Structural in Perovskite-Type Compounds. Phys. Rev. Lett. 1968, 21, 814–817. Doi: 10.1103/PhysRevLett.21.814. times cited: 267 10. Šimanek, E.; Müller, K.A. Covalency and Hyperfine Structure Constant-A of Iron Group Impurities in Crystals. J. Phys. Chem. 1970, 31, 1027–1040. Doi: 10.1016/0022-3697(70)90313-6. times cited: 266

5. Citation History of Selected Highly Cited Publications The citation record as a function of time of a particular article, colloquially called its ‘citation history’, is specific for every article. However, it often follows a similar pattern [20]. Typically, it takes about one or two years until the citation count markedly starts to rise. Often, the citation count reaches a peak after about three years with a subsequent decrease when newer research results gradually become available. Such exemplary pattern can be observed for three of the four next highest cited publications of Müller, namely those in collaboration with Müller, Takashige, Bednorz, PRL 1987, Zhao, Conder, Keller, Müller, Nature 1996, and Deutscher and Müller, PRL 1987 (see Figure4). However, the paper by Müller and Burkhard, Physical Review B (PRB) 1979 [21] (about 1200 total citations) entitled ‘SrTiO3—Intrinsic quantum para-electric below 4 K’ exhibits a remarkably different citation history. Published in 1979, it was barely referenced until about 1990. Afterwards, it picked up and kept on increasing until now. It appears that the research reported in this article was well ahead of its time. Publications showing such delayed recognition have sometimes been labeled as ‘sleeping beauties’. Condens. Matter 2018, 3, x FOR PEER REVIEW 6 of 9

Condens.ahead Matter of2020 its, time.5, 43 Publications showing such delayed recognition have sometimes been labeled as6 of 9 ‘sleeping beauties’.

FigureFigure 4. Citation 4. Citation history history of theof the next next four four highest highest cited cited articles articles ofof K.K. Alex Müller besides besides the the 1986 1986 ZeitschriftZeitschrift für Physik für Physik paper. paper. Note Note the the unusual unusual citation citation history characteristic characteristic of ofdelayed delayed recognition recognition of the Müller and Burkhard article on quantum para-electric behavior of SrTiO3. For the detailed of the Müller and Burkhard article on quantum para-electric behavior of SrTiO3. For the detailed referencesreferences see Table see Table2. 2.

6. The6. Co-AuthorThe Co-Author Network— Network—PeoplePeople FigureFigure5 displays 5 displays an overview an overview (generated (generated using using the software the software VOSviewer VOSviewer [ 22 ])[22]) of the of the co-authors co-authors with whomwith Müller whom published Müller morepublished than more five papers. than five Above papers. all, Above ranks J.all, Georg ranks Bednorz J. Georgfrom Bednorz IBM from Rüschlikon IBM Rüschlikon (58), with whom he shared the 1987 Nobel Prize. K. Alex Müller continuously (58), with whom he shared the 1987 Nobel Prize. K. Alex Müller continuously emphasized the amount of emphasized the amount of competent work and inspiration he received from the constant and fruitful competent work and inspiration he received from the constant and fruitful assistance of Walter Berlinger. assistance of Walter Berlinger. Even nowadays, it is by no means customary to include technical staff Evenas nowadays, a co-author it in is a by scientific no means paper. customary However, toK. includeAlex Müller technical never ceased staff as to aacknowledge co-author in Berlinger’s a scientific paper.work However, and, consequently, K. Alex Müller the number never ceasedof joint publications to acknowledge with Berlinger Berlinger’s (55) work ranks and,almost consequently, as high as the numberthat with of jointBednorz. publications K.W. Blazey, with also Berlinger from IBM (55) Rüschlikon, ranks almost is another as high regularly as that withnamed Bednorz. co-author. K.W. Blazey,Other also frequently from IBM appearing Rüschlikon, co-authors is another are regularlyH. Keller and named his postdoctoral co-author. Other fellow frequently A. Shengelaya appearing and co-authorstheir jointly are H. supervised Keller and PhD his student postdoctoral G. M. Zhao, fellow all A.affiliated Shengelaya with the and University their jointly in Zürich. supervised E. Kaldis, PhD studentK. G.Conder, M. Zhao, and allJ. Karpinski affiliated from with the the ETH University Zürich inwereZürich. frequent E. Kaldisand highly, K. Conder appreciated, and sources J. Karpinski of from theexquisite ETH Zürichsamples were and frequentcrystals. With and highlycolleagues appreciated from Germany sources and of exquisiteparticularly samples the Max-Planck- and crystals. With colleaguesInstitute (MPI) from for Germany Solid State and Research particularly in Stuttgart, the Max-Planck-Institute K. Alex Müller had (MPI)a long-lasting for Solid collaboration, State Research in Stuttgart,resulting K. in Alex several Müller joint publications had a long-lasting with A. collaboration,Bussmann-Holder resulting and A. Simon. in several joint publications Condens. Matter 2018, 3, x FOR PEER REVIEW 7 of 9 with A. Bussmann-Holder and A. Simon.

Figure 5.FigureK. Alex 5. K. Müller’s Alex Müller’s main main co-authors. co-authors. The The diameters diameter ofs of the the circles circles correspond correspond toto thethe number of of joint publications.joint publications. Only co-authorshipsOnly co-authorships with with at least at least five five joint joint papers papers are are included. included.

7. Countries Not surprisingly, K. Alex Müller’s main co-authors were affiliated with research institutions in his home country Switzerland (see Figure 6, also generated using the software VOSviewer [22]), which are by far dominating the co-authorship network. Outside Switzerland, the co-author network comprised colleagues from 16 countries (one disconnected country, The Netherlands, was removed). The immediate neighbors, France and Germany (note that Germany includes Western Germany before the reunification), rank second followed by the USA. However, there are also substantial numbers of co-authors from Russia (note that Russia includes the Soviet Union) and smaller countries like Georgia, Slovenia, and Estonia

Figure 6. Country of affiliation of K. Alex Müller’s main co-authors with whom he published at least one paper. The diameters of the circles correspond to the joint publication record with the named individual country.

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Figure 5. K. Alex Müller’s main co-authors. The diameters of the circles correspond to the number of joint publications. Only co-authorships with at least five joint papers are included.

7. Countries Condens. Matter 2020, 5, 43 7 of 9 Not surprisingly, K. Alex Müller’s main co-authors were affiliated with research institutions in 7.his Countries home country Switzerland (see Figure 6, also generated using the software VOSviewer [22]), whichNot are surprisingly, by far dominating K. Alex Müller’s the co-authorship main co-authors netw wereork. affiliated Outside with Switzerland, research institutions the co-author network incomprised his home country colleagues Switzerland from 16 (see countries Figure6, also (one generated disconnected using the country, software The VOSviewer Netherlands, [ 22]), was removed). whichThe immediate are by far dominating neighbors, the co-authorship France and network. Germany Outside (note Switzerland, that Germany the co-author includes network Western Germany comprisedbefore the colleagues reunification), from 16 countries rank second (one disconnected followed country,by the The USA. Netherlands, However, was there removed). are also substantial Thenumbers immediate of co-authors neighbors, France from and Russia Germany (note (note that that Russia Germany includes includes the Western Soviet Germany Union) before and smaller countries the reunification), rank second followed by the USA. However, there are also substantial numbers of co-authorslike Georgia, from RussiaSlovenia, (note and that RussiaEstonia includes the Soviet Union) and smaller countries like Georgia, Slovenia, and Estonia

FigureFigure 6. Country 6. Country of affi ofliation affiliation of K. Alex of Müller’sK. Alex main Müller’s co-authors main with co-authors whom he publishedwith whom at least he published at least oneone paper. paper. The The diameters diameters of the circlesof the correspond circles correspond to the joint publicationto the joint record publication with the namedrecord with the named individual country. individual country. 8. Conclusions The publication record and the citation history of K. Alex Müller’s publications are exceptional and, from the bibliometric point of view, very interesting: Before J. Georg Bednorz and K. Alex Müller published their seminal paper on possible high-Tc superconductivity in Zeitschrift für Physik in 1986, Müller’s scientific work focused on phase transition-related topics with an emphasis on perovskite materials, especially SrTiO3. Naturally, this changed after 1986 and the citation history of all his publications, subtracting out the high-Tc papers, increased by about a factor of four. Since its appearance, the Zeitschrift für Physik article itself has been cited more than 10,000 times. No other article on superconductivity has been cited more frequently. Forty of the 311 articles published by K. Alex Müller received more than 100 citations each. Müller’s h-index amounts to 64. Whereas most of his highest cited articles exhibit a citation history with a sharp peak shortly after publication and a steady decay thereafter (typical behavior), one noteworthy publication on SrTiO3, together with H. Burkhard, displays an unusual, delayed recognition-type citation history. Currently, this publication continuous to attract attention and reached approximately 70 citations in 2019.

Funding: This research received no external funding. Condens. Matter 2020, 5, 43 8 of 9

Acknowledgments: Very valuable discussions with W. Marx and Th. Scheidsteger as well as a careful reading of manuscript by S. Kremer are gratefully acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

References and Notes

1. Bednorz, J.G.; Müller, K.A. Possible high Tc superconductivity in the Ba La Cu O system. Phys. Z. − − − Condens. Matter 1986, 64, 189–193. [CrossRef] 2. Arrhenius, G.; Corenzwit, E.; Fitzgerald, R.; Hull, G.W.J.; Luo, H.L.; Matthias, B.T.; Zachariasen, W.H. Superconductivity of NB(3)(AL, GE) above 20.5 degrees K. Proc. Natl. Acad. Sci. USA 1968, 61, 621–628. [CrossRef][PubMed] 3. One of the authors (RKK) recollects a seminar given by B. T. Matthias at the TU Darmstadt in the Elschner/Steglich seminar in the very early eighties of the last century. At the end of his lecture Matthias was asked what he estimates could be the maximum superconducting Tc. Matthias responded with a smile: This question apparently did not come as a great surprise and he must certainly have heard it many times before—and answered that he thinks that Tc will somehow level off between 30 and 35 K. 4. The WoS as Provided by Clarivate Analytics was Used. Available online: https://apps.webofknowledge.com (accessed on 25 February 2020). 5. Birkle, C.; Pendlebury, D.A.; Schnell, J.; Adams, J. Web of Science as a data source for research on scientific and scholarly activity. Quant. Sci. Stud. 2020, 1, 363–376. [CrossRef] 6. At this occasion we want to point out that such bibliometric searches always are afflicted with some degree of uncertainty since they stick to the search terms. As an example, a publication containing the search term ‘high-Tc’ but not ‘superconduct ’ shows up in the high-Tc category only, whereas if it contains both search ∗ terms it appears in both categories. How intriguing bibliometric searches can be, may be realized from the fact that the term ‘high-temperature superconductors’ has already been used for some A15 compounds in 1953/1954 (see e.g., Ref. [7]). Bibliometric searches for authors and citations are comparatively easier and generally more precise. For a detailed treatment of bibliometric analysis and its aspects see e.g., Ref. [8]. 7. Gross, R.; Marx, A. Festkörperphysik; De Gruyter Mouton: Berlin, Germnay, 2012; p. 754. 8. Marx, W.; Bornmann, L. How to evaluate individual researchers working in the natural and life sciences meaningfully? A proposal of methods based on percentiles of citations. Scientometrics 2014, 89, 487–509. 9. Kamerling-Onnes, H. Further Experiments with Liquid Helium. C. On the Change of the Electric Resistance of Pure Metals at Very Low Temperature etc. IV. The Resistance of Pure Mercury at Helium Temperature. In Through Measurement to Knowledge; Springer: Dordrecht, the Netherlands, 1911; Volume 124. 10. Bardeen, J.; Cooper, L.N.; Schrieffer, J.R. Theory of Superconductivity. Phys. Rev. 1957, 108, 1175–1204. [CrossRef] 11. Dickson, P. Sputnik: The Shock of the Century; Walker Publishing Company, Inc.: New York, NY, USA, 2001. 12. See for Example the Historical Development of the US Federal Spending on Defense and Nondefense Research and Development at the Web-Page. Available online: https://www.aaas.org/programs/r-d-budget- and-policy/historical-trends-federal-rd (accessed on 20 February 2020). 13. Marx, W.; Cardona, M.; Lockwood, D.J. Rutherford’s impact on science over the last 110 years: A bibliometric analysis Physics in Canada. Phys. Can. 2011, 67, 35–40. 14. Marx, W.; Hoffmann, D. Bibliometric analysis of fifty years of physica status solidi. Phys. Status Solidi 2011, B248, 2762–2771. [CrossRef] 15. Müller, K.A. Paramagnetische Resonanz von Fe-3+ in SrTiO-3 Einkristallen. Helv. Phys. Acta 1958, 31, 173–433. 16. Müller, K.A. Electron Paramagnetic Resonance of Manganese IV in SrTiO3. Phys. Rev. Lett. 1959, 2, 341–343. [CrossRef] 17. Müller, K.A. Resonance paramagnetic du Cr3+ dans des monocristaux de SrTiO3. Publ. 7ème Colloq. Ampere Arch. Sci. 1958, 11, 150. 18. The Archives des Sciences éditées par la Société de Physique et d’Histoire Naturelle de Genève can be Accessed via. Available online: https://www.unige.ch/sphn/Publications/ArchivesSciences/ (accessed on 1 March 2020). 19. Shengelaya, A.; Zhao, G.; Keller, H.; Müller, K.A. EPR Evidence of Jahn-Teller Polaron Formation in La1 xCaxMnO3+y. Phys. Rev. Lett. 1996, 77, 5296–5299. [CrossRef][PubMed] − Condens. Matter 2020, 5, 43 9 of 9

20. Cardona, M.; Marx, W. Vitaly l. Ginzburg – A bibliometric study. J. Superconduct. Nov. Magn. 2006, 19, 459–466. [CrossRef] 21. Müller, K.A.; Burkhard, H. SrTiO3: An intrinsic quantum paraelectric below 4 K. Phys. Rev. B 1979, 19, 3593–3602. [CrossRef] 22. Waltman, L.; van Eck, N.J.; Noyons, E.C.M. A unified approach to mapping and clustering of bibliometric networks. J. Informetr. 2010, 4, 629–635. [CrossRef]

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