how warm superconductors were discovered

The Mechanism

In *late 1985 and early 1986*, at the *IBM Zurich Research Laboratory* in the village of *Rüschlikon* on the south shore of *Lake Zurich*, the German solid-state physicist *Johannes Georg Bednorz* (born Neuenkirchen, *16 May 1950*; aged 35 at the time of the discovery) and the Swiss-born physicist *Karl Alexander Müller* (born Basel, *20 April 1927*; died Zurich, *9 January 2023*, aged 95; age 58 at the time) discovered that a brittle ceramic compound of *lanthanum, barium, copper, and oxygen* — chemically a *barium-doped lanthanum cuprate*, La₂₋ₓBaₓCuO₄, with the perovskite-derived "K₂NiF₄" crystal structure — became *superconducting* (lost all measurable electrical resistance and expelled magnetic flux) at a temperature of approximately *35 K* (kelvin). That temperature — *minus 238 degrees Celsius* — was *roughly twelve degrees above* the previous record of *23 K*, held since 1973 by the *niobium-germanide compound Nb₃Ge*; the record had stood, unmoved, for thirteen years, and the consensus view of the superconductivity community in 1985 was that the *Bardeen-Cooper-Schrieffer (BCS) theory* of conventional superconductivity (1957) effectively imposed a *ceiling* of about *30 K* on superconducting transition temperatures, beyond which lattice-mediated phonon coupling could not be expected to bind electrons into Cooper pairs against thermal disruption. The Bednorz-Müller discovery showed that the ceiling was not real. Their breakthrough was the more remarkable for the field they had walked into. Bednorz had received his PhD at the *Eidgenössische Technische Hochschule Zürich (ETH Zurich)* in 1982 under Müller's joint supervision, on perovskite-structure oxides; he had then taken a position at IBM Zurich. Müller, then *Fellow* and group leader at IBM Zurich, was an established figure in perovskite oxide research (he had worked on *SrTiO₃* and *BaTiO₃* for two decades) but had no prior reputation as a superconductivity researcher. Their joint research programme — initiated in *1983* and pursued *in secret*, without funding, with no announcements to the IBM materials science department or the broader superconductivity community — was to *systematically screen perovskite oxides* for superconductivity. The rationale was idiosyncratic: Müller had a hunch, based on the Jahn-Teller distortion theory of certain transition-metal ions, that *strong electron-phonon coupling* could be engineered in mixed-valence copper-oxide perovskites. The community considered the idea unpromising. The decade-long search through binary, ternary, and quaternary oxides through 1983-85 found nothing. The breakthrough came when Bednorz, in *late 1985*, read a paper by the French group *Claude Michel*, *Bernard Raveau*, and colleagues at the *University of Caen* describing the synthesis of *La-Ba-Cu-O mixed-oxide* phases for catalytic studies. Bednorz prepared the compound following the Caen recipe and began resistance-versus-temperature measurements in the IBM laboratory's *gas-flow cryostat*. On *27 January 1986*, the resistance dropped sharply at approximately *30 K* — a sign, but not yet a proof, of superconductivity (the curve could also have been from a phase transition, a structural change, or a measurement artefact). Over the next two months, Bednorz and Müller refined the composition (raising barium content), worked out which of the three crystallographic phases in their multiphase sample was the superconducting one (the *K₂NiF₄*-type tetragonal phase, with copper-oxygen layers separated by lanthanum-barium-oxygen spacing layers), and pushed the transition temperature to about *35 K*. On *17 April 1986*, they submitted their paper — *"Possible high-Tc superconductivity in the Ba-La-Cu-O system"* — to the German physics journal *Zeitschrift für Physik B Condensed Matter*. The journal's review process accepted it without major revision, and the paper was published in volume 64, pp. 189-193, in *the summer issue of 1986*. The paper's title carefully said *"possible"* — Bednorz and Müller had not yet performed the gold-standard *Meissner-effect* measurement (the expulsion of magnetic flux from a superconductor, which alongside zero resistance is the diagnostic test). They confirmed the Meissner effect later in 1986 in collaboration with *Masaaki Takashige* at IBM Zurich and Tokyo, and reported it in *Europhysics Letters* in October 1986. The international community ignored the paper for about *six months* — *Zeitschrift für Physik* was a respectable but not high-profile venue, and *"possible high-Tc"* was a strong claim from an obscure ceramicist. The first independent confirmation came from the group of *Shoji Tanaka* at the *University of Tokyo* in *November 1986*, and immediately afterward from *Paul Chu's* group at the *University of Houston* in collaboration with *Maw-Kuen Wu* at the *University of Alabama Huntsville*. Chu and Wu then took the breakthrough further: in *January 1987*, they replaced lanthanum with the smaller rare-earth element *yttrium* and produced *yttrium-barium-copper-oxide (YBCO, YBa₂Cu₃O₇)*, which superconducted at *93 K* — *above the boiling point of liquid nitrogen* (77 K), which meant that for the first time in the history of the field a superconductor could be operated using cheap, abundant *liquid nitrogen* as its cryogen rather than the rare and industrially-expensive liquid helium. The implications detonated through the international physics community. The *American Physical Society's March 1987 meeting* in New York — which had been planned as a routine condensed-matter conference at the *New York Hilton* — was overwhelmed by what is now remembered as *"The Woodstock of Physics"*: an unscheduled all-night session beginning at *7:30 PM on 18 March 1987* and running through to morning, in which an estimated *2,500 physicists* packed a hotel ballroom to capacity and the spillover rooms beyond, listening to *51 hastily-prepared talks* on the new cuprate superconductors. The *Nobel Prize in Physics* was awarded to Bednorz and Müller the following year — *October 1987*, less than nineteen months after their paper was submitted, the *fastest-ever Nobel Prize* from discovery to award in the history of physics. The cuprate family has since grown to over *100 known compounds*, with transition temperatures up to *138 K* at ambient pressure (mercury-barium-calcium-copper-oxide, *HgBa₂Ca₂Cu₃O₈*, Schilling et al. 1993) and up to *164 K* under pressure. The mechanism of cuprate superconductivity — *why* copper-oxide planes pair electrons at temperatures so far above the BCS limit — has been studied intensively for four decades and remains the central unsolved problem in condensed-matter physics. Cuprate superconductors are now in commercial use as *MRI magnets*, *power-grid current limiters*, *fault-current protectors* in major utility substations (the *AmpaCity Essen* installation in Germany has used YBCO cables since 2014 to deliver 10 MW of urban distribution power), and the *toroidal field coils* of the *Tokamak Energy ST40* fusion reactor and the *Commonwealth Fusion Systems SPARC* reactor under construction in Massachusetts. The 35 K Bednorz-Müller transition has been studied so thoroughly that the *original LBCO compound* is now a teaching demonstration in solid-state physics undergraduate courses worldwide. The IBM Rüschlikon laboratory where the discovery happened — the small *low ceiling room with the gas-flow cryostat* — is still in operation; Bednorz remained at IBM until his retirement in 2014, and Müller until his death in January 2023, never moving from the laboratory in the Swiss village.