An unstable pathway to room temperature superconductivity?

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In PNAS, an international collaboration among groups at the University of Houston and Rice University in the United States and Jilin and Linyi Universities in China reports a discovery that in iron-selenide (FeSe) superconductivity survives up to the four times higher temperature if the sample is pressure quenched (1). In what follows, I will try to explain why (I believe) this is very exciting and eye opening. Kamerlingh Onnes, in 1911, observed that the electric resistivity of mercury vanished below T c = 4.2 K. Subsequently, many other materials were also found to show superconductivity, but for well over six decades, this remained a very low–temperature phenomenon. Why this is so was explained by Bardeen, Cooper, and Schrieffer. Their (“BCS”) theory tied the maximal temperature at which superconductivity can survive in a particular material to its phonon and electron spectra and the electron–phonon interaction. The (simplest version of) BCS expression for that “critical” temperature ( T c ) reads kB T c = 1.13ħ ω D × exp[−1/ N (0) V 0 ], where kB and ħ are the Boltzmann and Planck constants, respectively; ω D is the characteristic (Debye) phonon frequency; N (0) is the electronic density of states at the Fermi level; and V 0 is the electron–phonon coupling potential. Until the mideighties, if one inserted the parameters of any known superconducting material, one would infer that T c < 25 K, in agreement with what was observed experimentally. In principle, the BCS theory encodes a recipe of how to raise T c : increase ω D , N (0), and/or V . In practice, unfortunately, the parameters N (0) and V 0 are hard to change in a given material. One can increase ω D , however, by applying a (high) pressure, and hence, this line … [↵][1]1Email: bozovic{at}bnl.gov. [1]: #xref-corresp-1-1