Is Room‐temperature Superconductivity with Phonons Possible?

By recognizing the vital importance of two‐hole Cooper pairs (CPs) in addition to the usual two‐electron ones in a strongly‐interacting many‐electron system, the concept of CPs was re‐examined with striking conclusions: namely, they are gapped and linearly‐dispersive resonances with a finite lifetime—but provided the ideal‐gas Fermi sea is replaced by a BCS‐correlated unperturbed ground‐state “sea.” Based on this, Bose‐Einstein condensation (BEC) theory has been generalized to include not boson‐boson interactions (also neglected in BCS theory) but rather boson‐fermion (BF) interaction vertices reminiscent of the Fröhlich electron‐phonon interaction in metals. Instead of phonons, the bosons in the generalized BEC (GBEC) theory are now both particle and hole CPs. Unlike BCS theory, the GBEC model is not a mean‐field theory restricted to weak‐coupling as it can be diagonalized exactly. In weak coupling it reproduces the BCS condensation energy, and the next‐order‐in‐coupling term increases its magnitude with respect to BCS. Each kind of CP is responsible for only half the condensation energy. The GBEC theory reduces to all the old known statistical theories as special cases—including the so‐called “BCS‐Bose crossover” picture which in turn generalizes BCS theory by not assuming that the interelectronic chemical potential equals the Fermi energy. Indeed, a BCS condensate is precisely the weak‐coupling limit of a GBE condensate with equal numbers of both types of CPs. With feasible Cooper/BCS model interelectonic interaction parameter values, and even without BF interactions, the GBEC theory yields transition temperatures [including room‐temperature superconductivity (RTSC)] substantially higher than the BCS ceiling of around 45K, without relying on non‐phonon dynamics involving excitons, plasmons, magnons or otherwise purely‐electronic mechanisms. The results are expected to shed light on the experimental search for RTSC. © 2006 American Institute of Physics