Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High−Tc Superconductivity

Abstract

Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at opposite limits of a continuum of pairing interaction strength between fermions. A crossover between these limits is readily observed in a cold atomic Fermi gas. Whether it occurs in other systems such as the high temperature superconducting cuprates has remained an open question. We uncover here unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a universal magic gap ratio 2Δ/kBTc≈6.5 (where Δ is the pairing gap and Tc is the transition temperature) at which paired fermion condensates become optimally robust. At this gap ratio, corresponding to the unitary point in a cold atomic Fermi gas, the measured condensate fraction N0 and the height of the jump δγ(Tc) in the coefficient γ of the fermionic specific heat at Tc are strongly peaked. In the cuprates, δγ(Tc) is peaked at this gap ratio when Δ corresponds to the antinodal spectroscopic gap, thus reinforcing its interpretation as the pairing gap. We find the peak in δγ(Tc) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above Tc.

Source-reported materials — not catalogue approval

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