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/k_BT_c ~ 6.5 (where Δis 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 condensate fraction N_0 and the height of the jump δγ(Tc) in the coefficient γof the fermionic specific heat at Tc are strongly peaked. In the cuprates, δγ(Tc) is peaked at this gap ratio when Δcorresponds 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.

15 pages including 8 figures

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Cited by in corpus (8)

• When Superconductivity Crosses Over: From BCS to BEC
• Absence of a BCS-BEC crossover in the cuprate superconductors
• Andreev reflection spectroscopy in strongly paired superconductors
• Pseudogap formation in organic superconductors
• Test for BCS-BEC Crossover in the Cuprate Superconductors
• Topological two-band electron-hole superconductors with $d$-wave symmetry: Absence of Dirac quasiparticle annihilation in magic-angle twisted trilayer graphene
• Quantum Monte Carlo study of the metal-insulator crossover in the square-lattice Hubbard model
• Tunneling spectra of unconventional quasi-two-dimensional superconductors