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DOE OSTI · 1879408

Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High- $T_c$ Superconductivity

Harrison, Neil·Chan, Mun Keat
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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 B T c ≈ 6.5 (where Δ is the pairing gap and T c 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(T c ) in the coefficient γ of the fermionic specific heat at T c are strongly peaked. In the cuprates, δγ(T c ) 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(T c ) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T c .

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICSBEC-BCS crossovercupratesmagnetic susceptibility specific heatsuperconducting gapsuperconducting transitionsuperfluid density

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Harrison, Neil, Chan, Mun Keat. 2022-06-29. Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High- $T_c$ Superconductivity. https://doi.org/10.1103/physrevlett.129.017001

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