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.
Mun Keat Chan (Los Alamos National Lab) and Neil Harrison (Los Alamos National Lab) uncovered 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. The team found the peak in δγ(T𝒸) coincided with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T𝒸. Their research was recently published in Physical Review Letters.
Chan and Harrison sat down with the Physical Review Journal Club to discuss their research observing BCS to BEC crossover in high temperature superconducting cuprates. The first observed manifestation of this phenomenon in an experiment.
Chan and Harrison gave their presentation recounting their methods and data from their experiment, followed by a live question-and-answer session. This session was moderated by Sasa Dordevic, University of Akron, Ohio