Magic Gap Ratio at the "BCS Superconducting to Bose-Einstein Condensate" Crossover in the High-Tc Cuprates

Published November 14, 2022

BCS (left) refers to a conventional Bardeen-Schrieffer-Cooper state (i.e. weak pairing state) found in most superconductors, while BEC (right) refers to the strong pairing limit of a Bose-Einstein Condensate.

A defining experimental signature of a crossover in the strength of the pairing interactions from the weak coupling BCS to the strong coupling Bose-Einstein condensation limit has been discovered in high temperature superconductors.

What did scientists discover?

A Bose-Einstein condensate (BEC) is a macroscopic quantum state in which all of the particles acquire phase coherence at low temperatures. In doing so, they exhibit superfluidity. MagLab scientists compared data from seven different superconducting cuprates and found a magic gap ratio at which the robustness of the condensed state is a maximum. This indicates that the same underlying principles that cause pairs of fermions to acquire macroscopic phase coherence at ultracold temperatures also cause the same effect to occur in the high-T_c cuprates.

Why is this important?

Tunable interactions that lead to a crossover in the pairing from conventional (BCS) superconductivity to BEC are believed to be universal to all fermionic systems throughout the cosmos. Tunable interactions are believed to be important for understanding the formation of atomic nuclei, quark-gluon plasmas in the early universe, and color superconductivity in neutron stars. A similar magic gap ratio, at which superfluid states become optimally robust, is also predicted to occur in the iron-based superconductors and twisted graphene.

Who did the research?

Neil Harrison and Mun K. Chan

National MagLab - LANL

Why did they need the MagLab?

In addition to having unique high magnetic field magnets, the MagLab supports world-class in-house scientific research. This much-publicized in-house research benefited from interactions with many MagLab users and was supported in part by the “Science of 100 Tesla” grant from the Department of Energy’s Basic Energy Sciences.