The average kinetic energy density of Cooper pairs above T_c in YBa_2Cu_3O_{7-x}}, Bi_2Sr_2CaCu_2O_{8+\delta}, and Nb}

TL;DR

This study measures the kinetic energy density of Cooper pairs in various superconductors above T_c, revealing a persistent condensate in high-T_c materials that differs from conventional superconductors.

Contribution

It introduces a method to analyze the kinetic energy density above T_c using isofield magnetization curves and the virial theorem, highlighting differences in superconducting states.

Findings

High-T_c superconductors show a superconducting condensate above T_c.

Nb exhibits conventional behavior with no condensate above T_c.

Differences suggest a unique pairing mechanism in high-T_c materials.

Abstract

We have obtained isofield curves for the square root of the average kinetic energy density of the superconducting state for three single crystals of underdoped YBa_2Cu_3O_{7-x}, an optimally doped single crystal of Bi_2Sr_2CaCu_2O_{8+\delta}, and Nb. These curves, determined from isofield magnetization versus temperature measurements and the virial theorem of superconductivity, probe the order parameter amplitude near the upper critical field. The striking differences between the Nb and the high-T_c curves clearly indicate for the latter cases the presence of a unique superconducting condensate below and above T_c.