Specific heat measurements of the gap structure of the organic superconductors kappa-(ET)_2Cu[N(CN)_2]Br and kappa-(ET)_2Cu(NCS)_2

TL;DR

This study uses high-resolution heat capacity measurements to investigate the superconducting gap structure of two organic superconductors, revealing evidence for d-wave symmetry through temperature-dependent electronic specific heat analysis.

Contribution

The paper provides detailed heat capacity data in high magnetic fields, demonstrating d-wave superconductivity in kappa-(ET)_2Cu[N(CN)_2]Br and kappa-(ET)_2Cu(NCS)_2, challenging s-wave models.

Findings

Electronic specific heat varies as T^2 below 0.3 T_c.

Data inconsistent with s-wave symmetry.

Supports strong coupling d-wave superconductivity.

Abstract

We present high resolution heat capacity measurements for the organic superconductors kappa-(ET)_2Cu[N(CN)_2]Br and kappa-(ET)_2Cu(NCS)_2 in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat C_{el} in the superconducting state in zero and finite fields. We find that in both materials for T/T_c<0.3, C_{el}(H=0)\sim T^2 indicating d-wave superconductivity. Our data are inconsistent with s-wave behavior, but may be fitted to a strong coupling d-wave model over the full temperature range.