Symmetry change of $d$-wave superconductivity in $\kappa$-type organic superconductors

TL;DR

This study investigates the symmetry of $d$-wave superconductivity in $"kappa$-type organic superconductors using heat capacity measurements, revealing different gap symmetries linked to lattice structure and magnetic fluctuations.

Contribution

It identifies the specific $d$-wave gap symmetries in different $"kappa$-type organic superconductors through angle-resolved heat capacity analysis, highlighting the role of magnetic fluctuations.

Findings

Superconductivity exhibits line nodes on the Fermi surface.

Gap symmetry is $d_{x^{2}-y^{2}}$+$s_{ ext{pm}}$ for X=Cu[N(CN)$_2$]Br.

Gap symmetry is $d_{xy}$ for X=Ag(CN)$_2$H$_2$O.

Abstract

Magnetic-field-angle-resolved heat capacity of dimer-Mott organic superconductors $\kappa$-(BEDT-TTF)$_2$$X$ is reported. Temperature and field dependence of heat capacity indicates that the superconductivity has line nodes on the Fermi surface, which is a typical feature of $d$-wave superconductivity. In-plane field-angle dependence exhibits fourfold oscillations as well as twofold oscillations due to anisotropy of superconducting gap functions. From analyses of the fourfold term, the gap symmetry is determined as $d_{x^{2}-y^{2}}$+$s_{\pm}$ for $X$=Cu[N(CN)$_2$]Br and $d_{xy}$ for $X$=Ag(CN)$_2$H$_2$O. We suggest that the symmetry difference comes from the competition of $d_{xy}$ and $d_{x^{2}-y^{2}}$ antiferromagnetic fluctuations depending on lattice geometry.