Low Temperature Specific Heat of Optimally Doped BaFe$_{2-x}$$TM_x$As$_{2}$ ($TM$ = Co and Ni) Single Crystals: Constraint on the Pairing Gap

TL;DR

This study measures low temperature specific heat in doped BaFe$_{2-x}$TM$_x$As$_{2}$ crystals to investigate the superconducting gap structure, revealing evidence for nodal lines or point nodes in the pairing gap.

Contribution

It provides new insights into the pairing gap symmetry by analyzing the temperature dependence of electronic specific heat in optimally doped iron-based superconductors.

Findings

Electronic specific heat coefficient $oldsymbol{\gamma_e}$ follows a quadratic temperature dependence.

Small residual $oldsymbol{\gamma_0}$ indicates minimal impurity effects.

Results suggest the presence of nodal lines or point-like nodes in the superconducting gap.

Abstract

Low temperature specific heat has been measured in optimally doped and highly overdoped non-superconducting BaFe$_{2-x}$TM$_x$As$_{2}$ ($TM$ = Co and Ni) single crystals. By using the data of the overdoped samples, we successfully removed the phonon contribution of the optimally doped ones, and derived the electronic specific heat coefficient $\gamma_e$. Remarkably, we found a continuing temperature dependent $\gamma_e(T)$ which follows the quadratic relation $\gamma_e=\gamma_0+\alpha T^2$ in the low temperature limit. Together with the very small residual term $\gamma_0$, linear magnetic field dependence of $\gamma_e$, it is concluded that there are either small segments of nodal lines, or point-like nodes in these samples.