Effect of annealing on the specific heat of optimally doped Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$

TL;DR

This study investigates how annealing affects the low-temperature specific heat and superconducting properties of electron-doped Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$, revealing changes in residual heat capacity and evidence for anisotropic gap structures.

Contribution

It provides the first detailed comparison of specific heat in annealed and non-annealed samples of this superconductor, highlighting the impact of heat treatment on electronic excitations and gap symmetry.

Findings

Annealing increases $T_c$ from 20 K to 25 K.

Residual specific heat $ ext{$ ext{γ}_0$}$ decreases after annealing.

Data suggests an anisotropic superconducting gap, possibly with nodes.

Abstract

We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2}$As$_{2}$ superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows $T_{c}$=20 K, residual specific heat $\gamma_{0}$=3.6 mJ/mol K$^{2}$ and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 $^{o}C$ for two weeks and shows $T_{c}$ = 25 K and $\gamma_{0}$=1.4 mJ/mol~K$^{2}$. By subtracting the lattice specific heat, from pure BaFe$_{2}$As$_{2}$, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of $C_{el}(T)$ clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes