Evidence for line nodes in the energy gap of the overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ from low-temperature specific heat measurements

TL;DR

This study provides evidence of line nodes in the energy gap of overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ through low-temperature specific heat measurements, indicating doping-dependent changes in superconducting gap structure.

Contribution

It is the first to identify line nodes in the energy gap of overdoped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ using specific heat data across various doping levels.

Findings

Presence of a $T^2$ term in overdoped samples indicating line nodes.

Faster increase of field-induced electron specific heat coefficient in overdoped samples.

Superconducting gap structures vary significantly with doping.

Abstract

Low-temperature specific heat (SH) is measured on Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in a wide doping region under different magnetic fields. For the overdoped sample, we find the clear evidence for the presence of $T^2$ term in the data, which is absent both for the underdoped and optimal doped samples, suggesting the presence of line nodes in the energy gap of the overdoped samples. Moreover, the field induced electron specific heat coefficient $\Delta\gamma(H)$ increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, giving another support to our arguments. Our results suggest that the superconducting gap(s) in the present system may have different structures strongly depending on the doping regions.