Competition between the BCS superconductivity and ferromagnetic spin fluctuations in MgCNi$_3$

TL;DR

This study investigates the specific heat of MgCNi$_3$, revealing coexistence of BCS superconductivity and ferromagnetic spin fluctuations, with evidence supporting conventional s-wave pairing and the influence of magnetic fluctuations.

Contribution

It provides detailed experimental evidence of the competition between BCS superconductivity and ferromagnetic spin fluctuations in MgCNi$_3$, highlighting their impact on the superconducting state.

Findings

Additional specific heat contribution in MgCNi$_3$ is field-insensitive.

MgCNi$_3$'s specific heat aligns with BCS theory after subtracting the fluctuation contribution.

Evidence supports conventional s-wave pairing in MgCNi$_3$.

Abstract

The low temperature specific heat of the superconductor MgCNi$_3$ and a non-superconductor MgC$_{0.85}$Ni$_3$ is investigated in detail. An additional contribution is observed from the data of MgCNi$_3$ but absent in MgC$_{0.85}$Ni$_3$, which is demonstrated to be insensitive to the applied magnetic field even up to 12 Tesla. A detailed discussion on its origin is then presented. By subtracting this additional contribution, the zero field specific heat of MgCNi$_3$ can be well described by the BCS theory with the gap ratio ($\Delta/k_BT_c$) determined by the previous tunneling measurements. The conventional s-wave pairing state is further proved by the magnetic field dependence of the specific heat at low temperatures and the behavior of the upper critical field.