Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3

Abstract

We have measured upper-critical-field $H_{\text{c}2}$, specific heat C, and tunneling spectra of the intermetallic perovskite superconductor MgCNi${}_3$ with a superconducting transition temperature $T_{\text{c}}\approx 7.6$ K. Based on these measurements and relevant theoretical relations, we have evaluated various superconducting parameters for this material, including the thermodynamic critical field $H_{\text{c}}$(0), coherence length $\hat{I}¾$(0), penetration depth $\hat{I}»$(0), lower-critical-field $H_{\text{c}1}$(0), and Ginsberg-Landau parameter $\hat{I}º$(0). From the specific heat, we obtain the Debye temperature $\hat{I}{˜}_{\text{D}}\approx$ 280 K. We find a jump of $\hat{I}”C/{\hat{I}}^3{T}_{\text{c}}$=2.3 at $T_{\text{c}}$ (where ${\hat{I}}^3$ is the normal state electronic specific coefficient), which is much larger than the weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap feature in the tunneling spectra at $\hat{I}”$ with $2\hat{I}”/{\text{k}}_{\text{B}}{T}_{\text{c}}\approx$ 4.6, again larger than the weak-coupling value of 3.53. Both findings indicate that MgCNi${}_3$ is a strong-coupling superconductor. In addition, we observed a pronounced zero-bias conductance peak (ZBCP) in the tunneling spectra. We discuss the possible physical origins of the observed ZBCP, especially in the context of the pairing symmetry of the material.

5 pages, 4 figures