Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric $\eta$-Carbide Nb${}_4$Rh${}_2$C${}_{1-\delta }$

Abstract

The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as ${\mu }_0{H}_{\mathrm{Pauli}}\approx 1.86[T/K]\cdot T_{\mathrm{c}}$ for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic $\eta$-carbide-type compound Nb${}_4$Rh${}_2$C${}_{1-\delta }$, with a normalized specific heat jump of $\Delta C/\gamma T_{\mathrm{c}}=$ 1.64. We find that this material has a remarkably high upper critical field of ${\mu }_0{H}_{c2}(0)$ =~28.5~T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of ${\mu }_0{H}_{\mathrm{Pauli}}$~=~18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.

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• Topological States in Chevrel Phase Materials from First-principle Calculations
• Discovery of Superconductivity in Nb${}_4$SiSb${}_2$ with a V${}_4$SiSb${}_2$-Type Structure and Implications of Interstitial Doping on its Physical Properties
• Einstein and Debye temperatures, electron-phonon coupling constant and a probable mechanism for ambient-pressure room-temperature superconductivity in intercalated graphite
• Discovery of the Type-II Superconductor Ta${}_4$Rh${}_2$C${}_{1-\delta }$ with a High Upper Critical Field
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