Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric eta-Carbide Nb4Rh2C1-delta

The upper criticalfield is a fundamental measure of the strengthof superconductivity in a material. It is also a cornerstone for therealization of superconducting magnet applications. The critical fieldarises because of the Cooper pair breaking at a limiting field, whichis due to the Pauli paramagnetism of the electrons. The maximal possiblemagnetic field strength for this effect iscommonly known as the Pauli paramagnetic limit given as mu(0) H (Pauli) approximate to 1.86-[T/K]center dot T (c) for a weak-coupling Bardeen-Schrieffer-Cooper(BCS) superconductor. The violation of this limit is only rarely observed.Exceptions include some low-temperature heavy Fermion and some stronglyanisotropic superconductors. Here, we report on the superconductivityat 9.75 K in the centrosymmetric, cubic eta-carbide-type compoundNb(4)Rh(2)C(1-delta), with a normalizedspecific heat jump of Delta C/gamma T (c) = 1.64. We find that this material has a remarkablyhigh upper critical field of mu(0) H (c2)(0) = 28.5 T, which is exceeding by far its weak-couplingBCS Pauli paramagnetic limit of mu(0) H (Pauli) = 18.1 T. Determination of the origin and consequencesof this effect will represent a significant new direction in the studyof critical fields in superconductors.