Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric η-Carbide Nb4Rh2C1-δ

KeYuan Ma; Karolina Gornicka; Robin Lefèvre; Yikai Yang; Henrik M Rønnow; Harald O Jeschke; Tomasz Klimczuk; Fabian O von Rohr

doi:10.1021/acsmaterialsau.1c00011

Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric η-Carbide Nb₄Rh₂C_1-δ

ACS Mater Au. 2021 Jun 22;1(1):55-61. doi: 10.1021/acsmaterialsau.1c00011. eCollection 2021 Sep 8.

Authors

KeYuan Ma¹, Karolina Gornicka², Robin Lefèvre¹, Yikai Yang³, Henrik M Rønnow³, Harald O Jeschke⁴, Tomasz Klimczuk², Fabian O von Rohr¹

Affiliations

¹ Department of Chemistry, University of Zurich, CH-8057 Zürich, Switzerland.
² Faculty of Applied Physics and Mathematics, Gdansk University of Technology, Gdansk 80-233, Poland.
³ Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
⁴ Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan.

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 Cooper 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 μ₀ H _Pauli ≈ 1.86[T/K]·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 strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbide-type compound Nb₄Rh₂C_1-δ, with a normalized specific heat jump of ΔC/γT _c = 1.64. We find that this material has a remarkably high upper critical field of μ₀ H _c2(0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of μ₀ H _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.