Evidence of unconventional superconductivity in the Ni-doped NbB2 system

TL;DR

This study demonstrates that Ni-doping in NbB2 induces unconventional multiband superconductivity with increased critical temperature and anomalous critical field behaviors, supported by comprehensive structural, magnetic, and thermal analyses.

Contribution

It provides new evidence of unconventional multiband superconductivity in Ni-doped NbB2, highlighting the effects of Ni substitution on physical properties and critical fields.

Findings

Ni-doping induces bulk superconductivity in NbB2.

Critical temperature increases to ~6.0 K at 9% Ni doping.

Critical fields show linear dependence and specific heat deviates from BCS theory.

Abstract

In this paper, a comprehensive study of the effects of Ni-doping on structural, electrical, thermal and magnetic properties of the NbB2 is presented. Low amounts (\leq 10 %) of Ni substitution on Nb sites cause structural distortions and induce drastic changes in the physical properties, such as the emergence of a bulk superconducting state with anomalous behaviors in the critical fields (lower and upper) and in the specific heat. Ni-doping at the 9 at.% level, for instance, is able to increase the critical temperature (TC) in stoichiometric NbB2 (< 1.3 K) to approximately 6.0 K. Bulk superconductivity is confirmed by magnetization, electronic transport, and specific heat measurements. Both Hc1 and Hc2 critical fields exhibit a linear dependence with reduced temperature (T/TC), and the specific heat deviates remarkably from the conventional exponential temperature dependence of the single-band BCS theory. These findings suggest multiband superconductivity in the composition range from 0.01 \leq x \leq 0.10.