Absence of superconductivity in NbB

TL;DR

This study investigates the absence of intrinsic superconductivity in NbB, attributing observed superconducting signals to a minor Nb-rich phase rather than the main NbB phase.

Contribution

It provides evidence that superconductivity in NbB samples is due to a Nb-rich minority phase, not the primary NbB phase, clarifying previous ambiguous results.

Findings

Superconductivity originates from Nb-rich Nb$_{ss}$ filaments.

Main NbB phase shows no intrinsic superconductivity.

Superconducting volume fraction is due to minority phase filaments.

Abstract

A systematic study of the superconducting properties in a series of arc-melted Nb-B samples close to the 1:1 composition was carried out. Powder X-ray diffraction (XRD) shows that all samples are both non-stoichiometric, and comprising of two crystal phases: a majority orthorhombic NbB-type phase, and traces of a minor body-centered cubic Nb-rich phase Nb$_{ss}$ with stoichiometry close to Nb$_{0.98}$B$_{0.02}$. The emergence of superconductivity near T$_c \sim$ 9.0 K was inferred from magnetization data in chunk and powder samples. However, the very small superconducting volume fractions are inconsistent with superconductivity arising from the major NbB phase. On the other hand, micrographs of selected samples clearly show that the minority Nb$_{ss}$ forms a three-dimensional network of filaments that meander around the grains of the majority phase, forming a percolation path. Here we report the superconductivity of the Nb$_{ss}$ phase, and argue that the low superconducting volume fraction of non-stoichiometric NbB and zero resistance are due to the filaments of the minority phase. The electronic contribution to the entropy of the superconducting state, yielded from an analysis using the alpha model for single-band systems, indicates that the Sommerfeld constant of the arc-melted samples is close to the values found in non-superconducting NbB. Micrograph, XRD, and bulk measurements of magnetization, electrical resistivity, and specific heat suggest that the superconducting state in the NbB samples bearing some Nb$_{ss}$ minority phase is due to the latter.