Absence of superconductivity in NbB

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 nonstoichiometric and composed of two crystal phases: a majority orthorhombic NbB-type phase and traces of a minor body-centered-cubic Nb-rich phase ${\mathrm{Nb}}_{ss}$ with stoichiometry close to ${\mathrm{Nb}}_{0.98}{\mathrm{B}}_{0.02}$. The emergence of superconductivity near $T_c\sim 9.0\mathrm{K}$ 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 ${\mathrm{Nb}}_{ss}$ 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 ${\mathrm{Nb}}_{ss}$ phase and argue that the low superconducting volume fraction of nonstoichiometric 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 nonsuperconducting NbB. Micrograph, XRD, and bulk measurements of magnetization, electrical resistivity, and specific heat suggest that the superconducting state in the NbB samples bearing some ${\mathrm{Nb}}_{ss}$ minority phase is due to the latter.

Figure 1

Typical normalized XRD $\theta \text{−}2\theta$ scans for as-cast NbB (NbB#1). The red line represents a Rietveld refinement obtained with FULLPROF and the difference plots are shown at the bottom. The reflection corresponding to the minority phase solid solution ${\mathrm{Nb}}_{ss}$ is indicated with an asterisk. The inset displays an expanded view of XRD scans for five different samples near $2\theta \sim 38.5^{\circ }$, the most intense reflection for ${\mathrm{Nb}}_{ss}$.

Figure 2

(a) Temperature dependence of the electrical resistivity $\rho \left(T\right)$ for NbB (NbB#1) and the Nb foil material used as the starting material. The monotonic decrease of $\rho \left(T\right)$ with temperature indicates metallic behavior for both samples. The superconducting transition region is exhibited in the inset. Also shown are field-cooled and zero-field cooled magnetization $M\left(T\right)$ curves for an as-cast chunk [black symbols in (b)] and its powder [red symbols in (c)] of NbB (NbB#1) under an external dc magnetic field of 5 Oe.

Figure 3

Micrograph images for two NbB samples, (a) and (b) for NbB#1 and (c) and (d) for NbB#2. The red arrows point to white areas, which are identified with the solid solution ${\mathrm{Nb}}_{ss}$. The grayish areas are from the majority NbB phase.

Figure 4

(a) Reduced temperature $T/T_c$ dependence of the $C_{\text{ele}}/{\gamma }_n{T}_c$ ratio. The fit to the single-band model described in the text is represented by the red line. Also shown are plots of $C_p/T$ vs $T^2$ for samples (b) NbB#2 and (c) ${\mathrm{Nb}}_{0.95}{\mathrm{B}}_{0.05}$. The blue lines are extrapolations from the normal state data and their values at $T^2=0$ were taken as the Sommerfeld coefficients ${\gamma }_n$.