Superconductivity of non- stoichiometric intermetallic compound NbB2

TL;DR

This study investigates how non-stoichiometric NbB2+x compounds exhibit superconductivity, revealing that lattice parameter modifications due to boron content induce superconductivity in otherwise non-superconducting NbB2.

Contribution

It demonstrates that varying Nb/B ratios in NbB2+x induces superconductivity, highlighting the role of lattice parameter changes rather than processing conditions.

Findings

Superconductivity appears in NbB2+x samples with x>0.0.

Lattice c parameter increases with boron content up to x=0.4.

Critical fields Hc1 and Hc2 are approximately 590 Oe and 2000 Oe.

Abstract

We report the synthesis, magnetic susceptibility and crystal structure analysis for NbB2+x (x = 0.0 to 1.0) samples. The study facilitates in finding a correlation among the lattice parameters, chemical composition and the superconducting transition temperature Tc. Rietveld analysis is done on the X- ray diffraction patterns of all synthesized samples to determine the lattice parameters. The a parameter decreases slightly and has a random variation with increasing x, while c parameter increases from 3.26 for pure NbB2 to 3.32 for x=0.4 i.e. NbB2.4. With higher Boron content (x>0.4) the c parameter decreases slightly. The stretching of lattice in c direction induces superconductivity in the non- stoichiometric niobium boride. Pure NbB2 is non-superconductor while the other NbB2+x (x>0.0) samples show diamagnetic signal in the temperature range 8.9-11K. Magnetization measurements (M-H) at a fixed temperature of 5K are also carried out in both increasing and decreasing directions of field. The estimated lower and upper critical fields (Hc1 & Hc2) as viewed from M-H plots are around 590 and 2000Oe respectively for NbB2.6 samples. In our case, superconductivity is achieved in NbB2 by varying the Nb/B ratios, rather than changing the processing conditions as reported by others.