Specific heat and magnetization of the superconducting monoxides: NbO and TiO

Abstract

The specific heat of five $\mathrm{Nb}{\mathrm{O}}_x$ samples ($x$ between 0.96 and 1.02) is reported for the Meissner, mixed, and normal states. The specific heat of seven $\mathrm{Ti}{\mathrm{O}}_x$ samples, covering the range of composition from 0.91 to 1.17, is discussed and compared to previous work. For NbO, the transition temperature $T_0$ reaches a maximum of 1.61 K near $x=1.0\mathrm{}$; for TiO, the transition temperature is not a function of composition. The densities of states of both systems are derived from the coefficients of the electronic specific heat $\gamma$ and compared to the band-structure calculations. While NbO is found to be a low-$\kappa$ intrinsic type-II superconductor, TiO is a high -$\kappa$ dirty type-II material. Magnetization measurements on six $\mathrm{Nb}{\mathrm{O}}_x$ samples indicate that a deviation of stoichiometry by 1 at.% on the low oxygen side produces a multiphase system; the majority of such a sample is NbO and the minor phase is free Nb metal with a transition temperature around 6 K. Although magnetization data indicate too large a fraction for the minor phase, specific-heat data show a more reasonable value for this fraction.