Correlation between Superconducting Carrier Density and Transition Temperature in NbB 2+ x (original) (raw)

We report on the magnetic penetration depth, λ, in a type II superconductor NbB 2+x determined by muon spin rotation method. We show in the sample with x = 0.1 that λ at 2.0 K is independent of an applied magnetic field. This suggests that the superconducting order parameter in NbB 2+x is isotropic, as expected for conventional BCS superconductors. Meanwhile, the superconducting carrier density (∝ λ −2 ) exhibits an interesting tendency of increase with increasing T c (where T c varies with x). Possible origin of such behavior is discussed in comparison with the case of exotic superconductors. The recent revelation of superconductivity in MgB 2 with high transition temperature (T c = 39K) 1 has revived much interest in hexagonal diborides which were once subjected to active studies from 1950's through 70's. 2-4 The discovery of MgB 2 prompted various theoretical and experimental investigations to clarify the origin of the high T c seemingly exceeding the so-called 'BCS-barrier' (∼ 30 K). These compounds have a simple layered structure along c-axis, consisting of an alternative stack of metal ion layers forming a triangular lattice and boron atom layers in a honeycomb network. In MgB 2 , both theoretical and experimental studies suggested that the strong electron-phonon interaction in accordance with the light mass of the organized atoms and two-dimensional character of boron atom network are closely related to the relatively high-T c superconductivity. Moreover, it turned out that some of the unconventional properties in the superconducting phase can be attributed to the multiband superconductivity, where the presence of two energy gaps corresponding to p π and p σ orbits of boron atoms have been confirmed by experimental 5 and theoretical studies. 6 Among various diborides, NbB 2 is intriguing as it exhibits strong sensitivity of superconducting transition temperature to the stoichiometric imperfection; Cooper et al. reported T c = 3.87 K for boron-rich NbB 2 , 3 while Leyarovska et al. reported much lower T c ( = 0.62 K)