Correlations among superconductivity, structural instability, and band filling inNb1−xB2at the critical point

Abstract

We performed an extensive investigation on the correlations among superconductivity, structural instability and band filling in

NMR and Seebeck coefficient studies of the electronic band structure in Nb1−xB2

“…It clearly exhibits an increase in c but a reduction in a as x Ն 0.2 of Nb 1−x B 2 . Such an observation is consistent with that reported in the literature, [14][15][16]20,26 suggesting a similar quality of these samples.…”

“…Remarkably, such a feature is in good agreement with the prediction from band structure calculations based on the existence of the Nb vacancies in the Nbdeficient Nb 1−x B 2 samples. 22,26 Those calculations indicated a notable pseudogap, about 1 eV below E F , which locates at the antibonding states for NbB 2 . By removing Nb from stoichiometry, E F will shift toward the low energy side accompanied by the decrease in N͑E F ͒.…”

“…The corresponding T c gradually increases with x and tends to saturate as x Ն 0.2, being consistent with the previous observations. 14,15,17,20,26 NMR measurements were performed using a Varian 300 spectrometer, with a constant field of 7.05 T. Each specimen was put in a plastic vial that showed no observable 93 Nb NMR signal. Since the 93 Nb NMR resonance line is extremely quadrupolar broadened, the wide-line spectra were mapped out by integrating the spin echo signal of various excitations.…”

Mechanical and electronic properties of diborides of transition 3d–5d metals from first principles: Toward search of novel ultra-incompressible and superhard materials

“…Next, investigation of non-stoichiometric effects on the properties of borides, in particular, the influence of metal deficiency, seems to be fundamentally important. This effect (metal deficiency) was established [255][256][257][258][259] and simulated [65,[260][261][262][263][264] for a series of diborides of s, p, and d metals: Nb 1Àx B 2 , Ta 1Àx B 2 , Al 1Àx B 2 , Mg 1Àx B 2 , etc., review [66]. Other interesting directions involve ab initio simulations of elastic properties of metal diborides under pressure [242,265], as well as numerical calculations of lattice dynamical properties (phonon spectra) and the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy of these materials [266].…”

Valence band XPS and UPS studies of non-stoichiometric superconducting NbB2 +x

“…In order to explain the decrease of intensity of the valence band spectra of the compositions studied, we reviewed the results of band-structure calculations on the Nb 1−x B 2 [11,18]. The main results of these calculations show that the N(E F ) for this system decrease with respect to NbB 2 , whereas near the Fermi level the density of B 2p states increases while that for Nb 4d states decreases.…”