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

Abstract

With the aim of providing experimental information for the Nb deficiency enhanced superconducting temperature (TC) in the Nb1−xB2 samples, we carried out a study on Nb1−xB2 (x=0, 0.13, 0.20, and 0.26) by means of nuclear magnetic resonance (NMR) and Seebeck coefficient (S) measurements. From the N93b NMR spin-lattice relaxation rates, we can deduce the Nb 4d partial Fermi level density of states (DOS) Nd(EF) for each individual composition. The result indicates that Nb0.74B2 and Nb0.80B2 possess large Nd(EF) while the lowest one appears in Nb0.87B2. The Seebeck coefficient also shows smaller absolute values in Nb0.74B2 and Nb0.80B2, associated with higher Fermi level DOS in both compounds. Interestingly, the temperature variation in S exhibits a broad minimum in Nb0.87B2, which is attributed to the presence of a pseudogap near the Fermi level. These observations were found to be in good agreement with the prediction from band structure calculations based on the appearance of the Nb vacancies in Nb1−xB2. In addition, the present study clearly reveals that the observed Tc enhancement by Nb deficiency has no direct relevance to their electronic Fermi level DOS.

Evidence fors-wave superconductivity in noncentrosymmetric Re24Nb5fromLue1, Su2, Liu3 et al. 2011Phys. Rev. BSelf Cite

“…The specimen was put in a plastic vial that showed no observable 93 Nb NMR signal. 17 The 93 Nb NMR central transition lines were obtained by the Fourier transform of a half of the spin-echo signal using a standard π/2−τ − π sequence. Within the Re 24 Ti 5type crystal structure, there are two crystallographic Nb sites, Nb(1) and Nb(2), with population ratio of 1:4.…”

11B NMR Spectral and Nuclear Spin–Lattice Relaxation Analyses of ReB2

“…These properties are thought to arise from or at least be influenced by the boron bonding environments , found in these materials. As a result, numerous nuclear magnetic resonance (NMR) studies have been reported. ,− …”