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.
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