Doping-induced superconductivity of ZrB${}_2$ and HfB${}_2$

Abstract

Unlike the widely studied $s$-type two-gap superconductor MgB${}_2$, the chemically similar compounds ZrB${}_2$ and HfB${}_2$ do not superconduct above 1 K. Yet, it has been shown that small amounts of self- or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macro- and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation ($\hat{I}¼$SR), we present a comparative study of Zr${}_{0.96}$V${}_{0.04}$B${}_2$ and Hf${}_{0.97}$V${}_{0.03}$B${}_2$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for MgB${}_2$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between MgB${}_2$ and transition-metal diborides.

10 pages, 8 figures