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Specific Heat and Tunneling Spectroscopy Study of NbB2 with Maximum Tc∼10 K

We report on magnetic, transport, specific heat and tunneling measurements of NbB 2 with the maximum superconducting critical temperature T c ∼10 K. The essential superconducting properties have been investigated through mapping out the critical field phase diagram, from which it is turned out to be type II superconductor with a small Ginzbrug–Landau parameter κ∼2.2. Both the tunneling and specific heat measurements indicate that our NbB 2 specimen has a single gap and s -wave pairing symmetry with a weak-coupling limit. The electron–phonon coupling constant is estimated to be λ ep =0.71±0.02, suggesting that the maximum T c of NbB 2 can be explained by the McMillan formula.