Specific heat and tunneling spectroscopy study of NbB₂ with maximum T_c ∼ 10K

We report on magnetic, transport, specific heat and tunneling measurements of NbB2 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 NbB2 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 NbB₂ can be explained by the McMillan formula.