Magnetic-field-induced density of states inMgB2: Spin susceptibility measured by conduction-electron spin resonance

It is now generally accepted that MgB2 is a phononmediated superconductor 1–3 which owes its unusual properties to the strong differences in the electron-phonon coupling of its disconnected Fermi surface sheets. 4 The two-gap model assigns a large superconductor gap to the cylindrical Fermi surface sheets originating from B – B bonds and a much smaller gap to the sheets of electrons. The model successfully describes the temperature dependence of several properties like the specific heat 5 or the electron tunneling spectra 6 in zero magnetic field while predictions for finite magnetic fields are less tested. The strong electronphonon coupling on the sheets maintains superconductivity to about Hc2 c 2.5 T for fields perpendicular to and H c2 ab 16 T for fields lying in the crystalline a ,b boron planes. 7 It has been suggested that the weaker gap is closed by much smaller fields H c2 without destroying superconductivity. The actual value of Hc2 is rather uncertain: from single-crystal specific heat experiments 8 an isotropic Hc2 = 0.3– 0.4 T, from vortex imaging 9 0.13 T has been estimated, while point contact tunneling experiments 10–13 suggest values between 1 and 2 T.