Experimental support is found for the multiband model of the superconductivity in the recently discovered system MgB2 with the transition temperature Tc = 39 K. By means of Andreev reflection evidence is obtained for two distinct superconducting energy gaps. The sizes of the two gaps (∆S = 2.8 meV and ∆L = 7 meV) are respectively smaller and larger than the expected weak coupling value. Due to the temperature smearing of the spectra the two gaps are hardly distinguishable at elevated temperatures but when a magnetic field is applied the presence of two gaps can be demonstrated close to the bulk Tc in the raw data.PACS numbers: 74.50.+r, 74.60.Ec, Two decades of the boom in the field of superconductivity has recently been boosted by the surprising discovery of superconductivity in MgB 2 [1]. In contrast to the cuprates, the first tunneling [2][3][4] and point-contact [5,6] spectroscopy measurements have unequivocally shown that this system is a s-wave superconductor and isotope effects [7,8] have pointed towards a phonon mechanism. However, the size of the superconducting energy gap has remained unclear. We report here on experimental support for the multiband model of superconductivity recently proposed by Liu et al. [9] thus showing that MgB 2 belongs to an original class of superconductors in which two distinct 2D and 3D Fermi surfaces contribute to superconductivity. Indeed, our point-contact spectroscopy experiments clearly show the existence of two distinct superconducting gaps with ∆ S (0) = 2.8 meV and ∆ L (0) = 7 meV. Both gaps close near to the bulk transition temperature T c = 39 K. Our measurements in magnetic field show directly in the raw data the presence of two superconducting gaps at all temperatures up to the same bulk transition T c indicating that the two gaps are inherent to the superconductivity in MgB 2 .Although quite scattered, the first spectroscopy measurements [2-6,10] yielded to superconducting gap values surprisingly smaller than the BCS weak coupling limit 2∆/kT c = 3.52. Moreover simultaneous topographic imaging and quasiparticle density of states mapping [11] revealed substantial inhomogeneities at the surface of the sample as well as a large scattering of the energy gap values measured at different parts of the polycrystalline sample (with ∆ ranging from 3 to 7.5 meV). This energy gap distribution can be caused by sample inhomogeneities. However, Giubileo et al. also observed a superposition of two gaps (∆ S (0) = 3.9 meV and ∆ L (0) = 7.5 meV) in some of their local tunneling spectra. The same inhomogeneity argument could of course also explain such a superposition but a much more attractive scenario would be a two-gap model. Such a model has been first developed by Suhl et al. [12] in the case of overlapping s-an d-bands in conventional superconductors (such as V, Nb, Ta). Experimental evidence for the existence of two band superconductivity was obtained by tunneling spectroscopy in Nb-doped SrTiO 3 [13]. A similar model has been recently proposed by Liu et al. for MgB 2 . It ...

We report on specific heat, high magnetic field transport and ac−susceptibility measurments on magnesium diboride single crystals. The upper critical field Hc2 for magnetic fields perpendicular and parallel to the Mg and B planes is presented for the first time in the entire temperature range. A very different temperature dependence has been observed in the two directions which yields to a temperature dependent anisotropy with Γ ∼ 5 at low temperatures and about 2 near Tc. A peak effect is observed in the susceptibility measurments for µ0H ∼2 T parallel to the c−axis and the critical current density presents a sharp maximum for H parallel to the ab−plane.

A hysteresis loop is observed for the first time in the de Haas-van Alphen (dHvA) effect of beryllium at low temperatures and quantizing magnetic field applied parallel to the hexagonal axis of the single crystal. The irreversible behavior of the magnetization occurs at the paramagnetic part of the dHvA period in conditions of Condon domain formation arising by strong enough dHvA amplitude. The resulting extremely nonlinear response to a very small modulation field offers the possibility to find in a simple way the Condon domain phase diagram. From a harmonic analysis, the shape and size of the hysteresis loop is constructed.