Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

Abstract

The two-gap structure in the superconducting state of MgB_2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature Tc_pi ~ 13 K, and a critical temperature Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and Hc2_sigma_ab ~ 19 T, where the indexes πand σrefer to the 3D and 2D sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ~ 28 T when Tc = 30 K.

arXiv comment31 pages, 9 figures. Accepted in the Physica C special issue on MgB2

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• High-field superconductivity in alloyed MgB2 thin films
• Limits of the upper critical field in dirty two-gap superconductors
• Specific heat of Nb_3Sn: The case for a second energy gap
• Thermally-activated current transport in MgB_2 films
• Advances in point-contact spectroscopy: two-band superconductor MgB2 (A review)
• Effects of proton irradiation and ageing on the superconducting properties of single crystalline and polycrystalline MgB2
• Specific Heat Discontinuity in Impure Two-Band Superconductors
• Effect of density of state on isotope effect exponent of two-band superconductors
• Vortex motion and flux-flow resistivity in dirty multiband superconductors
• Magnetic Field Induced Density of States in Superconducting MgB${}_2$: Measurement of Conduction Electron Spin-Susceptibility
• Superconductivity in two-band systems with variable charge carrier density. The case of MgB2
• Investigate the effect of anisotropic order parameter on the specific heat of anisotropic two-band superconductors
• Magnetic behaviour of dirty multiband superconductors near the upper critical field
• Anisotropic properties of MgB2 by torque magnetometry