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Abstract: High-impedance contacts made on the surface of Sr0.88La0.12CuO2 superconducting thin films systematically display a zero-bias anomaly. We consider two-level systems (TLSs) as the origin of this anomaly. We observe that the contribution of some TLS to the contact resistance is weakened by a magnetic field. We show that this could result from the increase of the TLS relaxation rate in the superconducting state, due to its ability to create pairs of quasi-particles out of the condensate, when located close to the surface of the film. [Copyright &y& Elsevier]

The two-gap structure in the superconducting state of MgB2 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,π≅13 K, and a critical temperature Tc=Tc,σ≅38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ0Hc2,π≅0.35 T, and an anisotropic upper critical field with extreme values μ0Hc2,σ,c≅3.5 T and μ0Hc2,σ,ab≅19 T, where the indexes π and σ refer to the three-dimensional and two-dimensional 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. [Copyright &y& Elsevier]

We performed specific heat measurements to study the superconducting properties of the ∼40 K superconductor MgB2, up to 16 T, using polycrystal and single crystal samples. Our results establish the validity of the two-gap model. We tested the effect of disorder by irradiating our sample. This procedure decreased Tc down to ∼26 K, but did not suppress completely the smaller gap, at variance with theoretical expectations. A positive effect of the irradiation was the increase of Hc2 up to almost 30 T. Our results on the single crystal allow the anisotropy of each band to be determined independently, and show the existence of a cross-over field well below Hc2 characterizing the physics of the small-gapped band. We also present preliminary results on Nb3Sn, showing similar, but weaker effects. [Copyright &y& Elsevier]

CeRhIn5 is known to show an unusual transition at a critical pressure of ∼15 kbar. Specific-heat data show a gradual change in the zero-field “magnetic” specific-heat anomaly from one typical of antiferromagnetic (AF) ordering at ambient pressure to one more characteristic of a Kondo singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an AF ground state to a superconducting ground state, and evidence of a weak thermodynamic first-order transition. Above the critical pressure, the low-energy excitations are characteristic of superconductivity with line nodes in the energy gap, and, at intermediate pressures, of extended gaplessness. [Copyright &y& Elsevier]

Measurements of the specific heat of Mg11B2, from 1 to 50 K in magnetic fields to 9 T, are reported. The superconducting-state electron contribution is very different from that of other superconductors, but the general features are consistent with predictions for a two-gap superconductor, and a two-gap model based on BCS thermodynamics can quantitatively represent it. Parameters characterizing the gaps are in agreement with theoretical calculations. An unusually strong magnetic field dependence of the temperature-proportional term in the electron contribution to the vortex-state specific heat is another manifestation of the two gaps. [Copyright &y& Elsevier]

We report on specific heat measurements in high quality (K,Ba)BiO3 single crystals (Tc∼31.5 K). A well defined specific heat jump is clearly visible at TCp(H) in the entire investigated field range (up to 13 T). However, the corresponding TCp(H) exhibits an anomalous positive curvature and the amplitude of the jump rapidly decreases with field suggesting a non-linear increase of the Sommerfeld coefficient (γ(H)). This anomalous behaviour is confirmed by low temperature measurements which show that γ(H)∝Hα with α∼0.65. [Copyright &y& Elsevier]

The two-gap structure of MgB2 gives rise to unusual thermodynamic properties which depart markedly from the single-gap BCS model, both in their temperature- and field-dependence. We report measurements of the specific heat up to 16 T on ceramic and single crystal samples, which demonstrate these effects in bulk. The low-temperature mixed-state specific heat reveals a field-dependent anisotropy, and points to the existence of unusually large vortices, in agreement with local density-of-states measurements by scanning tunneling spectroscopy. It is finally shown that a suitable irradiation process nearly doubles Hc2 in the bulk. [Copyright &y& Elsevier]

We report specific-heat measurements on two samples of Mg11B2, one powder and one sintered, that give essentially identical results. Both samples are of exceptionally high quality: At the critical temperature the discontinuity in specific heat is higher than that of other samples, the transition is sharper than for most samples, and the signature feature of the small, non-BCS energy gap is particularly pronounced. The results are compared with a phenomenological model for a multi-gap superconductor, with band-structure calculations, and with spectroscopic determinations of the energy gaps. [Copyright &y& Elsevier]