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Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF) (TN ≃5.2 K) HoNi2B2C single crystals in the normal and two different superconducting (SC) states (Tc ≃8.5 K and T * c ≃ 5.6 K) are reported. The PC study of the electron-boson(phonon) interaction (EB(P)I) spectral function reveals pronounced phonon maxima at 16, 22 and 34 meV. For the first time the high energy maxima at about 50 meV and 100 meV are resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitations with a maximum near 10 meV and a 'magnetic' peak near 3 meV are observed. The contribution of the 10-meV peak in PC EPI constant λPC is evaluated as 20-30%, while contribution of the high energy modes at 50 and 100 meV amounts about 10% for each maxima, so the superconductivity might be affected by CEF excitations. The SC gap in HoNi2B2C exhibits a standard single-band BCS-like dependence, but vanishes at T * c ≃ 5.6 K< Tc, with 2∆/kBT * c ≃ 3.9. The strong coupling Eliashberg analysis of the low-temperature SC phase with T * c ≃ 5.6 K ∼ TN, coexisting with the commensurate AF structure, suggests a sizable value of the EPI constant λs ∼ 0.93. We also provide strong support for the recently proposed by us "Fermi surface (FS) separation" scenario for the coexistence of magnetism and superconductivity in magnetic borocarbides, namely, that the superconductivity in the commensurate AF phase survives at a special (nearly isotropic) FS sheet without an admixture of Ho 5d states. Above T * c the SC features in the PC characteristics are strongly suppressed pointing to a specific weakened SC state between T * c and Tc.

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In polycrystalline samples of superconducting MgCNi 3 , the critical temperature and the specific heat were measured as a function of magnetic field. A Wertheimer-Helfand-Hohenberg (WHH)-like shape of H c2 ðT Þ is observed. The H c2 ðT Þ and specific heat data are discussed on a qualitative level in terms of effective single-and multi-band models based on an orbital assignment of the disjoint Fermi surface sheets (FSS) derived from LDA full potential electronic structure calculations.

We examine the electronic structure of NaxCoO2.yH2O within the local density approximation. The parametrization of the band which forms the largest hole-Fermi surface centered at Γ shows significant deviations from what is frequently assumed in recent sophisticated theoretical studies. In particular, the commonly used nearest neighbor approaches in the framework of single band pictures are found to be unrealistic. The special role of H 2 O in screening the disorder in the charge reservoir is briefly discussed and compared with the case of Y1−xCaxCu3O 6+δ .

The influence of carbon deficiency on superconductivity of MgCNi 3 is investigated by specific heat measurements in the normal and superconducting state. In order to perform a detailed analysis of the normal state specific heat, a computer code is developed which allows for an instantaneous estimate of the main features of the lattice dynamics. By analyzing the evolution of the lattice vibrations within the series and simultaneously considering the visible mass enhancement, the loss in the electron-phonon coupling can be attributed to significant changes of the prominent Ni vibrations. The present data well supports the recently established picture of strong electron-phonon coupling and ferromagnetic spin fluctuations in this compound. The discovery of superconductivity in MgCNi 3 has caused much attention, 1 since the large Ni content suggests a magnetic state rather than superconductivity. Indeed, up to now a lot of experimental and theoretical publications point to a ferromagnetic instability at temperatures near and below the superconducting transition temperature T c ≈ 7 K.2,3,4,5,6 So far there is much discussion on the carbon content in MgCNi 3 , since only samples with carbon excess of ≈ 50 % show large T c values, whereas for the stoichiometric composition a strongly reduced T c is found, 7,8 which may be triggered by enhanced pair-breaking due to increasing spin fluctuations.9 However specific heat measurements indicate that this is not the case but instead the electron-phonon coupling is significantly reduced, 5 accompanied by a considerable hardening of low-energy vibrations.10 Band structure calculations predict a constant 9 or decreasing 5 electron density of states at the Fermi level (EDOS), leaving the interesting prospect of significantly changing lattice dynamics. Recent measurements of the carbon isotope effect support this picture.

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