No abstract

We present electronic-structure calculations, electrical resistivity data, and the first specific-heat measurements in the normal and superconducting states of quasi-one-dimensional M 2 Mo 6 Se 6 ͑M =Tl,In,Rb͒. Rb 2 Mo 6 Se 6 undergoes a metal-insulator transition at ϳ170 K: electronic-structure calculations indicate that this is likely to be driven by the formation of a dynamical charge-density wave. However, Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 remain metallic down to low temperature, with superconducting transitions at T c = 4.2 K and 2.85 K, respectively. The absence of any metal-insulator transition in these materials is due to a larger in-plane bandwidth, leading to increased interchain hopping which suppresses the density wave instability. Electronic heat-capacity data for the superconducting compounds reveal an exceptionally low density of states D E F = 0.055 states eV −1 atom −1 , with BCS fits showing 2⌬ / k B T c Ն 5 for Tl 2 Mo 6 Se 6 and 3.5 for In 2 Mo 6 Se 6 . Modeling the lattice specific heat with a set of Einstein modes, we obtain the approximate phonon density of states F͑͒. Deconvolving the resistivity for the two superconductors then yields their electron-phonon transport coupling function ␣ tr 2 F͑͒. In Tl 2 Mo 6 Se 6 and In 2 Mo 6 Se 6 , F͑͒ is dominated by an optical "guest ion" mode at ϳ5 meV and a set of acoustic modes from ϳ10 to 30 meV. Rb 2 Mo 6 Se 6 exhibits a similar spectrum; however, the optical phonon has a lower intensity and is shifted to ϳ8 meV. Electrons in Tl 2 Mo 6 Se 6 couple strongly to both sets of modes, whereas In 2 Mo 6 Se 6 only displays significant coupling in the 10-18 meV range. Although pairing is clearly not mediated by the guest ion phonon, we believe it has a beneficial effect on superconductivity in Tl 2 Mo 6 Se 6 , given its extraordinarily large coupling strength and higher T c compared to In 2 Mo 6 Se 6 .

A new superconducting state induced by an external magnetic field has been observed in the pseudoternary Eu-Sn molybdenum chalcogenides for different Eu concentrations. This phenomenon is explained in terms of the Jaccarino-Peter compensation effect which accounts correctly for the shape of the H c2 -T phase diagram.

No abstract

The critical current density of four MgB 2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current-voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magnetoresisitivity, indicative of a flux-flow regime.