Anisotropic critical fields of MgB2 single crystals

The recently discovered superconductivity in MgB2 has created the world sensation. In spite of the relatively high superconducting transition temperature Tc=39K, the superconductivity is understood in terms of rare two gap superconductor with energy gaps attached to the sigma- and pi-band. However, this simple model cannot describe the temperature dependent anisotropy in H_c2 or the temperature dependence of the anisotropic magnetic penetration depth. Here we propose a model with two anisotropic energy gaps with different shapes. Indeed the present model describes a number of pecularities of MgB2 which have been revealed only recently through single crystal MgB2.Comment: 4 pages, 1 figure, to appear in Acta Physica Polonica B, proceedings of the International Conference on Strongly Correlated Electron Systems, SCES2002, Krakow, Polan

Ginzburg-Landau Expansion and the Slope of the Upper Critical Field in Disordered Superconductors with Anisotropic Pairing

It is demonstrated that the slope of the upper critical field $|dH_{c2}/dT|_{T_{c}}$ in superconductors with $d$-wave pairing drops rather fast with concentration of normal impurities, while in superconductors with anisotropic $s$-wave pairing $|dH_{c2}/dT|_{T_{c}}$ grows, and in the limit of strong disorder is described by the known dependences of the theory of ``dirty'' superconductors. This allows to use the measurements of $H_{c2}$ in disordered superconductors to discriminate between these different types of pairing in high-temperature and heavy-fermion superconductors.Comment: 7 pages, 5 figures, RevTeX 3.0, 4 Postscript figures attached; Submitted to JETP Letter

Conductance of a spin-1 quantum dot: the two-stage Kondo effect

We discuss the physics of a of a spin-1 quantum dot, coupled to two metallic leads and develop a simple model for the temperature dependence of its conductance. Such quantum dots are described by a two-channel Kondo model with asymmetric coupling constants and the spin screening of the dot by the leads is expected to proceed via a two-stage process. When the Kondo temperatures of each channel are widely separated, on cooling, the dot passes through a broad cross-over regime dominated by underscreened Kondo physics. A singular, or non-fermi liquid correction to the conductance develops in this regime. At the lowest temperatures, destructive interference between resonant scattering in both channels leads to the eventual suppression of the conductance of the dot. We develop a model to describe the growth, and ultimate suppression of the conductance in the two channel Kondo model as it is screened successively by its two channels. Our model is based upon large-N approximation in which the localized spin degrees of freedom are described using the Schwinger boson formalism.Comment: 16 pages, 10 figure

Anomalous microwave conductivity coherence peak in c-axis MgB2 thin film

The temperature dependence of the real part of the microwave complex conductivity at 17.9 GHz obtained from surface impedance measurements of two c-axis oriented MgB2 thin films reveals a pronounced maximum at a temperature around 0.6 times the critical temperature. Calculations in the frame of a two-band model based on Bardeen-Cooper-Schrieffer (BCS) theory suggest that this maximum corresponds to an anomalous coherence peak resembling the two-gap nature of MgB2. Our model assumes there is no interband impurity scattering and a weak interband pairing interaction, as suggested by bandstructure calculations. In addition, the observation of a coherence peak indicates that the pi-band is in the dirty limit and dominates the total conductivity of our filmsComment: 10 pages, 4 figures, to be published in Phys. Rev. Let

Far-infrared and submillimeter-wave conductivity in electron-doped cuprate La_{2-x}Ce_xCuO_4

We performed far-infrared and submillimeter-wave conductivity experiments in the electron-doped cuprate La_{2-x}Ce_xCuO_4 with x = 0.081 (underdoped regime, T_c = 25 K). The onset of the absorption in the superconducting state is gradual in frequency and is inconsistent with the isotropic s-wave gap. Instead, a narrow quasiparticle peak is observed at zero frequency and a second peak at finite frequencies, clear fingerprints of the conductivity in a d-wave superconductor. A far-infrared conductivity peak can be attributed to 4Delta_0, or to 2Delta_0 + Delta_spin, where Delta_spin is the resonance frequency of the spin-fluctuations. The infrared conductivity as well as the suppression of the quasiparticle scattering rate below T_c are qualitatively similar to the results in the hole-doped cuprates.Comment: 5 pages, 4 figures include