55 research outputs found
Anisotropic s-wave superconductivity: comparison with experiments on MgB2 single crystals
The recently discovered superconductivity in MgB2 has captured world
attention due to its simple crystal structure and relatively high
superconducting transition temperature Tc=39K. It appears to be generally
accepted that it is phonon-mediated s-wave BCS-like superconductivity.
Surprisingly, the strongly temperature dependent anisotropy of the upper
critical field, observed experimentally in magnesium diboride single crystals,
is still lacking a consistent theoretical explanation. We propose a simple
single-gap anisotropic s-wave order parameter in order to compare its
implications with the prediction of a multi-gap isotropic s-wave model. The
quasiparticle density of states, thermodynamic properties, NMR spin-lattice
relaxation rate, optical conductivity, and Hc2 anisotropy have been analyzed
within this anisotropic s-wave model. We show that the present model can
capture many aspects of the unusual superconducting properties of MgB2
compound, though more experimental data appear to be necessary from single
crystal MgB2.Comment: 7 pages, 6 figures, some minor changes, to appear in Europhys. Let
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
in superconductors with -wave pairing drops rather
fast with concentration of normal impurities, while in superconductors with
anisotropic -wave pairing 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 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
Singular conductance of a spin 1 quantum dot
We interpret the recent observation of a zero-bias anomaly in spin-1 quantum
dots in terms of an underscreened Kondo effect. Although a spin-1 quantum dots
are expected to undergo a two-stage quenching effect, in practice the log
normal distribution of Kondo temperatures leads to a broad temperature region
dominated by underscreened Kondo physics. General arguments, based on the
asymptotic decoupling between the partially screened moment and the leads,
predict a singular temperature and voltage dependence of the conductance
and differential conductance , resulting in and . Using a Schwinger boson approach, we show how these qualitative
expectations are borne out in a detailed many body calculation.Comment: Four pages, four figures. Paper revised with additional references
added in response to feedback from reader
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
Ginzburg-Landau Expansion in a Toy Model of Superconductor with Pseudogap
We propose a toy model of electronic spectrum of two-dimensional system with
``hot-patches'' on the Fermi surface, which leads to essential renormalization
of spectral density (pseudogap). Within this model we derive Ginzburg-Landau
expansion for both s-wave and d-wave Cooper pairing and analyze the influence
of pseudogap formation on the basic properties of superconductors.Comment: 14 pages, 14 figures, RevTeX 3.0, Postscript figures attached, some
changes in the explanation of the model, published in JETP 115, No.2, (1999
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
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