64 research outputs found
Excess current in point contacts on two-band superconductor MgB in magnetic field
Series of characteristic and bias-dependent differential resistance
for point-contacts on the single crystals of two-band superconductor
MgB were measured in magnetic field up to 9 T. We have obtained magnetic
field dependences of the excess current on the curves and analyzed them
using Koshelev and Golubov (Phys. Rev. Lett. {\bf 90}, 177002 (2003))
theoretical results for the mixed state of a dirty two-band superconductor.
Introducing a simple model for the excess current in the point contact in the
mixed state our data can be qualitatively described utilizing theoretical
magnetic filed behavior of the and -band superconducting order
parameters and the corresponding averaged electronic density of states in
MgB.Comment: 4 pages, 3 figs, RevTex4; V2: plain text improved, BTK fit is added
to insets of Fig.
Carbon-substitution effect on the electronic properties of MgB single crystals
The electronic properties of the carbon substituted MgB single crystals
are reported. The carbon substitution drops T below 2 K. In-plane
resistivity shows a remarkable increase in residual resistivity by
C-substitution, while the change of in-plane/out-of-plane Hall coefficients is
rather small. Raman scattering spectra indicate that the E-phonon
frequency radically hardens with increasing the carbon-content, suggesting the
weakening of electron-phonon coupling. Another striking C-effect is the
increases of the second critical fields in both in-plane and out-of-plane
directions, accompanied by a reduction in the anisotropy ratio. The possible
changes in the electronic state and the origin of T-suppression by
C-substitution are discussed.Comment: 6 pages, 8 figure
Is LiPdB a self-doped hole superconductor ?
We propose that the electrons responsible for superconductivity in Li2Pd3B
come from the palladium 4d-electrons. So, its electronic properties are likely
to be dominated by strong electronic correlations. The basic unit in this
material are PdB octahedra which share vertices to form a 3-dimensional
network. Due to the highly distorted nature of the PdB octahedron, one far
stretched Pd atom per octahedra becomes almost inactive for electronic
conduction. Thus, the material escapes the fate of becoming a half- filled
insulating Mott antiferromagnet by hiding extra charges at these inactive Pd
sites and becomes a self-doped correlated metal. We propose a 3-dimensional
single band t-J model which could be the correct minimal model for this
material.Comment: 4 pages Revtex, 2 figures included in the text, some typos corrected,
some text and references adde
Upper critical field in dirty two-band superconductors: breakdown of the anisotropic Ginzburg-Landau theory
We investigate the upper critical field in a dirty two-band superconductor
within quasiclassical Usadel equations. The regime of very high anisotropy in
the quasi-2D band, relevant for MgB, is considered. We show that strong
disparities in pairing interactions and diffusion constant anisotropies for two
bands influence the in-plane in a different way at high and low
temperatures. This causes temperature-dependent anisotropy, in
accordance with recent experimental data in MgB. The three-dimensional
band most strongly influences the in-plane near , in the
Ginzburg-Landau (GL) region. However, due to a very large difference between
the c-axis coherence lengths in the two bands, the GL theory is applicable only
in an extremely narrow temperature range near . The angular dependence of
deviates from a simple effective-mass law even near .Comment: 12 pages, 5 figures, submitted to Phys.Rev.
Anisotropic Superconducting Properties of MgB2 Single Crystals
In-plane electrical transport properties of MgB2 single crystals grown under
high pressure of 4-6 GPa and temperature of 1400-1700oC in Mg-B-N system have
been measured. For all specimens we found sharp superconducting transition
around 38.1-38.3K with transition width within 0.2-0.3K. Estimated resistivity
value at 40K is about 1 mkOhmcm and resistivity ratio R(273K)/R(40K) of about
4.9. Results of measurements in magnetic field up to 5.5T perpendicular to Mg
and B planes and up to 9T in parallel orientation show temperature dependent
anisotropy of the upper critical field with anisotropy ratio increasing from
2.2 close to Tc up to about 3 below 30K. Strong deviation of the angular
dependence of Hc2 from anisotropic mass model has been also found.Comment: 10pages, including 5 figures,submitted to Physica C (in press
Vortex-lattice melting in magnesium diboride in terms of the elastic theory
In the framework of elastic theory, we study the vortex-lattice melting
transitions in magnesium diboride for magnetic fields both parallel and
perpendicular to the anisotropy axis. Using the parameters from experiments,
the vortex-lattice melting lines in the H-T diagram are located systematically
by various groups of Lindemann numbers and the anisotropic parameters. It is
observed that the theoretical result for the vortex melting with parallel and
perpendicular fields agrees well with the experimental data.Comment: 4 pages, 4 figure
Experimental study of intrinsic multiple Andreev reflections effect in GdO(F)FeAs superconductor array junctions
We report the first observation of the intrinsic multiple Andreev reflections
effect (IMARE) in S-n-S-...-S-arrays (S = superconductor, n = normal metal)
formed by "break-junction" technique in GdO(F)FeAs superconductor (Tc = 48 - 53
K). We show that superconducting gap peculiarities at dI/dV-spectra sharpen
dramatically in the arrays as compared with that in the single-contact spectra;
this enables to improve significantly accuracy of the bulk superconducting
parameters determination. Using IMARE, we determined the large and the small
gap values \Delta_L = 11 +- 1.1 meV and \Delta_S = 2.6 +- 0.4 meV. The
BCS-ratio 2\Delta_L/kTc^{local} = 5.0 - 5.9 > 3.52 (Tc^{local} is the contact
area critical temperature) evidences for a strong electron-boson coupling. The
results obtained agree well with our previous data by Andreev spectroscopy for
single SnS-contacts.Comment: 6 pages, 6 figure
- …