35 research outputs found
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
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
Crystal growth and characterization of MgB2: Relation between structure and superconducting properties
We discuss the important aspects of synthesis and crystal growth of MgB2
under high pressure (P) and temperature (T) in Mg-B-N system, including the
optimisation of P-T conditions for reproducible crystal growth, the role of
liquid phases in this process, the temperature dependence of crystal size and
the effect of growing instabilities on single crystals morphology. Extensive
experiments have been carried out on single crystals with slightly different
lattice constants and defects concentration, which revealed and possible
effects of Mg-deficiency and lattice strain on the superconducting properties
of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).Comment: 29 pages, 9 figures, 1 table, Proc.of International Workshop on
Superconductivity in Magnesium Diboride and Related Materials "BOROMAG",
17-19 June 2002, Genoa, Italy (submitted to Supercond.Sci and Technol.
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
Theory of fluctuations in a two-band superconductor
A theory of fluctuations in two-band superconductor MgB is developed.
Since the standard Ginzburg-Landau (GL) approach fails in description of its
properties, we generalize it basing on the microscopic theory of a two-band
superconductor. Calculating the microscopic fluctuation propagator, we build up
the nonlocal two-band GL functional and the corresponding time-dependent GL
equations. This allows us to calculate the main fluctuation observables such as
fluctuation specific heat and conductivity.Comment: 11 pages, 2 figures, subm. to Phys. Rev.
Anisotropic resistivity and Hall effect in MgB2 single crystals
We report resistivity and the Hall effect measurements in the normal and
superconducting states of MgB2 single crystal. The resistivity has been found
to be anisotropic with slightly temperature dependent resistivity ratio of
about 3.5. The Hall constant, with a magnetic field parallel to the Mg and B
sheets is negative in contrast to the hole-like Hall response with a field
directed along the c-axis indicating presence of both types of charge carriers
and, thus, multi-band electronic structure of MgB2. The Hall effect in the
mixed state shows no sign change anomaly reproducing the Hall effect behavior
in clean limit type-II superconductors.Comment: Minor changed content, 11 pages including 3 figure