28 research outputs found
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.
Systematic Study on Fluorine-doping Dependence of Superconducting and Normal State Properties in LaFePO1-xFx
We have investigated the fluorine-doping dependence of lattice constants,
transports and specific heat for polycrystalline LaFePO1-xFx. F doping slightly
and monotonically decreases the in-plane lattice parameter. In the normal
state, electrical resistivity at low temperature is proportional to the square
of temperature and the electronic specific heat coefficient has large value,
indicating the existence of moderate electron-electron correlation in this
system. Hall coefficient has large magnitude, and shows large temperature
dependence, indicating the low carrier density and multiple carriers in this
system. Temperature dependence of the upper critical field suggests that the
system is a two gap superconductor. The F-doping dependence of these properties
in this system are very weak, while in the FeAs system (LaFeAsO), the F doping
induces the large changes in electronic properties. This difference is probably
due to the different F-doping dependence of the lattice in these two systems.
It has been revealed that a pure effect of electron doping on electronic
properties is very weak in this Fe pnictide compound.Comment: 8 pages, 5 figures, accepted for publication in J. Phys. Soc. Jp
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.
Anomalous Flux Flow Resistivity in Two Gap Superconductor MgB_2
The flux flow resistivity associated with purely viscous motion of vortices
in high-quality MgB_2 was measured by microwave surface impedance. Flux flow
resistivity exhibits unusual field dependence with strong enhancement at low
field, which is markedly different to conventional s-wave superconductors. A
crossover field which separates two distinct flux flow regimes having different
flux flow resistivity slopes was clearly observed in H//ab-plane. The unusual
H-dependence indicates that two very differently sized superconducting gaps in
MgB_2 manifest in the vortex dynamics and almost equally contribute to energy
dissipation. The carrier scattering rate in two different bands is also
discussed with the present results, compared to heat capacity and thermal
conductivity results.Comment: 4 pages, 3figure
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
Dynamics of Flux Creep in Underdoped Single Crystals of Y_1-xPr_xBa_2Cu_3O_7-d
Transport as well as magnetic relaxation properties of the mixed state were
studied on strongly underdoped Y_1-xPr_xBa_2Cu_3O_7-d crystals. We observed two
correlated phenomena - a coupling transition and a transition to quantum creep.
The distribution of transport current below the coupling transition is highly
nonuniform, which facilitates quantum creep. We speculate that in the mixed
state below the coupling transition, where dissipation is nonohmic, the current
distribution may be unstable with respect to self-channeling resulting in the
formation of very thin current-carrying layers.Comment: 11 pages, 9 figures, Submitted to Phys. Rev.
The effect of magnetic field on the two superconducting gaps in MgB2
Double-gap superconductivity in an epitaxial MgB2 film has been studied by
means of point-contact spectroscopy in magnetic field up to 8 Tesla. The
relatively fast disappearance of the feature associated with the pi-band gap at
a field around 1 T is caused by the line broadening due to strong pair breaking
rather than to a collapse of the double-gap state. This pair breaking was found
to increase linearly with field. Field dependences of the order parameters
Delta_pi and Delta_sigma in the pi and sigma bands were measured in field
applied parallel and perpendicular to the film, at T = 4.2 K. In perpendicular
field, both order parameters survive to a common Hc2, which is about 6.5 T for
this direction. In parallel field, the decrease of Delta_sigma is much more
gradual, consistent with the Hc2 being about 4 times greater in this
ori-entation. The difference in Delta_pi measured in the two field orientations
is however smaller than the difference of Delta_sigma. We compare these results
with the data on tunnelling spectroscopy and specific heat measurements of MgB2
single crystals and find consistency between the different experimental
approaches.Comment: 4 pages, 5 figures Revised: discussion of the distribution of gap
value
Observation of Multi-Gap Superconductivity in GdO(F)FeAs by Andreev Spectroscopy
We have studied current-voltage characteristics of Andreev contacts in
polycrystalline GdOFFeAs samples with bulk critical
temperature = (52.5 \pm 1)K using break-junction technique. The data
obtained cannot be described within the single-gap approach and suggests the
existence of a multi-gap superconductivity in this compound. The large and
small superconducting gap values estimated at T = 4.2K are {\Delta}L = 10.5 \pm
2 meV and {\Delta}S = 2.3 \pm 0.4 meV, respectively.Comment: 5 pages, 4 figures, submitted to JETP Letter
Study of the Two-Gap Superconductivity in GdO(F)FeAs by ScS-Andreev Spectroscopy
Current-voltage characteristics and dynamic conductance of the superconductor
- constriction - superconductor junctions in GdO(F)FeAs polycrystalline samples
with critical temperatures Tc^local = 46 - 53 K were investigated. Two
superconducting gaps, the large Delta_L = 10.5 +- 2 meV, and the small one
Delta_S = 2.3 +- 0.4 meV were clearly observed at T = 4.2 K. The
2Delta_L/kTc^local = 5.5 +- 1 ratio gives support to the strong coupling
mechanism which is responsible for the high Tc value. Temperature dependence of
the large gap Delta_L(T) indicates the presence of intrinsic proximity effect
(in k-space) between two superconducting condensates.Comment: 7 pages, 6 figures. Submitted to Journal of Physics: Conference
Serie