641 research outputs found
Superconducting properties of the pyrochlore oxide Cd2Re2O7
We report the superconducting properties of the pyrochlore oxide Cd2Re2O7.
The bulk superconducting transition temperature Tc is about 1.0 K, and the
upper critical field Hc2 determined by the measurement of specific heat under
magnetic fields is 0.29 T. The superconducting coherence length is estimated to
be 34 nm. Specific heat data measured on single crystals suggest that the
superconducting gap of Cd2Re2O7 is nodeless.Comment: 6 pages, 6 figures, 1 table, to be published in J. Chem. Phys. Solid
Magnetotransport and the upper critical magnetic field in MgB2
Magnetotransport measurements are presented on polycrystalline MgB2 samples.
The resistive upper critical magnetic field reveals a temperature dependence
with a positive curvature from Tc = 39.3 K down to about 20 K, then changes to
a slightly negative curvature reaching 25 T at 1.5 K. The 25- Tesla upper
critical field is much higher than what is known so far on polycrystals of MgB2
but it is in agreement with recent data obtained on epitaxial MgB2 films. The
deviation of Bc2(T) from standard BCS might be due to the proposed two-gap
superconductivity in this compound. The observed quadratic normal-state
magnetoresistance with validity of Kohler's rule can be ascribed to classical
trajectory effects in the low-field limit.Comment: 6 pages, incl. 3 figure
Low-Temperature Rapid Synthesis and Superconductivity of Fe-Based Oxypnictide Superconductors
we were able to develop a novel method to synthesize Fe-based oxypnictide
superconductors. By using LnAs and FeO as the starting materials and a
ball-milling process prior to solid-state sintering, Tc as high as 50.7 K was
obtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering
at temperatures as low as 1173 K for times as short as 20 min.Comment: 2 pages,2 figures, 1 tabl
Temperature dependence of the superheating field for superconductors in the high-k London limit
We study the metastability of the superheated Meissner state in type II
superconductors with k >> 1 beyond Ginzburg-Landau theory, which is applicable
only in the vicinity of the critical temperature. Within Eilenberger's
semiclassical approximation, we use the local electrodynamic response of the
superconductor to derive a generalized thermodynamic potential valid at any
temperature. The stability analysis of this functional yields the temperature
dependence of the superheating field. Finally, we comment on the implications
of our results for superconducting cavities in particle accelerators.Comment: 7.5 pages, 2 figure
Two types of Hc2(T) dependences in Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+delta) with different Yttrium content
We reanalyze the magnetization data collected on
Bi_2Sr_2Ca_(1-x)Y_xCu_2O_(8+y) samples (Kim at al, Phys. Rev. B 72, 64525
(2005)) and argue that the method, which was used for the analysis of
equilibrium magnetization data, is not adequate to the experimental situation.
As a result, the temperature dependencies of the upper critical field Hc2(T)
and the magnetic field penetration depth lambda (T), obtained in this work, are
misinterpreted. Using a different approach to analysis, we demonstrate that the
normalizedHc2(T) curves are rather different from those presented in the
original publication and do not follow predictions of the
Werthamer-Helfand-Hohenberg theory. Another important observation is that the
Hc2(T) dependencies for two samples with different levels of doping are
qualitatively different.Comment: 10 pages, 3 figure
The BCS critical temperature in a weak external electric field via a linear two-body operator
We study the critical temperature of a superconductive material in a weak
external electric potential via a linear approximation of the BCS functional.
We reproduce a similar result as in [Frank, Hainzl, Seiringer, Solovej, 2016]
using the strategy introduced in [Frank, Hainzl, Langmann, 2018], where we
considered the case of an external constant magnetic field.Comment: Dedicated to Herbert Spohn on the occasion of his seventieth
birthday; 29 page
The BCS Critical Temperature in a Weak External Electric Field via a Linear Two-Body Operator
We study the critical temperature of a superconductive material in a weak external electric potential via a linear approximation of the BCS functional. We reproduce a similar result as in Frank et al. (Commun Math Phys 342(1):189–216, 2016, [5]) using the strategy introduced in Frank et al. (The BCS critical temperature in a weak homogeneous magnetic field, [2]), where we considered the case of an external constant magnetic field
Superconductivity at 11.3 K induced by cobalt doping in CeOFeAs
Pure phases of a new oxyarsenide superconductor of the nominal composition
CeOFe0.9Co0.1As was successfully synthesized by solid state reaction in sealed
silica ampoules at 1180 C. It crystallizes in the layered tetragonal ZrCuSiAs
type structure (sp gp P4/nmm) with lattice parameter of a = 3.9918(5) angstrom
and c = 8.603(1) angstrom. A sharp superconducting transition is observed at
11.31 K with an upper critical field of 45.22 T at ambient pressure. The
superconducting transition temperature is drastically lowered (~ 4.5, 4.9 K) on
increasing the concentration (x = 0.15, 0.2) of cobalt
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