308 research outputs found
Electrical and thermal transport properties in high T_c superconductors : effects of a magnetic field
Experimental studies of the electric and heat currents in the normal,
superconducting and mixed states of high T superconductors (HTcS) lead to
characterization, complementary to data obtained from equilibrium property
based techniques. A magnetic field superimposed on the superconducting sample
generates {\it magneto-transport phenomena}, from which an excess electrical
resistivity, an excess thermoelectric power, the Hall or the Nernst effect.
Different behavioral effects allow one to distinguish various dissipation
mechanisms, like quasi particle scattering, vortex motion dissipation and
superconductivity fluctuations, in particular when the Corbino geometry is
used. Moreover bulk measurements of the thermal conductivity and the
electrothermal conductivity in a magnetic field give us sure indications of the
order parameter symmetry. The location of the mixed state phase transition
lines in the technological phase diagram of HTcS are briefly pointed out
through precise measurements performed over broad temperature and magnetic
field ranges. The results are mainly reviewed with the aim of defining further
investigation lines.Comment: 9 pages, no figures; to appear in Physica
Aging process of electrical contacts in granular matter
The electrical resistance decay of a metallic granular packing has been
measured as a function of time. This measurement gives information about the
size of the conducting cluster formed by the well connected grains. Several
regimes have been encountered. Chronologically, the first one concerns the
growth of the conducting cluster and is identified to belong to diffusion
processes through a stretched exponential behavior. The relaxation time is
found to be simply related to the initial injected power. This regime is
followed by a reorganisation process due to thermal dilatation. For the long
term behavior of the decay, an aging process occurs and enhances the electrical
contacts between grains through microsoldering.Comment: 11 pages, 4 figure
Absence of singular superconducting fluctuation corrections to thermal conductivity
We evaluate the superconducting fluctuation corrections to thermal
conductivity in the normal state which diverge as T approaches T_c. We find
zero total contribution for one, two and three-dimensional superconductors for
arbitrary impurity concentration. The method used is diagrammatic many-body
theory, and all contributions -- Aslamazov-Larkin (AL), Maki-Thompson (MT), and
density-of-states (DOS) -- are considered. The AL contribution is convergent,
whilst the divergences of the DOS and MT diagrams exactly cancel.Comment: 4 pages text; 2 figure
Influence of a low magnetic field on the thermal diffusivity of Bi-2212
The thermal diffusivity of a Bi-2212 polycrystalline sample has been measured
under a 1T magnetic field applied perpendicularly to the heat flux. The
magnetic contribution to the heat carrier mean free path has been extracted and
is found to behave as a simple power law. This behavior can be attributed to a
percolation process of electrons in the vortex lattice created by the magnetic
field.Comment: 10 pages, 3 figures; to be published in Phys. Rev.
Study of the spin correlation and the percolation threshold in the B-site spinel
High-temperature series expansion of the correlation function on the B- spinel lattice are computed up to order 6 in β = 1/(kBT) for Heisenberg model including both the nearest and next-nearest-neighbor interactions J1 and J2 respectively. The behavior with the temperature and the site dilution is presented. The approach is applied to the experimental results of the B-spinel ZnCr2xAl2-2xS4. The critical temperature and the critical exponents for the susceptibility and the correlation length are deduced by applying the Padé approximant methods. The following estimates are obtained for the familiar critical exponents: γ=1.382±0.012 and ν = 0.691±0.011. These values are not sensitive to the dilution ratio x. The bond percolation threshold xp is determined by studying the disorder variation of the correlation length ξ. The xp is considered as the concentration at which ξ vanishes. The obtained values are xp =0.27 when only J1 is considered and 0.23 when both J1 and J2 are considered.High-temperature series expansion of the correlation function on the B- spinel lattice are computed up to order 6 in β = 1/(kBT) for Heisenberg model including both the nearest and next-nearest-neighbor interactions J1 and J2 respectively. The behavior with the temperature and the site dilution is presented. The approach is applied to the experimental results of the B-spinel ZnCr2xAl2-2xS4. The critical temperature and the critical exponents for the susceptibility and the correlation length are deduced by applying the Padé approximant methods. The following estimates are obtained for the familiar critical exponents: γ=1.382±0.012 and ν = 0.691±0.011. These values are not sensitive to the dilution ratio x. The bond percolation threshold xp is determined by studying the disorder variation of the correlation length ξ. The xp is considered as the concentration at which ξ vanishes. The obtained values are xp =0.27 when only J1 is considered and 0.23 when both J1 and J2 are considered
Study of critical properties in B-spinel ZnxCd1-xCr2Se4 (0.35 ≤ x ≤ 0.58)
The critical behaviour of the B-spinel ZnxCd1-xCr2Se4 systems is studied in the concentration range 0.35≤x≤0.58. The mean field theory and the ferromagnetic spin-wave theory at low temperatures are combined to calculate the exchange integrals up to the third nearest neighbours. The ferromagnetic critical region is studied by the high-temperature series expansion (H.T.S.) extrapolated with the Padé (P.A) approximants method. The critical temperatures T c and the critical exponents associated with the magnetic susceptibility (γ) and the correlation length (ν) are estimated. The obtained values of T c are in good agreement with those obtained by magnetic measurements. The values of γ and ν are sensitive to the dilution ratio x. For the compounds situated in the ferromagnetic region ( 0.35≤x≤0.41), they are close to those of 3D Heisenberg model. For the compounds presenting re-entrant behaviour ( 0.41≤x≤0.58 ), γ and ν deviate slowly from those of this model and approach the values found in re-entrant systems.The critical behaviour of the B-spinel ZnxCd1-xCr2Se4 systems is studied in the concentration range 0.35≤x≤0.58. The mean field theory and the ferromagnetic spin-wave theory at low temperatures are combined to calculate the exchange integrals up to the third nearest neighbours. The ferromagnetic critical region is studied by the high-temperature series expansion (H.T.S.) extrapolated with the Padé (P.A) approximants method. The critical temperatures T c and the critical exponents associated with the magnetic susceptibility (γ) and the correlation length (ν) are estimated. The obtained values of T c are in good agreement with those obtained by magnetic measurements. The values of γ and ν are sensitive to the dilution ratio x. For the compounds situated in the ferromagnetic region ( 0.35≤x≤0.41), they are close to those of 3D Heisenberg model. For the compounds presenting re-entrant behaviour ( 0.41≤x≤0.58 ), γ and ν deviate slowly from those of this model and approach the values found in re-entrant systems
Noninvasive Embedding of Single Co Atoms in Ge(111)2x1 Surfaces
We report on a combined scanning tunneling microscopy (STM) and density
functional theory (DFT) based investigation of Co atoms on Ge(111)2x1 surfaces.
When deposited on cold surfaces, individual Co atoms have a limited diffusivity
on the atomically flat areas and apparently reside on top of the upper
pi-bonded chain rows exclusively. Voltage-dependent STM imaging reveals a
highly anisotropic electronic perturbation of the Ge surface surrounding these
Co atoms and pronounced one-dimensional confinement along the pi-bonded chains.
DFT calculations reveal that the individual Co atoms are in fact embedded in
the Ge surface, where they occupy a quasi-stationary position within the big
7-member Ge ring in between the 3rd and 4th atomic Ge layer. The energy needed
for the Co atoms to overcome the potential barrier for penetration in the Ge
surface is provided by the kinetic energy resulting from the deposition
process. DFT calculations further demonstrate that the embedded Co atoms form
four covalent Co-Ge bonds, resulting in a Co4+ valence state and a 3d5
electronic configuration. Calculated STM images are in perfect agreement with
the experimental atomic resolution STM images for the broad range of applied
tunneling voltages.Comment: 19 pages, 15 figures, 3 table
Bilayered smectic phase polymorphism in the dipolar Gay-Berne liquid crystal model
We present computer simulations of the Gay–Berne model with a strong terminal dipole. We report
the existence of
different
stable antiferroelectric interdigitated bilayered phases in this model with
diverse in-plane organization. The occurrence of these phases depends crucially on the value of the
molecular elongation
. For
= 3 we find an interdigitated bilayered smectic-A phase
absent when
there is no dipole
and a bilayered smectic-T
or crystal
with positional in-plane tetragonal
ordering, different from the hexatic observed in the absence of the molecular dipole. For
=4,
bilayered smectic-A and in-plane hexatic-ordered smectic-B
or crystal
phases are observe
Quasiparticle contribution to heat carriers relaxation time in DyBaCuO from heat diffusivity measurements
It is shown that the controversy on phonons or electrons being the most
influenced heat carriers below the critical temperature of high-T
superconductors can be resolved. Electrical and thermal properties of the same
DyBaCuO monodomain have been measured for two highly different
oxygenation levels. While the oxygenated sample DyBaCuO has very
good superconducting properties ( K), the DyBaCuO
sample exhibits an insulator behavior. A careful comparison between
measurements of the {\bf thermal diffusivity} of both samples allows us to
extract the electronic contribution. This contribution to the relaxation time
of heat carriers is shown to be large below and more sensitive to the
superconducting state than the phonon contribution.Comment: 13 pages, 6 figure
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