36 research outputs found
Low Field Magnetic Response of the Granular Superconductor LaSrCuO
The properties of the low excitation field magnetic response of the granular
high temperature (HTSC) superconductor LaSrCuO have been analyzed at low
temperatures. The response of the Josephson currents has been extracted from
the data. It is shown that intergrain current response is fully irreversible,
producing shielding response, but do not carry Meissner magnetization. Analysis
of the data shows that the system of Josephson currents freezes into a glassy
state even in the absense of external magnetic field, which is argued to be a
consequence of the d-wave nature of superconductivity in LaSrCuO. The
macroscopic diamagnetic response to very weak variations of the magnetic field
is shown to be strongly irreversible but still qualitatively different from any
previously known kind of the critical-state behaviour in superconductors.
A phenomenological description of these data is given in terms of a newly
proposed ``fractal'' model of irreversibility in superconductors.Comment: LATEX, twocolumns, 22 pages including 20 eps-figure
Second harmonics and compensation effect in ceramic superconductors
A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model the ceramic superconductors. The
nonlinear ac susceptibility and the compensation effect are studied by Monte
Carlo simulations in this model. The compensation effect is shown to be due to
the existence of the chiral glass phase. We demonstrate, in agreement with
experiments, that this effect may be present in the ceramic superconductors
which show the paramagnetic Meissner effect.Comment: 6 pages, latex, 4 figures, Phys. Rev. B (accepted
Electric field dependence of thermal conductivity of a granular superconductor: Giant field-induced effects predicted
The temperature and electric field dependence of electronic contribution to
the thermal conductivity (TC) of a granular superconductor is considered within
a 3D model of inductive Josephson junction arrays. In addition to a
low-temperature maximum of zero-field TC K(T,0) (controlled by mutual
inductance L_0 and normal state resistivity R_n), the model predicts two major
effects in applied electric field: (i) decrease of the linear TC, and (ii)
giant enhancement of the nonlinear (i.e., grad T-dependent) TC with
[K(T,E)-K(T,0)]/K(T,0) reaching 500% for parallel electric fields E=E_T
(E_T=S_0|grad T| is an "intrinsic" thermoelectric field). A possiblity of
experimental observation of the predicted effects in granular superconductors
is discussed.Comment: 5 LaTeX pages (jetpl.sty included), 2 EPS figures. To be published in
JETP Letter
Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)PF salts
We report a study of the 16.5 GHz dielectric function of hydrogenated and
deuterated organic salts (TMTTF)PF. The temperature behavior of the
dielectric function is consistent with short-range polar order whose relaxation
time decreases rapidly below the charge ordering temperature. If this
transition has more a relaxor character in the hydrogenated salt, charge
ordering is strengthened in the deuterated one where the transition temperature
has increased by more than thirty percent. Anomalies in the dielectric function
are also observed in the spin-Peierls ground state revealing some intricate
lattice effects in a temperature range where both phases coexist. The variation
of the spin-Peierls ordering temperature under magnetic field appears to follow
a mean-field prediction despite the presence of spin-Peierls fluctuations over
a very wide temperature range in the charge ordered state of these salts.Comment: 7 pages, 6 figure
Nonlinear Seebeck Effect in a Model Granular Superconductor
The change of the Josephson supercurrent density of a weakly-connected
granular superconductor in response to externally applied arbitrary thermal
gradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D
Josephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to
be of the order of 10^4 K/m for YBCO ceramics with an average grain's size of
10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is
predicted to become strongly dT/dx-dependent.Comment: REVTEX, no figure
A new quantum fluid at high magnetic fields in the marginal charge-density-wave system -(BEDT-TTF)Hg(SCN) (where ~K and Rb)
Single crystals of the organic charge-transfer salts
-(BEDT-TTF)Hg(SCN) have been studied using Hall-potential
measurements (K) and magnetization experiments ( = K, Rb). The data show
that two types of screening currents occur within the high-field,
low-temperature CDW phases of these salts in response to time-dependent
magnetic fields. The first, which gives rise to the induced Hall potential, is
a free current (), present at the surface of the sample.
The time constant for the decay of these currents is much longer than that
expected from the sample resistivity. The second component of the current
appears to be magnetic (), in that it is a microscopic,
quasi-orbital effect; it is evenly distributed within the bulk of the sample
upon saturation. To explain these data, we propose a simple model invoking a
new type of quantum fluid comprising a CDW coexisting with a two-dimensional
Fermi-surface pocket which describes the two types of current. The model and
data are able to account for the body of previous experimental data which had
generated apparently contradictory interpretations in terms of the quantum Hall
effect or superconductivity.Comment: 13 pages, 11 figure
Charge ordering, symmetry and electronic structure issues and Wigner crystal structure of the quarter-filled band Mott insulators and high pressure metals δ-(EDT-TTF-CONMe2)2X, X = Br and AsF6
We report on the synthesis and application of an internal chemical pressure to effectively control, and reduce, the Mott gap in the system δ-(EDT-TTF-CONMe2)2X, X = Br, AsF6; the detailed accounts of its Pmna, averaged room temperature structure and reversible phase transition at ca. 190 K towards a low tem
Ground state and finite temperature behavior of 1/4-filled band zigzag ladders
We consider the simplest example of lattice frustration in the 1/4-filled
band, a one-dimensional chain with next-nearest neighbor interactions. For this
zigzag ladder with electron-electron as well as electron-phonon interactions we
present numerical results for ground state as well as thermodynamic properties.
In this system the ground state bond distortion pattern is independent of
electron-electron interaction strength. The spin gap in the ground state of the
zigzag ladder increases with the degree of frustration. Unlike in
one-dimension, where the spin-gap and charge ordering transitions can be
distinct, we show that in the ladder they occur simultaneously. We discuss spin
gap and charge ordering transitions in 1/4-filled materials with one, two, or
three dimensional crystal structures. We show empirically that regardless of
dimensionality the occurrence of simultaneous or distinct charge and magnetic
transitions can be correlated with the ground state bond distortion pattern.Comment: 12 pages, 8 eps figure
Electromagnetic response of a conductor with complex conductivity
The aim of this paper is to describe the electromagnetic response of a conductor with complex conductivity. We will show how the geometry of the measuring apparatus can modify the amplitude of this response. We will particularly emphasize the role that plays a complex conductivity, as we can find in granular superconductors, on the mesured magnetic susceptibility of the sample.Cet article a pour but de décrire la réponse électromagnétique d'un conducteur muni d'une conductivité complexe. Nous montrerons comment la géométrie du dispositif de mesure peut modifier l'amplitude de cette réponse. Nous insisterons particulièrement sur le rôle que joue une conductivité complexe, comme nous pouvons en trouver dans les supraconducteurs granulaires, sur la susceptibilité magnétique mesurée de l'échantillon