155 research outputs found
Tunnel and thermal c-axis transport in BSCCO in the normal and pseudogap state
We consider the problem of c-axis transport in double-layered cuprates, in
particular with reference to BiSrCaCuO
compounds. We exploit the effect of the two barriers on the thermal and tunnel
transport. The resulting model is able to describe accurately the normal state
c-axis resistivity in BiSrCaCuO, from the
underdoped side up to the strongly overdoped. We extend the model, without
introducing additional parameters, in order to allow for the decrease of the
barrier when an external voltage bias is applied. The extended model is found
to describe properly the c-axis resistivity for small voltage bias above the
pseudogap temperature , the c-axis resistivity for large voltage bias
even below , and the differential curves taken in mesa structures.Comment: 12 pages, 6 figures. Submitted to Superconductor Science and
Technolog
Mixed-state microwave response in superconducting cuprates
We report measurements of the magnetic-field induced microwave complex
resistivity in REBaCuO thin films, with RE = Y, Sm.
Measurements are performed at 48 GHz by means of a resonant cavity in the
end-wall-replacement configuration. The magnetic field dependence is
investigated by applying a moderate (0.8 T) magnetic field along the c-axis.
The measured vortex state complex resistivity in
YBaCuO and SmBaCuO is
analyzed within the well-known models for vortex dynamics. It is shown that
attributing the observed response to vortex motion alone leads to
inconsistencies in the as-determined vortex parameters (such as the vortex
viscosity and the pinning constant). By contrast, attributing the entire
response to field-induced pair breaking leads to a nearly quantitative
description of the data.Comment: 6 pages, 4 figures, to be published in J. Supercond. as proceedings
of 8th HTSHFF (May 26th-29th, 2004, Begur, Spain
Anisotropic renormalized fluctuations in the microwave resistivity in YBCO
We discuss the excess conductivity above Tc due to renormalized
order-parameter fluctuations in YBCO at microwave frequencies. We calculate the
effects of the uniaxial anisotropy on the renormalized fluctuations in the
Hartree approximation, extending the isotropic theory developed by Dorsey
[Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part of the microwave
resistivity at 24 and 48 GHz and of the dc resistivity are performed on
different YBCO films. The onset of the superconducting transition and the
deviation from the linear temperature behavior above Tc can be fully accounted
for by the extended theory. According to the theoretical calculation here
presented, a departure from gaussian toward renormalized fluctuations is
observed. Very consistent values of the fundamental parameters (critical
temperature, coherence lenghts, penetration depth) of the superconducting state
are obtained.Comment: RevTex, 8 pages with 5 figures included, to be published in Physical
Review
Nonlinear c-axis transport in Bi_2Sr_2CaCu_2O_(8+d) from two-barrier tunneling
Motivated by the peculiar features observed through intrinsic tunneling
spectroscopy of BiSrCaCuO mesas in the normal state,
we have extended the normal state two-barrier model for the c-axis transport
[M. Giura et al., Phys. Rev. B {\bf 68}, 134505 (2003)] to the analysis of
curves. We have found that the purely normal-state model reproduces all
the following experimental features: (a) the parabolic -dependence of
in the high- region (above the conventional pseudogap temperature),
(b) the emergence and the nearly voltage-independent position of the "humps"
from this parabolic behavior lowering the temperature, and (c) the crossing of
the absolute curves at a characteristic voltage . Our
findings indicate that conventional tunneling can be at the origin of most of
the uncommon features of the c axis transport in
BiSrCaCuO. We have compared our calculations to
experimental data taken in severely underdoped and slightly underdoped
BiSrCaCuO small mesas. We have found good agreement
between the data and the calculations, without any shift of the calculated
dI/dV on the vertical scale. In particular, in the normal state (above
) simple tunneling reproduces the experimental dI/dV quantitatively.
Below quantitative discrepancies are limited to a simple rescaling of
the voltage in the theoretical curves by a factor 2. The need for such
modifications remains an open question, that might be connected to a change of
the charge of a fraction of the carriers across the pseudogap opening.Comment: 7 pages, 5 figure
Instability due to long range Coulomb interaction in a liquid of polarizable particles (polarons, etc.)
The interaction Hamiltonian for a system of polarons a la Feynman in the
presence of long range Coulomb interaction is derived and the dielectric
function is computed in mean field. For large enough concentration a liquid of
such particles becomes unstable. The onset of the instability is signaled by
the softening of a collective optical mode in which all electrons oscillate in
phase in their respective self-trapping potential. We associate the instability
with a metallization of the system. Optical experiments in slightly doped
cuprates and doped nickelates are analyzed within this theory.
We discuss why doped cuprates matallize whereas nickelates do not.Comment: 5 pages,1 figur
c-axis transport and phenomenology of the pseudo-gap state in
We measure and analyze the resistivity of
crystals for different doping . We obtain the fraction of carrier
that do not participate to the c-axis
conductivity. All the curves collapse onto a universal curve
when plotted against a reduced temperature
. We find that at the superconducting
transition is doping independent. We also show that a magnetic field up
to 14 T does not affect the degree of localization in the (a,b) planes but
widens the temperature range of the x-scaling by suppressing the
superconducting phase coherence.Comment: 11 pages, 5 figures, submitted to Phys.Rev.
Evidence of anomalous dispersion of the generalized sound velocity in glasses
The dynamic structure factor, S(Q,w), of vitreous silica, has been measured
by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5
nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented
statistical accuracy in such an extended energy range allows to accurately
determine the longitudinal current spectra, and the energies of the vibrational
excitations. The simultaneous observation of two excitations in the acoustic
region, and the persistence of propagating sound waves up to Q values
comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive
dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from
6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass.Comment: 5 pages, 3 figures. To appear in Phys. Rev.
Microscopic dynamics and relaxation processes in liquid Hydrogen Fluoride
Inelastic x-ray scattering and Brillouin light scattering measurements of the
dynamic structure factor of liquid hydrogen fluoride have been performed in the
temperature range. The data, analysed using a viscoelastic
model with a two timescale memory function, show a positive dispersion of the
sound velocity between the low frequency value and the high
frequency value . This finding confirms the existence of
a structural () relaxation directly related to the dynamical
organization of the hydrogen bonds network of the system. The activation energy
of the process has been extracted by the analysis of the temperature
behavior of the relaxation time that follows an Arrhenius law.
The obtained value for , when compared with that observed in another
hydrogen bond liquid as water, suggests that the main parameter governing the
-relaxation process is the number of the hydrogen bonds per molecule.Comment: 9 pages and 12 figure
Small and large polarons in nickelates, manganites, and cuprates
By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO),
Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and
Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of
polarons in this cuprate family. While in LSNO and SLMO small polarons localize
into ordered structures below a transition temperature, in those cuprates the
polarons appear to be large, and at low T their binding energy decreases. This
reflects into an increase of the polaron radius, which may trigger coherent
transport.Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of
Superconductivity - Proc. "Stripes 1996" - Roma Dec 199
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