8 research outputs found
Quasi One-Dimensional Spin Fluctuations in YBa(2)Cu(3)O(6+x)
We study the spin fluctuation of the oxygen deficient planes of
YBa(2)Cu(3)O(6+x). The Cu-O chains that constitute these planes are described
by a model that includes antiferromagnetic interactions between spins and
Kondo-like scattering of oxygen holes. The spectrum of magnetic excitations
shows the presence of incommensurate dynamic fluctuations along the direction
of the chains. The presence of itinerant holes is responsible for the existence
of important differences between the spin dynamics of this system and that of a
quasi-one-dimensional localized antiferromagnet. We comment on the possibility
of experimental observation of these fluctuations.Comment: 22 pages, REVTEX, 3 figures, to appear in PRB55 (1May 1997
Bond-disordered spin systems: Theory and application to doped high-Tc compounds
We examine the stability of magnetic order in a classical Heisenberg model
with quenched random exchange couplings. This system represents the spin
degrees of freedom in high- compounds with immobile dopants.
Starting from a replica representation of the nonlinear -model, we
perform a renormalization-group analysis. The importance of cumulants of the
disorder distribution to arbitrarily high orders necessitates a functional
renormalization scheme. From the renormalization flow equations we determine
the magnetic correlation length numerically as a function of the impurity
concentration and of temperature. From our analysis follows that
two-dimensional layers can be magnetically ordered for arbitrarily strong but
sufficiently diluted defects. We further consider the dimensional crossover in
a stack of weakly coupled layers. The resulting phase diagram is compared with
experimental data for LaSrCuO.Comment: 12 pages, 5 figure
The Lower Critical Dimension of the XY Spin Glass
We investigate the XY spin-glass model in two and three dimensions using the
domain-wall renormalization-group method. The results for systems of linear
sizes up to L=12 (2D) and L=8 (3D) strongly suggest that the lower critical
dimension for spin-glass ordering may be rather than four as
is commonly believed. Our 3D data favor the scenario of a low but finite
spin-glass ordering temperature below the chiral transition but they are also
compatible with the system being at or slightly below its lower critical
dimension.Comment: 4 pages, 3 ps figures. Typos have been corrected, one reference has
been added and the concluding paragraph has been expanded. To appear in Phys.
Rev. Let
Diluted Josephson-junction arrays in a magnetic field: phase coherence and vortex glass thresholds
The effects of random dilution of junctions on a two-dimensional
Josephson-junction array in a magnetic field are considered. For rational
values of the average flux quantum per plaquette , the superconducting
transition temperature vanishes, for increasing dilution, at a critical value
, while the vortex ordering remains stable up to , much
below the value corresponding to the geometric percolation threshold. For
, the array behaves as a zero-temperature vortex-glass.
Numerical results for from defect energy calculations are presented
which are consistent with this scenario.Comment: 4 pages, 4 figures, to appear in Phys. Rev.
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
Non-Fermi liquid regime of a doped Mott insulator
We study the doping of a Mott insulator in the presence of quenched
frustrating disorder in the magnetic exchange. A low doping regime
is found, in which the quasiparticle coherent scale is low : with (the ratio of typical exchange to
hopping). In the ``quantum critical regime'' , several
physical quantities display Marginal Fermi Liquid behaviour : NMR relaxation
time , resistivity , optical lifetime
\tau_{opt}^{-1}\propto \omega/\ln(\omega/\epstar) and response functions obey
scaling, e.g. .
In contrast, single-electron properties display stronger deviations from Fermi
liquid theory in this regime with a dependence of the inverse
single-particle lifetime and a decay of the photoemission
intensity. On the basis of this model and of various experimental evidence, it
is argued that the proximity of a quantum critical point separating a glassy
Mott-Anderson insulator from a metallic ground-state is an important ingredient
in the physics of the normal state of cuprate superconductors (particularly the
Zn-doped materials). In this picture the corresponding quantum critical regime
is a ``slushy'' state of spins and holes with slow spin and charge dynamics
responsible for the anomalous properties of the normal state.Comment: 40 pages, RevTeX, including 13 figures in EPS. v2 : minor changes,
some references adde
Sr impurity effects on the magnetic correlations of LaSrCuO
We examine the low-temperature magnetic properties of moderately doped
LaSrCuO paying particular attention to the spin-glass (SG) phase and the C-IC
transition as they are affected by Sr impurity disorder. New measurements of
the low-temperature susceptibility in the SG phase show an increase of an
anomalously small Curie constant with doping. This behaviour is explained in
terms of our theoretical work that finds small clusters of AFM correlated
regions separated by disordered domain walls. The domain walls lead to a
percolating sequence of paths connecting the impurities. We predict that for
this spin morphology the Curie constant should scale as , a
result that is quantitatively in agreement with experiment. Also, we find that
the magnetic correlations in the ground states in the SG phase are
commensurate, and that this behaviour should persist at higher temperatures
where the holes should move along the domain walls. However, our results show
that incommensurate correlations develop continuously around 5 % doping,
consistent with recent measurements by Yamada.Comment: 30 pages, revtex, 8 .ps format figures (2 meant to be in colour), to
be published in Physical Review B