228 research outputs found
One-dimensional nature of the magnetic fluctuations in YBaCuO
There is increasing evidence that inhomogeneous distributions of charge and
spin--so-called "striped phases"--play an important role in determining the
properties of the high-temperature superconductors. For example, recent
neutron-scattering measurements on the YBaCuO family of
materials show both spin and charge fluctuations that are consistent with the
striped-phase picture. But the fluctuations associated with a striped phase are
expected to be one-dimensional, whereas the magnetic fluctuations observed to
date appear to display two-dimensional symmetry. We show here that this
apparent two-dimensionality results from measurements on twinned crystals, and
that similar measurements on substantially detwinned crystals of
YBaCuO reveal the one-dimensional character of the magnetic
fluctuations, thus greatly strengthening the striped-phase interpretation.
Moreover, our results also suggest that superconductivity originates in charge
stripes that extend along the b crystal axis, where the superfluid density is
found to be substantially larger than for the a direction.Comment: 3 pages, PDF onl
The connection between superconducting phase correlations and spin excitations in YBaCuO: A magnetic field study
One of the most striking universal properties of the
high-transition-temperature (high-) superconductors is that they are all
derived from the hole-doping of their insulating antiferromagnetic (AF) parent
compounds. From the outset, the intimate relationship between magnetism and
superconductivity in these copper-oxides has intrigued researchers. Evidence
for this link comes from neutron scattering experiments that show the
unambiguous presence of short-range AF correlations (excitations) in cuprate
superconductors. Even so, the role of such excitations in the pairing mechanism
and superconductivity is still a subject of controversy. For
YBaCuO, where controls the hole-doping level, the most
prominent feature in the magnetic excitations spectra is the ``resonance''.
Here we show that for underdoped YBaCuO, where and
are below the optimal values, modest magnetic fields suppress the resonance
significantly, much more so for fields approximately perpendicular rather than
parallel to the CuO planes. Our results indicate that the resonance
measures pairing and phase coherence, suggesting that magnetism plays an
important role in the superconductivity of cuprates. The persistence of a field
effect above favors mechanisms with preformed pairs in the normal state
of underdoped cuprates.Comment: 12 pages, 4 figures, Nature (in press
Spin and charge order in the vortex lattice of the cuprates: experiment and theory
I summarize recent results, obtained with E. Demler, K. Park, A. Polkovnikov,
M. Vojta, and Y. Zhang, on spin and charge correlations near a magnetic quantum
phase transition in the cuprates. STM experiments on slightly overdoped BSCCO
(J.E. Hoffman et al., Science 295, 466 (2002)) are consistent with the
nucleation of static charge order coexisting with dynamic spin correlations
around vortices, and neutron scattering experiments have measured the magnetic
field dependence of static spin order in the underdoped regime in LSCO (B. Lake
et al., Nature 415, 299 (2002)) and LaCuO_4+y (B. Khaykovich et al., Phys. Rev.
B 66, 014528 (2002)). Our predictions provide a semi-quantitative description
of these observations, with only a single parameter measuring distance from the
quantum critical point changing with doping level. These results suggest that a
common theory of competing spin, charge and superconducting orders provides a
unified description of all the cuprates.Comment: 18 pages, 7 figures; Proceedings of the Mexican Meeting on
Mathematical and Experimental Physics, Mexico City, September 2001, to be
published by Kluwer Academic/Plenum Press; (v2) added clarifications and
updated reference
Exact solutions of a restricted ballistic deposition model on a one-dimensional staircase
Surface structure of a restricted ballistic deposition(RBD) model is examined
on a one-dimensional staircase with free boundary conditions. In this model,
particles can be deposited only at the steps of the staircase. We set up
recurrence relations for the surface fluctuation width using generating
function method. Steady-state solutions are obtained exactly given system size
. In the infinite-size limit, diverges as with the scaling
exponent . The dynamic exponent
is also found to be by solving the recurrence relations
numerically. This model can be viewed as a simple variant of the model which
belongs to the Kardar-Parisi-Zhang (KPZ) universality class . Comparing its deposition time scale
with that of the single-step model, we argue that must be the same as
, which is consistent with our finding.Comment: 19 pages, REVTEX, 5 figures upon request, INHA-PHYS-94-00
Magnetic resonance peak and nonmagnetic impurities
Nonmagnetic Zn impurities are known to strongly suppress superconductivity.
We review their effects on the spin excitation spectrum in , as investigated by inelastic neutron scattering measurements.Comment: Proceedings of Mato Advanced Research Workshop BLED 2000. To appear
in Nato Science Series: B Physic
High-transition-temperature superconductivity in the absence of the magnetic-resonance mode
The fundamental mechanism that gives rise to high-transition-temperature
(high-Tc) superconductivity in the copper oxide materials has been debated
since the discovery of the phenomenon. Recent work has focussed on a sharp
'kink' in the kinetic energy spectra of the electrons as a possible signature
of the force that creates the superconducting state. The kink has been related
to a magnetic resonance and also to phonons. Here we report that infrared
spectra of Bi2Sr2CaCu2O(8+d), (Bi-2212) show that this sharp feature can be
separated from a broad background and, interestingly, weakens with doping
before disappearing completely at a critical doping level of 0.23 holes per
copper atom. Superconductivity is still strong in terms of the transition
temperature (Tc approx 55 K), so our results rule out both the magnetic
resonance peak and phonons as the principal cause of high-Tc superconductivity.
The broad background, on the other hand, is a universal property of the copper
oxygen plane and a good candidate for the 'glue' that binds the electrons.Comment: 4 pages, 3 figure
The Kondo effect in ferromagnetic atomic contacts
Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk,
electronic conduction in these materials takes place mainly through the and
electrons, whereas the magnetic moments are mostly in the narrow
-electron bands, where they tend to align. This general picture may change
at the nanoscale because electrons at the surfaces of materials experience
interactions that differ from those in the bulk. Here we show direct evidence
for such changes: electronic transport in atomic-scale contacts of pure
ferromagnets (iron, cobalt and nickel), despite their strong bulk
ferromagnetism, unexpectedly reveal Kondo physics, that is, the screening of
local magnetic moments by the conduction electrons below a characteristic
temperature. The Kondo effect creates a sharp resonance at the Fermi energy,
affecting the electrical properties of the system;this appears as a Fano-Kondo
resonance in the conductance characteristics as observed in other artificial
nanostructures. The study of hundreds of contacts shows material-dependent
lognormal distributions of the resonance width that arise naturally from Kondo
theory. These resonances broaden and disappear with increasing temperature,
also as in standard Kondo systems. Our observations, supported by calculations,
imply that coordination changes can significantly modify magnetism at the
nanoscale. Therefore, in addition to standard micromagnetic physics, strong
electronic correlations along with atomic-scale geometry need to be considered
when investigating the magnetic properties of magnetic nanostructures.Comment: 7 pages, 5 figure
Powerlaw optical conductivity with a constant phase angle in high Tc superconductors
In certain materials with strong electron correlations a quantum phase
transition (QPT) at zero temperature can occur, in the proximity of which a
quantum critical state of matter has been anticipated. This possibility has
recently attracted much attention because the response of such a state of
matter is expected to follow universal patterns defined by the quantum
mechanical nature of the fluctuations. Forementioned universality manifests
itself through power-law behaviours of the response functions. Candidates are
found both in heavy fermion systems and in the cuprate high Tc superconductors.
Although there are indications for quantum criticality in the cuprate
superconductors, the reality and the physical nature of such a QPT are still
under debate. Here we identify a universal behaviour of the phase angle of the
frequency dependent conductivity that is characteristic of the quantum critical
region. We demonstrate that the experimentally measured phase angle agrees
precisely with the exponent of the optical conductivity. This points towards a
QPT in the cuprates close to optimal doping, although of an unconventional
kind.Comment: pdf format, 9 pages, 4 color figures include
Aloe-induced Toxic Hepatitis
Aloe has been widely used in phytomedicine. Phytomedicine describes aloe as a herb which has anti-inflammatory, anti-proliferative, anti-aging effects. In recent years several cases of aloe-induced hepatotoxicity were reported. But its pharmacokinetics and toxicity are poorly described in the literature. Here we report three cases with aloe-induced toxic hepatitis. A 57-yr-old woman, a 62-yr-old woman and a 55-yr-old woman were admitted to the hospital for acute hepatitis. They had taken aloe preparation for months. Their clinical manifestation, laboratory findings and histologic findings met diagnostic criteria (RUCAM scale) of toxic hepatitis. Upon discontinuation of the oral aloe preparations, liver enzymes returned to normal level. Aloe should be considered as a causative agent in hepatotoxicity
Low Limit for Effective Signal Strength in the Stratus OCT in Imperative Low Signal Strength Cases
- …