2,958 research outputs found
Angle-resolved photoemission spectra in the cuprates from the d-density wave theory
Angle-resolved photoemission spectra present two challenges for the d-density
wave (DDW) theory of the pseudogap state of the cuprates: (1) hole pockets near
are not observed, in apparent contradiction with the assumption
of translational symmetry breaking, and (2) there are no well-defined
quasiparticles at the {\it antinodal} points, in contradiction with the
predictions of mean-field theory of this broken symmetry state. Here, we show
how these puzzles can be resolved.Comment: 4 pages, 3 eps figures, RevTex
Mechanistic Studies on the Reaction of cis-Diaquobisbiguanide-cobalt(III) Ion & Phenylbiguanide in Aqueous Acidic Solution: Formation of Mixed Chelate of Cobalt(III)
502-50
Supervised learning of an opto-magnetic neural network with ultrashort laser pulses
The explosive growth of data and its related energy consumption is pushing
the need to develop energy-efficient brain-inspired schemes and materials for
data processing and storage. Here, we demonstrate experimentally that Co/Pt
films can be used as artificial synapses by manipulating their magnetization
state using circularly-polarized ultrashort optical pulses at room temperature.
We also show an efficient implementation of supervised perceptron learning on
an opto-magnetic neural network, built from such magnetic synapses.
Importantly, we demonstrate that the optimization of synaptic weights can be
achieved using a global feedback mechanism, such that the learning does not
rely on external storage or additional optimization schemes. These results
suggest there is high potential for realizing artificial neural networks using
optically-controlled magnetization in technologically relevant materials, that
can learn not only fast but also energy-efficient.Comment: 9 pages, 4 figure
An Experimental Study of Parabolic Wire-reflectors on a Wave-length of About 3 Metres
Abstract not Availabl
An explanation for a universality of transition temperatures in families of copper oxide superconductors
A remarkable mystery of the copper oxide high-transition-temperature (Tc)
superconductors is the dependence of Tc on the number of CuO2 layers, n, in the
unit cell of a crystal. In a given family of these superconductors, Tc rises
with the number of layers, reaching a peak at n=3, and then declines: the
result is a bell-shaped curve. Despite the ubiquity of this phenomenon, it is
still poorly understood and attention has instead been mainly focused on the
properties of a single CuO2 plane. Here we show that the quantum tunnelling of
Cooper pairs between the layers simply and naturally explains the experimental
results, when combined with the recently quantified charge imbalance of the
layers and the latest notion of a competing order nucleated by this charge
imbalance that suppresses superconductivity. We calculate the bell-shaped curve
and show that, if materials can be engineered so as to minimize the charge
imbalance as n increases, Tc can be raised further.Comment: 15 pages, 3 figures. The version published in Natur
Dynamical simulation of current fluctuations in a dissipative two-state system
Current fluctuations in a dissipative two-state system have been studied
using a novel quantum dynamics simulation method. After a transformation of the
path integrals, the tunneling dynamics is computed by deterministic integration
over the real-time paths under the influence of colored noise. The nature of
the transition from coherent to incoherent dynamics at low temperatures is
re-examined.Comment: 4 pages, 4 figures; to appear in Phys. Rev. Letter
Theory of d-density wave viewed from a vertex model and its implications
The thermal disordering of the -density wave, proposed to be the origin of
the pseudogap state of high temperature superconductors, is suggested to be the
same as that of the statistical mechanical model known as the 6-vertex model.
The low temperature phase consists of a staggered order parameter of
circulating currents, while the disordered high temperature phase is a
power-law phase with no order. A special feature of this transition is the
complete lack of an observable specific heat anomaly at the transition. There
is also a transition at a even higher temperature at which the magnitude of the
order parameter collapses. These results are due to classical thermal
fluctuations and are entirely unrelated to a quantum critical point in the
ground state. The quantum mechanical ground state can be explored by
incorporating processes that causes transitions between the vertices, allowing
us to discuss quantum phase transition in the ground state as well as the
effect of quantum criticality at a finite temperature as distinct from the
power-law fluctuations in the classical regime. A generalization of the model
on a triangular lattice that leads to a 20-vertex model may shed light on the
Wigner glass picture of the metal-insulator transition in two-dimensional
electron gas. The power-law ordered high temperature phase may be generic to a
class of constrained systems and its relation to recent advances in the quantum
dimer models is noted.Comment: RevTex4, 10 pages, 11 figure
Incipient order in the t-J model at high temperatures
We analyze the high-temperature behavior of the susceptibilities towards a
number of possible ordered states in the t-J-V model using the high-temperature
series expansion. From all diagrams with up to ten edges, reliable results are
obtained down to temperatures of order J, or (with some optimism) to J/2. In
the unphysical regime, t<J, large superconducting susceptibilities are found,
which moreover increase with decreasing temperatures, but for t>J, these
susceptibilities are small and decreasing with decreasing temperature; this
suggests that the t-J model does not support high-temperature
superconductivity. We also find modest evidence of a tendency toward nematic
and d-density wave orders.
ERRATUM: Due to an error in the calculation, the series for d-wave
supeconducting and extended s-wave superconducting orders were incorrect. We
recalculate the series and give the replacement figures. In agreement with our
earlier findings, we still find no evidence of any strong enhancement of the
superconducting susceptibility with decreasing temperature. However, because
different Pade approximants diverge from each other at somewhat higher
temperatures than we originally found, it is less clear what this implies
concerning the presence or absence of high-temperature superconductivity in the
t-J model.Comment: 4 pages, 5 eps figures included; ERRATUM 2 pages, 3 eps figures
correcting the error in the series for superconducting susceptibilitie
Phases of the 2D Hubbard model at low doping
We show that the planar spiral phase of the 2D Hubbard model at low doping,
x, is unstable towards a noncoplanar spin configuration. The novel equilibrium
state we found at low doping is incommensurate with the inverse pitch of the
spiral varying as x^(1/2), but nevertheless has a dominant peak in the
susceptibility at (\pi,\pi). Relevance to the NMR and neutron scattering
experiments in La_2-xSr_xCuO_4 is disccussed.Comment: 12 pages, emtex v.3.
Random Frustration in two-dimensional spin-1/2 Heisenberg antiferromagnet
The square lattice spin-1/2 antiferromagnet containing a dilute
concentration, , of randomly placed ferromagnetic nearest-neighbor
bonds is studied at low-temperature via non-linear -model techniques
and by exact diagonalization. We generally find that long-range N\' eel order
is destroyed above a critical strength in the defective ferromagnetic exchange
coupling-constant given by . We also observe large
statistical fluctuations both in the spin-stiffness and in the
antiferromagnetic structure-factor near this critical point, suggesting the
onset of a spin-glass phase.Comment: 13 pgs, TeX, submitted to Phys. Rev. Feb. 22, 1994. (Correct set of
figures follow
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