3,108 research outputs found
A Schwinger-boson approach to the kagome with Dzyaloshinskii-Moriya interactions: phase diagram and dynamical structure factors
We have obtained the zero-temperature phase diagram of the kagome
antiferromagnet with Dzyaloshinskii-Moriya interactions in Schwinger-boson
mean-field theory. We find quantum phase transitions (first or second order)
between different topological spin liquids and Neel ordered phases (either the
state or the so-called Q=0 state). In the regime of
small Schwinger-boson density, the results bear some resemblances with exact
diagonalization results and we briefly discuss some issues of the mean-field
treatment. We calculate the equal-time structure factor (and its angular
average to allow for a direct comparison with experiments on powder samples),
which extends earlier work on the classical kagome to the quantum regime. We
also discuss the dynamical structure factors of the topological spin liquid and
the Neel ordered phase.Comment: 8 pages, 9 figure
Chern number spins of Mn acceptor magnets in GaAs
We determine the effective total spin of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from to , due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.Comment: 4 pages, 3 figure
Magnon bands of N-leg integer-spin antiferromagnetic systems in the weak interchain-coupling regime
Using the exact results of the O(3) nonlinear sigma model (NLSM) and a few
quantitative numerical data for integer-spin antiferromagnetic (AF) chains, we
systematically estimate all magnon excitation energies of N-leg integer-spin AF
ladders and tubes in the weak-interchain-coupling regime. Our method is based
on a first-order perturbation theory for the strength of the interchain
coupling. It can deal with any kind of interchain interactions, in principle.
We confirm that results of the perturbation theory are in good agreement with
those of a quantum Monte Carlo simulation and with our recent study based on a
saddle-point approximation of the NLSM [Phys. Rev. B 72, 104438 (2005)]. Our
theory further supports the existence of a Haldane (gapped) phase even in a
d-dimensional (d\geq 2) spatially anisotropic integer-spin AF model, if the
exchange coupling in one direction is sufficiently strong compared with those
in all the other directions. The strategy in this paper is applicable to other
N-leg systems consisting of gapped chains which low-energy physics is exactly
or quantitatively known.Comment: 11 pages, 4 figures, Revtex, published version, see also
cond-mat/0506049 (PRB72, 104438 (2005)
Neutrino-nucleus reactions on ^{12}C and ^{16}O
Exclusive and inclusive cross-sections and
-capture rates are calculated for ^{12}C and ^{16}O using the consistent
random phase approximation (RPA) and pairing model. After a pairing correction
is introduced to the RPA results the flux-averaged theoretical cross-sections and -capture rates in C are
in good agreement with experiment. In particular when one takes into account
the experimental error bars, the recently measured range of values for the
cross-section is in agreement with the present theoretical
results. Predictions of and cross-sections in
^{16}O are also presented.Comment: 13 pages, Revte
Tunneling-driven breakdown of the 331 state and the emergent Pfaffian and composite Fermi liquid phases
We examine the possibility of creating the Moore-Read Pfaffian in the lowest
Landau level when the multicomponent Halperin 331 state (believed to describe
quantum Hall bilayers and wide quantum wells at the filling factor )
is destroyed by the increase of tunneling. Using exact diagonalization of the
bilayer Hamiltonian with short-range and long-range (Coulomb) interactions in
spherical and periodic rectangular geometries, we establish that tunneling is a
perturbation that drives the 331 state into a compressible composite Fermi
liquid, with the possibility for an intermediate critical state that possesses
some properties of the Moore-Read Pfaffian. These results are interpreted in
the two-component BCS model for Cauchy pairing with a tunneling constraint. We
comment on the conditions to be imposed on a system with fluctuating density in
order to achieve the stable Pfaffian phase.Comment: 10 pages, 7 figure
Antiferromagnetic noise correlations in optical lattices
We analyze how noise correlations probed by time-of-flight (TOF) experiments
reveal antiferromagnetic (AF) correlations of fermionic atoms in
two-dimensional (2D) and three-dimensional (3D) optical lattices. Combining
analytical and quantum Monte Carlo (QMC) calculations using experimentally
realistic parameters, we show that AF correlations can be detected for
temperatures above and below the critical temperature for AF ordering. It is
demonstrated that spin-resolved noise correlations yield important information
about the spin ordering. Finally, we show how to extract the spin correlation
length and the related critical exponent of the AF transition from the noise.Comment: 4 pages, 4 figure
Superconductivity from purely repulsive interactions in the strong coupling approach : Application of the SU(2) slave-rotor theory to the Hubbard model
We propose a mechanism of superconductivity from purely repulsive
interactions in the strong coupling regime, where the BCS
(Bardeen-Cooper-Schrieffer) mechanism such as the spin-fluctuation approach is
difficult to apply. Based on the SU(2) slave-rotor representation of the
Hubbard model, we find that the single energy scale for the amplitude formation
of Cooper pairs and their phase coherence is separated into two energy scales,
allowing the so called pseudogap state where such Cooper pairs are coherent
locally but not globally, interpreted as realization of the density-phase
uncertainty principle. This superconducting state shows the temperature-linear
decreasing ratio of superfluid weight, resulting from strong phase
fluctuations
Nuclear Schiff moment and soft vibrational modes
The atomic electric dipole moment (EDM) currently searched by a number of
experimental groups requires that both parity and time-reversal invariance be
violated. According to current theoretical understanding, the EDM is induced by
the nuclear Schiff moment. The enhancement of the Schiff moment by the
combination of static quadrupole and octupole deformation was predicted
earlier. Here we study a further idea of the possible enhancement in the
absence of static deformation but in a nuclear system with soft collective
vibrations of two types. Both analytical approximation and numerical solution
of the simplified problem confirm the presence of the enhancement. We discuss
related aspects of nuclear structure which should be studied beyond mean-field
and random phase approximations.Comment: 14 pages, 4 figure
Nearby Doorways, Parity Doublets and Parity Mixing in Compound Nuclear States
We discuss the implications of a doorway state model for parity mixing in
compound nuclear states. We argue that in order to explain the tendency of
parity violating asymmetries measured in Th to have a common sign,
doorways that contribute to parity mixing must be found in the same energy
neighbourhood of the measured resonance. The mechanism of parity mixing in this
case of nearby doorways is closely related to the intermediate structure
observed in nuclear reactions in which compound states are excited. We note
that in the region of interest (Th) nuclei exhibit octupole
deformations which leads to the existence of nearby parity doublets. These
parity doublets are then used as doorways in a model for parity mixing. The
contribution of such mechanism is estimated in a simple model.Comment: 11 pages, REVTE
Nanoscale imaging of equilibrium quantum Hall edge currents and of the magnetic monopole response in graphene
The recently predicted topological magnetoelectric effect and the response to
an electric charge that mimics an induced mirror magnetic monopole are
fundamental attributes of topological states of matter with broken time
reversal symmetry. Using a SQUID-on-tip, acting simultaneously as a tunable
scanning electric charge and as ultrasensitive nanoscale magnetometer, we
induce and directly image the microscopic currents generating the magnetic
monopole response in a graphene quantum Hall electron system. We find a rich
and complex nonlinear behavior governed by coexistence of topological and
nontopological equilibrium currents that is not captured by the monopole
models. Furthermore, by utilizing a tuning fork that induces nanoscale
vibrations of the SQUID-on-tip, we directly image the equilibrium currents of
individual quantum Hall edge states for the first time. We reveal that the edge
states that are commonly assumed to carry only a chiral downstream current, in
fact carry a pair of counterpropagating currents, in which the topological
downstream current in the incompressible region is always counterbalanced by
heretofore unobserved nontopological upstream current flowing in the adjacent
compressible region. The intricate patterns of the counterpropagating
equilibrium-state orbital currents provide new insights into the microscopic
origins of the topological and nontopological charge and energy flow in quantum
Hall systems
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