4,039 research outputs found
Phase diagram of the frustrated, spatially anisotropic S=1 antiferromagnet on a square lattice
We study the S=1 square lattice Heisenberg antiferromagnet with spatially
anisotropic nearest neighbor couplings , frustrated by a
next-nearest neighbor coupling numerically using the density-matrix
renormalization group (DMRG) method and analytically employing the
Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of
the anisotropy, within both methods we find quantum fluctuations to stabilize
the N\'{e}el ordered state above the classically stable region. Whereas SBMFT
suggests a fluctuation-induced first order transition between the N\'{e}el
state and a stripe antiferromagnet for and an
intermediate paramagnetic region opening only for very strong anisotropy, the
DMRG results clearly demonstrate that the two magnetically ordered phases are
separated by a quantum disordered region for all values of the anisotropy with
the remarkable implication that the quantum paramagnetic phase of the spatially
isotropic - model is continuously connected to the limit of
decoupled Haldane spin chains. Our findings indicate that for S=1 quantum
fluctuations in strongly frustrated antiferromagnets are crucial and not
correctly treated on the semiclassical level.Comment: 10 pages, 10 figure
Contrast Interferometry Using Bose-Einstein Condensates to Measure h/m and the Fine Structure Constant
The kinetic energy of an atom recoiling due to absorption of a photon was
measured as a frequency using an interferometric technique called ``contrast
interferometry''. Optical standing wave pulses were used as atom-optical
elements to create a symmetric three-path interferometer with a Bose-Einstein
condensate. The recoil phase accumulated in different paths was measured using
a single-shot detection technique. The scheme allows for additional photon
recoils within the interferometer and its symmetry suppresses several random
and systematic errors including those from vibrations and ac Stark shifts. We
have measured the photon recoil frequency of sodium to ppm precision, using
a simple realization of this scheme. Plausible extensions should yield a
sufficient precision to bring within reach a ppb-level determination of
and the fine structure constant
Modeling near-field radiative heat transfer from sharp objects using a general 3d numerical scattering technique
We examine the non-equilibrium radiative heat transfer between a plate and
finite cylinders and cones, making the first accurate theoretical predictions
for the total heat transfer and the spatial heat flux profile for
three-dimensional compact objects including corners or tips. We find
qualitatively different scaling laws for conical shapes at small separations,
and in contrast to a flat/slightly-curved object, a sharp cone exhibits a local
\emph{minimum} in the spatially resolved heat flux directly below the tip. The
method we develop, in which a scattering-theory formulation of thermal transfer
is combined with a boundary-element method for computing scattering matrices,
can be applied to three-dimensional objects of arbitrary shape.Comment: 5 pages, 4 figures. Corrected background information in the
introduction, results and discussion unchange
Symmetry breaking in driven and strongly damped pendulum
We examine the conditions for appearance of symmetry breaking bifurcation in
damped and periodically driven pendulum in the case of strong damping. We show
that symmetry breaking, unlike other nonlinear phenomena, can exist at high
dissipation. We prove that symmetry breaking phases exist between phases of
symmetric normal and symmetric inverted oscillations. We find that symmetry
broken solutions occupy a sufficiently smaller region of pendulum's parameter
space in comparison to the statements made in earlier considerations [McDonald
and Plischke, Phys. Rev. B 27 (1983) 201]. Our research on symmetry breaking in
a strongly damped pendulum is relevant to an understanding of phenomena of
dynamic symmetry breaking and rectification in a pure ac driven semiconductor
superlattices.Comment: 11 pages, 4 color figures, RevTeX
Maximally-localized Wannier Functions in Antiferromagnetic MnO within the FLAPW Formalism
We have calculated the maximally-localized Wannier functions of MnO in its
antiferromagnetic (AFM) rhombohedral unit cell, which contains two formula
units. Electron Bloch functions are obtained with the linearized augmented
plane-wave method within both the LSD and the LSD+U schemes. The thirteen
uppermost occupied spin-up bands correspond in a pure ionic scheme to the five
Mn 3d orbitals at the Mn_1 (spin-up) site, and the four O 2s/2p orbitals at
each of the O_1 and O_2 sites. Maximal localization identifies uniquely four
Wannier functions for each O, which are trigonally-distorted sp^3-like
orbitals. They display a weak covalent bonding between O 2s/2p states and
minority-spin d states of Mn_2, which is absent in a fully ionic picture. This
bonding is the fingerprint of the interaction responsible for the AFM ordering,
and its strength depends on the one-electron scheme being used. The five Mn
Wannier functions are centered on the Mn_1 site, and are atomic orbitals
modified by the crystal field. They are not uniquely defined by the criterion
of maximal localization and we choose them as the linear combinations which
diagonalize the r^2 operator, so that they display the D_3d symmetry of the
Mn_1 site.Comment: 11 pages, 6 PostScript figures. Uses Revtex4. Hi-res figures
available from the author
Spin Stiffness in the Hubbard model
The spin stiffness of the repulsive Hubbard model that occurs
in the hydrodynamic theory of antiferromagnetic spin waves is shown to be the
same as the thermodynamically defined stiffness involved in twisting the order
parameter. New expressions for are derived, which enable easier
interpretation, and connections with superconducting weight and gauge
invariance are discussed.Comment: 21 Pages LaTeX2e, to be published in Journal of Physics
CP Violation in the Decay B \to X_d e^+ e^-
The decay has an amplitude containing comparable
contributions proportional to , and
. These pieces involve different unitarity phases produced by
and loops. The simultaneous presence of different CKM
phases and different dynamical phases leads to a calculable asymmetry in the
partial widths of and . Using the
effective Hamiltonian of the standard model, we calculate this asymmetry as a
function of the invariant mass. The effects of , and
resonances are taken into account in the vacuum polarization of the
and currents. As a typical result, an asymmetry of is predicted in the nonresonant domain , assuming and . The branching ratio
in this domain is . Results are also
obtained in the region of the resonance, where an asymmetry of
is expected, subject to certain theoretical uncertainties in
the amplitude.Comment: 20 pages, LaTeX, 3 eps figures, uses cite.sty, epsf.sty, and
amssym.sty. Version to appear in Phys. Rev. D, with some minor addition
Effect of 3d-doping on the electronic structure of BaFe2As2
The electronic structure of BaFe2As2 doped with Co, Ni, and Cu has been
studied by a variety of experimental and theoretical methods, but a clear
picture of the dopant 3d states has not yet emerged. Herein we provide
experimental evidence of the distribution of Co, Ni, and Cu 3d states in the
valence band. We conclude that the Co and Ni 3d states provide additional free
carriers to the Fermi level, while the Cu 3d states are found at the bottom of
the valence band in a localized 3d10 shell. These findings help shed light on
why superconductivity can occur in BaFe2As2 doped with Co and Ni but not Cu.Comment: 18 pages, 8 figure
Properties and Detection of Spin Nematic Order in Strongly Correlated Electron Systems
A spin nematic is a state which breaks spin SU(2) symmetry while preserving
translational and time reversal symmetries. Spin nematic order can arise
naturally from charge fluctuations of a spin stripe state. Focusing on the
possible existence of such a state in strongly correlated electron systems, we
build a nematic wave function starting from a t-J type model. The nematic is a
spin-two operator, and therefore does not couple directly to neutrons. However,
we show that neutron scattering and Knight shift experiments can detect the
spin anisotropy of electrons moving in a nematic background. We find the mean
field phase diagram for the nematic taking into account spin-orbit effects.Comment: 13 pages, 11 figures. (v2) References adde
Phenomenology of non-standard Z couplings in exclusive semileptonic b -> s transitions
The rare decays , and
are analyzed in a generic scenario where New Physics effects
enter predominantly via penguin contributions. We show that this
possibility is well motivated on theoretical grounds, as the vertex
is particularly susceptible to non-standard dynamics. In addition, such a
framework is also interesting phenomenologically since the coupling
is rather poorly constrained by present data. The characteristic features of
this scenario for the relevant decay rates and distributions are investigated.
We emphasize that both sign and magnitude of the forward-backward asymmetry of
the decay leptons in , , carry sensitive information on New Physics. The observable is proposed as a useful probe of
non-standard CP violation in couplings.Comment: Minor modifications; version to appear in Phys. Rev.
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