747 research outputs found
Effects of Domain Wall on Electronic Transport Properties in Mesoscopic Wire of Metallic Ferromagnets
We study the effect of the domain wall on electronic transport properties in
wire of ferromagnetic 3 transition metals based on the linear response
theory. We considered the exchange interaction between the conduction electron
and the magnetization, taking into account the scattering by impurities as
well. The effective electron-wall interaction is derived by use of a local
gauge transformation in the spin space. This interaction is treated
perturbatively to the second order. The conductivity contribution within the
classical (Boltzmann) transport theory turns out to be negligiblly small in
bulk magnets, due to a large thickness of the wall compared with the fermi
wavelength. It can be, however, significant in ballistic nanocontacts, as
indicated in recent experiments. We also discuss the quantum correction in
disordered case where the quantum coherence among electrons becomes important.
In such case of weak localization the wall can contribute to a decrease of
resistivity by causing dephasing. At lower temperature this effect grows and
can win over the classical contribution, in particular in wire of diameter
, being the inelastic diffusion
length. Conductance change of the quantum origin caused by the motion of the
wall is also discussed.Comment: 30 pages, 4 figures. Detailed paper of Phys. Rev. Lett. 78, 3773
(1997). Submitted to J. Phys. Soc. Jp
Deconfinement Transition of AdS/QCD at
We study the confinement/deconfinement phase transition of holographic
AdS/QCD models by using Ricci flat black holes up to , which corresponds to the expansion correction in the
dual field theory to , where is the 't Hooft coupling
constant. We consider two cases: one is the hard-wall AdS/QCD model where a
small radius region of the is removed; the other is the case where one
of spatial coordinates for the space is compactified, resulting in
Witten's QCD model in 2+1 dimensions. We find that in the hard-wall AdS/QCD
model, the deconfinement temperature decreases when the expansion
corrections are taken into account, while in Witten's QCD model, the
deconfinement transition always happens when the ratio of inverse temperature
to the period of the compactified coordinate decreases to
one, , the same as the case without the
correction.Comment: Latex, 14 pages; v2: the version to appear in PR
Geometry and cosmological perturbations in the bulk inflaton model
We consider a braneworld inflation model driven by the dynamics of a scalar
field living in the 5-dimensional bulk, the so-called ``bulk inflaton model'',
and investigate the geometry in the bulk and large scale cosmological
perturbations on the brane. The bulk gravitational effects on the brane are
described by a projection of the 5-dimensional Weyl tensor, which we denote by
. Focusing on a tachionic potential model, we take a perturbative
approach in the anti-de Sitter (AdS) background with a single de Sitter
brane. We first formulate the evolution equations for in the bulk.
Next, applying them to the case of a spatially homogeneous brane, we obtain two
different integral expressions for . One of them reduces to the
expression obtained previously when evaluated on the brane. The other is a new
expression that may be useful for analyzing the bulk geometry. Then we consider
superhorizon scale cosmological perturbations and evaluate the bulk effects
onto the brane. In the limit , where is the Hubble parameter
on the brane and is the bulk curvature radius, we find that the
effective theory on the brane is identical to the 4-dimensional Einstein-scalar
theory with a simple rescaling of the potential even under the presence of
inhomogeneities. % atleast on super-Hubble horizon scales. In particular, it is
found that the anticipated non-trivial bulk effect due to the spatially
anisotropic part of may appear only at %second order in the low
energy expansion, i.e., at .Comment: 21 pages including 6 pages for several appendixes, no figure
Bekenstein Bound, Holography and Brane Cosmology in Charged Black Hole Backgrounds
We obtain a Bekenstein entropy bound for the charged objects in arbitrary
dimensions () using the D-bound recently proposed by Bousso. With the
help of thermodynamics of CFTs corresponding to AdS Reissner-Norstr\"om (RN)
black holes, we discuss the relation between the Bekenstein and
Bekenstein-Verlinde bounds. In particular we propose a Bekenstein-Verlinde-like
bound for the charged systems. In the Einstein-Maxwell theory with a negative
cosmological constant, we discuss the brane cosmology with positive tension
using the Binetruy-Deffayet-Langlois approach. The resulting
Friedman-Robertson-Walker equation can be identified with the one obtained by
the moving domain wall approach in the AdS RN black hole background. Finally we
also address the holographic property of the brane universe.Comment: Latex, 17 pages, v2: minor changes, a reference adde
Hawking-Page Phase Transition of black Dp-branes and R-charged black holes with an IR Cutoff
We show that the confinement-deconfinement phase transition of supersymmetric
Yang-Mills theories with 16 supercharges in various dimensions can be realized
through the Hawking-Page phase transition between the near horizon geometries
of black Dp-branes and BPS Dp-branes by removing a small radius region in the
geometry in order to realize a confinement phase, which generalizes the
Herzog's discussion for the holographic hard-wall AdS/QCD model. Removing a
small radius region in the gravitational dual corresponds to introducing an IR
cutoff in the dual field theory. We also discuss the Hawking-Page phase
transition between thermal , , spaces and R-charged AdS
black holes coming from the spherical reduction of the decoupling limit of
rotating D3-, M2-, and M5- branes in type IIB supergravity and 11 dimensional
supergravity in grand canonical ensembles, where the IR cutoff also plays a
crucial role in the existence of the phase transition.Comment: 34 pages, 18 figures, JHEP3, v2, references added, v3, some
explanations adde
Resistivity due to a Domain Wall in Ferromagnetic Metal
The resistivity due to a domain wall in ferromagnetic metallic wire is
calculated based on the linear response theory. The interaction between
conduction electrons and the wall is expressed in terms of a classical gauge
field which is introduced by the local gauge transformation in the electron
spin space. It is shown that the wall contributes to the decoherence of
electrons and that this quantum correction can dominate over the Boltzmann
resisitivity, leading to a decrease of resisitivity by nucleation of a wall.
The conductance fluctuation due to the motion of the wall is also investigated.
The results are compared with recent experiments.Comment: 9 pages, 3 figure
Permanent current from non-commutative spin algebra
We show that a spontaneous electric current is induced in a nano-scale
conducting ring just by putting three ferromagnets. The current is a direct
consequence of the non-commutativity of the spin algebra, and is proportional
to the non-coplanarity (chirality) of the magnetization vectors. The
spontaneous current gives a natural explanation to the chirality-driven
anomalous Hall effect.Comment: 7 pages, 4 figures on separate pag
Black Holes in the Universe: Generalized Lemaitre-Tolman-Bondi Solutions
We present new exact solutions {which presumably describe} black holes in the
background of a spatially flat, pressureless dark matter (DM)-, or dark matter
plus dark energy (DM+DE)-, or quintom-dominated universe. These solutions
generalize Lemaitre-Tolman-Bondi metrics. For a DM- or (DM+DE)-dominated
universe, the area of the black hole apparent horizon (AH) decreases with the
expansion of the universe while that of the cosmic AH increases. However, for a
quintom-dominated universe, the black hole AH first shrinks and then expands,
while the cosmic AH first expands and then shrinks. A (DM+DE)-dominated
universe containing a black hole will evolve to the Schwarzschild-de Sitter
solution with both AHs approaching constant size. In a quintom-dominated
universe, the black hole and cosmic AHs will coincide at a certain time, after
which the singularity becomes naked, violating Cosmic Censorship.Comment: 13 pages, 4 figure
Rhenium oxyhalides: a showcase for anisotropic-triangular-lattice quantum antiferromagnets
The spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice
(ATL) is an archetypal spin system hosting exotic quantum magnetism and
dimensional crossover. However, the progress in experimental research on this
field has been limited due to the scarcity of ideal model materials. Here, we
show that rhenium oxyhalides ReO, where spin-1/2 Re
ions form a layered structure of ATLs, allow for flexible chemical substitution
in both cation ( = Ca, Sr, Ba, Pb) and anion ( = Cl, Br)
sites, leading to seven synthesizable compounds. By combining magnetic
susceptibility and high-field magnetization measurements with theoretical
calculations using the orthogonalized finite-temperature Lanczos method, we
find that the anisotropy ranges from 0.25 to 0.45 depending on the
chemical composition. Our findings demonstrate that ReO is an
excellent platform for realizing diverse effective spin Hamiltonians that
differ in the strength of the anisotropy as well as the relevance of
perturbation terms such as the Dzyaloshinskii-Moriya interaction and interlayer
exchange coupling.Comment: 11 pages, 5 figures, SM: 4 pages, 2 figure
Cosmological perturbations from braneworld inflation with a Gauss-Bonnet term
Braneworld inflation is a phenomenology related to string theory that
describes high-energy modifications to general relativistic inflation. The
observable universe is a braneworld embedded in 5-dimensional anti de Sitter
spacetime. Whe the 5-dimensional action is Einstein-Hilbert, we have a
Randall-Sundrum type braneworld. The amplitude of tensor and scalar
perturbations from inflation is strongly increased relative to the standard
results, although the ratio of tensor to scalar amplitudes still obeys the
standard consistency relation. If a Gauss-Bonnet term is included in the
action, as a high-energy correction motivated by string theory, we show that
there are important changes to the Randall-Sundrum case. We give an exact
analysis of the tensor perturbations. They satisfy the same wave equation and
have the same spectrum as in the Randall-Sundrum case, but the Gauss-Bonnet
change to the junction conditions leads to a modified amplitude of
gravitational waves. The amplitude is no longer monotonically increasing with
energy scale, but decreases asymptotically after an initial rise above the
standard level. Using an approximation that neglects bulk effects, we show that
the amplitude of scalar perturbations has a qualitatively similar behaviour to
the tensor amplitude. In addition, the tensor to scalar ratio breaks the
standard consistency relation.Comment: Minor alterations to match published versio
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