1,979 research outputs found
On the energy-momentum tensor in non-commutative gauge theories
We study the properties of the energy-momentum tensor in non-commutative
gauge theories by coupling them to a weak external gravitational field. In
particular, we show that the stress tensor of such a theory coincides exactly
with that derived from a theory where a Seiberg-Witten map has been implemented
(namely, the procedure is commutative). Various other interesting features are
also discussed.Comment: 3 page
Metallicity and its low temperature behavior in dilute 2D carrier systems
We theoretically consider the temperature and density dependent transport
properties of semiconductor-based 2D carrier systems within the RPA-Boltzmann
transport theory, taking into account realistic screened charged impurity
scattering in the semiconductor. We derive a leading behavior in the transport
property, which is exact in the strict 2D approximation and provides a zeroth
order explanation for the strength of metallicity in various 2D carrier
systems. By carefully comparing the calculated full nonlinear temperature
dependence of electronic resistivity at low temperatures with the corresponding
asymptotic analytic form obtained in the limit, both within the
RPA screened charged impurity scattering theory, we critically discuss the
applicability of the linear temperature dependent correction to the low
temperature resistivity in 2D semiconductor structures. We find quite generally
that for charged ionized impurity scattering screened by the electronic
dielectric function (within RPA or its suitable generalizations including local
field corrections), the resistivity obeys the asymptotic linear form only in
the extreme low temperature limit of . We point out the
experimental implications of our findings and discuss in the context of the
screening theory the relative strengths of metallicity in different 2D systems.Comment: We have substantially revised this paper by adding new materials and
figures including a detailed comparison to a recent experimen
The Effective Action in Gauged Supergravity on Hyperbolic Background and Induced Cosmological Constant
The one-loop effective action for 4-dimensional gauged supergravity with
negative cosmological constant, is investigated in space-times with compact
hyperbolic spatial section. The explicit expansion of the effective action as a
power series of the curvature on hyperbolic background is derived, making use
of heat-kernel and zeta-regularization techniques. The induced cosmological and
Newton constants are computed.Comment: 9 pages, UTF 23
New features of collective motion of intrinsic degrees of freedom. Toward a possible way to classify the intrinsic states
Three exactly solvable Hamiltonians of complex structure are studied in the
framework of a semi-classical approach. The quantized trajectories for
intrinsic coordinates correspond to energies which may be classified in
collective bands. For two of the chosen Hamiltonians the symmetry SU2xSU2 is
the appropriate one to classify the eigenvalues in the laboratory frame.
Connections of results presented here with the molecular spectrum and
Moszkowski model are pointed out. The present approach suggests that the
intrinsic states, which in standard formalisms are heading rotational bands,
are forming themselves "rotational" bands, the rotations being performed in a
fictious boson space.Comment: 33 pages, 9 figure
Wilson line correlators in two-dimensional noncommutative Yang-Mills theory
We study the correlator of two parallel Wilson lines in two-dimensional
noncommutative Yang-Mills theory, following two different approaches. We first
consider a perturbative expansion in the large-N limit and resum all planar
diagrams. The second approach is non-perturbative: we exploit the Morita
equivalence, mapping the two open lines on the noncommutative torus (which
eventually gets decompacted) in two closed Wilson loops winding around the dual
commutative torus. Planarity allows us to single out a suitable region of the
variables involved, where a saddle-point approximation of the general Morita
expression for the correlator can be performed. In this region the correlator
nicely compares with the perturbative result, exhibiting an exponential
increase with respect to the momentum p.Comment: 21 pages, 1 figure, typeset in JHEP style; some formulas corrected in
Sect.3, one reference added, results unchange
Distributed Minimum Cut Approximation
We study the problem of computing approximate minimum edge cuts by
distributed algorithms. We use a standard synchronous message passing model
where in each round, bits can be transmitted over each edge (a.k.a.
the CONGEST model). We present a distributed algorithm that, for any weighted
graph and any , with high probability finds a cut of size
at most in
rounds, where is the size of the minimum cut. This algorithm is based
on a simple approach for analyzing random edge sampling, which we call the
random layering technique. In addition, we also present another distributed
algorithm, which is based on a centralized algorithm due to Matula [SODA '93],
that with high probability computes a cut of size at most
in rounds for any .
The time complexities of both of these algorithms almost match the
lower bound of Das Sarma et al. [STOC '11], thus
leading to an answer to an open question raised by Elkin [SIGACT-News '04] and
Das Sarma et al. [STOC '11].
Furthermore, we also strengthen the lower bound of Das Sarma et al. by
extending it to unweighted graphs. We show that the same lower bound also holds
for unweighted multigraphs (or equivalently for weighted graphs in which
bits can be transmitted in each round over an edge of weight ),
even if the diameter is . For unweighted simple graphs, we show
that even for networks of diameter , finding an -approximate minimum cut
in networks of edge connectivity or computing an
-approximation of the edge connectivity requires rounds
Effect of the Generalized Uncertainty Principle on Post-Inflation Preheating
We examine effects of the Generalized Uncertainty Principle, predicted by
various theories of quantum gravity to replace the Heisenberg's uncertainty
principle near the Planck scale, on post inflation preheating in cosmology, and
show that it can predict either an increase or a decrease in parametric
resonance and a corresponding change in particle production. Possible
implications are considered.Comment: v1: 9 pages, revtex4, no figures, accepted for publication in JCAP;
v2: one reference added and various cosmetic (but no physics) changes to
match published versio
Sharp increase of the effective mass near the critical density in a metallic 2D electron system
We find that at intermediate temperatures, the metallic temperature
dependence of the conductivity \sigma(T) of 2D electrons in silicon is
described well by a recent interaction-based theory of Zala et al. (Phys. Rev.
B 64, 214204 (2001)). The tendency of the slope d\sigma/dT to diverge near the
critical electron density is in agreement with the previously suggested
ferromagnetic instability in this electron system. Unexpectedly, it is found to
originate from the sharp enhancement of the effective mass, while the effective
Lande g factor remains nearly constant and close to its value in bulk silicon
Characteristic length scales and formation of vortices in the Abelian Higgs model in the presence of a uniform background charge
In this brief report we consider a non-local Abelian Higgs model in the
presence of a neutralizing uniform background charge. We show that such a
system possesses vortices which key feature is a strong radial electric field.
We estimate the basic properties of such an object and characteristic length
scales in this model.Comment: Replaced with journal version. Some minor change
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