288 research outputs found
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
F-theory, Geometric Engineering and N=1 Dualities
We consider geometric engineering of N=1 supersymmetric QFTs with matter in
terms of a local model for compactification of F-theory on Calabi-Yau fourfold.
By bringing 3-branes near 7-branes we engineer N=1 supersymmetric
gauge theory with flavors in the fundamental. We identify the Higgs
branch of this system with the instanton moduli space on the compact four
dimensional space of the 7-brane worldvolume. Moreover we show that the
Euclidean 3-branes wrapped around the compact part of the 7-brane worldvolume
can generate superpotential for as well as lead to quantum
corrections to the moduli space for . Finally we argue that Seiberg's
duality for N=1 supersymmetric QCD may be mapped to T-duality exchanging
7-branes with 3-branes.Comment: 15 page
A Sequence of Duals for Sp(2N) Supersymmetric Gauge Theories with Adjoint Matter
We consider supersymmetric Sp(2N) gauge theories with F matter fields in the
defining representation, one matter field in the adjoint representation, and no
superpotential. We construct a sequence of dual descriptions of this theory
using the dualities of Seiberg combined with the ``deconfinement'' method
introduced by Berkooz. Our duals hint at a new non-perturbative phenomenon that
seems to be taking place at asymptotically low energies in these theories: for
small F some of the degrees of freedom form massless, non-interacting bound
states while the theory remains in an interacting non-Abelian Coulomb phase.
This phenomenon is the result of strong coupling gauge dynamics in the original
description, but has a simple classical origin in the dual descriptions. The
methods used for constructing these duals can be generalized to any model
involving arbitrary 2-index tensor representations of Sp(2N), SO(N), or SU(N)
groups.Comment: version (with additional references) to appear in Phys. Rev. D, 20
pages, LaTeX, one embedded eps figur
Entanglement entropy in lattice gauge theories
We report on the recent progress in theoretical and numerical studies of
entanglement entropy in lattice gauge theories. It is shown that the concept of
quantum entanglement between gauge fields in two complementary regions of space
can only be introduced if the Hilbert space of physical states is extended in a
certain way. In the extended Hilbert space, the entanglement entropy can be
partially interpreted as the classical Shannon entropy of the flux of the gauge
fields through the boundary between the two regions. Such an extension leads to
a reduction procedure which can be easily implemented in lattice simulations by
constructing lattices with special topology. This enables us to measure the
entanglement entropy in lattice Monte-Carlo simulations. On the simplest
example of Z2 lattice gauge theory in (2 + 1) dimensions we demonstrate the
relation between entanglement entropy and the classical entropy of the field
flux. For SU(2) lattice gauge theory in four dimensions, we find a signature of
non-analytic dependence of the entanglement entropy on the size of the region.
We also comment on the holographic interpretation of the entanglement entropy.Comment: Talk presented at the Confinement8 conference (Mainz, Germany,
September 1 - 6, 2008) and at the conference "Liouville Field Theory and
Statistical Models", dedicated to Alexey Zamolodchikov memory (Moscow,
Russia, June 21 - 24, 2008
Mirage Cosmology of U(1) Gauge Field on Unstable D3 Brane Universe
An unstable -brane universe governed by the DBI action of the tachyon
field minimally coupled to a U(1) gauge boson is examined. The cosmological
evolution of this coupled system, is further analyzed, in terms of the
expansion rate of the inflating brane, which is highly affected by the presence
of the tachyonic and gauge field charges. We show, that the minimal coupling
makes the effective brane density less divergent. However, for some sectors of
the theory the tachyon is not able to regulate it in an efficient fashion.
Also, a detailed analysis of the dependance of the effective brane density on
the scale factor of the universe is performed, which leads to various
cosmological models.Comment: ReVTeX format 20 pages; v2 1 figure added, one additional paragraph
with extra comments added, enlarged list of references, version to appear in
JHE
Higher Equations of Motion in Liouville Field Theory
An infinite set of operator-valued relations in Liouville field theory is
established. These relations are enumerated by a pair of positive integers
, the first representative being the usual Liouville equation of
motion. The relations are proven in the framework of conformal field theory on
the basis of exact structure constants in the Liouville operator product
expansions. Possible applications in 2D gravity are discussed.Comment: Contribution to the proceedings of the VI International Conference
``CFT and Integrable Models'', Chernogolovka, Russia, September 200
FZZ Scattering
We study the duality between the two dimensional black hole and the
sine-Liouville conformal field theories via exact operator quantization of a
classical scattering problem. The ideas are first illustrated in Liouville
theory, which is dual to itself under the interchange of the Liouville
parameter b by 1/b. In both cases, a classical scattering problem does not
determine uniquely the quantum reflection coefficient. The latter is only fixed
by assuming that the dual scattering problem has the same reflection
coefficient. We also discuss the relation of this approach to the method that
exploits the parafermionic symmetry of the model to compute the reflection
coefficient.Comment: 19 pages, JHEP style. v2: Minor changes in the proposed field of
sine-Liouville type, new section discussing the relation with parafermionic
symmetry, references adde
An Extension for Direct Gauge Mediation of Metastable Supersymmetry Breaking
We study the direct mediation of metastable supersymmetry breaking by a
\Phi^2-deformation to the ISS model and extend it by splitting both Tr\Phi and
Tr\Phi^2 terms in the superpotential and gauging the flavor symmetry. We find
that with such an extension the enough long-lived metastable vacua can be
obtained and the proper gaugino masses can be generated. Also, this allows for
constructing a kind of models which can avoid the Landau pole problem.
Especially, in our metastable vacua there exist a large region for the
parameter m_3 which can satisfy the phenomenology requirements and allow for a
low SUSY breaking scale (\sim 100 TeV).Comment: version in Europhys. Let
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