13,924 research outputs found
Lie group weight multiplicities from conformal field theory
Dominant weight multiplicities of simple Lie groups are expressed in terms of
the modular matrices of Wess-Zumino-Witten conformal field theories, and
related objects. Symmetries of the modular matrices give rise to new relations
among multiplicities. At least for some Lie groups, these new relations are
strong enough to completely fix all multiplicities.Comment: 12 pages, Plain TeX, no figure
Quantum probabilities as Bayesian probabilities
In the Bayesian approach to probability theory, probability quantifies a
degree of belief for a single trial, without any a priori connection to
limiting frequencies. In this paper we show that, despite being prescribed by a
fundamental law, probabilities for individual quantum systems can be understood
within the Bayesian approach. We argue that the distinction between classical
and quantum probabilities lies not in their definition, but in the nature of
the information they encode. In the classical world, maximal information about
a physical system is complete in the sense of providing definite answers for
all possible questions that can be asked of the system. In the quantum world,
maximal information is not complete and cannot be completed. Using this
distinction, we show that any Bayesian probability assignment in quantum
mechanics must have the form of the quantum probability rule, that maximal
information about a quantum system leads to a unique quantum-state assignment,
and that quantum theory provides a stronger connection between probability and
measured frequency than can be justified classically. Finally we give a
Bayesian formulation of quantum-state tomography.Comment: 6 pages, Latex, final versio
Mutually unbiased bases: tomography of spin states and star-product scheme
Mutually unbiased bases (MUBs) are considered within the framework of a
generic star-product scheme. We rederive that a full set of MUBs is adequate
for a spin tomography, i.e. knowledge of all probabilities to find a system in
each MUB-state is enough for a state reconstruction. Extending the ideas of the
tomographic-probability representation and the star-product scheme to
MUB-tomography, dequantizer and quantizer operators for MUB-symbols of spin
states and operators are introduced, ordinary and dual star-product kernels are
found. Since MUB-projectors are to obey specific rules of the star-product
scheme, we reveal the Lie algebraic structure of MUB-projectors and derive new
relations on triple- and four-products of MUB-projectors. Example of qubits is
considered in detail. MUB-tomography by means of Stern-Gerlach apparatus is
discussed.Comment: 11 pages, 1 table, partially presented at the 17th Central European
Workshop on Quantum Optics (CEWQO'2010), June 6-11, 2010, St. Andrews,
Scotland, U
Electron and Phonon Confinement and New Surface Phonon Modes in CdSe-CdS Core-Shell Nanocrystals
Optical and vibrational properties of bare and CdS shelled CdSe
nanocrystalline particles are investigated. To confirm the formation of such
nanocrystals in our samples we estimate their average particle sizes and size
distributions using TEM measurements. From the line profile analysis of the
images the core-shell structure in the particles has been confirmed. The blue
shift in optical absorption spectra, analyzed using theoretical estimates based
on the effective bond order model, establishes the electron confinement in the
nanoparticles. Unique characteristics of the nanocrystals (which are absent in
the corresponding bulk material), such as confinement of optical phonons and
the appearance of surface phonons, are then discussed. Making use of the
dielectric response function model we are able to match the experimental and
theoretical values of the frequencies of the surface phonons. We believe that
our studies using optical probes provide further evidence on the existence of
core-shell structures in CdSe-CdS type materials.Comment: 19 pages 8 figure
Temperature dependence of trapped magnetic field in MgB2 bulk superconductor
Based on DC magnetization measurements, the temperature dependencies of the
trapped magnetic field have been calculated for two MgB2 samples prepared by
two different techniques: the high-pressure sintering and the hot pressing.
Experimentally measured trapped field values for the first sample coincide
remarkably well with calculated ones in the whole temperature range. This
proves, from one side, the validity of the introduced calculation approach, and
demonstrates, from another side, the great prospects of the hot pressing
technology for large scale superconducting applications of the MgB2.Comment: 3 pages, 3 figures, submitted to AP
Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion
Epitaxially strained LaCoO3 (LCO) thin films were grown with different film
thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT)
substrates. After initial pseudomorphic growth the films start to relieve their
strain partly by the formation of periodic nano-twins with twin planes
predominantly along the direction. Nano-twinning occurs already at the
initial stage of growth, albeit in a more moderate way. Pseudomorphic grains,
on the other hand, still grow up to a thickness of at least several tenths of
nanometers. The twinning is attributed to the symmetry lowering of the
epitaxially strained pseudo-tetragonal structure towards the relaxed
rhombohedral structure of bulk LCO. However, the unit-cell volume of the
pseudo-tetragonal structure is found to be nearly constant over a very large
range of t. Only films with t > 130 nm show a significant relaxation of the
lattice parameters towards values comparable to those of bulk LCO.Comment: 31 pages, 10 figure
Interaction of Spatially Localized LHW with Banana Particles
The paper proposes a novel mechanism of LHW stochastic acceleration of electrons in a tokamak
Topological defects for the free boson CFT
Two different conformal field theories can be joined together along a defect
line. We study such defects for the case where the conformal field theories on
either side are single free bosons compactified on a circle. We concentrate on
topological defects for which the left- and right-moving Virasoro algebras are
separately preserved, but not necessarily any additional symmetries. For the
case where both radii are rational multiples of the self-dual radius we
classify these topological defects. We also show that the isomorphism between
two T-dual free boson conformal field theories can be described by the action
of a topological defect, and hence that T-duality can be understood as a
special type of order-disorder duality.Comment: 43 pages, 4 figure
A super-analogue of Kontsevich's theorem on graph homology
In this paper we will prove a super-analogue of a well-known result by
Kontsevich which states that the homology of a certain complex which is
generated by isomorphism classes of oriented graphs can be calculated as the
Lie algebra homology of an infinite-dimensional Lie algebra of symplectic
vector fields.Comment: 15 page
The superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy
FeSe single crystals have been studied by soft point-contact
Andreev-reflection spectroscopy. Superconducting gap features in the
differential resistance dV/dI(V) of point contacts such as a characteristic
Andreev-reflection double-minimum structure have been measured versus
temperature and magnetic field. Analyzing dV/dI within the extended two-gap
Blonder-Tinkham-Klapwijk model allows to extract both the temperature and
magnetic field dependence of the superconducting gaps. The temperature
dependence of both gaps is close to the standard BCS behavior. Remarkably, the
magnitude of the double-minimum structure gradually vanishes in magnetic field,
while the minima position only slightly shifts with field indicating a weak
decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25
point contacts results in the averaged gap values = 1.8+/-0.4meV and
=1.0+/-0.2 meV and reduced values 2/kTc=4.2+/-0.9 and
2/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively.
Additionally, the small gap contribution was found to be within tens of percent
decreasing with both temperature and magnetic field. No signatures in the dV/dI
spectra were observed testifying a gapless superconductivity or presence of
even smaller gaps.Comment: 8 pages, 4 figs., 3 tables. Shortened version without fig.4 and Table
3 is accepted for publication in Phys. Rev.
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