4,456 research outputs found
The spin-1/2 XXZ Heisenberg chain, the quantum algebra U_q[sl(2)], and duality transformations for minimal models
The finite-size scaling spectra of the spin-1/2 XXZ Heisenberg chain with
toroidal boundary conditions and an even number of sites provide a projection
mechanism yielding the spectra of models with a central charge c<1 including
the unitary and non-unitary minimal series. Taking into account the
half-integer angular momentum sectors - which correspond to chains with an odd
number of sites - in many cases leads to new spinor operators appearing in the
projected systems. These new sectors in the XXZ chain correspond to a new type
of frustration lines in the projected minimal models. The corresponding new
boundary conditions in the Hamiltonian limit are investigated for the Ising
model and the 3-state Potts model and are shown to be related to duality
transformations which are an additional symmetry at their self-dual critical
point. By different ways of projecting systems we find models with the same
central charge sharing the same operator content and modular invariant
partition function which however differ in the distribution of operators into
sectors and hence in the physical meaning of the operators involved. Related to
the projection mechanism in the continuum there are remarkable symmetry
properties of the finite XXZ chain. The observed degeneracies in the energy and
momentum spectra are shown to be the consequence of intertwining relations
involving U_q[sl(2)] quantum algebra transformations.Comment: This is a preprint version (37 pages, LaTeX) of an article published
back in 1993. It has been made available here because there has been recent
interest in conformal twisted boundary conditions. The "duality-twisted"
boundary conditions discussed in this paper are particular examples of such
boundary conditions for quantum spin chains, so there might be some renewed
interest in these result
Shape Changes of Self-Assembled Actin Bilayer Composite Membranes
We report the self-assembly of thin actin shells beneath the membranes of
giant vesicles. Ion-carrier mediated influx of Mg2+ induces actin
polymerization in the initially spherical vesicles. Buckling of the vesicles
and the formation of blisters after thermally induced bilayer expansion is
demonstrated. Bilayer flickering is dominated by tension generated by its
coupling to the actin cortex. Quantitative flicker analysis suggests the
bilayer and the actin cortex are separated by 0.4 \mum to 0.5 \mum due to
undulation forces.Comment: pdf-file, has been accepted by PR
Strong disorder renormalization group study of aperiodic quantum Ising chains
We employ an adaptation of a strong-disorder renormalization-group technique
in order to analyze the ferro-paramagnetic quantum phase transition of Ising
chains with aperiodic but deterministic couplings under the action of a
transverse field. In the presence of marginal or relevant geometric
fluctuations induced by aperiodicity, for which the critical behavior is
expected to depart from the Onsager universality class, we derive analytical
and asymptotically exact expressions for various critical exponents (including
the correlation-length and the magnetization exponents, which are not easily
obtainable by other methods), and shed light onto the nature of the ground
state structures in the neighborhood of the critical point. The main results
obtained by this approach are confirmed by finite-size scaling analyses of
numerical calculations based on the free-fermion method
Couplings of N=1 chiral spinor multiplets
We derive the action for chiral spinor multiplets coupled to vector and
scalar multiplets. We give the component form of the action, which contains
gauge invariant mass terms for the antisymmetric tensors in the spinor
superfield and additional Green-Schwarz couplings to vector fields. We observe
that supersymmetry provides mass terms for the scalars in the spinor multiplet
which do not arise from eliminating an auxiliary field. We construct the dual
action by explicitly performing the duality transformations in superspace and
give its component form.Comment: 17 pages, v2 small change
Quantum gases in trimerized kagom\'e lattices
We study low temperature properties of atomic gases in trimerized optical
kagom\'{e} lattices. The laser arrangements that can be used to create these
lattices are briefly described. We also present explicit results for the
coupling constants of the generalized Hubbard models that can be realized in
such lattices. In the case of a single component Bose gas the existence of a
Mott insulator phase with fractional numbers of particles per trimer is
verified in a mean field approach. The main emphasis of the paper is on an
atomic spinless interacting Fermi gas in the trimerized kagom\'{e} lattice with
two fermions per site. This system is shown to be described by a quantum spin
1/2 model on the triangular lattice with couplings that depend on the bond
directions. We investigate this model by means of exact diagonalization. Our
key finding is that the system exhibits non-standard properties of a quantum
spin-liquid crystal: it combines planar antiferromagnetic order in the ground
state with an exceptionally large number of low energy excitations. The
possibilities of experimental verification of our theoretical results are
critically discussed.Comment: 19 pages/14 figures, version to appear in Phys. Rev. A., numerous
minor corrections with respect to former lanl submissio
Calibration and performance of the photon sensor response of FACT -- The First G-APD Cherenkov telescope
The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of
the performance of silicon photo detectors in Cherenkov Astronomy. For more
than two years it is operated on La Palma, Canary Islands (Spain), for the
purpose of long-term monitoring of astrophysical sources. For this, the
performance of the photo detectors is crucial and therefore has been studied in
great detail. Special care has been taken for their temperature and voltage
dependence implementing a correction method to keep their properties stable.
Several measurements have been carried out to monitor the performance. The
measurements and their results are shown, demonstrating the stability of the
gain below the percent level. The resulting stability of the whole system is
discussed, nicely demonstrating that silicon photo detectors are perfectly
suited for the usage in Cherenkov telescopes, especially for long-term
monitoring purpose
On minimal coupling of the ABC-superparticle to supergravity background
By rigorous application of the Hamiltonian methods we show that the
ABC-formulation of the Siegel superparticle admits consistent minimal coupling
to external supergravity. The consistency check proves to involve all the
supergravity constraints.Comment: 8 pages RevTex file, to appear in Phys. Rev.
FACT -- Operation of the First G-APD Cherenkov Telescope
Since more than two years, the First G-APD Cherenkov Telescope (FACT) is
operating successfully at the Canary Island of La Palma. Apart from its purpose
to serve as a monitoring facility for the brightest TeV blazars, it was built
as a major step to establish solid state photon counters as detectors in
Cherenkov astronomy.
The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode
avalanche photo diodes (G-APD aka. MPPC or SiPM) for photon detection. Since
properties as the gain of G-APDs depend on temperature and the applied voltage,
a real-time feedback system has been developed and implemented. To correct for
the change introduced by temperature, several sensors have been placed close to
the photon detectors. Their read out is used to calculate a corresponding
voltage offset. In addition to temperature changes, changing current introduces
a voltage drop in the supporting resistor network. To correct changes in the
voltage drop introduced by varying photon flux from the night-sky background,
the current is measured and the voltage drop calculated. To check the stability
of the G-APD properties, dark count spectra with high statistics have been
taken under different environmental conditions and been evaluated.
The maximum data rate delivered by the camera is about 240 MB/s. The recorded
data, which can exceed 1 TB in a moonless night, is compressed in real-time
with a proprietary loss-less algorithm. The performance is better than gzip by
almost a factor of two in compression ratio and speed. In total, two to three
CPU cores are needed for data taking. In parallel, a quick-look analysis of the
recently recorded data is executed on a second machine. Its result is publicly
available within a few minutes after the data were taken.
[...]Comment: 19th IEEE Real-Time Conference, Nara, Japan (2014
Phase transitions in the boson-fermion resonance model in one dimension
We study 1D fermions with photoassociation or with a narrow Fano-Feshbach
resonance described by the Boson-Fermion resonance model. Using thebosonization
technique, we derive a low-energy Hamiltonian of the system. We show that at
low energy, the order parameters for the Bose Condensation and fermion
superfluidity become identical, while a spin gap and a gap against the
formation of phase slips are formed. As a result of these gaps, charge density
wave correlations decay exponentially in contrast with the phases where only
bosons or only fermions are present. We find a Luther-Emery point where the
phase slips and the spin excitations can be described in terms of
pseudofermions. This allows us to provide closed form expressions of the
density-density correlations and the spectral functions. The spectral functions
of the fermions are gapped, whereas the spectral functions of the bosons remain
gapless. The application of a magnetic field results in a loss of coherence
between the bosons and the fermion and the disappearance of the gap. Changing
the detuning has no effect on the gap until either the fermion or the boson
density is reduced to zero. Finally, we discuss the formation of a Mott
insulating state in a periodic potential. The relevance of our results for
experiments with ultracold atomic gases subject to one-dimensional confinement
is also discussed.Comment: 31 pages, 8 EPS figures, RevTeX 4, long version of cond-mat/050570
High resolution amplitude and phase gratings in atom optics
An atom-field geometry is chosen in which an atomic beam traverses a field
interaction zone consisting of three fields, one having frequency propagating in the direction and the other two having
frequencies and propagating in the
- direction. For and , where and are positive integers and
is the pulse duration in the atomic rest frame, the atom-field interaction
results in the creation of atom amplitude and phase gratings having period . In this manner, one can use optical fields having
wavelength to produce atom gratings having periodicity much less
than .Comment: 11 pages, 14 figure
- …