694 research outputs found
Resonating singlet valence plaquettes
We consider the simplest generalizations of the valence bond physics of SU(2)
singlets to SU(N) singlets that comprise objects with N sites -- these are
SU(N) singlet plaquettes with N=3 and N=4 in three spatial dimensions.
Specifically, we search for a quantum mechanical liquid of such objects -- a
resonating singlet valence plaquette phase that generalizes the celebrated
resonating valence bond phase for SU(2) spins. We extend the Rokhsar-Kivelson
construction of the quantum dimer model to the simplest SU(4) model for valence
plaquette dynamics on a cubic lattice. The phase diagram of the resulting
quantum plaquette model is analyzed both analytically and numerically. We find
that the ground state is solid everywhere, including at the Rokhsar-Kivelson
point where the ground state is an equal amplitude sum. By contrast, the equal
amplitude sum of SU(3) singlet triangular plaquettes on the face centered cubic
lattice is liquid and thus a candidate for describing a resonating single
valence plaquette phase, given a suitably defined local Hamiltonian.Comment: 12 pages, 15 figures, minor changes, references added, Phys Rev B
versio
Homoclinic standing waves in focussing DNLS equations --Variational approach via constrained optimization
We study focussing discrete nonlinear Schr\"{o}dinger equations and present a
new variational existence proof for homoclinic standing waves (bright
solitons). Our approach relies on the constrained maximization of an energy
functional and provides the existence of two one-parameter families of waves
with unimodal and even profile function for a wide class of nonlinearities.
Finally, we illustrate our results by numerical simulations.Comment: new version with revised introduction and improved condition (A3); 16
pages, several figure
Role of beam polarization in the determination of and couplings from
We evaluate the constraints on anomalous trilinear gauge-boson couplings that
can be obtained from the study of electron-positron annihilation into pairs
at a facility with either the electron beam longitudinally polarized or both
electron and positron beams transversely polarized. The energy ranges
considered in the analysis are the ones relevant to the next-linear collider
and to LEP~200. We discuss the possibilities of a model independent analysis of
the general conserving anomalous effective Lagrangian, as well as its
restriction to some specific models with reduced number of independent
couplings. The combination of observables with initial and final state
polarizations allows to separately constrain the different couplings and to
improve the corresponding numerical bounds.Comment: 24 pages, LaTeX, 9 figures (available on request from the authors
Properties of Al-doped ZnS films grown by chemical bath deposition
Zinc sulphide (ZnS) buffer layers are a cadmium free, wider energy band gap, alternative to the cadmium sulphide(CdS) buffer layers commonly used in copper indium gallium diselenide (CuInGaSe2)-based solar cells. However extrinsic doping of the ZnS is important to lower the resistivity of the layers and to improve flexibility of device design. In this work, Al-doped ZnS nanocrystalline films have been produced on glass substrates using a chemical bath deposition (CBD) method. The Al- concentration was varied from 0 at. % to 10 at. %, keeping other deposition parameters constant. The elemental composition of a typical sample with 6 at. % ‘Al’ in ZnS was Zn=44.9 at. %, S=49.8 at. % and Al=5.3 at.%. The X-ray diffraction data taken on these samples showed a broad peak corresponding to the (111) plane of ZnS while the crystallite size varied in the range, 8 – 15 nm, depending on the concentration of Al in the layers. The films with a Al-doping content of 6 at. % had an optical transmittance of 75 % in the visible range and the energy band gap evaluated from the data was 3.66 eV. The films n-type electrical conductivities and the electrical resistivity varied in the range, 107-103 Ωcm, it decreasing with an increase of the Al-concentration in the solution
Automorphism groups of Grassmann codes
We use a theorem of Chow (1949) on line-preserving bijections of
Grassmannians to determine the automorphism group of Grassmann codes. Further,
we analyze the automorphisms of the big cell of a Grassmannian and then use it
to settle an open question of Beelen et al. (2010) concerning the permutation
automorphism groups of affine Grassmann codes. Finally, we prove an analogue of
Chow's theorem for the case of Schubert divisors in Grassmannians and then use
it to determine the automorphism group of linear codes associated to such
Schubert divisors. In the course of this work, we also give an alternative
short proof of MacWilliams theorem concerning the equivalence of linear codes
and a characterization of maximal linear subspaces of Schubert divisors in
Grassmannians.Comment: revised versio
Identification of extra neutral gauge bosons at the International Linear Collider
Heavy neutral gauge bosons, Z's, are predicted by many theoretical schemes of
physics beyond the Standard Model, and intensive searches for their signatures
will be performed at present and future high energy colliders. It is quite
possible that Z's are heavy enough to lie beyond the discovery reach expected
at the CERN Large Hadron Collider LHC, in which case only indirect signatures
of Z' exchanges may occur at future colliders, through deviations of the
measured cross sections from the Standard Model predictions. We here discuss in
this context the foreseeable sensitivity to Z's of fermion-pair production
cross sections at an e^+e^- linear collider, especially as regards the
potential of distinguishing different Z' models once such deviations are
observed. Specifically, we assess the discovery and identification reaches on
Z' gauge bosons pertinent to the E_6, LR, ALR and SSM classes of models, that
should be attained at the planned International Linear Collider (ILC). With the
high experimental accuracies expected at the ILC, the discovery and the
identification reaches on the Z' models under consideration could be increased
substantially. In particular, the identification among the different models
could be achieved for values of Z' masses in the discovery (but beyond the
identification) reach of the LHC. An important role in enhancing such reaches
is played by the electron (and possibly the positron) longitudinally polarized
beams. Also, although the purely leptonic processes are experimentally cleaner,
the measurements of c- and b-quark pair production cross sections are found to
carry important, and complementary, information on these searches.Comment: 21 page
Tunka-Rex: the Cost-Effective Radio Extension of the Tunka Air-Shower Observatory
Tunka-Rex is the radio extension of the Tunka cosmic-ray observatory in
Siberia close to Lake Baikal. Since October 2012 Tunka-Rex measures the radio
signal of air-showers in coincidence with the non-imaging air-Cherenkov array
Tunka-133. Furthermore, this year additional antennas will go into operation
triggered by the new scintillator array Tunka-Grande measuring the secondary
electrons and muons of air showers. Tunka-Rex is a demonstrator for how
economic an antenna array can be without losing significant performance: we
have decided for simple and robust SALLA antennas, and we share the existing
DAQ running in slave mode with the PMT detectors and the scintillators,
respectively. This means that Tunka-Rex is triggered externally, and does not
need its own infrastructure and DAQ for hybrid measurements. By this, the
performance and the added value of the supplementary radio measurements can be
studied, in particular, the precision for the reconstructed energy and the
shower maximum in the energy range of approximately eV. Here
we show first results on the energy reconstruction indicating that radio
measurements can compete with air-Cherenkov measurements in precision.
Moreover, we discuss future plans for Tunka-Rex.Comment: Proceeding of UHECR 2014, Springdale, Utah, USA, accepted by JPS
Conference Proceeding
Semiclassical Analysis of Extended Dynamical Mean Field Equations
The extended Dynamical Mean Field Equations (EDMFT) are analyzed using
semiclassical methods for a model describing an interacting fermi-bose system.
We compare the semiclassical approach with the exact QMC (Quantum Montecarlo)
method. We found the transition to an ordered state to be of the first order
for any dimension below four.Comment: RevTex, 39 pages, 16 figures; Appendix C added, typos correcte
Radio measurements of the energy and the depth of the shower maximum of cosmic-ray air showers by Tunka-Rex
We reconstructed the energy and the position of the shower maximum of air
showers with energies PeV applying a method using radio
measurements performed with Tunka-Rex. An event-to-event comparison to
air-Cherenkov measurements of the same air showers with the Tunka-133
photomultiplier array confirms that the radio reconstruction works reliably.
The Tunka-Rex reconstruction methods and absolute scales have been tuned on
CoREAS simulations and yield energy and values consistent
with the Tunka-133 measurements. The results of two independent measurement
seasons agree within statistical uncertainties, which gives additional
confidence in the radio reconstruction. The energy precision of Tunka-Rex is
comparable to the Tunka-133 precision of , and exhibits a
uncertainty on the absolute scale dominated by the amplitude calibration of the
antennas. For , this is the first direct experimental
correlation of radio measurements with a different, established method. At the
moment, the resolution of Tunka-Rex is approximately g/cm. This resolution can probably be improved by deploying additional
antennas and by further development of the reconstruction methods, since the
present analysis does not yet reveal any principle limitations.Comment: accepted for publication by JCA
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