576 research outputs found
Unitary Positive-Energy Representations of Scalar Bilocal Quantum Fields
The superselection sectors of two classes of scalar bilocal quantum fields in
D>=4 dimensions are explicitly determined by working out the constraints
imposed by unitarity. The resulting classification in terms of the dual of the
respective gauge groups U(N) and O(N) confirms the expectations based on
general results obtained in the framework of local nets in algebraic quantum
field theory, but the approach using standard Lie algebra methods rather than
abstract duality theory is complementary. The result indicates that one does
not lose interesting models if one postulates the absence of scalar fields of
dimension D-2 in models with global conformal invariance. Another remarkable
outcome is the observation that, with an appropriate choice of the Hamiltonian,
a Lie algebra embedded into the associative algebra of observables completely
fixes the representation theory.Comment: 27 pages, v3: result improved by eliminating redundant assumptio
Finite Size Analysis of the Structure Factors in the Antiferromagnetic XXZ Model
We perform a finite size analysis of the longitudinal and transverse
structure factors in the groundstate of the
spin- XXZ model. Comparison with the exact results of Tonegawa for
the XX model yields excellent agreement. Comparison with the conjecture of
M\"uller, Thomas, Puga and Beck reveals discrepancies in the momentum
dependence of the longitudinal structure factors.Comment: 9 pages RevTex 3.0 and 17 figures as uuencoded fil
Exchange interactions and Curie temperature in (GaMn)As
We use supercell and frozen-magnon approaches to study the dependence of the
magnetic interactions in (Ga,Mn)As on the Mn concentration. We report the
parameters of the exchange interaction between Mn spins and the estimates of
the Curie temperature within the mean-field and random-phase approximations. In
agreement with experiment we obtain a nonmonotonous dependence of the Curie
temperature on the Mn concentration. We estimate the dependence of the Curie
temperature on the concentration of the carries in the system and show that the
decrease of the number of holes in the valence band leads to fast decrease of
the Curie temperature. We show that the hole states of the valence band are
more efficient in mediating the exchange interaction between Mn spins than the
electron states of the conduction band
CCD BV and 2MASS photometric study of the open cluster NGC 1513
We present CCD BV and JHK 2MASS photometric data for the open cluster
NGC 1513. We observed 609 stars in the direction of the cluster up to a
limiting magnitude of mag. The star count method shows that the
centre of the cluster lies at ,
and its angular size is arcmin.
The optical and near-infrared two-colour diagrams reveal the colour excesses in
the direction of the cluster as , and
mag. These results are consistent with normal
interstellar extinction values. Optical and near-infrared Zero Age
Main-Sequences (ZAMS) provided an average distance modulus of
mag, which can be translated into a distance of
pc. Finally, using Padova isochrones we determined the metallicity
and age of the cluster as ( dex) and
, respectively.Comment: 15 pages, 12 figures and 4 tables, accepted for publication in
Astrophysics & Space Scienc
Aspects of Nucleon Compton Scattering
We consider the spin-averaged nucleon forward Compton scattering amplitude in
heavy baryon chiral perturbation theory including all terms to order . The chiral prediction for the spin-averaged forward Compton scattering
amplitude is in good agreement with the data for photon energies MeV. We also evaluate the nucleon electric and magnetic Compton
polarizabilities to this order and discuss the uncertainties of the various
counter terms entering the chiral expansion of these quantities.Comment: 17 pp, TeX, 7 figures available from the authors, preprint CRN-93/5
Characterization of human cytomegalovirus genome diversity in immunocompromised hosts by whole genomic sequencing directly from clinical specimens
Background:
Advances in next-generation sequencing (NGS) technologies allow comprehensive studies of genetic diversity over the entire genome of human cytomegalovirus (HCMV), a significant pathogen for immunocompromised individuals.
Methods:
NGS was performed on target-enriched sequence libraries prepared directly from a variety of clinical specimens (blood, urine, breast-milk, respiratory samples, biopsies and vitreous humor) obtained longitudinally or from different anatomical compartments from 20 HCMV-infected patients (renal transplant recipients, stem cell transplant recipients and congenitally infected children).
Results:
De novo assembled HCMV genome sequences were obtained for 57/68 sequenced samples. Analysis of longitudinal or compartmental HCMV diversity revealed various patterns: no major differences were detected among longitudinal, intra-individual blood samples from 9/15 patients and in most of the patients with compartmental samples, whereas a switch of the major HCMV population was observed in six individuals with sequential blood samples and upon compartmental analysis of one patient with HCMV retinitis. Variant analysis revealed additional aspects of minor virus population dynamics and antiviral resistance mutations.
Conclusions:
In immunosuppressed patients, HCMV can remain relatively stable or undergo drastic genomic changes that are suggestive of the emergence of minor resident strains or de novo infection
Critical properties of 1-D spin 1/2 antiferromagnetic Heisenberg model
We discuss numerical results for the 1-D spin 1/2 antiferromagnetic
Heisenberg model with next-to-nearest neighbour coupling and in the presence of
an uniform magnetic field. The model develops zero frequency excitations at
field dependent soft mode momenta. We compute critical quantities from finite
size dependence of static structure factors.Comment: talk given by H. Kr{\"o}ger at Heraeus Seminar Theory of Spin
Lattices and Lattice Gauge Models, Bad Honnef (1996), 20 pages, LaTeX + 18
figures, P
Nonequilibrium Magnetization Dynamics of Gadolinium Studied by Magnetic Linear Dichroismin Time-Resolved 4f Core-Level Photoemission
The magnetic linear dichroism of the gadolinium 4f core level is studied in a time-resolved photoemission experiment employing laser pump- and synchrotron-radiation probe pulses. Upon optical excitation of the 5d6s valence electrons with femtosecond laser pulses, the magnetic order in the 4f spin system is reduced. Remarkably, the linear dichroism remains at 80% of the equilibrium contrast while the lattice temperature reaches the Curie temperature due to electron-phonon scattering. Contrasting itinerant ferromagnets, this shows that equilibration between the lattice and spin subsystems takes in Gd about 80 ps and is established in parallel with heat diffusion
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