283 research outputs found
A pulsed, mono-energetic and angular-selective UV photo-electron source for the commissioning of the KATRIN experiment
The KATRIN experiment aims to determine the neutrino mass scale with a
sensitivity of 200 meV/c^2 (90% C.L.) by a precision measurement of the shape
of the tritium -spectrum in the endpoint region. The energy analysis of
the decay electrons is achieved by a MAC-E filter spectrometer. To determine
the transmission properties of the KATRIN main spectrometer, a mono-energetic
and angular-selective electron source has been developed. In preparation for
the second commissioning phase of the main spectrometer, a measurement phase
was carried out at the KATRIN monitor spectrometer where the device was
operated in a MAC-E filter setup for testing. The results of these measurements
are compared with simulations using the particle-tracking software
"Kassiopeia", which was developed in the KATRIN collaboration over recent
years.Comment: 19 pages, 16 figures, submitted to European Physical Journal
Distribution of spectral weight in a system with disordered stripes
The ``band-structure'' of a disordered stripe array is computed and compared,
at a qualitative level, to angle resolved photoemission experiments on the
cuprate high temperature superconductors. The low-energy states are found to be
strongly localized transverse to the stripe direction, so the electron dynamics
is strictly one-dimensional (along the stripe). Despite this, aspects of the
two dimensional band-structure Fermi surface are still vividly apparent.Comment: 10 pages, 11 figure
One particle interchain hopping in coupled Hubbard chains
Interchain hopping in systems of coupled chains of correlated electrons is
investigated by exact diagonalizations and Quantum-Monte-Carlo methods. For two
weakly coupled Hubbard chains at commensurate densities (e.g. n=1/3) the
splitting at the Fermi level between bonding and antibonding bands is strongly
reduced (but not suppressed) by repulsive interactions extending to a few
lattice spacings. The magnitude of this reduction is directly connected to the
exponent of the 1D Luttinger liquid. However, we show that the
incoherent part of the single particle spectral function is much less affected
by the interchain coupling. This suggests that incoherent interchain hopping
could occur for intermediate values.Comment: 4 pages, LaTeX 3.0, 7 PostScript figures in uuencoded for
Stripes in Doped Antiferromagnets: Single-Particle Spectral Weight
Recent photoemission (ARPES) experiments on cuprate superconductors provide
important guidelines for a theory of electronic excitations in the stripe
phase. Using a cluster perturbation theory, where short-distance effects are
accounted for by exact cluster diagonalization and long-distance effects by
perturbation (in the hopping), we calculate the single-particle Green's
function for a striped t-J model. The data obtained quantitatively reproduce
salient (ARPES-) features and may serve to rule out "bond-centered" in favor of
"site-centered" stripes.Comment: final version as appeared in PRL; (c) 2000 The American Physical
Society; 4 pages, 4 figure
RA-specific expression profiles and new candidate genes
Objective:
To identify rheumatoid arthritis- (RA)-specific profiles of differentially expressed genes.
Methods:
Synovial tissues from RA and osteoarthritis (OA) patients and from normal joints were selected according to their disease-characteristic histology. Gene expression was analyzed using DNA microarrays (GeneChip; Unigene-array) and representational difference analysis (RDA). Data were validated on larger cohorts of patients by RT-PCR.
Results:
Nine hundred and eighty genes were significantly regulated in RA synovial tissue as compared with non-RA. Specialized cluster analysis identified a set of 312 genes as sufficient of unequivocally discriminating RA from non-RA patterns (class discovery). Genes of highest regulation were associated with leukocyte activation (chemokines, chemokine receptors, B- and T-cell genes), endothelial and angiogenic
activation, tissue destruction and remodelling [MMP-3, BMP-4, TIMPs]. Interestingly, a large set of genes was down-regulated in RA (TGF-β superfamily, apoptosis-related genes, transcription factors). Osteopontin-like genes (n=46) — up-regulated in RA — and glutathione peroxidase-3-like genes (n=85) — down-regulated in RA — yielded the highest correlation coefficients (>0.94). Megakaryocyte stimulating factor (MSF), down-regulated in a subset of RA, may hold the key to subclassification: a loss-of-function mutation in the MSF-encoding gene leads to synovial hyperplasia in camptodactyly–arthropathy–coxa vara–pericarditis syndrome, and, as in RA, also to pericardial involvement. A further candidate, vitamin-D3-up-regulated protein-1 (VDUP-1), is regulated like MSF and predisposes to premature coronary artery disease when mutated, again a feature of a subset of RA.
Conclusion:
RA specific gene profiles were identified and are useful to improve diagnostics of the disease. Novel gene candidates not yet in the focus of RA pathogenesis have been identified that are likely to further the understanding of RA
Critical Properties in Dynamical Charge Correlation Function for the One-Dimensional Mott Insulator
Critical properties in the dynamical charge correlation function for the
one-dimensional Mott insulator are studied. By properly taking into account
{\it the final-state interaction} between the charge and spin degrees of
freedom, we find that the edge singularity in the charge correlation function
is governed by massless spinon excitations, although it is naively expected
that spinons do not directly contribute to the charge excitation over the
Hubbard gap. We obtain the momentum-dependent anomalous critical exponent by
applying the finite-size scaling analysis to the Bethe ansatz solution of the
half-filled Hubbard model.Comment: 7 pages, REVTe
Dual Nature of the Electronic Structure of the Stripe Phase
High resolution angle-resolved photoemission measurements have been carried
out on (La_1.4-xNd_0.6Sr_x)CuO_4, a model system with static stripes, and
(La_1.85Sr_0.15)CuO_4, a high temperature superconductor (T_c=40K) with dynamic
stripes. In addition to the straight segments near (pi, 0) and (0, pi)
antinodal regions, we have identified the existence of nodal spectral weight
and its associated Fermi surface in the electronic structure of both systems.
The ARPES spectra in the nodal region show well-defined Fermi cut-off,
indicating a metallic character of this charge-ordered state. This observation
of nodal spectral weight, together with the straight segments near antinodal
regions, reveals dual nature of the electronic structure of the stripes due to
the competition of order and disorder
Critical Properties in Photoemmision Spectra for One Dimensional Orbitally Degenerate Mott Insulator
Critical properties in photoemission spectra for the one-dimensional Mott
insulator with orbital degeneracy are studied by exploiting the integrable {\it
t-J} model, which is a supersymmetric generalization of the SU() degenerate
spin model. We discuss the critical properties for the holon dispersion as well
as the spinon dispersions, by applying the conformal field theory analysis to
the exact finite-size energy spectrum. We study the effect of orbital-splitting
on the spectra by evaluating the momentum-dependent critical exponents.Comment: 8 pages, REVTeX, 2 figures(available upon request), accepted for
publication in JPSJ 68 (1999) No.
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