67 research outputs found
Parity and the Spin-Statistics Connection
The spin-statistics connection is obtained in a simple and elementary way for
general causal fields by using the parity operation to exchange spatial
coordinates in the scalar product of a locally commuting field operator,
evaluated at position x, with the same field operator evaluated at -x, at equal
times.Comment: 6 page
Robustness of the quantum Hall effect, sample size versus sample topology, and quality control management of III-V molecular beam epitaxy
We measure the IQHE on macroscopic (1.5cm x 1.5cm) "quick 'n' dirty" prepared
III-V heterostructure samples with van der Pauw and modified Corbino geometries
at 1.3 K. We compare our results with (i) data taken on smaller specimens,
among them samples with a standard Hall bar geometry, (ii) results of our
numerical analysis taking inhomogenities of the 2DEG into account. Our main
finding is a confirmation of the expected robustness of the IQHE which favours
the development of wide plateaux for small filling factors and very large
sample sizes (here with areas 10,000 times larger than in standard
arrangements).Comment: 51 pages, 27 figures, 3 tables, 49 references. This paper is
intimately related to the set-up decribed in physics/980400
Density functional electronic spectrum of the cluster and possible local Jahn-Teller distorsions in the La-Ba-Cu-O superconductor
We present a density functional theory (DFT) calculation in the generalized
gradient approximation to study the possibility for the existence of
Jahn-Teller (JT) or pseudo Jahn-Teller (PJT) type local distortions in the
La-Ba-Cu-O superconducting system. We performed the calculation and
correspondingly group theory classification of the electronic ground state of
the CuO elongated octahedra cluster, immersed in a background
simulating the superconductor. Part of the motivation to do this study is that
the origin of the apical deformation of the CuO cluster is not
due to a pure JT effect, having therefore a non {\it a priori} condition to
remove the degeneracy of the electronic ground state of the parent regular
octahedron. We present a comparative analysis of the symmetry classified
electron spectrum with previously reported results using unrestricted
Hartree-Fock calculations (UHF). Both the DFT and UHF calculations produced a
non degenerate electronic ground state, not having therefore the necessary
condition for a pure JT effect. However, the appearance of a degenerate E
state near to the highest occupied molecular orbital in the DFT calculation,
suggests the possibility for a PJT effect responsible for a local distortion of
the oxidized CuO cluster.Comment: 12 pages, 3 figures, submitted to International Journal of Modern
Physics B (IJMPB
Oligomerization and Nitration of the Grass Pollen Allergen Phl p 5 by Ozone, Nitrogen Dioxide, and Peroxynitrite: Reaction Products, Kinetics, and Health Effects
The allergenic and inflammatory potential of proteins can be enhanced by chemical modification upon exposure to atmospheric or physiological oxidants. The molecular mechanisms and kinetics of such modifications, however, have not yet been fully resolved. We investigated the oligomerization and nitration of the grass pollen allergen Phl p 5 by ozone (O(3)), nitrogen dioxide (NO(2)), and peroxynitrite (ONOO(–)). Within several hours of exposure to atmospherically relevant concentration levels of O(3) and NO(2), up to 50% of Phl p 5 were converted into protein oligomers, likely by formation of dityrosine cross-links. Assuming that tyrosine residues are the preferential site of nitration, up to 10% of the 12 tyrosine residues per protein monomer were nitrated. For the reaction with peroxynitrite, the largest oligomer mass fractions (up to 50%) were found for equimolar concentrations of peroxynitrite over tyrosine residues. With excess peroxynitrite, the nitration degrees increased up to 40% whereas the oligomer mass fractions decreased to 20%. Our results suggest that protein oligomerization and nitration are competing processes, which is consistent with a two-step mechanism involving a reactive oxygen intermediate (ROI), as observed for other proteins. The modified proteins can promote pro-inflammatory cellular signaling that may contribute to chronic inflammation and allergies in response to air pollution
Many-body spin related phenomena in ultra-low-disorder quantum wires
Zero length quantum wires (or point contacts) exhibit unexplained conductance
structure close to 0.7 X 2e^2/h in the absence of an applied magnetic field. We
have studied the density- and temperature-dependent conductance of
ultra-low-disorder GaAs/AlGaAs quantum wires with nominal lengths l=0 and 2 mu
m, fabricated from structures free of the disorder associated with modulation
doping. In a direct comparison we observe structure near 0.7 X 2e^2/h for l=0
whereas the l=2 mu m wires show structure evolving with increasing electron
density to 0.5 X 2e^2/h in zero magnetic field, the value expected for an ideal
spin-split sub-band. Our results suggest the dominant mechanism through which
electrons interact can be strongly affected by the length of the 1D region.Comment: 5 Pages, 4 figure
Bias and temperature dependence of the 0.7 conductance anomaly in Quantum Point Contacts
The 0.7 (2e^2/h) conductance anomaly is studied in strongly confined, etched
GaAs/GaAlAs quantum point contacts, by measuring the differential conductance
as a function of source-drain and gate bias as well as a function of
temperature. We investigate in detail how, for a given gate voltage, the
differential conductance depends on the finite bias voltage and find a
so-called self-gating effect, which we correct for. The 0.7 anomaly at zero
bias is found to evolve smoothly into a conductance plateau at 0.85 (2e^2/h) at
finite bias. Varying the gate voltage the transition between the 1.0 and the
0.85 (2e^2/h) plateaus occurs for definite bias voltages, which defines a gate
voltage dependent energy difference . This energy difference is
compared with the activation temperature T_a extracted from the experimentally
observed activated behavior of the 0.7 anomaly at low bias. We find \Delta =
k_B T_a which lends support to the idea that the conductance anomaly is due to
transmission through two conduction channels, of which the one with its subband
edge \Delta below the chemical potential becomes thermally depopulated as the
temperature is increased.Comment: 9 pages (RevTex) with 9 figures (some in low resolution
Scattering States of Plektons (PARTICLES with Braid Group Statistics) in 2+1 Dimensional Quantum Field Theory
A Haag-Ruelle scattering theory for particles with braid group statistics is
developed, and the arising structure of the Hilbert space of multiparticle
states is analyzed.Comment: 18 pages, LATEX, DAMTP-94-9
Unusual Thermodynamics on the Fuzzy 2-Sphere
Higher spin Dirac operators on both the continuum sphere() and its fuzzy
analog() come paired with anticommuting chirality operators. A
consequence of this is seen in the fermion-like spectrum of these operators
which is especially true even for the case of integer-spin Dirac operators.
Motivated by this feature of the spectrum of a spin 1 Dirac operator on
, we assume the spin 1 particles obey Fermi-Dirac statistics. This
choice is inspite of the lack of a well defined spin-statistics relation on a
compact surface such as . The specific heats are computed in the cases of
the spin and spin 1 Dirac operators. Remarkably the specific heat
for a system of spin particles is more than that of the spin 1
case, though the number of degrees of freedom is more in the case of spin 1
particles. The reason for this is inferred through a study of the spectrums of
the Dirac operators in both the cases. The zero modes of the spin 1 Dirac
operator is studied as a function of the cut-off angular momentum and is
found to follow a simple power law. This number is such that the number of
states with positive energy for the spin 1 and spin system become
comparable. Remarks are made about the spectrums of higher spin Dirac operators
as well through a study of their zero-modes and the variation of their spectrum
with degeneracy. The mean energy as a function of temperature is studied in
both the spin and spin 1 cases. They are found to deviate from
the standard ideal gas law in 2+1 dimensions.Comment: 19 pages, 7 figures. The paper has been significantly modified. Main
results are unchange
Interaction Effects in a One-Dimensional Constriction
We have investigated the transport properties of one-dimensional (1D)
constrictions defined by split-gates in high quality GaAs/AlGaAs
heterostructures. In addition to the usual quantized conductance plateaus, the
equilibrium conductance shows a structure close to , and in
consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135
(1996)] this 0.7 structure has been investigated in a wide range of samples as
a function of temperature, carrier density, in-plane magnetic field
and source-drain voltage . We show that the 0.7
structure is not due to transmission or resonance effects, nor does it arise
from the asymmetry of the heterojunction in the growth direction. All the 1D
subbands show Zeeman splitting at high , and in the wide channel
limit the -factor is , close to that of bulk GaAs.
As the channel is progressively narrowed we measure an exchange-enhanced
-factor. The measurements establish that the 0.7 structure is related to
spin, and that electron-electron interactions become important for the last few
conducting 1D subbands.Comment: 8 pages, 7 figures (accepted in Phys. Rev. B
Spinning Particles, Braid Groups and Solitons
We develop general techniques for computing the fundamental group of the
configuration space of identical particles, possessing a generic internal
structure, moving on a manifold . This group generalizes the -string
braid group of which is the relevant object for structureless particles. In
particular, we compute these generalized braid groups for particles with an
internal spin degree of freedom on an arbitrary . A study of their unitary
representations allows us to determine the available spectrum of spin and
statistics on in a certain class of quantum theories. One interesting
result is that half-integral spin quantizations are obtained on certain
manifolds having an obstruction to an ordinary spin structure. We also compare
our results to corresponding ones for topological solitons in
-invariant nonlinear sigma models in -dimensions, generalizing
recent studies in two spatial dimensions. Finally, we prove that there exists a
general scalar quantum theory yielding half-integral spin for particles (or
solitons) on a closed, orientable manifold if and only if
possesses a structure.Comment: harvmac, 34 pages, HUTP-93/A037; UICHEP-TH/93-18; BUHEP-93-2
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