40 research outputs found
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
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
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
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
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
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
Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics
The atmospheric greenhouse effect, an idea that many authors trace back to
the traditional works of Fourier (1824), Tyndall (1861), and Arrhenius (1896),
and which is still supported in global climatology, essentially describes a
fictitious mechanism, in which a planetary atmosphere acts as a heat pump
driven by an environment that is radiatively interacting with but radiatively
equilibrated to the atmospheric system. According to the second law of
thermodynamics such a planetary machine can never exist. Nevertheless, in
almost all texts of global climatology and in a widespread secondary literature
it is taken for granted that such mechanism is real and stands on a firm
scientific foundation. In this paper the popular conjecture is analyzed and the
underlying physical principles are clarified. By showing that (a) there are no
common physical laws between the warming phenomenon in glass houses and the
fictitious atmospheric greenhouse effects, (b) there are no calculations to
determine an average surface temperature of a planet, (c) the frequently
mentioned difference of 33 degrees Celsius is a meaningless number calculated
wrongly, (d) the formulas of cavity radiation are used inappropriately, (e) the
assumption of a radiative balance is unphysical, (f) thermal conductivity and
friction must not be set to zero, the atmospheric greenhouse conjecture is
falsified.Comment: 115 pages, 32 figures, 13 tables (some typos corrected
Fragmentation of single-particle strength around the doubly-magic nucleus 132Sn and the position of the 0f5/2 proton-hole state in 131In
Spectroscopic factors of neutron-hole and proton-hole states in 131Sn and 131In, respectively, were
measured using one-nucleon removal reactions from doubly magic 132Sn at relativistic energies. For 131In, a
2910(50)-keV Îł ray was observed for the first time and tentatively assigned to a decay from a 5=2â state
at 3275(50) keV to the known 1=2â level at 365 keV. The spectroscopic factors determined for this new
excited state and three other single-hole states provide first evidence for a strong fragmentation of singlehole strength in 131Sn and 131In. The experimental results are compared to theoretical calculations based on
the relativistic particle-vibration couplin
Intruder configurations in Ne at the transition into the island of inversion: Detailed structure study of Ne
Detailed -ray spectroscopy of the exotic neon isotope Ne has
been performed for the first time using the one-neutron removal reaction from
Ne on a liquid hydrogen target at 240~MeV/nucleon. Based on an analysis
of parallel momentum distributions, a level scheme with spin-parity assignments
has been constructed for Ne and the negative-parity states are
identified for the first time. The measured partial cross sections and momentum
distributions reveal a significant intruder -wave strength providing
evidence of the breakdown of the and shell gaps. Only a weak,
possible -wave strength was observed to bound final states. Large-scale
shell-model calculations with different effective interactions do not reproduce
the large -wave and small -wave strength observed experimentally,
indicating an ongoing challenge for a complete theoretical description of the
transition into the island of inversion along the Ne isotopic chain