52 research outputs found
Nuclear Masses, Chaos, and the Residual Interaction
We interpret the discrepancy between semiempirical nuclear mass formulas and
actual nuclear masses in terms of the residual interaction. We show that
correlations exist among all binding energies and all separation energies
throughout the valley of stability. We relate our approach to chaotic motion in
nuclei.Comment: 9 page
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Characteristics of a 9mm triple-beam tuning fork resonant sensor
This paper describes the design and testing of the first miniaturised metallic triple-beam tuning fork resonant sensors for use in force, pressure and torque measurement applications. The new devices with 9mm length vibrating tines have resulted in over a 40% reduction in size when compared to previously tested resonators. The four fold increase in operating frequency to 26 kHz, with Q factors in air up to 4000, provides additional benefits for resolution, accuracy, range and overload capability. Measurement repeatability of at least 0.02% of span levels for torque transducers employing the sensors are quoted. Results of characterisation over the temperature range -30oC to +90oC are given
Crossover from Orthogonal to Unitary Symmetry for Ballistic Electron Transport in Chaotic Microstructures
We study the ensemble-averaged conductance as a function of applied magnetic
field for ballistic electron transport across few-channel microstructures
constructed in the shape of classically chaotic billiards. We analyse the
results of recent experiments, which show suppression of weak localization due
to magnetic field, in the framework of random-matrix theory. By analysing a
random-matrix Hamiltonian for the billiard-lead system with the aid of
Landauer's formula and Efetov's supersymmetry technique, we derive a universal
expression for the weak-localization contribution to the mean conductance that
depends only on the number of channels and the magnetic flux. We consequently
gain a theoretical understanding of the continuous crossover from orthogonal
symmetry to unitary symmetry arising from the violation of time-reversal
invariance for generic chaotic systems.Comment: 49 pages, latex, 9 figures as tar-compressed uuencoded fil
Conformal Sigma-Models on Supercoset Targets
We investigate the quantum behaviour of sigma models on coset superspaces G/H
defined by Z_{2n} gradings of G. We find that, whenever G has vanishing Killing
form, there is a choice of WZ term which renders the model quantum conformal,
at least to one loop. The choice coincides with that for which the model is
known to be classically integrable. This generalizes results for models
associated to Z_4 gradings, including IIB superstrings in AdS_5\times S^5.Comment: 16 pages, corrected footnote 4 and minor typos, added reference
Towards a Field Theory of the Plateau Transition
We suggest a procedure for calculating correlation functions of the local
densities of states (DOS) at the plateau transitions in the Integer Quantum
Hall effect (IQHE). We argue that their correlation functions are appropriately
described in terms of the SL()/SU(2) WZNW model (at the usual Ka{\v
c}--Moody point and with the level ). In this model we have
identified the operators corresponding to the local DOS, and derived the
partial differential equation determining their correlation functions. The OPEs
for powers of the local DOS obtained from this equation are in agreement with
available results.Comment: typos corrected, a revised versio
Weak Charge Quantization as an Instanton of Interacting sigma-model
Coulomb blockade in a quantum dot attached to a diffusive conductor is
considered in the framework of the non-linear sigma-model. It is shown that the
weak charge quantization on the dot is associated with instanton configurations
of the Q-field in the conductor. The instantons have a finite action and are
replica non--symmetric. It is argued that such instantons may play a role in
the transition regime to the interacting insulator.Comment: 4 pages. The 2D case substantially modifie
Energy averages and fluctuations in the decay out of superdeformed bands
We derive analytic formulae for the energy average (including the energy
average of the fluctuation contribution) and variance of the intraband decay
intensity of a superdeformed band. Our results may be expressed in terms of
three dimensionless variables: , ,
and . Here is
the spreading width for the mixing of a superdeformed (SD) state with the
normally deformed (ND) states whose spin is the same as 's. The
have mean level spacing and mean electromagnetic decay width
whilst has electromagnetic decay width .
The average decay intensity may be expressed solely in terms of the variables
and or, analogously to statistical
nuclear reaction theory, in terms of the transmission coefficients and
describing transmission from the to the SD band via and
to lower ND states.
The variance of the decay intensity, in analogy with Ericson's theory of
cross section fluctuations depends on an additional variable, the correlation
length
\Gamma_N/(\Gamma_S+\Gamma^{\downarrow})=\frac{d}{2\pi}T_N/(\Gamma_S+\Gamma^{\d
ownarrow}). This suggests that analysis of an experimentally obtained variance
could yield the mean level spacing as does analysis of the cross section
autocorrelation function in compound nuclear reactions.
We compare our results with those of Gu and Weidenm\"uller.Comment: revtex4, 14 pages, 4 figures, to appear in Physical Review
Interacting electrons in disordered potentials: Conductance versus persistent currents
An expression for the conductance of interacting electrons in the diffusive
regime as a function of the ensemble averaged persistent current and the
compressibility of the system is presented. This expression involves only
ground-state properties of the system. The different dependencies of the
conductance and persistent current on the electron-electron interaction
strength becomes apparent. The conductance and persistent current of a small
system of interacting electrons are calculated numerically and their variation
with the strength of the interaction is compared. It is found that while the
persistent current is enhanced by interactions, the conductance is suppressed.Comment: REVTeX, 4 pages, 3 figures, all uuencoded, accepted for publication
in PR
Disordered Dirac Fermions: the Marriage of Three Different Approaches
We compare the critical multipoint correlation functions for two-dimensional
(massless) Dirac fermions in the presence of a random su(N) (non-Abelian) gauge
potential, obtained by three different methods. We critically reexamine
previous results obtained using the replica approach and in the limit of
infinite disorder strength and compare them to new results (presented here)
obtained using the supersymmetric approach to the N=2 case. We demonstrate that
this menage a trois of different approaches leads to identical results.
Remarkable relations between apparently different conformal field theories
(CFTs) are thereby obtained. We further establish a connection between the
random Dirac fermion problem and the c=-2 theory of dense polymers. The
presence of the c=-2 theory may be seen in all three different treatments of
the disorder.Comment: 38 pages, extended version submitted to NP
Weak Localization and Integer Quantum Hall Effect in a Periodic Potential
We consider magnetotransport in a disordered two-dimensional electron gas in
the presence of a periodic modulation in one direction. Existing quasiclassical
and quantum approaches to this problem account for Weiss oscillations in the
resistivity tensor at moderate magnetic fields, as well as a strong
modulation-induced modification of the Shubnikov-de Haas oscillations at higher
magnetic fields. They do not account, however, for the operation at even higher
magnetic fields of the integer quantum Hall effect, for which quantum
interference processes are responsible. We then introduce a field-theory
approach, based on a nonlinear sigma model, which encompasses naturally both
the quasiclassical and quantum-mechanical approaches, as well as providing a
consistent means of extending them to include quantum interference corrections.
A perturbative renormalization-group analysis of the field theory shows how
weak localization corrections to the conductivity tensor may be described by a
modification of the usual one-parameter scaling, such as to accommodate the
anisotropy of the bare conductivity tensor. We also show how the two-parameter
scaling, conjectured as a model for the quantum Hall effect in unmodulated
systems, may be generalized similarly for the modulated system. Within this
model we illustrate the operation of the quantum Hall effect in modulated
systems for parameters that are realistic for current experiments.Comment: 15 pages, 4 figures, ReVTeX; revised version with condensed
introduction; two figures taken out; reference adde
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