1,950 research outputs found
Ground state spin and Coulomb blockade peak motion in chaotic quantum dots
We investigate experimentally and theoretically the behavior of Coulomb
blockade (CB) peaks in a magnetic field that couples principally to the
ground-state spin (rather than the orbital moment) of a chaotic quantum dot. In
the first part, we discuss numerically observed features in the magnetic field
dependence of CB peak and spacings that unambiguously identify changes in spin
S of each ground state for successive numbers of electrons on the dot, N. We
next evaluate the probability that the ground state of the dot has a particular
spin S, as a function of the exchange strength, J, and external magnetic field,
B. In the second part, we describe recent experiments on gate-defined GaAs
quantum dots in which Coulomb peak motion and spacing are measured as a
function of in-plane magnetic field, allowing changes in spin between N and N+1
electron ground states to be inferred.Comment: To appear in Proceedings of the Nobel Symposium 2000 (Physica
Scripta
Flight design system requirements evaluation program
Flight planning requirements were analyzed and a computer system architecture was defined. An evaluation method of the proposed flight design system is also included
Model C critical dynamics of random anisotropy magnets
We study the relaxational critical dynamics of the three-dimensional random
anisotropy magnets with the non-conserved n-component order parameter coupled
to a conserved scalar density. In the random anisotropy magnets the structural
disorder is present in a form of local quenched anisotropy axes of random
orientation. When the anisotropy axes are randomly distributed along the edges
of the n-dimensional hypercube, asymptotical dynamical critical properties
coincide with those of the random-site Ising model. However structural disorder
gives rise to considerable effects for non-asymptotic critical dynamics. We
investigate this phenomenon by a field-theoretical renormalization group
analysis in the two-loop order. We study critical slowing down and obtain
quantitative estimates for the effective and asymptotic critical exponents of
the order parameter and scalar density. The results predict complex scenarios
for the effective critical exponent approaching an asymptotic regime.Comment: 8 figures, style files include
Randomly dilute Ising model: A nonperturbative approach
The N-vector cubic model relevant, among others, to the physics of the
randomly dilute Ising model is analyzed in arbitrary dimension by means of an
exact renormalization-group equation. This study provides a unified picture of
its critical physics between two and four dimensions. We give the critical
exponents for the three-dimensional randomly dilute Ising model which are in
good agreement with experimental and numerical data. The relevance of the cubic
anisotropy in the O(N) model is also treated.Comment: 4 pages, published versio
Large spin-orbit effects in small quantum dots
We consider small ballistic quantum dots weakly coupled to the leads in the
chaotic regime and look for significant spin-orbit effects. We find that these
effects can become quite prominent in the vicinity of degeneracies of many-body
energies. We illustrate the idea by considering a case where the intrinsic
exchange term -JS^2 brings singlet and triplet many-body states near each
other, while an externally tunable Zeeman term then closes the gap between the
singlet and the one of the triplet states (with spin projection parallel the
external field). Near this degeneracy, the spin-orbit coupling leads to a
striking temperature dependence of the conductance, with observable effects of
order unity at temperatures lower than the strength of the spin-orbit coupling.
Under favorable circumstances, spelled out in the paper, these order unity
effects in the conductance persist to temperatures much higher than the
spin-orbit coupling strength. Our conclusions are unaffected by the presence of
non-universal perturbations. We suggest a class of experiments to explore this
regime.Comment: 13 pages, 8 figure
Numerical study of resonant spin relaxation in quasi-1D channels
Recent experiments demonstrate that a ballistic version of spin resonance,
mediated by spin-orbit interaction, can be induced in narrow channels of a
high-mobility GaAs two-dimensional electron gas by matching the spin precession
frequency with the frequency of bouncing trajectories in the channel. Contrary
to the typical suppression of Dyakonov-Perel' spin relaxation in confined
geometries, the spin relaxation rate increases by orders of magnitude on
resonance. Here, we present Monte Carlo simulations of this effect to explore
the roles of varying degrees of disorder and strength of spin-orbit
interaction. These simulations help to extract quantitative spin-orbit
parameters from experimental measurements of ballistic spin resonance, and may
guide the development of future spintronic devices
Gauge dependenceof the order parameter anomalous dimension in the Ginzburg-Landau model and the critical fluctuations in superconductors
The critical fluctuations of superconductors are discussed in a fixed
dimension scaling suited to describe the type II regime. The gauge dependence
of the anomalous dimension of the scalar field is stablished exactly from the
Ward-Takahashi identities. Its fixed point value gives the critical
exponent and it is shown that is gauge independent, as expected on
physical grounds. In the scaling considered, is found to be zero at
1-loop order, while . This result is just the 1-loop values
for the XY model obtained in the fixed dimension renormalization group
approach. It is shown that this XY behavior holds at all orders. The result
should be contrasted with the negative values frequently
reported in the literature.Comment: EuroLaTex, 7 pages, 2 figures, reference updated; version to be
published in Europhysics Letter
XY Spin Fluid in an External Magnetic Field
A method of integral equations is developed to study inhomogeneous fluids
with planar spins in an external field. As a result, the calculations for these
systems appear to be no more difficult than those for ordinary homogeneous
liquids. The approach proposed is applied to the ferromagnetic XY spin fluid in
a magnetic field using a soft mean spherical closure and the Born-Green-Yvon
equation. This provides an accurate reproduction of the complicated phase
diagram behavior obtained by cumbersome Gibbs ensemble simulation and multiple
histogram reweighting techniques.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Detecting Spin-Polarized Currents in Ballistic Nanostructures
We demonstrate a mesoscopic spin polarizer/analyzer system that allows the
spin polarization of current from a quantum point contact in an in-plane
magnetic field to be measured. A transverse focusing geometry is used to couple
current from an emitter point contact into a collector point contact. At large
in-plane fields, with the point contacts biased to transmit only a single spin
(g < e^2/h), the voltage across the collector depends on the spin polarization
of the current incident on it. Spin polarizations of greater than 80% are found
for both emitter and collector at 300mK and 7T in-plane field.Comment: related papers at http://marcuslab.harvard.ed
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