1,447 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
A nearly closed ballistic billiard with random boundary transmission
A variety of mesoscopic systems can be represented as a billiard with a
random coupling to the exterior at the boundary. Examples include quantum dots
with multiple leads, quantum corrals with different kinds of atoms forming the
boundary, and optical cavities with random surface refractive index. The
specific example we study is a circular (integrable) billiard with no internal
impurities weakly coupled to the exterior by a large number of leads with one
channel open in each lead. We construct a supersymmetric nonlinear
-model by averaging over the random coupling strengths between bound
states and channels. The resulting theory can be used to evaluate the
statistical properties of any physically measurable quantity in a billiard. As
an illustration, we present results for the local density of states.Comment: 5 pages, 1 figur
Scalar and vector Keldysh models in the time domain
The exactly solvable Keldysh model of disordered electron system in a random
scattering field with extremely long correlation length is converted to the
time-dependent model with extremely long relaxation. The dynamical problem is
solved for the ensemble of two-level systems (TLS) with fluctuating well depths
having the discrete Z_2 symmetry. It is shown also that the symmetric TLS with
fluctuating barrier transparency may be described in terms of the planar
Keldysh model with dime-dependent random planar rotations in xy plane having
continuous SO(2) symmetry. The case of simultaneous fluctuations of the well
depth and barrier transparency is subject to non-abelian algebra. Application
of this model to description of dynamic fluctuations in quantum dots and
optical lattices is discussed.Comment: 6 pages, 5 eps figures. Extended version of the paper to be published
in JETP Lett 89 (2009
Longitudinal Data with Follow-up Truncated by Death: Match the Analysis Method to Research Aims
Diverse analysis approaches have been proposed to distinguish data missing due to
death from nonresponse, and to summarize trajectories of longitudinal data truncated
by death. We demonstrate how these analysis approaches arise from factorizations of
the distribution of longitudinal data and survival information. Models are illustrated
using cognitive functioning data for older adults. For unconditional models, deaths do
not occur, deaths are independent of the longitudinal response, or the unconditional
longitudinal response is averaged over the survival distribution. Unconditional mod-
els, such as random effects models fit to unbalanced data, may implicitly impute data
beyond the time of death. Fully conditional models stratify the longitudinal response
trajectory by time of death. Fully conditional models are effective for describing in-
dividual trajectories, in terms of either aging (age, or years from baseline) or dying
(years from death). Causal models (principal stratification) as currently applied are
fully conditional models, since group differences at one timepoint are described for a
cohort that will survive past a later timepoint. Partly conditional models summarize
the longitudinal response in the dynamic cohort of survivors. Partly conditional models
are serial cross-sectional snapshots of the response, reflecting the average response in
survivors at a given timepoint rather than individual trajectories. Joint models of sur-
vival and longitudinal response describe the evolving health status of the entire cohort.
Researchers using longitudinal data should consider which method of accommodating
deaths is consistent with research aims, and use analysis methods accordingly
Finite Size Corrections for the Pairing Hamiltonian
We study the effects of superconducting pairing in small metallic grains. We
show that in the limit of large Thouless conductance one can explicitly
determine the low energy spectrum of the problem as an expansion in the inverse
number of electrons on the grain. The expansion is based on the formal exact
solution of the Richardson model. We use this expansion to calculate finite
size corrections to the ground state energy, Matveev-Larkin parameter, and
excitation energies.Comment: 22 pages, 1 figur
VACUUM CASTING OF ALUMINUM-SILICON COATING ON TUBALLOY URANIUM . Final Report on a part of P.A. No. 390-ML-54-S F.S. 17
Welding is used to fabricate titanium and titanium-alloy components for air-frames, Jet engines, missiles, and chemical equipment. Annong the most important considerations in adapting titanium and its alloys to welded components is to use proper welding procedures and to select alloys that have the required weld-joint properties. The chemical and metallurgical characteristics that affect the selection of welding processes and alloys are discussed. Also, information is presented on surface preparation, welding procedures, and quality control. In addition, detailed data on the mechanical properties of welded joints in the commercial grades of titanium and titanium alloys and how these properties are affected by heat treatment and elevated temperatures are presented. (auth
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