1,904 research outputs found
Analysis of a communication satellite for lunar far-side exploration
Analysis of communication satellite for lunar far-side exploration relaying color television, voice, high-bit-rate telemetry data, and ranging code from command and service, and lunar module
Radiation testing of composite materials, in situ versus ex situ effects
The effect of post irradiation test environments on tensile properties of representative advanced composite materials (T300/5208, T300/934, C6000/P1700) was investigated. Four ply (+ or - 45 deg/+ or - 45 deg) laminate tensile specimens were exposed in vacuum up to a bulk dose of 1 x 10 to the 10th power rads using a mono-energetic fluence of 700 keV electrons from a Van de Graaff accelerator. Post irradiation testing was performed while specimens were being irradiated (in situ data), in vacuum after cessation of irradiation (in vacuo data), and after exposure to air (ex situ data). Room temperature and elevated temperature effects were evaluated. The radiation induced changes to the tensile properties were small. Since the absolute changes in tensile properties were small, the existance of a post irradiation test environment effect was indeterminate
Measuring the Impact of the COVID-19 Lockdown on Crime in a Medium-Sized City in China
Objectives:
The study examines the variation in the daily incidence of eight acquisitive crimes: automobile theft, electromobile theft, motorcycle theft, bicycle theft, theft from automobiles, pickpocketing, residential burglary, and cyber-fraud before the lockdown and the duration of the lockdown for a medium-sized city in China.
Methods:
Regression discontinuity in time (RDiT) models are used to test the effect of the lockdown measures on crime by examining the daily variation of raw counts and rate.
Results:
It is indicated that in contrast to numerous violent crime categories such as domestic violence where findings have repeatedly found increases during the COVID-19 pandemic, acquisitive crimes in this city were reduced during the lockdown period for all categories, while “cyber-fraud” was found more resilient in the sense that its decrease was not as salient as for most other crime types, possibly due to people’s use of the internet during the lockdown period.
Conclusions:
The findings provide further support to opportunity theories of crime that are contingent upon the need for a motivated offender to identify a suitable target in physical space
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
Quantum dots with two electrons: Singlet-triplet transitions
The magnetic character of the ground-state of two electrons on a double
quantum dot, connected in series to left and right single-channel leads, is
considered. By solving exactly for the spectrum of the two interacting
electrons, it is found that the coupling to the continuum of propagating states
on the leads, in conjunction with the electron-electron interactions, may
result in a delocalization of the bound state of the two electrons. This, in
turn, reduces significantly the range of the Coulomb interaction parameters
over which singlet-triplet transitions can be realized. It is also found that
the coupling to the leads favors the singlet ground-state.Comment: 8 pages, submitted to Phys. Rev.
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
Role of a parallel magnetic field in two dimensional disordered clusters containing a few correlated electrons
An ensemble of 2d disordered clusters with a few electrons is studied as a
function of the Coulomb energy to kinetic energy ratio r_s. Between the Fermi
system (small r_s) and the Wigner molecule (large r_s), an interaction induced
delocalization of the ground state takes place which is suppressed when the
spins are aligned by a parallel magnetic field. Our results confirm the
existence of an intermediate regime where the Wigner antiferromagnetism
defavors the Stoner ferromagnetism and where the enhancement of the Lande g
factor observed in dilute electron systems is reproduced.Comment: 4 pages, 3 figure
Interactions and Disorder in Quantum Dots: Instabilities and Phase Transitions
Using a fermionic renormalization group approach we analyse a model where the
electrons diffusing on a quantum dot interact via Fermi-liquid interactions.
Describing the single-particle states by Random Matrix Theory, we find that
interactions can induce phase transitions (or crossovers for finite systems) to
regimes where fluctuations and collective effects dominate at low energies.
Implications for experiments and numerical work on quantum dots are discussed.Comment: 4 pages, 1 figure; version to appear in Phys Rev Letter
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