2,220 research outputs found
Spin-Blockade in Single and Double Quantum Dots in Magnetic Fields: a Correlation Effect
The total spin of correlated electrons in a quantum dot changes with magnetic
field and this effect is generally linked to the change in the total angular
momentum from one magic number to another, which can be understood in terms of
an `electron molecule' picture for strong fields. Here we propose to exploit
this fact to realize a spin blockade, i.e., electrons are prohibited to tunnel
at specific values of the magnetic field. The spin-blockade regions have been
obtained by calculating both the ground and excited states. In double dots the
spin-blockade condition is found to be less stringent than in single dots.Comment: 4pages, to be published in Phys. Rev. B (Rapid Communication
A Tidal Disruption Flare in Abell 1689 from an Archival X-ray Survey of Galaxy Clusters
Theory suggests that a star making a close passage by a supermassive black
hole at the center of a galaxy can under most circumstances be expected to emit
a giant flare of radiation as it is disrupted and a portion of the resulting
stream of shock-heated stellar debris falls back onto the black hole itself. We
examine the first results of an ongoing archival survey of galaxy clusters
using Chandra and XMM-selected data, and report a likely tidal disruption flare
from SDSS J131122.15-012345.6 in Abell 1689. The flare is observed to vary by a
factor of >30 over at least 2 years, to have maximum L_X(0.3-3.0 keV)> 5 x
10^{42} erg s^{-1} and to emit as a blackbody with kT~0.12 keV. From the galaxy
population as determined by existing studies of the cluster, we estimate a
tidal disruption rate of 1.2 x 10^{-4} galaxy^{-1} year^{-1} if we assume a
contribution to the observable rate from galaxies whose range of luminosities
corresponds to a central black hole mass (M_bh) between 10^6 and 10^8 M_sun.Comment: 24 pages, including 6 figures and 2 tables Accepted for publication
in the Astrophysical Journa
Possibilities to Improve the Therapy of the Patients with Ureteral Concretions
The aim of the given study was to improve the results of treatment of patients with concretions of the upper third of the ureter.Materials and methods. The clinical trial was carried out as a non-interventional open, controlled, in two groups of patients with baseline control. The inclusion criteria concerned the patients with concretions of the upper third of the ureter 0.7-0.9 mm in size, which had one session of an extracorporeal shock-wave lithotripsy (ESWL), and after the procedure the size of the concretions fragments was up to3 mm. The study involved 60 patients, all patients in the main group (n = 30) were treated using phytotherapy, within 1 month starting with basic therapy under hospital conditions and continuing with preventative treatment in an outpatient setting. The control group (n = 30) received baseline therapy (up to 10 days) at the stationary stage. Results and discussion. In patients of the main group, fragments of concretions came out significantly faster, namely: in the main group, the fragments came out in 21 patients (70.0 %), while in the control group in 15 patients (50.0 %) (p1- p2, p <0.05) to the 7th day, and in the main group, the fragments came out in one patient more than 14 days, and in the control group – in 5 patients, p <0.05. The "stone path" in the bottom third of the ureter was in 1 patient (3.3 %) of the main group and in 3 patients (10.0 %) in the control group (p1-p2, p <0.05). The "stone path" departure time in patients of the main group was 2 days, and in patients of the control group it was 4.1 days from the time of its formation (p1-p2, p <0.05). Bacteriuria was observed in the main group on the 10th day 6.6 % less relative to the control group.Conclusion. At comparing the obtained results in patients of both groups in 1 month it was noted that high efficacy of the treatment was registered in 6 (20 %) patients of the main group and in 3 patients (10 %) of the control group (p <0.05), moderate efficiency was registered in 23 (76.7 %) patients of the main group and in 22 (73.3 %) patients of the control group respectively, low efficacy was registered in 1 (3.33 %) patient in the main group and 5 (16.7 %) patients of the control group (p <0,05), which points to the effectiveness of Urolesan capsules use in complex therapy of patients with concretions of the one third of ureter
Currents in a many-particle parabolic quantum dot under a strong magnetic field
Currents in a few-electron parabolic quantum dot placed into a perpendicular
magnetic field are considered. We show that traditional ways of investigating
the Wigner crystallization by studying the charge density correlation function
can be supplemented by the examination of the density-current correlator.
However, care must be exercised when constructing the correct projection of the
multi-dimensional wave function space. The interplay between the magnetic field
and Euler-liquid-like behavior of the electron liquid gives rise to persistent
and local currents in quantum dots. We demonstrate these phenomena by collating
a quasi-classical theory valid in high magnetic fields and an exact numerical
solution of the many-body problem.Comment: Uses RevTeX4, figures included in the tex
Effective interactions and large-scale diagonalization for quantum dots
The widely used large-scale diagonalization method using harmonic oscillator
basis functions (an instance of the Rayleigh-Ritz method, also called a
spectral method, configuration-interaction method, or ``exact diagonalization''
method) is systematically analyzed using results for the convergence of Hermite
function series. We apply this theory to a Hamiltonian for a one-dimensional
model of a quantum dot. The method is shown to converge slowly, and the
non-smooth character of the interaction potential is identified as the main
problem with the chosen basis, while on the other hand its important advantages
are pointed out. An effective interaction obtained by a similarity
transformation is proposed for improving the convergence of the diagonalization
scheme, and numerical experiments are performed to demonstrate the improvement.
Generalizations to more particles and dimensions are discussed.Comment: 7 figures, submitted to Physical Review B Single reference error
fixe
Re-entrant pinning of Wigner molecules in a magnetic field due to a Coulomb impurity
Pinning of magnetic-field induced Wigner molecules (WMs) confined in
parabolic two-dimensional quantum dots by a charged defect is studied by an
exact diagonalization approach. We found a re-entrant pinning of the WMs as
function of the magnetic field, a magnetic field induced re-orientation of the
WMs and a qualitatively different pinning behaviour in the presence of a
positive and negative Coulomb impurity
A Tidal Flare Candidate in Abell 1795
As part of our ongoing archival X-ray survey of galaxy clusters for tidal
flares, we present evidence of an X-ray transient source within 1 arcmin of the
core of Abell 1795. The extreme variability (a factor of nearly 50), luminosity
(> 2 x 10^42 erg s^{-1}), long duration (> 5 years) and supersoft X-ray
spectrum (< 0.1 keV) are characteristic signatures of a stellar tidal
disruption event according to theoretical predictions and to existing X-ray
observations, implying a massive >~10^5 M_sun black hole at the centre of that
galaxy. The large number of X-ray source counts (~700) and long temporal
baseline (~12 years with Chandra and XMM-Newton) make this one of the
best-sampled examples of any tidal flare candidate to date. The transient may
be the same EUV source originally found contaminating the diffuse ICM
observations of Bowyer et al. (1999), which would make it the only tidal flare
candidate with reported EUV observations and implies an early source luminosity
1-2 orders of magnitude greater. If the host galaxy is a cluster member then it
must be a dwarf galaxy, an order of magnitude less massive than the quiescent
galaxy Henize 2-10 which hosts a massive black hole that is difficult to
reconcile with its low mass. The unusual faintness of the host galaxy may be
explained by tidal stripping in the cluster core.Comment: Accepted by MNRAS 2013 July 23. 27 pages, 10 figure
- …