2,377 research outputs found
Spin Readout and Initialization in a Semiconductor Quantum Dot
Electron spin qubits in semiconductors are attractive from the viewpoint of
long coherence times. However, single spin measurement is challenging. Several
promising schemes incorporate ancillary tunnel couplings that may provide
unwanted channels for decoherence. Here, we propose a novel spin-charge
transduction scheme, converting spin information to orbital information within
a single quantum dot by microwave excitation. The same quantum dot can be used
for rapid initialization, gating, and readout. We present detailed modeling of
such a device in silicon to confirm its feasibility.Comment: Published versio
Theory of valley-orbit coupling in a Si/SiGe quantum dot
Electron states are studied for quantum dots in a strained Si quantum well,
taking into account both valley and orbital physics. Realistic geometries are
considered, including circular and elliptical dot shapes, parallel and
perpendicular magnetic fields, and (most importantly for valley coupling) the
small local tilt of the quantum well interface away from the crystallographic
axes. In absence of a tilt, valley splitting occurs only between pairs of
states with the same orbital quantum numbers. However, tilting is ubiquitous in
conventional silicon heterostructures, leading to valley-orbit coupling. In
this context, "valley splitting" is no longer a well defined concept, and the
quantity of merit for qubit applications becomes the ground state gap. For
typical dots used as qubits, a rich energy spectrum emerges, as a function of
magnetic field, tilt angle, and orbital quantum number. Numerical and
analytical solutions are obtained for the ground state gap and for the mixing
fraction between the ground and excited states. This mixing can lead to valley
scattering, decoherence, and leakage for Si spin qubits.Comment: 18 pages, including 4 figure
The Initial Conditions of Clustered Star Formation III. The Deuterium Fractionation of the Ophiuchus B2 Core
We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of
the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular
cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+
data reveal the deuterium fractionation in the high density gas across Oph B2.
The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2,
with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is
consistent with previous results in isolated starless and protostellar cores.
The column density distributions of both H2D+ and N2D+ show no correlation with
total H2 column density. We find, however, an anticorrelation in deuterium
fractionation with proximity to the embedded protostars in Oph B2 to distances
>= 0.04 pc. Destruction mechanisms for deuterated molecules require gas
temperatures greater than those previously determined through NH3 observations
of Oph B2 to proceed. We present temperatures calculated for the dense core gas
through the equating of non-thermal line widths for molecules (i.e., N2D+ and
H2D+) expected to trace the same core regions, but the observed complex line
structures in B2 preclude finding a reasonable result in many locations. This
method may, however, work well in isolated cores with less complicated velocity
structures. Finally, we use R_D and the H2D+ column density across Oph B2 to
set a lower limit on the ionization fraction across the core, finding a mean
x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using
deuterated species as a probe of the physical conditions of dense gas in
star-forming regions.Comment: ApJ accepte
Abundant cyanopolyynes as a probe of infall in the Serpens South cluster-forming region
We have detected bright HC7N J = 21-20 emission toward multiple locations in
the Serpens South cluster-forming region using the K-Band Focal Plane Array at
the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold
filamentary structures that have yet to form stars, largely avoiding the dense
gas associated with small protostellar groups and the main central cluster of
Serpens South. Where detected, the HC7N abundances are similar to those found
in other nearby star forming regions. Toward some HC7N `clumps', we find
consistent variations in the line centroids relative to NH3 (1,1) emission, as
well as systematic increases in the HC7N non-thermal line widths, which we
argue reveal infall motions onto dense filaments within Serpens South with
minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of
NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n
~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are
rarely co-located with those of either NH3 or continuum, it is likely that
Serpens South is not particularly remarkable in its abundance of HC7N, but
instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed
us to detect HC7N at low abundances in regions where it otherwise may not have
been looked for. This result extends the known star-forming regions containing
significant HC7N emission from typically quiescent regions, like the Taurus
molecular cloud, to more complex, active environments.Comment: 19 pages, 13 figures, accepted to MNRAS. Version with full resolution
figures available at http://www.dunlap.utoronto.ca/~friesen/Friesen_HC7N.pd
Beyond the technical: The role of emotion regulation in lacrosse officiating.
Emotions can influence the performance of referees leading to a need to examine emotions experienced, and regulation strategies used by referees. The present study assessed emotions and emotion regulation strategies of 19 referees officiating at an Under-19 Lacrosse World Championship. Using survey methods and focus group interviews, officials responded to five questions: (a) What emotions were experienced? (b) What events elicited emotions? (c) How did lacrosse officials manage their own emotional states prior to, throughout, and following a competitive game? (d) How did officials manage others' emotional states? (e) What were the perceived consequences of these strategies? Results indicate that emotions fluctuated throughout the tournament as referees encountered intrapersonal and interpersonal emotion-eliciting events. These fluctuations are suggested to come from a progressively diminished capacity for emotion regulation. Participants used emotion regulation strategies that could be classified into Gross' (1999) families of emotion regulation strategies, often relying on suppression, emotion contagion, and preventative refereeing. Collectively, the results offer new insights into referee emotion regulation at international events
Can we always get the entanglement entropy from the Kadanoff-Baym equations? The case of the T-matrix approximation
We study the time-dependent transmission of entanglement entropy through an
out-of-equilibrium model interacting device in a quantum transport set-up. The
dynamics is performed via the Kadanoff-Baym equations within many-body
perturbation theory. The double occupancy , needed to determine the entanglement entropy, is obtained from
the equations of motion of the single-particle Green's function. A remarkable
result of our calculations is that can become negative, thus not permitting to evaluate the
entanglement entropy. This is a shortcoming of approximate, and yet conserving,
many-body self-energies. Among the tested perturbation schemes, the -matrix
approximation stands out for two reasons: it compares well to exact results in
the low density regime and it always provides a non-negative . For the second part of this statement, we
give an analytical proof. Finally, the transmission of entanglement across the
device is diminished by interactions but can be amplified by a current flowing
through the system.Comment: 6 pages, 6 figure
Non-Gaussian statistics of electrostatic fluctuations of hydration shells
We report the statistics of electric field fluctuations produced by SPC/E
water inside a Kihara solute given as a hard-sphere core with a Lennard-Jones
layer at its surface. The statistics of electric field fluctuations, obtained
from numerical simulations, are studied as a function of the magnitude of a
point dipole placed close to the solute-water interface. The free energy
surface as a function of the electric field projected on the dipole direction
shows a cross-over with the increasing dipole magnitude. While it is a
single-well harmonic function at low dipole values, it becomes a double-well
surface at intermediate dipole moment magnitudes, transforming to a single-well
surface, with a non-zero minimum position, at still higher dipoles. A broad
intermediate region where the interfacial waters fluctuate between the two
minima is characterized by intense field fluctuations, with non-Gaussian
statistics and the variance far exceeding the linear-response expectations. The
excited state of the surface water is found to be lifted above the ground state
by the energy required to break approximately two hydrogen bonds. This state is
pulled down in energy by the external electric field of the solute dipole,
making it readily accessible to thermal excitations. The excited state is a
localized surface defect in the hydrogen-bond network creating a stress in the
nearby network, but otherwise relatively localized in the region closest to the
solute dipole
VLA Survey of Dense Gas in Extended Green Objects: Prevalence of 25 GHz Methanol Masers
We present resolution Very Large Array (VLA) observations of four
CHOH - 25~GHz transitions (=3, 5, 8, 10) along with 1.3~cm
continuum toward 20 regions of active massive star formation containing
Extended Green Objects (EGOs), 14 of which we have previously studied with the
VLA in the Class~I 44~GHz and Class~II 6.7~GHz maser lines (Cyganowski et al.
2009). Sixteen regions are detected in at least one 25~GHz line (=5), with
13 of 16 exhibiting maser emission. In total, we report 34 new sites of
CHOH maser emission and ten new sites of thermal CHOH emission,
significantly increasing the number of 25~GHz Class I CHOH masers observed
at high angular resolution. We identify probable or likely maser counterparts
at 44~GHz for all 15 of the 25~GHz masers for which we have complementary data,
providing further evidence that these masers trace similar physical conditions
despite uncorrelated flux densities. The sites of thermal and maser emission of
CHOH are both predominantly associated with the 4.5 m emission from
the EGO, and the presence of thermal CHOH emission is accompanied by 1.3~cm
continuum emission in 9 out of 10 cases. Of the 19 regions that exhibit 1.3~cm
continuum emission, it is associated with the EGO in 16 cases (out of a total
of 20 sites), 13 of which are new detections at 1.3~cm. Twelve of the 1.3~cm
continuum sources are associated with 6.7~GHz maser emission and likely trace
deeply-embedded massive protostars
Effective mass theory of monolayer \delta-doping in the high-density limit
Monolayer \delta-doped structures in silicon have attracted renewed interest
with their recent incorporation into atomic-scale device fabrication strategies
as source and drain electrodes and in-plane gates. Modeling the physics of
\delta-doping at this scale proves challenging, however, due to the large
computational overhead associated with ab initio and atomistic methods. Here,
we develop an analytical theory based on an effective mass approximation. We
specifically consider the Si:P materials system, and the limit of high donor
density, which has been the subject of recent experiments. In this case,
metallic behavior including screening tends to smooth out the local disorder
potential associated with random dopant placement. While smooth potentials may
be difficult to incorporate into microscopic, single-electron analyses, the
problem is easily treated in the effective mass theory by means of a jellium
approximation for the ionic charge. We then go beyond the analytic model,
incorporating exchange and correlation effects within a simple numerical model.
We argue that such an approach is appropriate for describing realistic,
high-density, highly disordered devices, providing results comparable to
density functional theory, but with greater intuitive appeal, and lower
computational effort. We investigate valley coupling in these structures,
finding that valley splitting in the low-lying \Gamma band grows much more
quickly than the \Gamma-\Delta band splitting at high densities. We also find
that many-body exchange and correlation corrections affect the valley splitting
more strongly than they affect the band splitting
Phase Diagram of the Two Dimensional Lattice Coulomb Gas
We use Monte Carlo simulations to map out the phase diagram of the two
dimensional Coulomb gas on a square lattice, as a function of density and
temperature. We find that the Kosterlitz-Thouless transition remains up to
higher charge densities than has been suggested by recent theoretical
estimates.Comment: 4 pages, including 6 in-line eps figure
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