63,582 research outputs found
Fabrication and Characterization of Electrostatic Quantum Dots in a Si/SiGe 2D Electron Gas, Including an Integrated Read-out Channel
A new fabrication technique is used to produce quantum dots with read-out
channels in silicon/silicon-germanium two-dimensional electron gases. The
technique utilizes Schottky gates, placed on the sides of a shallow etched
quantum dot, to control the electronic transport process. An adjacent quantum
point contact gate is integrated to the side gates to define a read-out channel
and thus allow for noninvasive detection of the electronic occupation of the
quantum dot. Reproducible and stable Coulomb oscillations and the corresponding
jumps in the read-out channel resistance are observed at low temperatures. The
fabricated dot combined with the read-out channel represent a step towards the
spin-based quantum bit in Si/SiGe heterostructures.Comment: 3 pages, 4 fig
Sputtered Gold as an Effective Schottky Gate for Strained Si/SiGe Nanostructures
Metallization of Schottky surface gates by sputtering Au on strained Si/SiGe
heterojunctions enables the depletion of the two dimensional electron gas
(2DEG) at a relatively small voltage while maintaining an extremely low level
of leakage current. A fabrication process has been developed to enable the
formation of sub-micron Au electrodes sputtered onto Si/SiGe without the need
of a wetting layer.Comment: 3 pages, 3 figure
Cavity QED with atomic mirrors
A promising approach to merge atomic systems with scalable photonics has
emerged recently, which consists of trapping cold atoms near tapered
nanofibers. Here, we describe a novel technique to achieve strong, coherent
coupling between a single atom and photon in such a system. Our approach makes
use of collective enhancement effects, which allow a lattice of atoms to form a
high-finesse cavity within the fiber. We show that a specially designated
"impurity" atom within the cavity can experience strongly enhanced interactions
with single photons in the fiber. Under realistic conditions, a "strong
coupling" regime can be reached, wherein it becomes feasible to observe vacuum
Rabi oscillations between the excited impurity atom and a single cavity
quantum. This technique can form the basis for a scalable quantum information
network using atom-nanofiber systems.Comment: 20 pages, 4 figure
Limits to solar cycle predictability: Cross-equatorial flux plumes
Within the Babcock-Leighton framework for the solar dynamo, the strength of a
cycle is expected to depend on the strength of the dipole moment or net
hemispheric flux during the preceding minimum, which depends on how much flux
was present in each hemisphere at the start of the previous cycle and how much
net magnetic flux was transported across the equator during the cycle. Some of
this transport is associated with the random walk of magnetic flux tubes
subject to granular and supergranular buffeting, some of it is due to the
advection caused by systematic cross-equatorial flows such as those associated
with the inflows into active regions, and some crosses the equator during the
emergence process.
We aim to determine how much of the cross-equatorial transport is due to
small-scale disorganized motions (treated as diffusion) compared with other
processes such as emergence flux across the equator. We measure the
cross-equatorial flux transport using Kitt Peak synoptic magnetograms,
estimating both the total and diffusive fluxes. Occasionally a large sunspot
group, with a large tilt angle emerges crossing the equator, with flux from the
two polarities in opposite hemispheres. The largest of these events carry a
substantial amount of flux across the equator (compared to the magnetic flux
near the poles). We call such events cross-equatorial flux plumes. There are
very few such large events during a cycle, which introduces an uncertainty into
the determination of the amount of magnetic flux transported across the equator
in any particular cycle. As the amount of flux which crosses the equator
determines the amount of net flux in each hemisphere, it follows that the
cross-equatorial plumes introduce an uncertainty in the prediction of the net
flux in each hemisphere. This leads to an uncertainty in predictions of the
strength of the following cycle.Comment: A&A, accepte
Development of high critical current density in multifilamentary round-wire Bi2Sr2CaCu2O8+x by strong overdoping
Bi2Sr2CaCu2O8+x is the only cuprate superconductor that can be made into a
round-wire conductor form with a high enough critical current density Jc for
applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed
filamentary wires can be doubled to more than 1.4x10^5 A/cm^2 by low
temperature oxygenation. Careful analysis shows that the improved performance
is associated with a 12 K reduction in transition temperature Tc to 80 K and a
significant enhancement in intergranular connectivity. In spite of the
macroscopically untextured nature of the wire, overdoping is highly effective
in producing high Jc values.Comment: 4 figure
The effect of activity-related meridional flow modulation on the strength of the solar polar magnetic field
We studied the effect of the perturbation of the meridional flow in the
activity belts detected by local helioseismology on the development and
strength of the surface magnetic field at the polar caps. We carried out
simulations of synthetic solar cycles with a flux transport model, which
follows the cyclic evolution of the surface field determined by flux emergence
and advective transport by near-surface flows. In each hemisphere, an
axisymmetric band of latitudinal flows converging towards the central latitude
of the activity belt was superposed onto the background poleward meridional
flow. The overall effect of the flow perturbation is to reduce the latitude
separation of the magnetic polarities of a bipolar magnetic region and thus
diminish its contribution to the polar field. As a result, the polar field
maximum reached around cycle activity minimum is weakened by the presence of
the meridional flow perturbation. For a flow perturbation consistent with
helioseismic observations, the polar field is reduced by about 18% compared to
the case without inflows. If the amplitude of the flow perturbation depends on
the cycle strength, its effect on the polar field provides a nonlinearity that
could contribute to limiting the amplitude of a Babcock-Leighton type dynamo.Comment: 16 pages, 5 figures, accepted by Ap
Local Ferroelectricity in SrTiO_3 Thin Films
The temperature-dependent polarization of SrTiO_3 thin films is investigated
using confocal scanning optical microscopy. A homogeneous out-of-plane and
inhomogeneous in-plane ferroelectric phase are identified from images of the
linear electrooptic response. Both hysteretic and non-hysteretic behavior are
observed under a dc bias field. Unlike classical transitions in bulk
ferroelectrics, local ferroelectricity is observed at temperatures far above
the dielectric permittivity maximum. The results demonstrate the utility of
local probe experiments in understanding inhomogeneous ferroelectrics.Comment: 8 pages, 3 figures, accepted for publication in PR
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