562 research outputs found
Single electron magneto-conductivity of a nondegenerate 2D electron system in a quantizing magnetic field
We study transport properties of a non-degenerate two-dimensional system of
non-interacting electrons in the presence of a quantizing magnetic field and a
short-range disorder potential. We show that the low-frequency
magnetoconductivity displays a strongly asymmetric peak at a nonzero frequency.
The shape of the peak is restored from the calculated 14 spectral moments, the
asymptotic form of its high-frequency tail, and the scaling behavior of the
conductivity for omega -> 0. We also calculate 10 spectral moments of the
cyclotron resonance absorption peak and restore the corresponding
(non-singular) frequency dependence using the continuous fraction expansion.
Both expansions converge rapidly with increasing number of included moments,
and give numerically accurate results throughout the region of interest. We
discuss the possibility of experimental observation of the predicted effects
for electrons on helium.Comment: RevTeX 3.0, 14 pages, 8 eps figures included with eps
Trapping electrons in electrostatic traps over the surface of helium
We have observed trapping of electrons in an electrostatic trap formed over
the surface of liquid helium-4. These electrons are detected by a Single
Electron Transistor located at the centre of the trap. We can trap any desired
number of electrons between 1 and . By repeatedly (
times) putting a single electron into the trap and lowering the electrostatic
barrier of the trap, we can measure the effective temperature of the electron
and the time of its thermalisation after heating up by incoherent radiation.Comment: Presented at QFS06 - Kyoto, to be published in J. Low Temp. Phys., 6
pages, 3 figure
Magnetoresistance of nondegenerate quantum electron channels formed on the surface of superfluid helium
Transport properties of quasi-one-dimensional nondegenerate quantum wires
formed on the surface of liquid helium in the presence of a normal magnetic
field are studied using the momentum balance equation method and the memory
function formalism. The interaction with both kinds of scatterers available
(vapor atoms and capillary wave quanta) is considered. We show that unlike
classical wires, quantum nondegenerate channels exhibit strong
magnetoresistance which increases with lowering the temperature.Comment: 8 pages, 7 figure
Observation of audio-frequency edge magnetoplasmons in the classical two-dimensional electron gas
The electric admittance of a two-dimensional electron gas on liquid helium measured at audio frequencies ¿ is observed to oscillate as a function of magnetic field at strong magnetic fields. The oscillations can be attributed to the propagation of very-low-frequency (¿t~10-6, t scattering time; ¿/¿c~10-8, ¿c cyclotron frequency) edge magnetoplasmons. The directly determined dispersion relation agrees with theory and quantitatively with measurements in the collisionless regime (¿t»1). The attenuation, theoretically obtained by incorporating the screening in a simple local-capacitance model, agrees well with experiments
Tunneling from a correlated 2D electron system transverse to a magnetic field
We show that, in a magnetic field parallel to the 2D electron layer, strong
electron correlations change the rate of tunneling from the layer
exponentially. It results in a specific density dependence of the escape rate.
The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum
of the tunneling electron is partly transferred to the whole electron system,
depending on the interrelation between the rate of interelectron momentum
exchange and the tunneling duration. We also show that, in a certain
temperature range, magnetic field can enhance rather than suppress the
tunneling rate. The effect is due to the magnetic field induced energy exchange
between the in-plane and out-of-plane motion. Magnetic field can also induce
switching between intra-well states from which the system tunnels, and a
transition from tunneling to thermal activation. Explicit results are obtained
for a Wigner crystal. They are in qualitative and quantitative agreement with
the relevant experimental data, with no adjustable parameters.Comment: 16 pages, 9 figure
Mental health and substance use screening in HIV primary care before and during the early COVID-19 pandemic
Background: Mental health and substance use disorders disproportionately affect people with HIV (PWH), and may have been exacerbated during COVID-19. The Promoting Access to Care Engagement (PACE) trial was designed to assess the effectiveness of electronic screening for mental health and substance use in HIV primary care and enrolled PWH from October 2018 to July 2020. Our objective here was to compare screening rates and results for PWH before (October 2018 – February 2020) and early in the COVID-19 pandemic (March-July 2020). Methods: Adult (≥ 18 years) PWH from 3 large HIV primary care clinics in a US-based integrated healthcare system were offered electronic screening online or via in-clinic tablet computer every 6 months. Screening completion and results (for depression, suicidal ideation, anxiety, and substance use) were analyzed using logistic regression with generalized estimating equations to estimate prevalence ratios (PR) before and after the start of the regional COVID-19 shelter-in-place orders on March 17, 2020. Models adjusted for demographics (age, sex, race/ethnicity), HIV risk factors (men who have sex with men, injection drug use, heterosexual, other), medical center, and modality of screening completion (online or tablet). We conducted qualitative interviews with providers participating in the intervention to evaluate how the pandemic impacted patient care. Results: Of 8,954 eligible visits, 3,904 completed screenings (420 during COVID, 3,484 pre-COVID), with lower overall completion rates during COVID (38% vs. 44%). Patients completing screening during COVID were more likely to be White (63% vs. 55%), male (94% vs. 90%), and MSM (80% vs., 75%). Adjusted PRs comparing COVID and pre-COVID (reference) were 0.70 (95% CI), 0.92 (95% CI), and 0.54 (95% CI) for tobacco use, any substance use, and suicidal ideation, respectively. No significant differences were found by era for depression, anxiety, alcohol, or cannabis use. These results were in contrast to provider-reported impressions of increases in substance use and mental health symptoms. Conclusion: Findings suggest PWH had modest declines in screening rates early in the COVID-19 pandemic which may have been affected by the shift to telemedicine. There was no evidence that mental health problems and substance use increased for PWH in primary care. Trial registration: NCT03217058 (First registration date: 7/13/2017); https://clinicaltrials.gov/ct2/show/NCT03217058
Using the past to constrain the future: how the palaeorecord can improve estimates of global warming
Climate sensitivity is defined as the change in global mean equilibrium
temperature after a doubling of atmospheric CO2 concentration and provides a
simple measure of global warming. An early estimate of climate sensitivity,
1.5-4.5{\deg}C, has changed little subsequently, including the latest
assessment by the Intergovernmental Panel on Climate Change.
The persistence of such large uncertainties in this simple measure casts
doubt on our understanding of the mechanisms of climate change and our ability
to predict the response of the climate system to future perturbations. This has
motivated continued attempts to constrain the range with climate data, alone or
in conjunction with models. The majority of studies use data from the
instrumental period (post-1850) but recent work has made use of information
about the large climate changes experienced in the geological past.
In this review, we first outline approaches that estimate climate sensitivity
using instrumental climate observations and then summarise attempts to use the
record of climate change on geological timescales. We examine the limitations
of these studies and suggest ways in which the power of the palaeoclimate
record could be better used to reduce uncertainties in our predictions of
climate sensitivity.Comment: The final, definitive version of this paper has been published in
Progress in Physical Geography, 31(5), 2007 by SAGE Publications Ltd, All
rights reserved. \c{opyright} 2007 Edwards, Crucifix and Harriso
Inferring superposition and entanglement from measurements in a single basis
We discuss what can be inferred from measurements on one- and two-qubit
systems using a single measurement basis at various times. We show that, given
reasonable physical assumptions, carrying out such measurements at
quarter-period intervals is enough to demonstrate coherent oscillations of one
or two qubits between the relevant measurement basis states. One can thus infer
from such measurements alone that an approximately equal superposition of two
measurement basis states has been created in a coherent oscillation experiment.
Similarly, one can infer that a near maximally entangled state of two qubits
has been created in an experiment involving a putative SWAP gate. These results
apply even if the relevant quantum systems are only approximate qubits. We
discuss applications to fundamental quantum physics experiments and quantum
information processing investigations.Comment: Final published versio
Anisotropic flow of charged hadrons, pions and (anti-)protons measured at high transverse momentum in Pb-Pb collisions at TeV
The elliptic, , triangular, , and quadrangular, , azimuthal
anisotropic flow coefficients are measured for unidentified charged particles,
pions and (anti-)protons in Pb-Pb collisions at TeV
with the ALICE detector at the Large Hadron Collider. Results obtained with the
event plane and four-particle cumulant methods are reported for the
pseudo-rapidity range at different collision centralities and as a
function of transverse momentum, , out to GeV/.
The observed non-zero elliptic and triangular flow depends only weakly on
transverse momentum for GeV/. The small dependence
of the difference between elliptic flow results obtained from the event plane
and four-particle cumulant methods suggests a common origin of flow
fluctuations up to GeV/. The magnitude of the (anti-)proton
elliptic and triangular flow is larger than that of pions out to at least
GeV/ indicating that the particle type dependence persists out
to high .Comment: 16 pages, 5 captioned figures, authors from page 11, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/186
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