5,342 research outputs found
The Linear Boltzmann Equation as the Low Density Limit of a Random Schrodinger Equation
We study the evolution of a quantum particle interacting with a random
potential in the low density limit (Boltzmann-Grad). The phase space density of
the quantum evolution defined through the Husimi function converges weakly to a
linear Boltzmann equation with collision kernel given by the full quantum
scattering cross section.Comment: 74 pages, 4 figures, (Final version -- typos corrected
Broadband Tuning (170nm) of InGaAs Quantum Well Lasers
The wavelength tuning properties of strained InGaAs quantum well lasers using an external grating for feedback is reported. Tunable laser oscillation has been observed over a range of 170 nm, between 840 and 1010 nm, under pulsed current excitation. The optimal conditions for broadband tunability for the InGaAs lasers are different from GaAs lasers, which is attributed to a difference in spectral gain curves. Together with an optimised GaAs quantum well laser the entire region between 740 and 1010 nm is spanned
Characteristics of ferroelectric-ferroelastic domains in N{\'e}el-type skyrmion host GaVS
GaVS is a multiferroic semiconductor hosting N{\'e}el-type magnetic
skyrmions dressed with electric polarization. At T = 42K, the compound
undergoes a structural phase transition of weakly first-order, from a
non-centrosymmetric cubic phase at high temperatures to a polar rhombohedral
structure at low temperatures. Below T, ferroelectric domains are formed
with the electric polarization pointing along any of the four axes. Although in this material the size and the shape of the
ferroelectric-ferroelastic domains may act as important limiting factors in the
formation of the N{\'e}el-type skyrmion lattice emerging below T=13\:K, the
characteristics of polar domains in GaVS have not been studied yet.
Here, we report on the inspection of the local-scale ferroelectric domain
distribution in rhombohedral GaVS using low-temperature piezoresponse
force microscopy. We observed mechanically and electrically compatible lamellar
domain patterns, where the lamellae are aligned parallel to the (100)-type
planes with a typical spacing between 100 nm-1.2 m. We expect that the
control of ferroelectric domain size in polar skyrmion hosts can be exploited
for the spatial confinement and manupulation of N{\'e}el-type skyrmions
Ultrabroadband tunable external-cavity quantum well lasers
The spectral tunability of properly designed quantum well lasers is demonstrated to equal the range of dye lasers. Theoretical and measured gain spectra are presented to support this statement, and stepwise wavelength tuning over a 125 nm span in GaAs/AlGaAs lasers and 175 nm in InGaAs/AlGaAs lasers are shown
Perceptions of the Use of Alcohol and Drugs after Sudden Bereavement by Unnatural Causes: Analysis of Online Qualitative Data
Bereavement is associated with an increased risk of psychiatric morbidity and all-cause mortality, particularly in younger people and after unnatural deaths. Substance misuse is implicated but little research has investigated patterns of drug or alcohol use after bereavement. We used a national online survey to collect qualitative data describing whether and how substance use changes after sudden bereavement. We conducted thematic analysis of free-text responses to a question probing use of alcohol and drugs after the sudden unnatural (non-suicide) death of a family member or a close friend. We analysed data from 243 adults in British Higher Education Institutions aged 18-40, identifying two main themes describing post-bereavement alcohol or drug use: (1) sense of control over use of drugs or alcohol (loss of control versus self-discipline), (2) harnessing the specific effects of drugs or alcohol. Across themes we identified age patterning in relation to substance misuse as a form of rebellion among those bereaved in childhood, and gender patterning in relation to men using alcohol to help express their emotions. The limitations of our sampling mean that these findings may not be generalizable from highly-educated settings to young people in the general population. Our findings describe how some young bereaved adults use drugs and alcohol to help them cope with traumatic loss, and suggest how clinicians might respond to any difficulties controlling substance use
Use of Alcohol and Unprescribed Drugs after Suicide Bereavement: Qualitative Study
Studies describing the impact of suicide bereavement report an excess risk of suicide, suicide attempt, psychiatric illness, and drug and alcohol use disorders compared with the general population. However, the nature of patterns of drug and alcohol use after suicide bereavement is unclear. We used an online survey to collect qualitative data to understand whether and how drug and alcohol use changes after suicide bereavement. We conducted thematic analysis of free-text responses to a question capturing their use of alcohol and drugs after the suicide of a family member or a close friend. Analysing data from 346 adults in Britain aged 18â40, we identified three main themes describing the relationship of suicide bereavement to alcohol or drug use: (1) control over drug or alcohol use, (2) the perceived purpose of using drugs or alcohol, and (3) the attribution of drug or alcohol misuse to external factors. Overlying these themes were dimensions of control and of awareness of potential harms. This study highlights that increased use of drugs and alcohol after suicide bereavement may form part of a bereaved personâs coping strategies, and that sensitive approaches are needed when judging whether and when to intervene
Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry
We present transport measurements of a tunable silicon
metal-oxide-semiconductor double quantum dot device with lateral geometry.
Experimentally extracted gate-to-dot capacitances show that the device is
largely symmetric under the gate voltages applied. Intriguingly, these gate
voltages themselves are not symmetric. Comparison with numerical simulations
indicates that the applied gate voltages serve to offset an intrinsic asymmetry
in the physical device. We also show a transition from a large single dot to
two well isolated coupled dots, where the central gate of the device is used to
controllably tune the interdot coupling.Comment: 4 pages, 3 figures, to be published in Applied Physics Letter
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