47 research outputs found
Real-time electron dynamics with exact-exchange time-dependent density-functional theory
The exact exchange potential in time-dependent density-functional theory is
defined as an orbital functional through the time-dependent optimized effective
potential (TDOEP) method. We numerically solve the TDOEP integral equation for
the real-time nonlinear intersubband electron dynamics in a semiconductor
quantum well with two occupied subbands. By comparison with adiabatic
approximations, it is found that memory effects in the exact exchange potential
become significant when the electron dynamics takes place in the vicinity of
intersubband resonances.Comment: 5 pages, 5 figure
Coherent control of intersubband optical bistability in quantum wells
We present a study of the nonlinear intersubband (ISB) response of conduction
electrons in a GaAs/AlGaAs quantum well to strong THz radiation, using a
density-matrix approach combined with time-dependent density-functional theory.
We demonstrate coherent control of ISB optical bistability, using THz control
pulses to induce picosecond switching between the bistable states. The
switching speed is determined by the ISB relaxation and decoherence times, T1
and T2.Comment: 3 pages, 3 figure
Time-dependent Kohn-Sham theory with memory
In time-dependent density-functional theory, exchange and correlation (xc)
beyond the adiabatic local density approximation can be described in terms of
viscoelastic stresses in the electron liquid. In the time domain, this leads to
a velocity-dependent xc vector potential with a memory containing short- and
long-range components. The resulting time-dependent Kohn-Sham formalism
describes the dynamics of electronic systems including decoherence and
relaxation. For the example of collective charge-density oscillations in a
quantum well, we illustrate the xc memory effects, clarify the dissipation
mechanism, and extract intersubband relaxation rates for weak and strong
excitations.Comment: 4 pages, 4 figure
Time-Dependent Kohn-Sham Theory with Memory
URL:http://link.aps.org/doi/10.1103/PhysRevLett.95.086401
DOI:10.1103/PhysRevLett.95.086401In time-dependent density-functional theory, exchange and correlation (xc) beyond the adiabatic approximation can be described by viscoelastic stresses in the electron liquid. In the time domain, the resulting velocity-dependent xc vector potential has a memory containing short- and long-range components, leading to decoherence and energy relaxation. We solve the associated time-dependent Kohn-Sham equations, including the dependence on densities and currents at previous times, for the case of charge-density oscillations in a quantum well. We illustrate xc memory effects, clarify the dissipation mechanism, and extract intersubband relaxation rates for weak and strong excitations.Acknowledgment is made to Research Corporation and to the donors of the Petroleum Research Fund, administered by the ACS
Nonlinear Intersubband Photoabsorption in Asymmetric Single Quantum Wells
doi:10.1063/1.1994513A density-matrix approach combined with time-dependent density-functional theory is used to calculate the intersubband photoabsorption in a strongly driven, DC-biased GaAs/AlGaAs single quantum well. For certain frequencies and intensities of the driving field, optical bistability is observed. Compared to a full time propagation of the density matrix, the commonly used two-level rotating wave approximation becomes less and less accurate for increasing asymmetry.The authors acknowledge support from the donors of the Petroleum Research Fund, administered by the ACS. C.A.U. is a Cottrell Scholar of the Research Corporation
Physical signatures of discontinuities of the time-dependent exchange-correlation potential
The exact exchange-correlation (XC) potential in time-dependent
density-functional theory (TDDFT) is known to develop steps and discontinuities
upon change of the particle number in spatially confined regions or isolated
subsystems. We demonstrate that the self-interaction corrected adiabatic
local-density approximation for the XC potential has this property, using the
example of electron loss of a model quantum well system. We then study the
influence of the XC potential discontinuity in a real-time simulation of a
dissociation process of an asymmetric double quantum well system, and show that
it dramatically affects the population of the resulting isolated single quantum
wells. This indicates the importance of a proper account of the discontinuities
in TDDFT descriptions of ionization, dissociation or charge transfer processes.Comment: 17 pages, 6 figure
Plasmonically Enhanced Reflectance of Heat Radiation from Low-Bandgap Semiconductor Microinclusions
Increased reflectance from the inclusion of highly scattering particles at
low volume fractions in an insulating dielectric offers a promising way to
reduce radiative thermal losses at high temperatures. Here, we investigate
plasmonic resonance driven enhanced scattering from microinclusions of
low-bandgap semiconductors (InP, Si, Ge, PbS, InAs and Te) in an insulating
composite to tailor its infrared reflectance for minimizing thermal losses from
radiative transfer. To this end, we compute the spectral properties of the
microcomposites using Monte Carlo modeling and compare them with results from
Fresnel equations. The role of particle size-dependent Mie scattering and
absorption efficiencies, and, scattering anisotropy are studied to identify the
optimal microinclusion size and material parameters for maximizing the
reflectance of the thermal radiation. For composites with Si and Ge
microinclusions we obtain reflectance efficiencies of 57 - 65% for the incident
blackbody radiation from sources at temperatures in the range 400 - 1600
{\deg}C. Furthermore, we observe a broadbanding of the reflectance spectra from
the plasmonic resonances due to charge carriers generated from defect states
within the semiconductor bandgap. Our results thus open up the possibility of
developing efficient high-temperature thermal insulators through use of the
low-bandgap semiconductor microinclusions in insulating dielectrics.Comment: Main article (8 Figures and 2 Tables) + Supporting Information (8
Figures
Effects of fluoxetine on functional outcomes after acute stroke (FOCUS): a pragmatic, double-blind, randomised, controlled trial
Background
Results of small trials indicate that fluoxetine might improve functional outcomes after stroke. The FOCUS trial aimed to provide a precise estimate of these effects.
Methods
FOCUS was a pragmatic, multicentre, parallel group, double-blind, randomised, placebo-controlled trial done at 103 hospitals in the UK. Patients were eligible if they were aged 18 years or older, had a clinical stroke diagnosis, were enrolled and randomly assigned between 2 days and 15 days after onset, and had focal neurological deficits. Patients were randomly allocated fluoxetine 20 mg or matching placebo orally once daily for 6 months via a web-based system by use of a minimisation algorithm. The primary outcome was functional status, measured with the modified Rankin Scale (mRS), at 6 months. Patients, carers, health-care staff, and the trial team were masked to treatment allocation. Functional status was assessed at 6 months and 12 months after randomisation. Patients were analysed according to their treatment allocation. This trial is registered with the ISRCTN registry, number ISRCTN83290762.
Findings
Between Sept 10, 2012, and March 31, 2017, 3127 patients were recruited. 1564 patients were allocated fluoxetine and 1563 allocated placebo. mRS data at 6 months were available for 1553 (99·3%) patients in each treatment group. The distribution across mRS categories at 6 months was similar in the fluoxetine and placebo groups (common odds ratio adjusted for minimisation variables 0·951 [95% CI 0·839–1·079]; p=0·439). Patients allocated fluoxetine were less likely than those allocated placebo to develop new depression by 6 months (210 [13·43%] patients vs 269 [17·21%]; difference 3·78% [95% CI 1·26–6·30]; p=0·0033), but they had more bone fractures (45 [2·88%] vs 23 [1·47%]; difference 1·41% [95% CI 0·38–2·43]; p=0·0070). There were no significant differences in any other event at 6 or 12 months.
Interpretation
Fluoxetine 20 mg given daily for 6 months after acute stroke does not seem to improve functional outcomes. Although the treatment reduced the occurrence of depression, it increased the frequency of bone fractures. These results do not support the routine use of fluoxetine either for the prevention of post-stroke depression or to promote recovery of function.
Funding
UK Stroke Association and NIHR Health Technology Assessment Programme
How can we turn heat into useful energy using nanotechnology?
In 1821, the German physicist Thomas Seebeck made a groundbreaking discovery that revealed the direct conversion of heat energy into electricity. He did so by bringing two different metals together and holding one end heated and the other end cooled. This process is famously known as the Seebeck effect. William Thomson, a British physicist later known as Lord Kelvin, further developed the concept of thermoelectric circuits and introduced the idea of a temperature-dependent voltage in a circuit made of two dissimilar metals. Lord Kelvin’s contribution to the field of thermoelectricity paved the way for various applications in temperature sensors, power generators, refrigeration, and cooling systems
Analysis of the heat column formation in a 10 storey building
This paper aims to analyse the formation of heat column and its effects on the performance of the air conditioning systems. The computational fluid dynamic techniques combined with experimental validation were used to investigate the temperature distribution along the heat column under different loading conditions. Further the influence of wind and the humidity were also predicted. Results show that a significant loss of performance of the AC system can be expected under the influence of the heat column