1,043 research outputs found
Radiative Lifetimes of Single Excitons in Semiconductor Quantum Dots- Manifestation of the Spatial Coherence Effect
Using time correlated single photon counting combined with temperature
dependent diffraction limited confocal photoluminescence spectroscopy we
accurately determine, for the first time, the intrinsic radiative lifetime of
single excitons confined within semiconductor quantum dots. Their lifetime is
one (two) orders of magnitude longer than the intrinsic radiative lifetime of
single excitons confined in semiconductor quantum wires (wells) of comparable
confining dimensions. We quantitatively explain this long radiative time in
terms of the reduced spatial coherence between the confined exciton dipole
moment and the radiation electromagnetic field.Comment: 4 pages, 3 figure
Thermal and Surface Core-Electron Binding-Energy Shifts in Metals
High-resolution photoemission spectra from the shallow core levels of alkali metals and of In have been obtained between 78 K and room temperature. The data yield values for the alkali-metal surface-atom core-level shift and show thermal shifts of comparable size for bulk and surface. The positive surface shifts are due to the spill-out of conduction-electron charge, which is responsible for the surface dipole layer. The surface shifts are in good agreement with values obtained from a Born-Haber cycle expressed in terms of surface energies. The thermal shifts are proportional to the lattice expansion, and arise from both initial-state and final-state effects. As the lattice expands, the Fermi level decreases, decreasing the core-electron binding energy. At the same time, the expansion of the conduction-electron charge increases rs, thereby decreasing the potential at the core level and increasing the binding energy. The expansion also decreases the relaxation energy, further increasing the core-electron binding energy. In the alkali metals, the combined potential- and relaxation-energy terms dominate the Fermi-level term, making the shifts positive. In divalent metals the three terms tend to cancel, while in trivalent metals it is the Fermi-level term that dominates, making the shifts negative
Crystal Field Splitting and Charge Flow in the Buckled-Dimer Reconstruction of Si(100)—2× 1
The effect of the 2×1 reconstruction on the core-electron binding energies of the outermost Si(100) layers has been determined using high-resolution photoemission data. A previously unobserved 190-meV crystal-field splitting is resolved for the up-atoms of the asymmetric surface dimers, whose average core-level shift is -400 meV. The signal from the down-atoms is clearly identified and has a shift of +220 meV. These new findings indicate a charge flow of ∼0.05e from the subsurface to the surface layers, with a substantially larger difference of ∼0.34e between the up-atoms and down-atoms in the dimer
Optical absorption in semiconductor quantum dots: Nonlocal effects
The optical absorption of a single spherical semiconductor quantum dot in an
electrical field is studied taking into account the nonlocal coupling between
the field of the light and the polarizability of the semiconductor. These
nonlocal effects lead to a small size anf field dependent shift and broadening
of the excitonic resonance which may be of interest in future high precision
experiments.Comment: 6 pages, 4 figure
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Developing a scalable training model in global mental health: pilot study of a video-assisted training Program for Generalist Clinicians in Rural Nepal.
BackgroundIn low- and middle-income countries, mental health training often includes sending few generalist clinicians to specialist-led programs for several weeks. Our objective is to develop and test a video-assisted training model addressing the shortcomings of traditional programs that affect scalability: failing to train all clinicians, disrupting clinical services, and depending on specialists.MethodsWe implemented the program -video lectures and on-site skills training- for all clinicians at a rural Nepali hospital. We used Wilcoxon signed-rank tests to evaluate pre- and post-test change in knowledge (diagnostic criteria, differential diagnosis, and appropriate treatment). We used a series of 'Yes' or 'No' questions to assess attitudes about mental illness, and utilized exact McNemar's test to analyze the proportions of participants who held a specific belief before and after the training. We assessed acceptability and feasibility through key informant interviews and structured feedback.ResultsFor each topic except depression, there was a statistically significant increase (Δ) in median scores on knowledge questionnaires: Acute Stress Reaction (Δ = 20, p = 0.03), Depression (Δ = 11, p = 0.12), Grief (Δ = 40, p < 0.01), Psychosis (Δ = 22, p = 0.01), and post-traumatic stress disorder (Δ = 20, p = 0.01). The training received high ratings; key informants shared examples and views about the training's positive impact and complementary nature of the program's components.ConclusionVideo lectures and on-site skills training can address the limitations of a conventional training model while being acceptable, feasible, and impactful toward improving knowledge and attitudes of the participants
Tax evasion and exchange equity: a reference-dependent approach
The standard portfolio model of tax evasion with a public good produces the perverse conclusion that when taxpayers perceive the public good to be under-/overprovided, an increase in the tax rate increases/decreases evasion. The author treats taxpayers as thinking in terms of gains and losses relative to an endogenous reference level, which reflects perceived exchange equity between the value of taxes paid and the value of public goods supplied. With these alternative behavioral assumptions, the author overturns the aforementioned result in a direction consistent with the empirical evidence. The author also finds a role for relative income in determining individual responses to a change in the marginal rate of tax
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
Teratogenic risk and contraceptive counselling in psychiatric practice: analysis of anticonvulsant therapy
<p>Background:
Anticonvulsants have been used to manage psychiatric conditions for over 50 years. It is recognised that some, particularly valproate, carbamazepine and lamotrigine, are human teratogens, while others including topiramate require further investigation. We aimed to appraise the documentation of this risk by psychiatrists and review discussion around contraceptive issues.</p>
<p>Methods:
A retrospective review of prescribing patterns of four anticonvulsants (valproate, carbamazepine, lamotrigine and topiramate) in women of child bearing age was undertaken. Documented evidence of discussion surrounding teratogenicity and contraceptive issues was sought.</p>
<p>Results:
Valproate was most commonly prescribed (n=67). Evidence of teratogenic risk counselling at medication initiation was sub-optimal – 40% of individuals prescribed carbamazepine and 22% of valproate. Documentation surrounding contraceptive issues was also low- 17% of individuals prescribed carbamazepine and 13% of valproate.</p>
<p>Conclusion:
We found both low rates of teratogenic risk counselling and low rates of contraception advice in our cohort. Given the high rates of unplanned pregnancies combined with the relatively high risk of major congenital malformations, it is essential that a detailed appraisal of the risks and benefits associated with anticonvulsant medication occurs and is documented within patients’ psychiatric notes.</p>
Free induction signal from biexcitons and bound excitons
A theory of the free induction signal from biexcitons and bound excitons is
presented. The simultaneous existence of the exciton continuum and a bound
state is shown to result in a new type of time dependence of the free
induction. The optically detected signal increases in time and oscillates with
increasing amplitude until damped by radiative or dephasing processes.
Radiative decay is anomalously fast and can result in strong picosecond pulses.
The expanding area of a coherent exciton polarization (inflating antenna),
produced by the exciting pulse, is the underlying physical mechanism. The
developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1
Fermi-Edge Singularities in AlxGa1-xAs Quantum Wells : Extrinsic Versus Many-Body Scattering Processes
A Fano resonance mechanism is evidenced to control the formation of optical
Fermi-edge singularities in multi-subband systems such as remotely doped
AlxGa1-xAs heterostructures. Using Fano parameters, we probe the physical
nature of the interaction between Fermi-sea electrons and empty conduction
subbands. We show that processes of extrinsic origin like alloy-disorder
prevail easily at 2D over multiple diffusions from charged valence holes
expected by many-body scenarios.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
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