316 research outputs found
Dynamic instabilities in resonant tunneling induced by a magnetic field
We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Letter
Dynamical description of the buildup process in resonant tunneling: Evidence of exponential and non-exponential contributions
The buildup process of the probability density inside the quantum well of a
double-barrier resonant structure is studied by considering the analytic
solution of the time dependent Schr\"{o}dinger equation with the initial
condition of a cutoff plane wave. For one level systems at resonance condition
we show that the buildup of the probability density obeys a simple charging up
law, where is the
stationary wave function and the transient time constant is exactly
two lifetimes. We illustrate that the above formula holds both for symmetrical
and asymmetrical potential profiles with typical parameters, and even for
incidence at different resonance energies. Theoretical evidence of a crossover
to non-exponential buildup is also discussed.Comment: 4 pages, 2 figure
Dynamic instability in resonant tunneling
We show that an instability may be present in resonant tunneling through a
quantum well in one, two and three dimensions, when the resonance lies near the
emitter Fermi level. A simple semiclassical model which simulates the resonance
and the projected density of states by a nonlinear conductor, the Coulomb
barrier by a capacitance, and the time evolution by an iterated map, is used.
The model reproduces the observed hysteresis in such devices, and exhibits a
series of bifurcations leading to fast chaotic current fluctuations.Comment: 7 pages, 2 figure
Coherent and sequential photoassisted tunneling through a semiconductor double barrier structure
We have studied the problem of coherent and sequential tunneling through a
double barrier structure, assisted by light considered to be present All over
the structure, i,e emitter, well and collector as in the experimental evidence.
By means of a canonical transformation and in the framework of the time
dependent perturbation theory, we have calculated the transmission coefficient
and the electronic resonant current. Our calculations have been compared with
experimental results turning out to be in good agreement. Also the effect on
the coherent tunneling of a magnetic field parallel to the current in the
presence of light, has been considered.Comment: Revtex3.0, 8figures uuencoded compressed tar-fil
Coherent resonant tunneling in ac fields
We have analyzed the tunneling transmission probability and electronic
current density through resonant heterostructures in the presence of an
external electromagnetic field. In this work, we compare two different models
for a double barrier : In the first case the effect of the external field is
taken into account by spatially dependent AC voltages and in the second one the
electromagnetic field is described in terms of a photon field that irradiates
homogeneously the whole sample. While in the first description the tunneling
takes place mainly through photo sidebands in the case of homogeneous
illumination the main effective tunneling channels correspond to the coupling
between different electronic states due to photon absorption and emission. The
difference of tunneling mechanisms between these configurations is strongly
reflected in the transmission and current density which present very different
features in both cases. In order to analyze these effects we have obtained,
within the Transfer Hamiltonian framework, a general expression for the
transition probability for coherent resonant tunneling in terms of the Green's
function of the system.Comment: 16 pages,Figures available upon request,to appear in Phys.Rev B (15
April 1996
AC-conductance of a quantum wire with electron-electron interaction
The complex ac-response of a quasi-one dimensional electron system in the
one-band approximation with an interaction potential of finite range is
investigated. It is shown that linear response is exact for this model. The
influence of the screening of the electric field is discussed. The complex
absorptive conductance is analyzed in terms of resistive, capacitive and
inductive behaviors.Comment: 13 pages, REVTeX, 7 eps figures, to appear in Phys. Rev.
The usefulness and feasibility of a screening instrument to identify psychosocial problems in patients receiving curative radiotherapy: a process evaluation
<p>Abstract</p> <p>Background</p> <p>Psychosocial problems in cancer patients are often unrecognized and untreated due to the low awareness of the existence of these problems or pressures of time. The awareness of the need to identify psychosocial problems in cancer patients is growing and has affected the development of screening instruments. This study explored the usefulness and feasibility of using a screening instrument (SIPP: Screening Inventory of Psychosocial Problems) to identify psychosocial problems in cancer patients receiving curative radiotherapy treatment (RT).</p> <p>Methods</p> <p>The study was conducted in a radiation oncology department in the Netherlands. Several methods were used to document the usefulness and feasibility of the SIPP. Data were collected using self-report questionnaires completed by seven radiotherapists and 268 cancer patients.</p> <p>Results</p> <p>Regarding the screening procedure 33 patients were offered to consult a psychosocial care provider (e.g. social worker, psychologist) during the first consultation with their radiotherapist. Of these patients, 31 patients suffered from at least sub-clinical symptoms and two patients hardly suffered from any symptoms. Patients' acceptance rate 63.6% (21/33) was high. Patients were positive about the content of the SIPP (mean scores vary from 8.00 to 8.88, out of a range between 0 and 10) and about the importance of discussing items of the SIPP with their radiotherapist (mean score = 7.42). Radiotherapists' perspectives about the contribution of the SIPP to discuss the different psychosocial problems were mixed (mean scores varied from 3.17 to 4.67). Patients were more positive about discussing items of the SIPP if the radiotherapists had positive attitudes towards screening and discussing psychosocial problems.</p> <p>Conclusions</p> <p>The screening procedure appeared to be feasible in a radiotherapy department. In general, patients' perspectives were at least moderate. Radiotherapists considered the usefulness and feasibility of the SIPP generally to be lower, but their evaluations were mixed. A positive attitude to using screening instruments like the SIPP needs to be encouraged among radiotherapists, as this may not only improve the usefulness of a screening instrument, but also patients' satisfaction with care.</p> <p>Trial Registration</p> <p>ClinicalTrials.gov: <a href="http://www.clinicaltrials.gov/ct2/show/NCT00859768">NCT00859768</a></p
Variational Methods for Biomolecular Modeling
Structure, function and dynamics of many biomolecular systems can be
characterized by the energetic variational principle and the corresponding
systems of partial differential equations (PDEs). This principle allows us to
focus on the identification of essential energetic components, the optimal
parametrization of energies, and the efficient computational implementation of
energy variation or minimization. Given the fact that complex biomolecular
systems are structurally non-uniform and their interactions occur through
contact interfaces, their free energies are associated with various interfaces
as well, such as solute-solvent interface, molecular binding interface, lipid
domain interface, and membrane surfaces. This fact motivates the inclusion of
interface geometry, particular its curvatures, to the parametrization of free
energies. Applications of such interface geometry based energetic variational
principles are illustrated through three concrete topics: the multiscale
modeling of biomolecular electrostatics and solvation that includes the
curvature energy of the molecular surface, the formation of microdomains on
lipid membrane due to the geometric and molecular mechanics at the lipid
interface, and the mean curvature driven protein localization on membrane
surfaces. By further implicitly representing the interface using a phase field
function over the entire domain, one can simulate the dynamics of the interface
and the corresponding energy variation by evolving the phase field function,
achieving significant reduction of the number of degrees of freedom and
computational complexity. Strategies for improving the efficiency of
computational implementations and for extending applications to coarse-graining
or multiscale molecular simulations are outlined.Comment: 36 page
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