12,740 research outputs found
The Use of Dramatic Demonstrations to Enhance the Motivation and Learning of Chemistry Students
As part of the Virginia Collaborative for Excellence in the Preparation of Teachers (VCEPT) project, a series of demonstrations was incorporated into Chemistry 100: Man and Environment, a science course taken by non-science majors including many prospective K-12 teachers. Dramatic chemical demonstrations were ïŹrst presented to the undergraduate students by the instructor, and then they used demonstration activities to teach each other during the semester. Finally, these undergraduates presented to the K-6 students in the Norfolk Statue University (NSU) Summer Childrenâs College. The perceptions of science by the undergraduates at the beginning and end of the course were assessed using a questionnaire. The responses of the K-6 students in the Childrenâs College were assessed through informal interviews and audience response. The use of these demonstrations seemed to change the perception of science held by the undergraduate students. In addition, this limited assessment indicated that these demonstrations may have helped more of the undergraduates consider teaching as a career option
On giant piezoresistance effects in silicon nanowires and microwires
The giant piezoresistance (PZR) previously reported in silicon nanowires is
experimentally investigated in a large number of surface depleted silicon nano-
and micro-structures. The resistance is shown to vary strongly with time due to
electron and hole trapping at the sample surfaces. Importantly, this time
varying resistance manifests itself as an apparent giant PZR identical to that
reported elsewhere. By modulating the applied stress in time, the true PZR of
the structures is found to be comparable with that of bulk silicon
Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density
Carrier and spin recombination are investigated in p-type GaAs of acceptor
concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence
spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly
neutral and photoelectrons can either recombine with holes bound to acceptors
(e-A0 line) or form excitons which are mostly trapped on neutral acceptors
forming the (A0X) complex. It is found that the spin lifetime is shorter for
electrons that recombine through the e-A0 transition due to spin relaxation
generated by the exchange scattering of free electrons with either trapped or
free holes, whereas spin flip processes are less likely to occur once the
electron forms with a free hole an exciton bound to a neutral acceptor. An
increase of exci- tation power induces a cross-over to a regime where the
bimolecular band-to-band (b-b) emission becomes more favorable due to screening
of the electron-hole Coulomb interaction and ionization of excitonic complexes
and free excitons. Then, the formation of excitons is no longer possible, the
carrier recombination lifetime increases and the spin lifetime is found to
decrease dramatically with concentration due to fast spin relaxation with free
photoholes. In this high density regime, both the electrons that recombine
through the e-A0 transition and through the b-b transition have the same spin
relaxation time.Comment: 4 pages, 5 figure
Collective states of the odd-mass nuclei within the framework of the Interacting Vector Boson Model
A supersymmetric extension of the dynamical symmetry group of
the Interacting Vector Boson Model (IVBM), to the orthosymplectic group
is developed in order to incorporate fermion degrees of
freedom into the nuclear dynamics and to encompass the treatment of odd mass
nuclei. The bosonic sector of the supergroup is used to describe the complex
collective spectra of the neighboring even-even nuclei and is considered as a
core structure of the odd nucleus. The fermionic sector is represented by the
fermion spin group .
The so obtained, new exactly solvable limiting case is applied for the
description of the nuclear collective spectra of odd mass nuclei. The
theoretical predictions for different collective bands in three odd mass
nuclei, namely , and from rare earth region are
compared with the experiment. The transition probabilities for the
and between the states of the ground band are also
studied. The important role of the symplectic structure of the model for the
proper reproduction of the behavior is revealed. The obtained results
reveal the applicability of the models extension.Comment: 18 pages, 8 figure
Quantum communication between trapped ions through a dissipative environment
We study two trapped ions coupled to the axial phonon modes of a
one-dimensional Coulomb crystal. This system is formally equivalent to the "two
spin-boson" model. We propose a scheme to dynamically generate a maximally
entangled state of two ions within a decoherence-free subspace. Here the
phononic environment of the trapped ions, whatever its temperature and number
of modes, serves as the entangling bus. The efficient production of the pure
singlet state can be exploited to perform short-ranged quantum communication
which is essential in building up a large-scale quantum computer.Comment: 4 pages, 2 figure
Effect of the Pauli principle on photoelectron spin transport in GaAs
In p+ GaAs thin films, the effect of photoelectron degeneracy on spin
transport is investigated theoretically and experimentally by imaging the spin
polarization profile as a function of distance from a tightly-focussed light
excitation spot. Under degeneracy of the electron gas (high concentration, low
temperature), a dip at the center of the polarization profile appears with a
polarization maximum at a distance of about from the center. This
counterintuitive result reveals that photoelectron diffusion depends on spin,
as a direct consequence of the Pauli principle. This causes a concentration
dependence of the spin stiffness while the spin dependence of the mobility is
found to be weak in doped material. The various effects which can modify spin
transport in a degenerate electron gas under local laser excitation are
considered. A comparison of the data with a numerical solution of the coupled
diffusion equations reveals that ambipolar coupling with holes increases the
steady-state photo-electron density at the excitation spot and therefore the
amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents
are predicted to depend on spin under degeneracy (spin Soret currents), but
these currents are negligible except at very high excitation power where they
play a relatively small role. Coulomb spin drag and bandgap renormalization are
negligible due to electrostatic screening by the hole gas
Geometrical dependence of decoherence by electronic interactions in a GaAs/GaAlAs square network
We investigate weak localization in metallic networks etched in a two
dimensional electron gas between mK and mK when electron-electron
(e-e) interaction is the dominant phase breaking mechanism. We show that, at
the highest temperatures, the contributions arising from trajectories that wind
around the rings and trajectories that do not are governed by two different
length scales. This is achieved by analyzing separately the envelope and the
oscillating part of the magnetoconductance. For K we find
\Lphi^\mathrm{env}\propto{T}^{-1/3} for the envelope, and
\Lphi^\mathrm{osc}\propto{T}^{-1/2} for the oscillations, in agreement with
the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first
experimental confirmation of the geometry dependence of decoherence due to e-e
interaction.Comment: LaTeX, 5 pages, 4 eps figure
Dissipation enhanced vibrational sensing in an olfactory molecular switch
Motivated by a proposed olfactory mechanism based on a
vibrationally-activated molecular switch, we study electron transport within a
donor-acceptor pair that is coupled to a vibrational mode and embedded in a
surrounding environment. We derive a polaron master equation with which we
study the dynamics of both the electronic and vibrational degrees of freedom
beyond previously employed semiclassical (Marcus-Jortner) rate analyses. We
show: (i) that in the absence of explicit dissipation of the vibrational mode,
the semiclassical approach is generally unable to capture the dynamics
predicted by our master equation due to both its assumption of one-way
(exponential) electron transfer from donor to acceptor and its neglect of the
spectral details of the environment; (ii) that by additionally allowing strong
dissipation to act on the odorant vibrational mode we can recover exponential
electron transfer, though typically at a rate that differs from that given by
the Marcus-Jortner expression; (iii) that the ability of the molecular switch
to discriminate between the presence and absence of the odorant, and its
sensitivity to the odorant vibrational frequency, are enhanced significantly in
this strong dissipation regime, when compared to the case without mode
dissipation; and (iv) that details of the environment absent from previous
Marcus-Jortner analyses can also dramatically alter the sensitivity of the
molecular switch, in particular allowing its frequency resolution to be
improved. Our results thus demonstrate the constructive role dissipation can
play in facilitating sensitive and selective operation in molecular switch
devices, as well as the inadequacy of semiclassical rate equations in analysing
such behaviour over a wide range of parameters.Comment: 12 pages, 6 figures, close to published version, comments welcom
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