682 research outputs found
Modality-specific tracking of attention and sensory statistics in the human electrophysiological spectral exponent
A hallmark of electrophysiological brain activity is its 1/f-like spectrum â power decreases with increasing frequency. The steepness of this âroll-offâ is approximated by the spectral exponent, which in invasively recorded neural populations reflects the balance of excitatory to inhibitory neural activity (E:I balance). Here, we first establish that the spectral exponent of non-invasive electroencephalography (EEG) recordings is highly sensitive to general (i.e., anaesthesia-driven) changes in E:I balance. Building on the EEG spectral exponent as a viable marker of E:I, we then demonstrate its sensitivity to the focus of selective attention in an EEG experiment during which participants detected targets in simultaneous audio-visual noise. In addition to these endogenous changes in E:I balance, EEG spectral exponents over auditory and visual sensory cortices also tracked auditory and visual stimulus spectral exponents, respectively. Individualsâ degree of this selective stimulusâbrain coupling in spectral exponents predicted behavioural performance. Our results highlight the rich information contained in 1/f-like neural activity, providing a window into diverse neural processes previously thought to be inaccessible in non-invasive human recordings
Bloch oscillations, Zener tunneling and Wannier-Stark ladders in the time-domain
We present a time-domain analysis of carrier dynamics in a semiconductor
superlattice with two minibands. Integration of the density-matrix equations of
motion reveals a number of new features: (i) for certain values of the applied
static electric field strong interband transitions occur; (ii) in static fields
the complex time-dependence of the density-matrix displays a sequence of stable
plateaus in the low field regime, and (iii) for applied fields with a periodic
time-dependence the dynamic response can be understood in terms of the
quasienergy spectra.Comment: 4 pages, 6 PostScript figures available from [email protected], REVTEX
3.
Zener transitions between dissipative Bloch bands. II: Current Response at Finite Temperature
We extend, to include the effects of finite temperature, our earlier study of
the interband dynamics of electrons with Markoffian dephasing under the
influence of uniform static electric fields. We use a simple two-band
tight-binding model and study the electric current response as a function of
field strength and the model parameters. In addition to the Esaki-Tsu peak,
near where the Bloch frequency equals the damping rate, we find current peaks
near the Zener resonances, at equally spaced values of the inverse electric
field. These become more prominenent and numerous with increasing bandwidth (in
units of the temperature, with other parameters fixed). As expected, they
broaden with increasing damping (dephasing).Comment: 5 pages, LateX, plus 5 postscript figure
Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport
A non-equilibrium Green's function method is applied to model high-field
quantum transport and electron-phonon resonances in semiconductor
superlattices. The field-dependent density of states for elastic (impurity)
scattering is found non-perturbatively in an approach which can be applied to
both high and low electric fields. I-V curves, and specifically electron-phonon
resonances, are calculated by treating the inelastic (LO phonon) scattering
perturbatively. Calculations show how strong impurity scattering suppresses the
electron-phonon resonance peaks in I-V curves, and their detailed sensitivity
to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl
Superfluid Dynamics of a Bose-Einstein Condensate in a Periodic Potential
We investigate the superfluid properties of a Bose-Einstein condensate (BEC)
trapped in a one dimensional periodic potential. We study, both analytically
(in the tight binding limit) and numerically, the Bloch chemical potential, the
Bloch energy and the Bogoliubov dispersion relation, and we introduce {\it two}
different, density dependent, effective masses and group velocities. The
Bogoliubov spectrum predicts the existence of sound waves, and the arising of
energetic and dynamical instabilities at critical values of the BEC
quasi-momentum which dramatically affect its coherence properties. We
investigate the dependence of the dipole and Bloch oscillation frequencies in
terms of an effective mass averaged over the density of the condensate. We
illustrate our results with several animations obtained solving numerically the
time-dependent Gross-Pitaevskii equation.Comment: 13 pages, 7 figures, movies and published paper available at
http://www.iop.org/EJ/abstract/1367-2630/5/1/11
Schrodinger cat states prepared by Bloch oscillation in a spin-dependent optical lattice
We propose to use Bloch oscillation of ultra-cold atoms in a spin-dependent
optical lattice to prepare schrodinger cat states. Depending on its internal
state, an atom feels different periodic potentials and thus has different
energy band structures for its center-of-mass motion. Consequently, under the
same gravity force, the wave packets associated with different internal states
perform Bloch oscillation of different amplitudes in space and in particular
they can be macroscopically displaced with respect to each other. In this way,
a cat state can be prepared.Comment: 4 pages, 3 figures; slightly modifie
Time Periodic Behavior of Multiband Superlattices in Static Electric Fields
We use an analytic perturbation expansion for the two-band system of tight
binding electrons to discuss Bloch oscillations and Zener tunneling within this
model. We make comparison with recent numerical results and predict
analytically the frequency of radiation expected from Zener tunneling,
including its disappearance, as a function of the system parameters.Comment: 12 pages, no figure include
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