931 research outputs found
Strong coupling regime in coherent electron transport in periodic quantum nanostructures
We study coherent transport in a system of a periodic linear chain of quantum
dots placed between two parallel quantum wires. We show that resonant-tunneling
conductance between the wires exhibits a Rabi splitting of the resonance peak
as a function of Fermi energy in the wires indicating the emergence of strong
coupling between the system constituents. The underlying mechanism of the
strong coupling regime is conservation of the quasimomentum in a periodic
system that leads to transition resonances between electron states in a quantum
dot chain and quantum wires. A perpendicular magnetic field, by breaking the
system's left-right symmetry, gives rise to a fine structure of the conductance
lineshape.Comment: 6 pages, 5 figure
Tracking electron pathways with magnetic field: Aperiodic Aharonov-Bohm oscillations in coherent transport through a periodic array of quantum dots
We study resonant tunneling through a periodic square array of quantum dots
sandwiched between modulation-doped quantum wells. If a magnetic field is
applied parallel to the quantum dot plane, the tunneling current exhibits a
highly complex Aharonov-Bohm oscillation pattern due to the interference of
multiple pathways traversed by a tunneling electron. Individual pathways
associated with conductance beats can be enumerated by sweeping the magnetic
field at various tilt angles. Remarkably, Aharonov-Bohm oscillations are
aperiodic unless the magnetic field slope relative to the quantum dot lattice
axes is a rational number.Comment: 5 page
Solid Continuum with Thermofluctuation Kinetics of Microcracks. Phase Transition
Thermodynamic equations for a solid and a solid continuum under stress are
derived on the basis of a multicomponent mean field Markov process for
thermofluctuation kinetics of microcracks. The resulting continuum is viscous
elastoplastic continuum with damage. It can radiate elastic waves . The
existence of phase transitions with microcrack density as an order parameter is
proved for a stationary state of a special model of solid. For a finite large
system the distribution of the logarithmic power of acoustic emission at a
critical point is similar to the distribution of the logarithmic energy of
earthquakes.Comment: 23 pages, 4 figure
Spin-dependent coherent transport in a double quantum dot system
We study spin-resolved resonant tunneling in a system of two quantum dots
sandwiched between doped quantum wells. In the coherent (Dicke) regime, i.e.,
when quantum dot separation is smaller than the Fermi wavelength in a
two-dimensional electron gas in quantum wells, application of an in-plane
magnetic field leads to a pronounced spin-resolved structure of the conductance
peak lineshape even for very small Zeeman splitting of the quantum dots'
resonant levels. In the presence of electron-gas spin-orbit coupling, this
spin-resolved structure gets washed out due to Fermi surface deformation in the
momentum space. We also show that Aharonov-Bohm flux penetrating the area
enclosed by electron tunneling pathways completely destroys conductance spin
structure.Comment: 8 pages, 6 figure
Synchrotron Radiation as the Source of Gamma-Ray Burst Spectra
We investigate synchrotron emission models as the source of gamma-ray burst spectra. We show that including the possibility for synchrotron self-absorption, a ``smooth cutoff'' to the electron energy distribution, and an anisotropic distribution for the electron pitch angles produces a whole range of low energy spectral behavior. In addition, we show that the procedure of spectral fitting to GRB data over a finite bandwidth can introduce a spurious correlation between spectral parameters - in particular, the value of the peak of the nu F_nu spectrum, E_p, and the low energy photon spectral index alpha (the lower E_p is, the lower (softer) the fitted value of alpha will be). From this correlation and knowledge of the E_p distribution, we show how to derive the expected distribution of alpha. We show that optically thin synchrotron models with an isotropic electron pitch angle distribution can explain the distribution of alpha below alpha=-2/3. This agreement is achieved if we relax the unrealistic assumption of the presence of a sharp low energy cutoff in the spectrum of accelerated electrons, and allow for a more gradual break. We show that this low energy portion of the electron spectrum can be at most flat. We also show that optically thin synchrotron models with an anisotropic electron pitch angle distribution can explain all bursts with -2/3 < alpha <= 0$. The very few bursts with low energy spectral indices that fall above alpha=0 may be due the presence of a the synchrotron self-absorption frequency entering the lower end of the BATSE window. Our results also predict a particular relationship between alpha and E_p during the temporal evolution of a GRB. We give examples of spectral evolution in GRBs and discuss how the behavior are consistent with the above models
Discrete Filters for Large Eddy Simulation of Forced Compressible MHD Turbulence
In present study, we discuss results of applicability of discrete filters for
large eddy simulation (LES) method of forced compressible magnetohydrodynamic
(MHD) turbulent flows with the scale-similarity model. Influences and effects
of discrete filter shapes on the scale-similarity model are examined in
physical space using a finite-difference numerical schemes. We restrict
ourselves to the Gaussian filter and the top-hat filter. Representations of
this subgrid-scale model which correspond to various 3- and 5-point
approximations of both Gaussian and top-hat filters for different values of
parameter (the ratio of the mesh size to the cut-off lengthscale of
the filter) are investigated. Discrete filters produce more discrepancies for
magnetic field. It is shown that the Gaussian filter is more sensitive to the
parameter than the top-hat filter in compressible forced MHD
turbulence. The 3-point filters at and give the least
accurate results and the 5-point Gaussian filter shows the best results at
.Comment: 9 pages, 4 figure
Oxygen rich cool stars in the Cepheus region, New observations. III
We present moderate resolution CCD spectra and R photometry for seven KP2001
stars. We revised the spectral classification of the stars in the range 3900 -
8500 A. On the bases of light curves of the NSVS (Northern Sky Variability
Survey) database we classify KP2001-18 as a semi regular and KP2001-176 as Mira
type variables. For all observed objects NSVS phase - dependence light curve
analysis and variability type classification was performed with the VStar
Software. Using period luminosity relation we computed MK magnitudes and the
distances to variables.Comment: Nine pages, six figures, in press on Astrophysics (Astrofizika
The Evolution and Luminosity Function of Quasars from Complete Optical Surveys
We use several quasar samples to determine the density and luminosity evolution of quasars. Combining different samples and accounting for varying selection criteria require tests of correlation and the determination of distributions for multiply truncated data. We describe new non-parametric techniques for accomplishing these tasks, which have been developed recently by Efron and Petrosian (1998). We use matter dominated cosmological models with either zero cosmological constant or zero spatial curvature. Of the two most common models of luminosity evolution, L = exp(k t(z)) and L = (1+z)^k', we find the second model to be a better description of the data at all luminosities; we find k' = 2.58 ([2.14,2.91] one sigma region) for the Einstein - de Sitter model. Using this form of luminosity evolution we determine a global luminosity function and the evolution of the co-moving density for the two types of cosmological models. For the Einstein - de Sitter cosmological model we find a relatively strong increase in co-moving density up to a redshift of about 2, at which point the density peaks and begins to decrease rapidly. This is in agreement with results from high redshift surveys. We find some co-moving density evolution for all cosmological models, with the rate of evolution lower for models with lower matter density. We find that the local cumulative luminosity function exhibits the usual double power law behavior. The luminosity density (i.e. the total rate of energy output of quasars) is found to increase rapidly at low redshift and to peak at around z = 2. Our results for the luminosity density are compared to results from high redshift surveys and to the variation of the star formation rate with redshift
Eigenfunction expansions in the imaginary Lobachevsky space
Eigenfunctions of the Schrodinger equation with the Coulomb potential in the
imaginary Lobachevsky space are studied in two coordinate systems admitting
solutions in terms of hypergeometric functions. Normalization and coefficients
of mutual expansions for some sets of solutions are found
Tunable self-assembled spin chains of strongly interacting cold atoms
We have developed an efficient computational method to treat long,
one-dimensional systems of strongly-interacting atoms forming self-assembled
spin chains. Such systems can be used to realize many spin chain model
Hamiltonians tunable by the external confining potential. As a concrete
demonstration, we consider quantum state transfer in a Heisenberg spin chain
and we show how to determine the confining potential in order to obtain
nearly-perfect state transfer.Comment: 16 pages, 7 figures, 1 appendix, final versio
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