494 research outputs found
Surface Phonons and Other Localized Excitations
The diatomic linear chain of masses coupled by harmonic springs is a
textboook model for vibrational normal modes (phonons) in crystals. In addition
to propagating acoustic and optic branches, this model is known to support a
``gap mode'' localized at the surface, provided the atom at the surface has
light rather than heavy mass. An elementary argument is given which explains
this mode and provides values for the frequency and localization length. By
reinterpreting this mode in different ways, we obtain the frequency and
localization lengths for three other interesting modes: (1) the surface
vibrational mode of a light mass impurity at the surface of a monatomic chain;
(2) the localized vibrational mode of a stacking fault in a diatomic chain; and
(3) the localized vibrational mode of a light mass impurity in a monatomic
chain.Comment: 5 pages with 4 embedded postscript figures. This paper will appear in
the American Journal of Physic
Localized Modes in Open One-Dimensional Dissipative Random Systems
We consider, both theoretically and experimentally, the excitation and
detection of the localized quasi-modes (resonances) in an open dissipative 1D
random system. We show that even though the amplitude of transmission drops
dramatically so that it cannot be observed in the presence of small losses,
resonances are still clearly exhibited in reflection. Surprisingly, small
losses essentially improve conditions for the detection of resonances in
reflection as compared with the lossless case. An algorithm is proposed and
tested to retrieve sample parameters and resonances characteristics inside the
random system exclusively from reflection measurements.Comment: 5 pages, 3 figures, to appear in Phys. Rev. Let
Magnetotransport of coupled electron-holes
The carriers in InAs-GaSb double quantum wells are hybrid ``electron-holes''.
We study the magnetotransport properties of such particles using a
two-component Keldysh technique, which results in a semi-analytic expression
for the small-field current. We show that zero temperature current can be large
even when the Fermi energy lies within the hybridization gap, a result which
cannot be understood within a semiclassical (Boltzmann) approach. Magnetic
field dependence of the conductance is also affected significantly by the
hybridization of electrons and holes.Comment: 4 pages, 2 figure
Quantum oscillations in graphene in the presence of disorder and interactions
Quantum oscillations in graphene is discussed. The effect of interactions are
addressed by Kohn's theorem regarding de Haas-van Alphen oscillations, which
states that electron-electron interactions cannot affect the oscillation
frequencies as long as disorder is neglected and the system is sufficiently
screened, which should be valid for chemical potentials not very close to the
Dirac point. We determine the positions of Landau levels in the presence of
potential disorder from exact transfer matrix and finite size diagonalization
calculations. The positions are shown to be unshifted even for moderate
disorder; stronger disorder, can, however, lead to shifts, but this also
appears minimal even for disorder width as large as one-half of the bare
hopping matrix element on the graphene lattice. Shubnikov-de Haas oscillations
of the conductivity are calculated analytically within a self-consistent Born
approximation of impurity scattering. The oscillatory part of the conductivity
follows the widely invoked Lifshitz-Kosevich form when certain mass and
frequency parameters are properly interpreted.Comment: Appendix A was removed, as the content of it is already contained in
Ref. 17. Thanks to M. A. H. Vozmedian
Recent developments in the determination of the amplitude and phase of quantum oscillations for the linear chain of coupled orbits
De Haas-van Alphen oscillations are studied for Fermi surfaces (FS)
illustrating the model proposed by Pippard in the early sixties, namely the
linear chain of orbits coupled by magnetic breakdown. This FS topology is
relevant for many multiband quasi-two dimensional (q-2D) organic metals such as
-(BEDT-TTF)Cu(NCS) and
-(BEDT-TTF)CoBr(CHCl) which are considered in
detail. Whereas the Lifshits-Kosevich model only involves a first order
development of field- and temperature-dependent damping factors, second order
terms may have significant contribution on the Fourier components amplitude for
such q-2D systems at high magnetic field and low temperature. The strength of
these second order terms depends on the relative value of the involved damping
factors, which are in turns strongly dependent on parameters such as the
magnetic breakdown field, effective masses and, most of all, effective
Land\'{e} factors. In addition, the influence of field-dependent Onsager phase
factors on the oscillation spectra is considered.Comment: arXiv admin note: text overlap with arXiv:1304.665
Extended quasimodes within nominally localized random waveguides
We have measured the spatial and spectral dependence of the microwave field
inside an open absorbing waveguide filled with randomly juxtaposed dielectric
slabs in the spectral region in which the average level spacing exceeds the
typical level width. Whenever lines overlap in the spectrum, the field exhibits
multiple peaks within the sample. Only then is substantial energy found beyond
the first half of the sample. When the spectrum throughout the sample is
decomposed into a sum of Lorentzian lines plus a broad background, their
central frequencies and widths are found to be essentially independent of
position. Thus, this decomposition provides the electromagnetic quasimodes
underlying the extended field in nominally localized samples. When the
quasimodes overlap spectrally, they exhibit multiple peaks in space.Comment: 4 pages, submitted to PRL (23 December 2005
The Localization Length of Stationary States in the Nonlinear Schreodinger Equation
For the nonlinear Schreodinger equation (NLSE), in presence of disorder,
exponentially localized stationary states are found. In the present Letter it
is demonstrated analytically that the localization length is typically
independent of the strength of the nonlinearity and is identical to the one
found for the corresponding linear equation. The analysis makes use of the
correspondence between the stationary NLSE and the Langevin equation as well as
of the resulting Fokker-Planck equation. The calculations are performed for the
``white noise'' random potential and an exact expression for the exponential
growth of the eigenstates is obtained analytically. It is argued that the main
conclusions are robust
Non-meanfield deterministic limits in chemical reaction kinetics far from equilibrium
A general mechanism is proposed by which small intrinsic fluctuations in a
system far from equilibrium can result in nearly deterministic dynamical
behaviors which are markedly distinct from those realized in the meanfield
limit. The mechanism is demonstrated for the kinetic Monte-Carlo version of the
Schnakenberg reaction where we identified a scaling limit in which the global
deterministic bifurcation picture is fundamentally altered by fluctuations.
Numerical simulations of the model are found to be in quantitative agreement
with theoretical predictions.Comment: 4 pages, 4 figures (submitted to Phys. Rev. Lett.
Matter-wave analog of an optical random laser
The accumulation of atoms in the lowest energy level of a trap and the
subsequent out-coupling of these atoms is a realization of a matter-wave analog
of a conventional optical laser. Optical random lasers require materials that
provide optical gain but, contrary to conventional lasers, the modes are
determined by multiple scattering and not a cavity. We show that a
Bose-Einstein condensate can be loaded in a spatially correlated disorder
potential prepared in such a way that the Anderson localization phenomenon
operates as a band-pass filter. A multiple scattering process selects atoms
with certain momenta and determines laser modes which represents a matter-wave
analog of an optical random laser.Comment: 4 pages, 3 figures version accepted for publication in Phys. Rev. A;
minor changes, the present title substituted for "Atom Random Laser
Condensation and vortex formation in Bose-gas upon cooling
The mechanism for the transition of a Bose gas to the superfluid state via
thermal fluctuations is considered. It is shown that in the process of external
cooling some critical fluctuations (instantons) are formed above the critical
temperature. The probability of the instanton formation is calculated in the
three and two-dimensional cases. It is found that this probability increases as
the system approaches the transition temperature. It is shown that the
evolution of an individual instanton is impossible without the formation of
vortices in its superfluid part
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