72 research outputs found
Discrete soliton mobility in two-dimensional waveguide arrays with saturable nonlinearity
We address the issue of mobility of localized modes in two-dimensional
nonlinear Schr\"odinger lattices with saturable nonlinearity. This describes
e.g. discrete spatial solitons in a tight-binding approximation of
two-dimensional optical waveguide arrays made from photorefractive crystals. We
discuss numerically obtained exact stationary solutions and their stability,
focussing on three different solution families with peaks at one, two, and four
neighboring sites, respectively. When varying the power, there is a repeated
exchange of stability between these three solutions, with symmetry-broken
families of connecting intermediate stationary solutions appearing at the
bifurcation points. When the nonlinearity parameter is not too large, we
observe good mobility, and a well defined Peierls-Nabarro barrier measuring the
minimum energy necessary for rendering a stable stationary solution mobile.Comment: 19 pages, 4 figure
Radiationless Travelling Waves In Saturable Nonlinear Schr\"odinger Lattices
The longstanding problem of moving discrete solitary waves in nonlinear
Schr{\"o}dinger lattices is revisited. The context is photorefractive crystal
lattices with saturable nonlinearity whose grand-canonical energy barrier
vanishes for isolated coupling strength values. {\em Genuinely localised
travelling waves} are computed as a function of the system parameters {\it for
the first time}. The relevant solutions exist only for finite velocities.Comment: 5 pages, 4 figure
Spin singlet small bipolarons in Nb-doped BaTiO3
The magnetic susceptibility and electrical resistivity of n-type
BaTi{1-x}Nb{x}O3 have been measured over a wide temperature range. It is found
that, for 0 < x < 0.2, dopant electrons form immobile spin singlet small
bipolarons with binding energy around 110 meV. For x = 0.2, a maximum in the
electrical resistivity around 15 K indicates a crossover from band to hopping
transport of the charge carriers, a phenomenon expected but rarely observed in
real polaronic systems.Comment: 5 pages, 4 figure
Polaron and bipolaron dispersion curves in one dimension for intermediate coupling
Bipolaron energies are calculated as a function of wave vector by a
variational method of Gurari appropriate for weak or intermediate coupling
strengths, for a model with electron-phonon interactions independent of phonon
wave vectors and a short-ranged Coulomb repulsion. It is assumed that the bare
electrons have a constant effective mass. A two-parameter trial function is
taken for the relative motion of the two electrons in the bipolaron. Energies
of bipolarons are compared with those of two single polarons as a function of
wave vector for various parameter values. Results for effective masses at the
zone center are also obtained. Comparison is made with data of other authors
for bipolarons in the Hubbard-Holstein model, which differs mainly from the
present model in that it has a tight-binding band structure for the bare
electrons.Comment: 11 pages including six figures. Physical Review B, to be publishe
Solitons in nonlinear lattices
This article offers a comprehensive survey of results obtained for solitons
and complex nonlinear wave patterns supported by purely nonlinear lattices
(NLs), which represent a spatially periodic modulation of the local strength
and sign of the nonlinearity, and their combinations with linear lattices. A
majority of the results obtained, thus far, in this field and reviewed in this
article are theoretical. Nevertheless, relevant experimental settings are
surveyed too, with emphasis on perspectives for implementation of the
theoretical predictions in the experiment. Physical systems discussed in the
review belong to the realms of nonlinear optics (including artificial optical
media, such as photonic crystals, and plasmonics) and Bose-Einstein
condensation (BEC). The solitons are considered in one, two, and three
dimensions (1D, 2D, and 3D). Basic properties of the solitons presented in the
review are their existence, stability, and mobility. Although the field is
still far from completion, general conclusions can be drawn. In particular, a
novel fundamental property of 1D solitons, which does not occur in the absence
of NLs, is a finite threshold value of the soliton norm, necessary for their
existence. In multidimensional settings, the stability of solitons supported by
the spatial modulation of the nonlinearity is a truly challenging problem, for
the theoretical and experimental studies alike. In both the 1D and 2D cases,
the mechanism which creates solitons in NLs is principally different from its
counterpart in linear lattices, as the solitons are created directly, rather
than bifurcating from Bloch modes of linear lattices.Comment: 169 pages, 35 figures, a comprehensive survey of results on solitons
in purely nonlinear and mixed lattices, to appear in Reviews of Modern
Physic
Bipolaron Binding in Quantum Wires
A theory of bipolaron states in quantum wires with a parabolic potential well
is developed applying the Feynman variational principle. The basic parameters
of the bipolaron ground state (the binding energy, the number of phonons in the
bipolaron cloud, the effective mass, and the bipolaron radius) are studied as a
function of sizes of the potential well. Two cases are considered in detail: a
cylindrical quantum wire and a planar quantum wire. Analytical expressions for
the bipolaron parameters are obtained at large and small sizes of the quantum
well. It is shown that at [where means the radius (halfwidth) of a
cylindrical (planar) quantum wire, expressed in Feynman units], the influence
of confinement on the bipolaron binding energy is described by the function
for both cases, while at small sizes this influence is different
in each case. In quantum wires, the bipolaron binding energy increases
logarithmically with decreasing radius. The shapes and the sizes of a
nanostructure, which are favorable for observation of stable bipolaron states,
are determined.Comment: 17 pages, 6 figures, E-mail addresses: [email protected];
[email protected]
Optical Absorption Spectra of Bipolarons
The absorption of large bipolarons is investigated using the path-integral
method. The response of a bipolaron to an external electromagnetic field is
derived in the framework of the memory-function approach. The bipolaron optical
absorption spectrum consists of a series of relatively narrow peaks. The
peculiarities of the bipolaron optical absorption as a function of the
frequency of the electromagnetic field may be attributed to the transitions
involving relaxed excited states and scattering states of a bipolaron.Comment: 14 pages, 3 figures, E-mail addresses: [email protected],
[email protected]; to be published in Phys. Rev.
Electron-lattice interaction and its impact on high Tc superconductivity
In this Colloquium, the main features of the electron-lattice interaction are
discussed and high values of the critical temperature up to room temperature
could be provided. While the issue of the mechanism of superconductivity in the
high Tc cuprates continues to be controversial, one can state that there have
been many experimental results demonstrating that the lattice makes a strong
impact on the pairing of electrons. The polaronic nature of the carriers is
also a manifestation of strong electron-lattice interaction. One can propose an
experiment that allows an unambiguous determination of the intermediate boson
(phonon, magnon, exciton, etc.) which provides the pairing. The
electron-lattice interaction increases for nanosystems, and this is due to an
effective increase in the density of states
Bulk Damage Effects in Irradiated Silicon Detectors due to Clustered Divacancies
High resistivity silicon particle detectors will be used extensively in experiments at the future CERN Large Hadron Collider where the enormous particle fluences give rise to significant atomic displacement damage. A model has been developed to estimate the evolution of defect concentrations during irradiation and their electrical behaviour according to Shockley-Read-Hall (SRH) semiconductor statistics. The observed increases in leakage current and doping concentration changes can be described well after gamma irradiation but less well after fast neutron irradiation. A possible non-SRH mechanism is considered, based on the hypothesis of charge transfer between clustered divacancy defects in neutron damaged silicon detectors. This leads to a large enhancement over the SRH prediction for V2 acceptor state occupancy and carrier generation rate which may resolve the discrepancy
Froehlich Polaron and Bipolaron: Recent Developments
It is remarkable how the Froehlich polaron, one of the simplest examples of a
Quantum Field Theoretical problem, as it basically consists of a single fermion
interacting with a scalar Bose field of ion displacements, has resisted full
analytical or numerical solution at all coupling since 1950, when its
Hamiltonian was first written. The field has been a testing ground for
analytical, semi-analytical, and numerical techniques, such as path integrals,
strong-coupling perturbation expansion, advanced variational, exact
diagonalisation (ED), and quantum Monte Carlo (QMC) techniques. This article
reviews recent developments in the field of continuum and discrete (lattice)
Froehlich (bi)polarons starting with the basics and covering a number of active
directions of research.Comment: 131 pages, 17 figures, 409 references, appear in Reports on Progress
in Physic
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