97 research outputs found
Effects of dimensionality and anisotropy on the Holstein polaron
We apply weak-coupling perturbation theory and strong-coupling perturbation
theory to the Holstein molecular crystal model in order to elucidate the
effects of anisotropy on polaron properties in D dimensions. The ground state
energy is considered as a primary criterion through which to study the effects
of anisotropy on the self-trapping transition, the self-trapping line
associated with this transition, and the adiabatic critical point. The effects
of dimensionality and anisotropy on electron-phonon correlations and polaronic
mass enhancement are studied, with particular attention given to the polaron
radius and the characteristics of quasi-1D and quasi-2D structures.
Perturbative results are confirmed by selected comparisons with variational
calculations and quantum Monte Carlo data
Mass Renormalization in the Su-Schrieffer-Heeger Model
This study of the one dimensional Su-Schrieffer-Heeger model in a weak
coupling perturbative regime points out the effective mass behavior as a
function of the adiabatic parameter , is the
zone boundary phonon energy and is the electron band hopping integral.
Computation of low order diagrams shows that two phonons scattering processes
become appreciable in the intermediate regime in which zone boundary phonons
energetically compete with band electrons. Consistently, in the intermediate
(and also moderately antiadiabatic) range the relevant mass renormalization
signals the onset of a polaronic crossover whereas the electrons are
essentially undressed in the fully adiabatic and antiadiabatic systems. The
effective mass is roughly twice as much the bare band value in the intermediate
regime while an abrupt increase (mainly related to the peculiar 1D dispersion
relations) is obtained at .Comment: To be published in Phys.Rev.B - 3 figure
Calculation of excited polaron states in the Holstein model
An exact diagonalization technique is used to investigate the low-lying
excited polaron states in the Holstein model for the infinite one-dimensional
lattice. For moderate values of the adiabatic ratio, a new and comprehensive
picture, involving three excited (coherent) polaron bands below the phonon
threshold, is obtained. The coherent contribution of the excited states to both
the single-electron spectral density and the optical conductivity is evaluated
and, due to the invariance of the Hamiltonian under the space inversion, the
two are shown to contain complementary information about the single-electron
system at zero temperature. The chosen method reveals the connection between
the excited bands and the renormalized local phonon excitations of the
adiabatic theory, as well as the regime of parameters for which the electron
self-energy has notable non-local contributions. Finally, it is shown that the
hybridization of two polaron states allows a simple description of the ground
and first excited state in the crossover regime.Comment: 12 pages, 9 figures, submitted to PR
Repulsive Casimir-Polder forces from cosmic strings
We investigate the Casimir-Polder force acting on a polarizable microparticle
in the geometry of a straight cosmic string. In order to develop this analysis
we evaluate the electromagnetic field Green tensor on the imaginary frequency
axis. The expression for the Casimir-Polder force is derived in the general
case of anisotropic polarizability. In dependence of the eigenvalues for the
polarizability tensor and of the orientation of its principal axes, the
Casimir-Polder force can be either repulsive or attractive. Moreover, there are
situations where the force changes the sign with separation. We show that for
an isotropic polarizability tensor the force is always repulsive. At large
separations between the microparticle and the string, the force varies
inversely as the fifth power of the distance. In the non-retarded regime,
corresponding to separations smaller than the relevant transition wavelengths,
the force decays as the inverse fourth power of the distance. In the case of
anisotropic polarizability, the dependence of the Casimir-Polder potential on
the orientation of the polarizability tensor principal axes also leads to the
moment of force acting on the particle.Comment: 16 pages, 2 figure
Semiclassical Strings, Dipole Deformations of N=1 SYM and Decoupling of KK Modes
In this paper we investigate the recently found -deformed
Maldacena-Nunez background by studying the behavior of different semiclassical
string configurations. This background is conjectured to be dual to dipole
deformations of SYM. We compare our results to those in the pure
Maldacena-Nunez background and show that the energies of our string
configurations are higher than in the undeformed background. Thinking in the
lines of (hep-th/0505100) we argue that this is an evidence for better
decoupling of the Kaluza-Klein modes from the pure SYM theory excitations.
Moreover we are able to find a limit of the background in which the string
energy is independent of , these strings are interpreted as
corresponding to pure gauge theory effects.Comment: 31 pages, references added, new solutions in Section 7 presented, an
appendix added, to appear in JHE
Polaron features of the one-dimensional Holstein Molecular Crystal Model
The polaron features of the one-dimensional Holstein Molecular Crystal Model
are investigated by improving a variational method introduced recently and
based on a linear superposition of Bloch states that describe large and small
polaron wave functions. The mean number of phonons, the polaron kinetic energy,
the electron-phonon local correlation function, and the ground state spectral
weight are calculated and discussed. A crossover regime between large and small
polaron for any value of the adiabatic parameter is found and a
polaron phase diagram is proposed.Comment: 12 pages, 2 figure
On the perturbative chiral ring for marginally deformed N=4 SYM theories
For \cal{N}=1 SU(N) SYM theories obtained as marginal deformations of the
\cal{N}=4 parent theory we study perturbatively some sectors of the chiral ring
in the weak coupling regime and for finite N. By exploiting the relation
between the definition of chiral ring and the effective superpotential we
develop a procedure which allows us to easily determine protected chiral
operators up to n loops once the superpotential has been computed up to (n-1)
order. In particular, for the Lunin-Maldacena beta-deformed theory we determine
the quantum structure of a large class of operators up to three loops. We
extend our procedure to more general Leigh-Strassler deformations whose chiral
ring is not fully understood yet and determine the weight-two and weight-three
sectors up to two loops. We use our results to infer general properties of the
chiral ring.Comment: LaTex, 40 pages, 4 figures, uses JHEP3; v2: minor correction
Metal-insulator transition in the one-dimensional Holstein model at half filling
We study the one-dimensional Holstein model with spin-1/2 electrons at
half-filling. Ground state properties are calculated for long chains with great
accuracy using the density matrix renormalization group method and extrapolated
to the thermodynamic limit. We show that for small electron-phonon coupling or
large phonon frequency, the insulating Peierls ground state predicted by
mean-field theory is destroyed by quantum lattice fluctuations and that the
system remains in a metallic phase with a non-degenerate ground state and
power-law electronic and phononic correlations. When the electron-phonon
coupling becomes large or the phonon frequency small, the system undergoes a
transition to an insulating Peierls phase with a two-fold degenerate ground
state, long-range charge-density-wave order, a dimerized lattice structure, and
a gap in the electronic excitation spectrum.Comment: 6 pages (LaTex), 10 eps figure
Quantum Monte Carlo and variational approaches to the Holstein model
Based on the canonical Lang-Firsov transformation of the Hamiltonian we
develop a very efficient quantum Monte Carlo algorithm for the Holstein model
with one electron. Separation of the fermionic degrees of freedom by a
reweighting of the probability distribution leads to a dramatic reduction in
computational effort. A principal component representation of the phonon
degrees of freedom allows to sample completely uncorrelated phonon
configurations. The combination of these elements enables us to perform
efficient simulations for a wide range of temperature, phonon frequency and
electron-phonon coupling on clusters large enough to avoid finite-size effects.
The algorithm is tested in one dimension and the data are compared with
exact-diagonalization results and with existing work. Moreover, the ideas
presented here can also be applied to the many-electron case. In the
one-electron case considered here, the physics of the Holstein model can be
described by a simple variational approach.Comment: 18 pages, 11 Figures, v2: one typo correcte
Polaron formation for a non-local electron-phonon coupling: A variational wave-function study
We introduce a variational wave-function to study the polaron formation when
the electronic transfer integral depends on the relative displacement between
nearest-neighbor sites giving rise to a non-local electron-phonon coupling with
optical phonon modes. We analyze the ground state properties such as the
energy, the electron-lattice correlation function, the phonon number and the
spectral weight. Variational results are found in good agreement with analytic
weak-coupling perturbative calculations and exact numerical diagonalization of
small clusters. We determine the polaronic phase diagram and we find that the
tendency towards strong localization is hindered from the pathological sign
change of the effective next-nearest-neighbor hopping.Comment: 11 page
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