22,811 research outputs found

### Electron dynamics in the normal state of cuprates: spectral function, Fermi surface and ARPES data

An influence of the electron-phonon interaction on excitation spectrum and
damping in a narrow band electron subsystem of cuprates has been investigated.
Within the framework of the t-J model an approach to solving a problem of
account of both strong electron correlations and local electron-phonon binding
with characteristic Einstein mode $\omega _0$ in the normal state has been
presented. In approximation Hubbard-I it was found an exact solution to the
polaron bands. We established that in the low-dimensional system with a pure
kinematic part of Hamiltonian a complicated excitation spectrum is realized. It
is determined mainly by peculiarities of the lattice Green's function. In the
definite area of the electron concentration and hopping integrals a correlation
gap may be possible on the Fermi level. Also, in specific cases it is observed
a doping evolution of the Fermi surface. We found that the strong
electron-phonon binding enforces a degree of coherence of electron-polaron
excitations near the Fermi level and spectrum along the nodal direction depends
on wave vector module weakly. It corresponds to ARPES data. A possible origin
of the experimentally observed kink in the nodal direction of cuprates is
explained by fine structure of the polaron band to be formed near the mode
-$\omega _0$

### Tunable graphene bandgaps from superstrate mediated interactions

A theory is presented for the strong enhancement of graphene-on-substrate
bandgaps by attractive interactions mediated through phonons in a polarizable
superstrate. It is demonstrated that gaps of up to 1eV can be formed for
experimentally achievable values of electron-phonon coupling and phonon
frequency. Gap enhancements range between 1 and 4, indicating possible benefits
to graphene electronics through greater bandgap control for digital
applications, lasers, LEDs and photovoltaics through the relatively simple
application of polarizable materials such as SiO2 and Si3N4.Comment: 4 pages, 4 figures, to appear in Phys. Rev.

### Bipolaron-SO(5) Non-Fermi Liquid in a Two-channel Anderson Model with Phonon-assisted Hybridizations

We analyze non-Fermi liquid (NFL) properties along a line of critical points
in a two-channel Anderson model with phonon-assisted hybridizations. We succeed
in identifying hidden nonmagnetic SO(5) degrees of freedom for
valence-fluctuation regime and analyze the model on the basis of boundary
conformal field theory. We find that the NFL spectra along the critical line,
which is the same as those in the two-channel Kondo model, can be alternatively
derived by a fusion in the nonmagnetic SO(5) sector. The leading irrelevant
operators near the NFL fixed points vary as a function of Coulomb repulsion U;
operators in the spin sector dominate for large U, while those in the SO(5)
sector do for small U, and we confirm this variation in our numerical
renormalization group calculations. As a result, the thermodynamic singularity
for small U differs from that of the conventional two-channel Kondo problem.
Especially, the impurity contribution to specific heat is proportional to
temperature and bipolaron fluctuations, which are coupled electron-phonon
fluctuations, diverge logarithmically at low temperatures for small U.Comment: 16 pages, 4 figures, 3 table

### Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells

Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure

### Effect of the Spatial Dispersion on the Shape of a Light Pulse in a Quantum Well

Reflectance, transmittance and absorbance of a symmetric light pulse, the
carrying frequency of which is close to the frequency of interband transitions
in a quantum well, are calculated. Energy levels of the quantum well are
assumed discrete, and two closely located excited levels are taken into
account. A wide quantum well (the width of which is comparable to the length of
the light wave, corresponding to the pulse carrying frequency) is considered,
and the dependance of the interband matrix element of the momentum operator on
the light wave vector is taken into account. Refractive indices of barriers and
quantum well are assumed equal each other. The problem is solved for an
arbitrary ratio of radiative and nonradiative lifetimes of electronic
excitations. It is shown that the spatial dispersion essentially affects the
shapes of reflected and transmitted pulses. The largest changes occur when the
radiative broadening is close to the difference of frequencies of interband
transitions taken into account.Comment: 7 pages, 5 figure

### Elastic Light Scattering by Semiconductor Quantum Dots

Elastic light scattering by low-dimensional semiconductor objects is
investigated theoretically. The differential cross section of resonant light
scattering on excitons in quantum dots is calculated. The polarization and
angular distribution of scattered light do not depend on the quantum-dot form,
sizes and potential configuration if light wave lengths exceed considerably the
quantum-dot size. In this case the magnitude of the total light scattering
cross section does not depend on quantum-dot sizes. The resonant total light
scattering cross section is about a square of light wave length if the exciton
radiative broadening exceeds the nonradiative broadening. Radiative broadenings
are calculated

### Transmission of a Symmetric Light Pulse through a Wide QW

The reflection, transmission and absorption of a symmetric electromagnetic
pulse, which carrying frequency is close to the frequency of an interband
transition in a QW (QW), are obtained. The energy levels of a QW are assumed
discrete, one exited level is taken into account. The case of a wide QW is
considered when a length of the pulse wave, appropriate to the carrying
frequency, is comparable to the QW's width. In figures the time dependencies of
the dimensionless reflection, absorption are transmission are represented. It
is shown, that the spatial dispersion and a distinction in refraction indexes
influence stronger reflection.Comment: 8 pages,8 figures with caption

### Quantum quenches and driven dynamics in a single-molecule device

The nonequilibrium dynamics of molecular devices is studied in the framework
of a generic model for single-molecule transistors: a resonant level coupled by
displacement to a single vibrational mode. In the limit of a broad level and in
the vicinity of the resonance, the model can be controllably reduced to a form
quadratic in bosonic operators, which in turn is exactly solvable. The response
of the system to a broad class of sudden quenches and ac drives is thus
computed in a nonperturbative manner, providing an asymptotically exact
solution in the limit of weak electron-phonon coupling. From the analytic
solution we are able to (1) explicitly show that the system thermalizes
following a local quantum quench, (2) analyze in detail the time scales
involved, (3) show that the relaxation time in response to a quantum quench
depends on the observable in question, and (4) reveal how the amplitude of
long-time oscillations evolves as the frequency of an ac drive is tuned across
the resonance frequency. Explicit analytical expressions are given for all
physical quantities and all nonequilibrium scenarios under study.Comment: 23 pages, 13 figure

### Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations

The bases of the theory of light reflection and absorption by low-dimensional
semiconductor objects (quantum wells, wires and dots) at both monochromatic and
pulse irradiations and at any form of light pulses are developed. The
semiconductor object may be placed in a stationary quantizing magnetic field.
As an example the case of normal light incidence on a quantum well surface is
considered. The width of the quantum well may be comparable to the light wave
length and number of energy levels of electronic excitations is arbitrary. For
Fourier-components of electric fields the integral equation (similar to the
Dyson-equation) and solutions of this equation for some individual cases are
obtained.Comment: 14 page

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