317 research outputs found
Photoluminescence of tetrahedral quantum-dot quantum wells
Taking into account the tetrahedral shape of a quantum dot quantum well
(QDQW) when describing excitonic states, phonon modes and the exciton-phonon
interaction in the structure, we obtain within a non-adiabatic approach a
quantitative interpretation of the photoluminescence (PL) spectrum of a single
CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW
is bright, in contrast to the dark ground state for a spherical QDQW.Comment: 4 pages, 2 figure
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.
Ground state and optical conductivity of interacting polarons in a quantum dot
The ground-state energy, the addition energies and the optical absorption
spectra are derived for interacting polarons in parabolic quantum dots in three
and two dimensions. A path integral formalism for identical particles is used
in order to take into account the fermion statistics. The approach is applied
to both closed-shell and open-shell systems of interacting polarons. Using a
generalization of the Jensen-Feynman variational principle, the ground-state
energy of a confined N-polaron system is analyzed as a function of N and of the
electron-phonon coupling constant. As distinct from the few-electron systems
without the electron-phonon interaction, three types of spin polarization are
possible for the ground state of the few-polaron systems: (i) a spin-polarized
state, (ii) a state where the spin is determined by Hund's rule, (iii) a state
with the minimal possible spin. A transition from a state fulfilling Hund's
rule, to a spin-polarized state occurs when decreasing the electron density. In
the strong-coupling limit, the system of interacting polarons turns into a
state with the minimal possible spin. These transitions should be
experimentally observable in the optical absorption spectra of quantum dots.Comment: 33 pages, 9 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
accepted for Phys. Rev.
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
Nucleation of superconductivity in mesoscopic star-shaped superconductors
We study the phase transition of a star-shaped superconductor, which covers
smoothly the range from zero to two dimensions with respect to the
superconducting coherence length. Detailed measurements and numerical
calculations show that the nucleation of superconductivity in this device is
very inhomogeneous, resulting in rich structure in the superconducting
transition as a function of temperature and magnetic field. The superconducting
order parameter is strongly enhanced and mostly robust in regions close to
multiple boundaries.Comment: 4 pages, 5 figures, E-mail addresses:
[email protected] (V. Chandrasekhar), [email protected]
(J. T. Devreese
Optical Absorption of an Interacting Many-Polaron Gas
The optical absorption of a many (continuum) polaron gas is derived in the
framework of a variational approach at zero temperature and weak or
intermediate electron-phonon coupling strength. We derive a compact formula for
the optical conductivity of the many-polaron system taking into account
many-body effects in the electron or hole system. Within the method presented
here, these effects are contained completely in the dynamical structure factor
of the electron or hole system. This allows to build on well-established
studies of the interacting electron gas. Based on this approach a novel feature
in the absorption spectrum of the many-polaron gas, related to the emission of
a plasmon together with a phonon, is identified. As an application and
illustration of the technique, we compare the theoretical many-polaron optical
absorption spectrum as derived in the present work with the `d-band' absorption
feature in NdCuO. Similarities are shown between the theoretically
and the experimentally derived first frequency moment of the optical absorption
of a family of differently doped NdCeCuO materials.Comment: 24 pages, 5 figures; revised and expanded versio
Photoluminescence of tetrahedral quantum-dot quantum wells
Taking into account the tetrahedral shape of a quantum dot quantum well
(QDQW) when describing excitonic states, phonon modes and the exciton-phonon
interaction in the structure, we obtain within a non-adiabatic approach a
quantitative interpretation of the photoluminescence spectrum of a single
CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW
is bright, in contrast to the dark ground state for a spherical QDQW. The
position of the phonon peaks in the photoluminescence spectrum is attributed to
interface optical phonons. We also show that the experimental value of the
Huang-Rhys parameter can be obtained only within the nonadiabatic theory of
phonon-assisted transitions.Comment: 4 pages, 4 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
to be published in Phys. Rev. Letter
Model ab initio study of charge carrier solvation and large polaron formation on conjugated carbon chains
Using long C_{N}H_{2} conjugated carbon chains with the polyynic structure as
prototypical examples of one-dimensional (1D) semiconductors, we discuss
self-localization of excess charge carriers into 1D large polarons in the
presence of the interaction with a surrounding polar solvent. The solvation
mechanism of self-trapping is different from the polaron formation due to
coupling with bond-length modulations of the underlying atomic lattice
well-known in conjugated polymers. Model ab initio computations employing the
hybrid B3LYP density functional in conjunction with the polarizable continuum
model are carried out demonstrating the formation of both electron- and
hole-polarons. Polarons can emerge entirely due to solvation but even larger
degrees of charge localization occur when accompanied by atomic displacements
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