508 research outputs found
Frenkel Excitons in Random Systems With Correlated Gaussian Disorder
Optical absorption spectra of Frenkel excitons in random one-dimensional
systems are presented. Two models of inhomogeneous broadening, arising from a
Gaussian distribution of on-site energies, are considered. In one case the
on-site energies are uncorrelated variables whereas in the second model the
on-site energies are pairwise correlated (dimers). We observe a red shift and a
broadening of the absorption line on increasing the width of the Gaussian
distribution. In the two cases we find that the shift is the same, within our
numerical accuracy, whereas the broadening is larger when dimers are
introduced. The increase of the width of the Gaussian distribution leads to
larger differences between uncorrelated and correlated disordered models. We
suggest that this higher broadening is due to stronger scattering effects from
dimers.Comment: 9 pages, REVTeX 3.0, 3 ps figures. To appear in Physical Review
Bound states in the continuum driven by AC fields
We report the formation of bound states in the continuum driven by AC fields.
This system consists of a quantum ring connected to two leads. An AC side-gate
voltage controls the interference pattern of the electrons passing through the
system. We model the system by two sites in parallel connected to two
semi-infinite lattices. The energy of these sites change harmonically with
time. We obtain the transmission probability and the local density of states at
the ring sites as a function of the parameters that define the system. The
transmission probability displays a Fano profile when the energy of the
incoming electron matches the driving frequency. Correspondingly, the local
density of states presents a narrow peak that approaches a Dirac delta function
in the weak coupling limit. We attribute these features to the presence of
bound states in the continuum.Comment: 5 pages, 3 figure
Transport in random quantum dot superlattices
We present a novel model to calculate single-electron states in random
quantum dot superlattices made of wide-gap semiconductors. The source of
disorder comes from the random arrangement of the quantum dots (configurational
disorder) as well as spatial inhomogeneities of their shape (morphological
disorder). Both types of disorder break translational symmetry and prevent the
formation of minibands, as occurs in regimented arrays of quantum dots. The
model correctly describes channel mixing and broadening of allowed energy bands
due to elastic scattering by disorder
Non-equilibrium transport through a disordered molecular nanowire
We investigate the non-equilibrium transport properties of a disordered
molecular nanowire. The nanowire is regarded as a quasi-one-dimensional organic
crystal composed of self-assembled molecules. One orbital and a single random
energy are assigned to each molecule while the intermolecular coupling does not
fluctuate. Consequently, electronic states are expected to be spatially
localized. We consider the regime of strong localization, namely, the
localization length is smaller than the length of the molecular wire.
Electron-vibron interaction, taking place in each single molecule, is also
taken into account. We investigate the interplay between disorder and
electron-vibron interaction in response to either an applied electric bias or a
temperature gradient. To this end, we calculate the electric and heat currents
when the nanowire is connected to leads, using the Keldysh non-equilibrium
Green's function formalism. At intermediate temperature, scattering by disorder
dominates both charge and heat transport. We find that the electron-vibron
interaction enhances the effect of the disorder on the transport properties due
to the exponential suppression of tunneling
Enhancing thermoelectric properties of graphene quantum rings
We study the thermoelectric properties of rectangular graphene rings
connected symmetrically or asymmetrically to the leads. A side-gate voltage
applied across the ring allows for the precise control of the electric current
flowing through the system. The transmission coefficient of the rings manifests
Breit-Wigner line-shapes and/or Fano line-shapes, depending on the connection
configuration, the width of nanoribbons forming the ring and the side-gate
voltage. We find that the thermopower and the figure of merit are greatly
enhanced when the chemical potential is tuned close to resonances. Such
enhancement is even more pronounced in the vicinity of Fano like
anti-resonances which can be induced by a side-gate voltage independently of
the geometry. This opens a possibility to use the proposed device as a tunable
thermoelectric generator.Comment: 6 pages, 5 figures, accepted for publication in Physical Review
Transmission resonances and supercritical states in a one dimensional cusp potential
We solve the two-component Dirac equation in the presence of a spatially one
dimensional symmetric cusp potential. We compute the scattering and bound
states solutions and we derive the conditions for transmission resonances as
well as for supercriticality.Comment: 10 pages. Revtex 4. To appear in Phys Rev.
FIBONACCI SUPERLATTICES OF NARROW-GAP III-V SEMICONDUCTORS
We report theoretical electronic structure of Fibonacci superlattices of
narrow-gap III-V semiconductors. Electron dynamics is accurately described
within the envelope-function approximation in a two-band model.
Quasiperiodicity is introduced by considering two different III-V semiconductor
layers and arranging them according to the Fibonacci series along the growth
direction. The resulting energy spectrum is then found by solving exactly the
corresponding effective-mass (Dirac-like) wave equation using tranfer-matrix
techniques. We find that a self-similar electronic spectrum can be seen in the
band structure. Electronic transport properties of samples are also studied and
related to the degree of spatial localization of electronic envelope-functions
via Landauer resistance and Lyapunov coefficient. As a working example, we
consider type II InAs/GaSb superlattices and discuss in detail our results in
this system.Comment: REVTeX 3.0, 16 pages, 8 figures available upon request. To appear in
Semiconductor Science and Technolog
Fluorescence decay in aperiodic Frenkel lattices
We study motion and capture of excitons in self-similar linear systems in
which interstitial traps are arranged according to an aperiodic sequence,
focusing our attention on Fibonacci and Thue-Morse systems as canonical
examples. The decay of the fluorescence intensity following a broadband pulse
excitation is evaluated by solving the microscopic equations of motion of the
Frenkel exciton problem. We find that the average decay is exponential and
depends only on the concentration of traps and the trapping rate. In addition,
we observe small-amplitude oscillations coming from the coupling between the
low-lying mode and a few high-lying modes through the topology of the lattice.
These oscillations are characteristic of each particular arrangement of traps
and they are directly related to the Fourier transform of the underlying
lattice. Our predictions can be then used to determine experimentally the
ordering of traps.Comment: REVTeX 3.0 + 3PostScript Figures + epsf.sty (uuencoded). To appear in
Physical Review
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