210 research outputs found
On dynamical localization corrections to band transport
Bloch-Boltzmann transport theory fails to describe the carrier diffusion in
current crystalline organic semiconductors, where the presence of
large-amplitude thermal molecular motions causes substantial dynamical
disorder. The charge transport mechanism in this original situation is now
understood in terms of a transient localization of the carriers' wavefunctions,
whose applicability is however limited to the strong disorder regime. In order
to deal with the ever-improving performances of new materials, we develop here
a unified theoretical framework that includes transient localization theory as
a limiting case, and smoothly connects with the standard band description when
molecular disorder is weak. The theory, which specifically adresses the
emergence of dynamical localization corrections to semiclassical transport, is
used to determine a "transport phase diagram" of high-mobility organic
semiconductors.Comment: 14 pages, 6 figures completely revised versio
Hopping dynamics of interacting polarons
We derive an effective cluster model to address the transport properties of
mutually interacting small polarons. We propose a decoupling scheme where the
hopping dynamics of any given particle is determined by separating out
explicitly the degrees of freedom of its environment, which are treated as a
statistical bath. The general cavity method developed here shows that the
long-range Coulomb repulsion between the carriers leads to a net increase of
the thermal activation barrier for electrical transport, and hence to a sizable
reduction of the carrier mobility. A mean-field calculation of this effect is
provided, based on the known correlation functions of the interacting liquid in
two and three dimensions. The present theory gives a natural explanation of
recent experiments performed in organic field-effect transistors with highly
polarizable gate dielectrics, and might well find application in other classes
of polaronic systems such as doped transition-metal oxides
Band dispersion and electronic lifetimes in crystalline organic semiconductors
The consequences of several microscopic interactions on the photoemission
spectra of crystalline organic semiconductors (OSC) are studied theoretically.
It is argued that their relative roles can be disentangled by analyzing both
their temperature and their momentum/energy dependence. Our analysis shows that
the polaronic thermal band narrowing, that is the foundation of most theories
of electrical transport in OSC, is inconsistent in the range of microscopic
parameters appropriate for these materials. An alternative scenario is proposed
to explain the experimental trends.Comment: 4+ pages, revised conclusions; accepted for publication in Phys. Rev.
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Dynamical mean field theory of small polaron transport
We present a unified view of the transport properties of small-polarons in
the Holstein model at low carrier densities, based on the Dynamical Mean Field
Theory. The nonperturbative nature of the approach allows us to study the
crossover from classical activated motion at high temperatures to coherent
motion at low temperatures. Large quantitative discrepancies from the standard
polaronic formulae are found. The scaling properties of the resistivity are
analysed, and a simple interpolation formula is proposed in the nonadiabatic
regime
Electronic transport and quantum localization effects in organic semiconductors
We explore the charge transport mechanism in organic semiconductors based on
a model that accounts for the thermal intermolecular disorder at work in pure
crystalline compounds, as well as extrinsic sources of disorder that are
present in current experimental devices. Starting from the Kubo formula, we
develop a theoretical framework that relates the time-dependent quantum
dynamics of electrons to the frequency-dependent conductivity. The electron
mobility is then calculated through a relaxation time approximation that
accounts for quantum localization corrections beyond Boltzmann theory, and
allows us to efficiently address the interplay between highly conducting states
in the band range and localized states induced by disorder in the band tails.
The emergence of a "transient localization" phenomenon is shown to be a general
feature of organic semiconductors, that is compatible with the bandlike
temperature dependence of the mobility observed in pure compounds. Carrier
trapping by extrinsic disorder causes a crossover to a thermally activated
behavior at low temperature, which is progressively suppressed upon increasing
the carrier concentration, as is commonly observed in organic field-effect
transistors. Our results establish a direct connection between the localization
of the electronic states and their conductive properties, formalizing
phenomenological considerations that are commonly used in the literature
Polaron Crossover and Bipolaronic Metal-Insulator Transition in the half- filled Holstein model
The formation of a finite density multipolaronic state is analyzed in the
context of the Holstein model using the Dynamical Mean-Field Theory. The
spinless and spinful fermion cases are compared to disentangle the polaron
crossover from the bipolaron formation. The exact solution of Dynamical
Mean-Field Theory is compared with weak-coupling perturbation theory,
non-crossing (Migdal), and vertex correction approximations. We show that
polaron formation is not associated to a metal-insulator transition, which is
instead due to bipolaron formation.Comment: 4 pages, 5 figure
Polaron Crossover and Bipolaronic Metal-Insulator Transition in the Holstein model at half-filling
The evolution of the properties of a finite density electronic system as the
electron-phonon coupling is increased are investigated in the
Holstein model using the Dynamical Mean-Field Theory (DMFT).
We compare the spinless fermion case, in which only isolated polarons can be
formed, with the spinful model in which the polarons can bind and form
bipolarons. In the latter case, the bipolaronic binding occurs through a
metal-insulator transition. In the adiabatic regime in which the phonon energy
is small with respect to the electron hopping we compare numerically exact DMFT
results with an analytical scheme inspired by the Born-Oppenheimer procedure.
Within the latter approach,a truncation of the phononic Hilbert space leads to
a mapping of the original model onto an Anderson spin-fermion model. In the
anti-adiabatic regime (where the phonon energy exceeds the electronic scales)
the standard treatment based on Lang-Firsov canonical transformation allows to
map the original model on to an attractive Hubbard model in the spinful case.
The separate analysis of the two regimes supports the numerical evidence that
polaron formation is not necessarily associated to a metal-insulator
transition, which is instead due to pairing between the carriers. At the
polaron crossover the Born-Oppenheimer approximation is shown to break down due
to the entanglement of the electron-phonon state.Comment: 19 pages, 15 figure
Spectral properties and isotope effect in strongly interacting systems: Mott-Hubbard insulator and polaronic semiconductor
We study the electronic spectral properties in two examples of strongly
interacting systems: a Mott-Hubbard insulator with additional electron-boson
interactions, and a polaronic semiconductor. An approximate unified framework
is developed for the high energy part of the spectrum, in which the electrons
move in a random field determined by the interplay between magnetic and bosonic
fluctuations. When the boson under consideration is a lattice vibration, the
resulting isotope effect on the spectral properties is similar in both cases,
being strongly temperature and energy dependent, in qualitative agreement with
recent photoemission experiments in the cuprates.Comment: Refs. added, revised introduction and conclusio
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