31 research outputs found
A density matrix approach to the dynamical properties of a two-site Holstein model
The two-site Holstein model represents a first non-trivial paradigm for the
interaction between an itinerant charge with a quantum oscillator, a very
common topic in different ambits. Exact results can be achieved both
analytically and numerically, nevertheless it can be useful to compare them
with approximate, semi-classical techniques in order to highlight the role of
quantum effects. In this paper we consider the adiabatic limit in which the
oscillator is very much slow than the electron. A density matrix approach is
introduced for studying the charge dynamics and the exact results are compared
with two different approximations: a Born-Oppenheimer-based Static
Approximation for the oscillator (SA) and a Quantum-classical (QC) dynamics
Strong interplay between electron-phonon interaction and disorder in low doped systems
The effects of doping on the spectral properties of low doped systems are
investigated by means of Coherent Potential Approximation to describe the
distributed disorder induced by the impurities and Phonon-Phonon Non-Crossing
Approximation to characterize a wide class of electron-phonon interactions
which dominate the low-energy spectral features. When disorder and
electron-phonon interaction work on comparable energy scales, a strong
interplay between them arises, the effect of disorder can no more be described
as a mere broadening of the spectral features and the phonon signatures are
still visible despite the presence of strong disorder. As a consequence, the
disorder-induced metal-insulator transition, is strongly affected by a weak or
moderate electron-phonon coupling which is found to stabilize the insulating
phase.Comment: New version with improved bibliography and discussio
Phenomenological model for charge dynamics and optical response of disordered systems: application to organic semiconductors
We provide a phenomenological formula which describes the low-frequency
optical absorption of charge carriers in disordered systems with localization.
This allows to extract, from experimental data on the optical conductivity, the
relevant microscopic parameters determining the transport properties, such as
the carrier localization length and the elastic and inelastic scattering times.
This general formula is tested and applied here to organic semiconductors,
where dynamical molecular disorder is known to play a key role in the transport
properties. The present treatment captures the basic ideas underlying the
recently proposed transient localization scenario for charge transport,
extending it from the d.c. mobility to the frequency domain. When applied to
existing optical measurements in rubrene FETs, our analysis provides
quantitative evidence for the transient localization phenomenon. Possible
applications to other disordered electronic systems are briefly discussed.Comment: extended version with optical conductivity formulas for both
non-degenerate and degenerate electron system
Strange metal behavior from incoherent carriers scattered by local moments
We study metallic transport in an effective model that describes the coupling
of electrons to fluctuating magnetic moments with full SU(2) symmetry,
exhibiting characteristic behavior of metals at the approach of the Mott
transition. We show that scattering by fluctuating local moments causes a fully
incoherent regime of electron transport with linear T-dependent resistivities.
This strange metal regime is characterized by almost universal, "Planckian"
slope and a finite intercept at , that we can associate respectively to
the fluctuations in orientation and amplitude of the local moments. Our results
indicate a route for understanding the microscopic origin of strange metal
behavior that is unrelated to quantum criticality and does not rely on the
existence of quasiparticles.Comment: 5 pages, 3 figure
Impact of quantized vibrations on the efficiency of interfacial charge separation in photovoltaic devices
We demonstrate that charge separation at donor-acceptor interfaces is a
complex process that is controlled by the combined action of Coulomb binding
for electron-hole pairs and partial relaxation due to quantized phonons. A
joint electron-vibration quantum dynamical study reveals that high energy
vibrations sensitively tune the charge transfer probability as a function of
time and injection energy, due to polaron formation. These results have
bearings for the optimization of energy transfer both in organic and quantum
dot photovoltaics, as well as in biological light harvesting complexes.Comment: 5 pages, 3 figures. v2 contains additional discussion of experiments,
and extra physical motivatio
Pairing and polarization in systems with retarded interactions
In a system where a boson (e.g, a phonon) of finite frequency is
coupled to electrons, two phenomena occur as the coupling is increased:
electron pairing and polarization of the boson field. Within a path integral
formalism and a Dynamical Mean-Field approach, we introduce {\it ad hoc}
distribution function which allow us to pinpoint the two effects. When
is smaller than the bandwidth , pairing and polarization occur
for fairly similar couplings for all considered temperatures. When , the two phenomena tend to coincide only for , but are no
longer tied for low temperatures so that a state of paired particles without
finite polarization is stabilized.Comment: 4 pages, 2 figure
Universal scaling near band-tuned metal-insulator phase transitions
We present a theory for band-tuned metal-insulator transitions based on the
Kubo formalism. Such a transition exhibits scaling of the resistivity curves,
in the regime where or , where is the scattering
time and the chemical potential. At the critical value of the chemical
potential, the resistivity diverges as a power law, .
Consequently, on the metallic side there is a regime with negative ,
which is often misinterpreted as insulating. We show that scaling and this
`fake insulator' regime is observed in a wide range of experimental systems. In
particular, we show that Mooij correlations in high-temperature metals with
negative can be quantitatively understood with our scaling theory in
the presence of -linear scattering.Comment: 10 pages, 7 figure
The origin of Mooij correlations in disordered metals
Sufficiently disordered metals display systematic deviations from the
behavior predicted by semi-classical Boltzmann transport theory. Here the
scattering events from impurities or thermal excitations can no longer be
considered as additive independent processes, as asserted by Matthiessen's rule
following from this picture. In the intermediate region between the regime of
good conduction and that of insulation, one typically finds a change of sign of
the temperature coefficient of resistivity (TCR), even at elevated temperature
spanning ambient conditions, a phenomenology that was first identified by Mooij
in 1973. Traditional weak coupling approaches to identify relevant corrections
to the Boltzmann picture focused on long distance interference effects such as
"weak localization", which are especially important in low dimensions (1D, 2D)
and close to the zero temperature limit. Here we formulate a strong-coupling
approach to tackle the interplay of strong disorder and lattice deformations
(phonons) in bulk three-dimensional metals at high temperatures. We identify a
polaronic mechanism of strong disorder renormalization, which describes how a
lattice locally responds to the relevant impurity potential. This mechanism,
which quantitatively captures the Mooij regime, is physically distinct and
unrelated to Anderson localization, but realizes early seminal ideas of
Anderson himself, concerning the interplay of disorder and lattice
deformations