1,721,009 research outputs found
Interplay between spin and phonon fluctuations in the double-exchange model for the manganites
No full textWe present exact solutions, mainly analytical, for the two-site double-exchange-Holstein model, that allow us to draw a complete picture of the role of both phonon and spin quantum fluctuations in determining the short-range correlations in the manganites. We provide analytical solutions of the model for arbitrary electron-phonon coupling and phonon frequency, for S=1/2 and for the classical spin limit S=infinity, and compare these results with numerical diagonalization of the realistic S=3/2 case. The comparison reveals that the realistic case S=3/2 is not well described by the classical spin limit, which is often used in literature. On the other hand, the phonon fluctuations, parametrized by the phonon frequency omega(0), stabilize ferromagnetic phases with respect to the adiabatic limit. We also provide a complete analysis on the polaron crossover in this model. RI Capone, Massimo/A-7762-200
Polaron crossover and bipolaronic metal-insulator transition in the half-filled Holstein model
No full textThe formation of a finite-density polaronic 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, noncrossing (Migdal), and vertex correction approximations. We show that polaron formation is not associated with a metal-insulator transition, which is instead due to bipolaron formation. RI Capone, Massimo/A-7762-200
Dynamical mean field theory of polarons and bipolarons in the half-filled Holstein model
No full textThe evolution of the properties of a finite density electronic system as a function of the electron-phonon coupling is investigated in the Holstein model using the dynamical mean-field theory (DMFT) that becomes exact in infinite dimensions. 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 within DMFT as 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 one 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 the presence of well-defined polaronic lattice displacements is not necessarily associated to a metal-insulator transition, which is instead due to pairing between the carriers. The finite-dimensionality effects neglected in DMFT may lead to a finite conductivity in the bipolaronic state which is, however, not always associated with polaronic distortions. At the polaron crossover the Born-Oppenheimer approximation is shown to break down due to the entanglement of the electron-phonon state. RI Capone, Massimo/A-7762-200
Pairing and polarization in electron-boson systems with retarded interactions via dynamical mean-field theory
No full textIn a system where a boson (e.g., a phonon) of finite frequency omega(0) 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 ad hoc distribution functions which allow us to pinpoint the two effects. When omega(0) is smaller than the bandwidth D, pairing and polarization occur for fairly similar couplings for all considered temperatures. When omega(0)>D, the two phenomena tend to coincide only for T >omega(0), but are no longer tied for low temperatures so that a state of paired particles with significant fluctuations but zero average polarization is stabilized. RI Capone, Massimo/A-7762-200
Electronic Structure, Electron-Phonon Coupling, and Charge Transport in Crystalline Rubrene under Mechanical Strain
No full textMotivated by the potential for the application of organic semiconductors in flexible electronics, we present a theoretical study aiming at elucidating the interplay between mechanical strain and electronic, vibrational, and charge transport properties of the prototypical high-mobility molecular semiconductor rubrene. Our study considers several factors that can play a role in the electro-mechanical response of a soft, van-der-Waals bonded molecular crystal, such as intermolecular charge transfer integrals, lattice dynamics, and electron phonon coupling. We find that compressive strain leads to an increase in magnitude of charge transfer integrals but also of the energetic disorder hampering the mobility. Charge transport simulations, based on the transient localization framework and fed with first-principles inputs, reveal a remarkably different response to strain applied along different crystal axes, in line with the most recent experiments. The critical interplay between the energetic disorder of intrinsic and extrinsic nature on the mobility-strain relationship is also discussed. The theoretical approach proposed in this work paves the way for the systematic study of the electro-mechanical response molecular semiconductors. of different classes of high-mobility molecular semiconductors
Strong interplay between electron-phonon interaction and disorder in low-doped systems
No full textThe effects of doping on the spectral properties of low-doped systems are investigated by means of the coherent potential approximation to describe the distributed disorder induced by the impurities and the phonon-phonon noncrossing approximation to characterize a wide class of electron-phonon interactions that 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 longer 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
Interplay between local response and vertex divergences in many-fermion systems with on-site attraction
No full textWe investigate the divergences appearing in the two-particle-irreducible vertex functions of many-fermion systems with attractive on-site interactions. By means of dynamical mean-field theory calculations, we determine the location of singularity lines in the phase diagram of the attractive Hubbard model at half-filling, where the local Bethe-Salpeter equations are noninvertible. We find that divergences appear both in the magnetic and in the density scattering channels. The former affect a sector of suppressed fluctuations and comply with the mapping of the physical susceptibilities of the repulsive case. At the same time, the appearance of singularities in the density channel of the attractive model demonstrates that vertex divergences can also plague the dominant scattering sectors associated with enhanced local susceptibilities. This constitutes a counterexample to previously proposed interpretations. Eventually, by exploiting the underlying physical symmetries and a spectral representation of the susceptibilities, we clarify the relation between vertex divergences and the local response of the system in different channels
Detecting pairing and polarization crossovers in systems with retarded interactions
No full textIn a system where a boson (e.g., a phonon) of finite frequency omega(0) is coupled to electrons, two phenomena occur as the coupling is increased: local electron pairing via multiple boson exchanges and polarization of the boson field. Within a path integral formalism in a dynamical mean-field approach, we introduce two distribution functions which allow us to pinpoint the two effects. When coo is smaller than the bandwidth D, pairing and polarization occur for fairly similar couplings for all considered temperatures. When omega(0) > D, the two phenomena tend to coincide only for T >> omega(0), but are no longer tied for low temperatures so that a state of local pairs without finite polarization is stabilized. (c) 2007 Elsevier B.V. All rights reserved. RI Capone, Massimo/A-7762-200
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