S. Ciuchi - Academia.edu (original) (raw)

Papers by S. Ciuchi

Physical Review B, 1988

The growth of a spontaneous lattice average magnetization in a magnetic system which is suddenly ... more The growth of a spontaneous lattice average magnetization in a magnetic system which is suddenly brought below the transition temperature is a stochastic process in which the very small fluctuations of the initial magnetization are amplified to a macroscopic size. The initial magnetization fluctuates in time around the zero average value because of the finite size of the system. As a consequence of the fluctuation-amplification phenomenon the nonlinear relaxation of the finite system is qualitatively different from that of the infinite one. The present paper studies this feature of phase-transition kinetics in the framework of a very simple model: the dynamical generalization of the spherical model in the molecular-field approximation.

Physical Review B, 2010

A model describing the low density carrier state in an organic single crystal FET with high-κ gat... more A model describing the low density carrier state in an organic single crystal FET with high-κ gate dielectrics is studied. The interplay between charge carrier coupling with inter-molecular vibrations in the bulk of the organic material and the long-range interaction induced at the interface with a polar dielectric is investigated. This interplay is responsible for the stabilization of a polaronic state with an internal structure extending on few lattice sites, at much lower coupling strengths than expected from the polar interaction alone. This effect could give rise to polaron self-trapping in high-κ organic FET's without invoking unphysically large values of the carrier interface interaction.

Nature Materials, 2006

In organic field-effect transistors (FETs), charges move near the surface of an organic semicondu... more In organic field-effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers--which determines the device performance--has been related to the quality of the organic semiconductor. Recently, it was discovered that the nearby dielectric also has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms, we have studied transport through organic single-crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Fröhlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime. This represents a considerable step forward in our understanding of transport through organic transistors, and identifies a microscopic physical process with a large influence on device performance.

Journal of Physics: Condensed Matter, 1990

The authors study the kinetics of the formation of antiferromagnetic order in a finite spin syste... more The authors study the kinetics of the formation of antiferromagnetic order in a finite spin system. The system is initially forced to be an unstable ferromagnetic state by means of an external field. When the field is switched off the system decays toward equilibrium. The finite size of the system has relevant consequences on the time of persistence of the initial unstable state. The fluctuations driving the system to equilibrium become anomalously large in the transient. The authors obtain analytic results for the transient fluctuation statistics in the limit where the average squared magnetisation relaxes much more quickly than relative orientation of the sublattices. The results have been obtained in the framework of spherical model and of molecular field approximation. The relaxation dynamics are ruled by the nonlinear Langevin equations.

PHYSICAL REVIEW B, 2002

We present exact solutions, mainly analytical, for the two-site double-exchange-Holstein model, t... more We 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 = ∞, 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 ω0, stabilize ferromagnetic phases with respect to the adiabatic limit. We also provide a complete analysis on the polaron crossover in this model. 71.38.Ht, 75.50.Dd, 75.50.Ee

Physical Review B, 1997

A dynamical mean-field theory of the small polaron problem is presented, which becomes exact in t... more A dynamical mean-field theory of the small polaron problem is presented, which becomes exact in the limit of infinite dimensions. The ground state properties and the one-electron spectral function are obtained for a single electron interacting with Einstein phonons by a mapping of the lattice problem onto a polaronic impurity model. The one-electron propagator of the impurity model is calculated through a continued fraction expansion (CFE), both at zero and finite temperature, for any electron-phonon coupling and phonon energy. In contrast to the ground state properties such as the effective polaron mass, which show a continuous behaviour as the coupling is increased, spectral properties exhibit a sharp qualitative change at low enough phonon frequency: beyond a critical coupling, one energy gap and then more and more open in the density of states at low energy, while the high energy part of the spectrum is broad and can be qualitatively explained by a strong coupling adiabatic approximation. As a consequence narrow and coherent low-energy subbands coexist with an incoherent featureless structure at high energy. The subbands denote the formation of quasiparticle polaron states. Also, divergencies of the self-energy may occur in the gaps. At finite temperature such effect triggers an important damping and broadening of the polaron subbands. On the other hand, in the large phonon frequency regime such a separation of energy scales does not exist and the spectrum has always a multipeaked structure.

Physical Review B, 2000

Within the path integral formalism, we derive exact expressions for correlation functions measuri... more Within the path integral formalism, we derive exact expressions for correlation functions measuring the lattice charge induced by an electron and associated polarization in Fröhlich polaron problem. We prove that a sum rule for the total induced charge, already obtained within approximated approaches is indeed exact. As a consequence the total induced charge is shown rigorously to be temperature independent. In addition we perform path integral Monte Carlo calculations of the correlation functions and we compare with variational results based on Feynman method. As the temperature increases the polaron radius decreases. On the other hand at high temperatures the electron motion is not hindered by the lattice. These apparently contradictory results are discussed.

Physical Review B, 2002

We investigate the interplay between the formation of lattice and magnetic polaron in the case of... more We investigate the interplay between the formation of lattice and magnetic polaron in the case of a single hole in the antiferromagnetic background. We present an exact analytical solution of the Holstein-t-J model in infinite dimensions. Ground state energy, electron-lattice correlation function, spin bag dimension as well as spectral properties are calculated. The magnetic and holelattice correlations sustain each other, i.e. the presence of antiferromagnetic correlations favors the formation of the lattice polaron at lower value of the electron-phonon coupling while the polaronic effect contributes to reduce the number of spin defects in the antiferromagnetic background. The crossover towards a spin-lattice small polaron region of the phase diagram becomes a discontinuous transition in the adiabatic limit. PACS number(s): 71.10. Fd, 75.30.Kz, 71.38.Ht wherec † iσ are the electron operators in the presence of infinite on-site repulsion that prevents double occupancy

Physical Review Letters, 2005

Isotope effects (IEs) are powerful tool to probe directly the dependence of many physical propert... more Isotope effects (IEs) are powerful tool to probe directly the dependence of many physical properties on the lattice dynamics. In this paper we invenstigate the onset of anomalous IEs in the spinless Holstein model by employing the dynamical mean field theory. We show that the isotope coefficients of the electron effective mass and of the dressed phonon frequency are sizeable also far away from the strong coupling polaronic crossover and mark the importance of nonadiabatic lattice fluctuations in the weak to moderate coupling region. We characterize the polaronic regime by the appearence of huge IEs. We draw a nonadiabatic phase diagram in which we identify a novel crossover, not related to polaronic features, where the IEs attain their largest anomalies.

Physical Review Letters, 2002

The evidence for the key role of the σ bands in the electronic properties of MgB2 points to the p... more The evidence for the key role of the σ bands in the electronic properties of MgB2 points to the possibility of nonadiabatic effects in the superconductivity of these materials. These are governed by the small value of the Fermi energy due to the vicinity of the hole doping level to the top of the σ bands. We show that the nonadiabatic theory leads to a coherent interpretation of Tc = 39 K and the boron isotope coefficient αB = 0.30 without invoking very large couplings and it naturally explains the role of the disorder on Tc. It also leads to various specific predictions for the properties of MgB2 and for the material optimization of these type of compounds.

Physical Review Letters, 2001

We present a novel Path Integral Monte Carlo scheme to solve the Fröhlich polaron model. At inter... more We present a novel Path Integral Monte Carlo scheme to solve the Fröhlich polaron model. At intermediate and strong electron-phonon coupling, the polaron self-trapping is properly taken into account at the level of an effective action obtained by a preaveraging procedure with a retarded trial action. We compute the free energy at several couplings and temperatures in three and two dimensions. Our results show that the accuracy of the Feynman variational upper bound for the free energy is always better than 5% although the thermodynamics derived from it is not correct. Our estimates of the ground state energies demonstrate that the second cumulant correction to the variational upper bound predicts the self energy to better than 1% at intermediate and strong coupling.

Physical Review Letters, 2012

By comparing photoemission spectroscopy with a nonperturbative dynamical mean field theory extens... more By comparing photoemission spectroscopy with a nonperturbative dynamical mean field theory extension to many-body ab initio calculations, we show in the prominent case of pentacene crystals that an excellent agreement with experiment for the bandwidth, dispersion, and lifetime of the hole carrier bands can be achieved in organic semiconductors, provided that one properly accounts for the coupling to molecular vibrational modes and the presence of disorder. Our findings rationalize the growing experimental evidence that even the best band structure theories based on a many-body treatment of electronic interactions cannot reproduce the experimental photoemission data in this important class of materials.

Physical Review B, 2009

We investigate the temperature and doping dependence of the optical conductivity σ(ω) of Nd2−xCex... more We investigate the temperature and doping dependence of the optical conductivity σ(ω) of Nd2−xCexCuO4 in terms of magnetic/lattice polaron formation. We employ dynamical mean-field theory in the context of the Holstein-t-J model where an exact analytical solution is available in the limit of infinite connectivity. We show that the pseudogap features in the optical conductivity of this compound can be associated to the formation of lattice polarons assisted by the magnetic interaction.

Physical Review B, 2006

The evolution of the properties of a finite density electronic system as a function of the electr... more The 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.

Physica C: Superconductivity, 2004

Superconductivity at T c ' 40 K in MgB 2 is thought to origin from the strong electron-phonon (el... more Superconductivity at T c ' 40 K in MgB 2 is thought to origin from the strong electron-phonon (el-ph) coupling of the r-bands, whereas the residual interband scattering gives rise to the multigap phenomenology. The extremely low charge density of the r-bands is reflected in the small Fermi energy E r F , a fraction of eV, a common feature which is shared also by cuprates and fullerides. In our contribution we discuss the anomalous effects arising from the small Fermi energy phenomenology, when E r F becomes comparable with the other energy scales of the systems. In particular we analyze the nonadiabatic effects arising from the finite adiabatic ratio x ph =E r F ; the anharmonic character of the E 2g phonon mode, which is shown to be related to the smallness of the Fermi energy with respect to the electron-phonon coupling g E 2 g : E r F g E 2 g ; the anomalous effects of disorder when impurity scattering rate c imp is compared with E r F . We discuss also the possibility of an enhancement of the nonadiabatic character due to zero point quantum fluctuations.

Physica B: Condensed Matter, 2005

The evolution of the properties of a finite density electronic system as the electron-phonon coup... more 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).

Physica B: Condensed Matter, 2008

We employ dynamical mean-field theory to study the optical conductivity σ(ω) of one hole in the H... more We employ dynamical mean-field theory to study the optical conductivity σ(ω) of one hole in the Holstein-t-J model. We provide an exact solution for σ(ω) in the limit of infinite connectivity. We apply our analysis to Nd2−xCexCuO4. We show that our model can explain many features of the optical conductivity in this compounds in terms of magnetic/lattice polaron formation.

Journal of Physics and Chemistry of Solids, 2008

Recent experiments have demonstrated that the performances of organic FETs strongly depend on the... more Recent experiments have demonstrated that the performances of organic FETs strongly depend on the dielectric properties of the gate insulator. In particular, it has been shown that the temperature dependence of the mobility evolves from a metallic-like to an insulating behavior upon increasing the dielectric constant of the gate material. This phenomenon can be explained in terms of the formation of small polarons, due to the polar interaction of the charge carriers with the phonons at the organic/dielectric interface. Building on this model, the possible consequences of the Coulomb repulsion between the carriers at high concentrations are analyzed.