arXiv subjects

S. Ciuchi

Publications and source records attributed to S. Ciuchi.

46 records · Page 3Linked to original sources

arXiv2003

Band-filling effects on electron-phonon properties of normal and superconducting state

We address the effect of band filling on the effective electron mass $m^*$ and the superconducting critical temperature $T_c$ in a electron-phonon system. We compare the vertex corrected theory with the non-crossing approximation of the Holstein model within a local approximation. We identify two regions of the electron density where $m^*$ and $T_c$ are enhanced or decreased by the inclusion of the vertex diagrams. We show that the crossover between the enhancement at low density and the decrease towards half filling is almost independent of the microscopic electron-phonon parameters. These different behaviors are explained in terms of the net sign of the vertex diagrams which is positive at low densities and negative close to half filling. Predictions of the present theory for doped MgB$_2$, which is argued to be in the low density regime, are discussed.

cond-mat.supr-con↗

arXiv2003

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.

cond-mat.str-el↗

arXiv2002

Magnetic and lattice polaron in Holstein-t-J model

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 hole-lattice 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.

cond-mat.str-el↗

arXiv2002

High $T_c$ superconductivity in MgB$_2$ by nonadiabatic pairing

The evidence for the key role of the $σ$ bands in the electronic properties of MgB$_2$ 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 $T_c = 39$ K and the boron isotope coefficient $α_{\rm B} = 0.30$ without invoking very large couplings and it naturally explains the role of the disorder on $T_c$. It also leads to various specific predictions for the properties of MgB$_2$ and for the material optimization of these type of compounds.

cond-mat.supr-con↗

arXiv2001

Interplay between Spin and Phonon Fluctuations in the double-exchange model for the manganites

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 $S=\infty$, 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 $ω_0$, stabilize ferromagnetic phases with respect to the adiabatic limit. We also provide a complete analysis on the polaron crossover in this model.

cond-mat.str-el↗

arXiv2000

Free energy of the Fröhlich polaron in two and three dimensions

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.

cond-mat.str-el↗

arXiv2000

Different Scenarios for Critical Glassy Dynamics

We study the role of different terms in the $N$-body potential of glass forming systems on the critical dynamics near the glass transition. Using a simplified spin model with quenched disorder, where the different terms of the real $N$-body potential are mapped into multi-spin interactions, we identified three possible scenarios. For each scenario we introduce a ``minimal'' model representative of the critical glassy dynamics near, both above and below, the critical transition lin e. For each ``minimal'' model we discuss the low temperature equilibrium dynamics.

cond-mat.soft↗

arXiv1998

The Charge Ordered State from Weak to Strong Coupling

We apply the Dynamical Mean Field Theory to the problem of charge ordering. In the normal state as well as in the Charge Ordered (CO) state the existence of polarons, i.e. electrons strongly coupled to local lattice deformation, is associated to the qualitative properties of the Lattice Polarization Distribution Function (LPDF). At intermediate and strong coupling a CO state characterized by a certain amount of thermally activated defects arise from the spatial ordering of preexisting randomly distributed polarons. Properties of this particular CO state gives a qualitative understanding of the low frequency behavior of optical conductivity of $Ni$ perovskites.

cond-mat.str-el↗

arXiv1998

The Small Polaron Crossover: Role of Dimensionality

The crossover from quasi free electron to small polaron in the Holstein model for a single electron is studied by means of both exact and self-consistent calculations in one dimension and on an infinite coordination lattice, in order to understand the role of dimensionality in such a crossover. We show that a small polaron ground-state occurs when both strong coupling ($λ>1$) and multiphonon ($α^2 >1$) conditions are fulfilled leading to different relevant coupling constants ($λ$) in adiabatic and ($α^2$) anti adiabatic region of the parameters space. We also show that the self-consistent calculations obtained by including the first electron-phonon vertex correction give accurate results in a sizeable region of the phase diagram well separated from the polaronic crossover.

cond-mat.str-el↗

arXiv1997

Dynamical mean-field theory of the small polaron

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 have a smooth behaviour, 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 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.

cond-mat↗