In this paper we investigate the optical spectra of one hole in the Holstein-t-J model. We employ a dynamical mean-field theory which becomes exact in the limit of infinite connectivity. This allow ...us to investigate the local (incoherent) features which are related to the internal structure of the polaron, disregarding coherent motion associated with the Drude-like peak. We show that magnetic and electron-phonon interactions sustain each other in establishing polaronic regime. Polaron formation is reflected in a peculiar mid-infrared (MIR) band which is however notably different in the case of a lattice or magnetic origin. The dependence of σ(ω) on the electron-phonon coupling constant λ, on the exchange interaction J and on temperature T is investigated. We compare our results with experimental data in Nd2-xCexCuO4 showing that the doping and temperature dependences of the optical conductivity in this compounds is naturally reproduced by a spin/lattice polaronic model.
We present a novel path integral Monte Carlo scheme to solve the Fröhlich polaron model. At intermediate and strong electron-phonon couplings, 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 within a few percent. The method can be generalized to the N-polarons case.
The formation of a finite density multipolaronic state in the Holstein model is analyzed by means of the Dynamical mean-field theory at zero and finite temperature. The spinless and spinful fermion ...cases are compared to disentangle the polaron crossover from the bipolaron formation. We show that polaron formation is not associated to a metal–insulator transition, which is instead due to bipolaron formation. In the spinful case a region of bound pairs without significant lattice distortion appears between the weak-coupling regime and the strong-coupling bipolaronic phase. This region shrinks with increasing temperature and with decreasing phonon frequency.
Hopping dynamics of interacting polarons Ciuchi, S.; Fratini, S.
Physical review. B, Condensed matter and materials physics,
01/2009, Letnik:
79, Številka:
3
Journal Article
Recenzirano
Odprti dostop
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 in the thermal activation barrier for electrical transport and hence to a sizable reduction in 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.
We apply dynamical mean field theory to study a prototypical model that describes charge ordering in the presence of both electron-lattice interactions and intersite electrostatic repulsion between ...electrons. We calculate the optical and dc conductivity and derive approximate formulas valid in the limiting electron-lattice coupling regimes. In the weak-coupling regime, we recover the usual behavior of charge density waves, characterized by a transfer of spectral weight due to the opening of a gap in the excitation spectrum. In the opposite limit of very strong electron-lattice coupling, instead, the charge ordering transition is signaled by a global enhancement of the optical absorption, with no appreciable spectral weight transfer. Such behavior is related to the progressive suppression of thermally activated charge defects taking place below the critical temperature. At intermediate values of the coupling within the polaronic regime, a complex behavior is obtained where both mechanisms of transfer and enhancement of spectral weight coexist.
One possible scenario for the critical properties of high-temperature superconductors in the underdoped regime takes into account the critical slowing down of two fluctuation fields associated to ...density and to Cooper pairs. The first is due to the instability toward a CDW ordering and the second to the instability to superconductivity. We develop a nonperturbative theory of this regime in the framework of the attractive Hubbard model. Above a critical temperature, the particles move under the action of a static field whose direction fixes the relative weight of density and Cooper pair fluctuations.
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