Article

# Effect of Electron-Phonon Interaction Range for a Half-Filled Band in One Dimension

Institut für Theoretische Physik und Astrophysik, Universität Würzburg, 97074 Würzburg, Germany.

Physical Review Letters (Impact Factor: 7.73). 05/2012; 109(11). DOI: 10.1103/PhysRevLett.109.116407 Source: arXiv

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**ABSTRACT:**We use continuous-time quantum Monte Carlo simulations to study retardation effects in the metallic, quarter-filled Holstein model in one dimension. Based on results which include the one- and two-particle spectral functions as well as the optical conductivity, we conclude that with increasing phonon frequency the ground state evolves from one with dominant diagonal order-2k(F) charge correlations-to one with dominant off-diagonal fluctuations, namely s-wave pairing correlations. In the parameter range of this crossover, our numerical results support the existence of a spin gap for all phonon frequencies. The crossover can hence be interpreted in terms of preformed pairs corresponding to bipolarons, which are essentially localized in the Peierls phase, and 'condense' with increasing phonon frequency to generate dominant pairing correlations.Journal of Physics Condensed Matter 12/2012; 25(1):014005. · 2.22 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**We determine the phase diagram of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger couplings and show that it has two sharp transitions, in contrast to pure models which exhibit one (for Su-Schrieffer-Heeger coupling) or no (for breathing-mode coupling) transition. We then show that ultracold molecules trapped in optical lattices can be used as a quantum simulator to study precisely this mixed Hamiltonian, and that the relative contributions of the two couplings can be tuned with external electric fields. The parameters of current experiments place them in the region where one of the transitions occurs. We also propose a scheme to measure the polaron dispersion using stimulated Raman spectroscopy.Physical Review Letters 05/2013; 110(22):223002. · 7.73 Impact Factor - [Show abstract] [Hide abstract]

**ABSTRACT:**Employing the recently developed self-consistent variational basis generation scheme, we have investigated the bipolaron-bipolaron interaction within the purview of Holstein-Hubbard and the Froehlich-Hubbard model on a discrete one-dimensional lattice. The density-matrix renormalization group (DMRG) method has also been used for the Holstein-Hubbard model. We have shown that there exists no bipolaron-bipolaron attraction in the Holstein-Hubbard model. In contrast, we have obtained clear-cut bipolaron-bipolaron attraction in the Froehlich-Hubbard model. Composite bipolarons are formed above a critical electron-phonon coupling strength, which can survive the finite Hubbard U effect. We have constructed the phase diagram of Froehlich-Hubbard polarons and bipolarons, and discussed the phase separation in terms of the formation of composite bipolarons.Physical Review B. 01/2014; 89:035146.

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