Light and stable triplet bipolarons on square and triangular lattices

We compute the properties of singlet and triplet bipolarons on two-dimensional lattices using the continuous time quantum Monte Carlo algorithm. Properties of the bipolaron including the total energy, inverse mass, bipolaron radius and number of phonons associated with the bipolaron demonstrate the qualitative difference between models of electron phonon interaction with long-range interaction (screened Fr\"ohlich) and those with purely local (Holstein) interaction. A major result of our survey of the parameter space is the existence of extra-light hybrid singlet bipolarons consisting of an on-site and an off-site component on both square and triangular lattices. We also compute triplet properties of the bipolarons and the pair dispersion. For pair momenta on the edge of the Brillouin zone of the triangular lattice, we find that triplet states are more stable than singlets

Isotope effect on the electron band structure of doped insulators

Applying a continuous-time quantum Monte-Carlo algorithm we calculate the exact coherent band dispersion and the density of states of a two dimensional lattice polaron in the region of parameters where any approximation might fail. We find an isotope effect on the band structure, which is different for different wave-vectors of the Brillouin zone and depends on the radius and strength of the electron-phonon interaction. An isotope effect on the electron spectral function is also discussed.Comment: 4 pages, 3 figure

Bipolarons from long range interactions: Singlet and triplet pairs in the screened Hubbard-Froehlich model on the chain

We present details of a continuous-time quantum Monte-Carlo algorithm for the screened Hubbard-Froehlich bipolaron. We simulate the bipolaron in one dimension with arbitrary interaction range in the presence of Coulomb repulsion, computing the effective mass, binding energy, total number of phonons associated with the bipolaron, mass isotope exponent and bipolaron radius in a comprehensive survey of the parameter space. We discuss the role of the range of the electron-phonon interaction, demonstrating the evolution from Holstein to Froehlich bipolarons and we compare the properties of bipolarons with singlet and triplet pairing. Finally, we present simulations of the bipolaron dispersion. The band width of the Froehlich bipolaron is found to be broad, and the decrease in bandwidth as the two polarons bind into a bipolaron is found to be far less rapid than in the case of the Holstein interaction. The properties of bipolarons formed from long range electron-phonon interactions, such as light strongly bound bipolarons and intersite pairing when Coulomb repulsion is large, are found to be robust against screening, with qualitative differences between Holstein and screened Froehlich bipolarons found even for interactions screened within a single lattice site.Comment: 20 pages, 17 figure

High temperature superconductivity and charge segregation in a model with strong long-range electron-phonon and Coulomb interactions

An analytical method of studying strong long-range electron-phonon and Coulomb interactions in complex lattices is presented. The method is applied to a perovskite layer with anisotropic coupling of holes to the vibrations of apical atoms. Depending on the relative strength of the polaronic shift, Ep, and the inter-site Coulomb repulsion, Vc, the system is either a polaronic Fermi liquid, Vc > 1.23 Ep, a bipolaronic superconductor, 1.16 Ep < Vc < 1.23 Ep, or a charge segregated insulator, Vc < 1.16 Ep. In the superconducting window, the carriers are mobile bipolarons with a remarkably low effective mass. The model describes the key features of the underdoped superconducting cuprates.Comment: 5 pages, 2 figures (1 color

Enhanced stability of bound pairs at nonzero lattice momenta

A two-body problem on the square lattice is analyzed. The interaction potential consists of strong on-site repulsion and nearest-neighbor attraction. Exact pairing conditions are derived for s-, p-, and d-symmetric bound states. The pairing conditions are strong functions of the total pair momentum K. It is found that the stability of pairs increases with K. At weak attraction, the pairs do not form at the $\Gamma$-point but stabilize at lattice momenta close to the Brillouin zone boundary. The phase boundaries in the momentum space, which separate stable and unstable pairs are calculated. It is found that the pairs are formed easier along the $(\pi,0)$ direction than along the $(\pi,\pi)$ direction. This might lead to the appearance of ``hot pairing spots" on the Kx and Ky axes.Comment: 7 RevTEX pages, 5 figure

Polaron effective mass from Monte Carlo simulations

A new Monte Carlo algorithm for calculating polaron effective mass is proposed. It is based on the path-integral representation of a partial partition function with fixed total quasi-momentum. Phonon degrees of freedom are integrated out analytically resulting in a single-electron system with retarded self-interaction and open boundary conditions in imaginary time. The effective mass is inversely proportional to the covariance of total energy calculated on an electron trajectory and the square distance between ends of the trajectory. The method has no limitations on values of model parameters and on the size and dimensionality of the system although large statistics is required for stable numerical results. The method is tested on the one-dimensional Holstein model for which simulation results are presented.Comment: 4 pages + 1 figure, RevTeX. Accepted for publication as a Rapid Communication in Phys.Rev.

Three-body scattering problem and two-electron tunneling in molecular wires

We solve the Lippmann-Schwinger equation describing elastic scattering of preformed pairs (e.g. bipolarons) off a short-range scattering center and find the two-particle transmission through a thin potential barrier. While the pair transmission is smaller than the single-electron transmission in the strong-coupling limit, it is remarkably larger in the weak coupling limit. We also calculate current-voltage characteristics of a molecule - barrier - molecule junction. They show unusual temperature and voltage behavior which are experimentally verifiable at low temperatures.Comment: 5 pages, 2 figure