117 research outputs found
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 -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 direction than along the
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
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