3,519 research outputs found
Yet Another Poincare's Polyhedron Theorem
Poincar\'e's Polyhedron Theorem is a widely known valuable tool in
constructing manifolds endowed with a prescribed geometric structure. It is one
of the few criteria providing discreteness of groups of isometries. This work
contains a version of Poincar\'e's Polyhedron Theorem that is applicable to
constructing fibre bundles over surfaces and also suits geometries of
nonconstant curvature. Most conditions of the theorem, being as local as
possible, are easy to verify in practice.Comment: 9 pages, 2 figures, 5 references. Final versio
Superlight small bipolarons from realistic long-range Coulomb and Fr\"ohlich interactions
We report analytical and numerical results on the two-particle states of the
polaronic t-Jp model derived recently with realistic Coulomb and
electron-phonon (Frohlich) interactions in doped polar insulators. Eigenstates
and eigenvalues are calculated for two different geometries. Our results show
that the ground state is a bipolaronic singlet, made up of two polarons. The
bipolaron size increases with increasing ratio of the polaron hopping integral
t to the exchange interaction Jp but remains small compared to the system size
in the whole range 0<t/Jp<1. Furthermore, the model exhibits a phase transition
to a superconducting state with a critical temperature well in excess of 100K.
In the range t/Jp<1, there are distinct charge and spin gaps opening in the
density of states, specific heat, and magnetic susceptibility well above Tc.Comment: Calculation section and discussion of gap have been updated. Revised
calculations now enhance the predicted T_c in our model to over 200 K at
large hoppin
Hall effect and resistivity in underdoped cuprates
The behaviour of the Hall ratio as a function of temperature is
one of the most intriguing normal state properties of cuprate superconductors.
One feature of all the data is a maximum of in the normal state that
broadens and shifts to temperatures well above with decreasing doping. We
show that a model of preformed pairs-bipolarons provides a selfconsistent
quantitative description of together with in-plane resistivity and
uniform magnetic susceptibility for a wide range of doping.Comment: 4 pages, 2 figures, the model and fits were refine
Pseudogap in high-temperature superconductors from realistic Fr\"ohlich and Coulomb interactions
It has been recently shown that the competition between unscreened Coulomb
and Fr\"{o}hlich electron-phonon interactions can be described in terms of a
short-range spin exchange and an effective on-site interaction
in the framework of the polaronic -- model. This
model, that provides an explanation for high temperature superconductivity in
terms of Bose-Einstein condensation (BEC) of small and light bipolarons, is now
studied as a charged Bose-Fermi mixture. Within this approximation, we show
that a gap between bipolaron and unpaired polaron bands results in a strong
suppression of low-temperature spin susceptibility, specific heat and tunneling
conductance, signaling the presence of normal state pseudogap without any
assumptions on preexisting orders or broken symmetries in the normal state of
the model.Comment: 5 pages, 5 figure
Coherent `ab' and `c' transport theory of high- cuprates
We propose a microscopic theory of the `'-axis and in-plane transport of
copper oxides based on the bipolaron theory and the Boltzmann kinetics. The
fundamental relationship between the anisotropy and the spin susceptibility is
derived, . The
temperature and doping dependence of the in-plane, and
out-of-plane, resistivity and the spin susceptibility,
are found in a remarkable agreement with the experimental data in underdoped,
optimally and overdoped for the entire temperature
regime from up to . The normal state gap is explained and its
doping and temperature dependence is clarified.Comment: 12 pages, Latex, 3 figures available upon reques
Bipolarons in the Extended Holstein Hubbard Model
We numerically and analytically calculate the properties of the bipolaron in
an extended Hubbard Holstein model, which has a longer range electron-phonon
coupling like the Fr\" ohlich model. In the strong coupling regime, the
effective mass of the bipolaron in the extended model is much smaller than the
Holstein bipolaron mass. In contrast to the Holstein bipolaron, the bipolaron
in the extended model has a lower binding energy and remains bound with
substantial binding energy even in the large-U limit. In comparison with the
Holstein model where only a singlet bipolaron is bound, in the extended
Holstein model a triplet bipolaron can also form a bound state. We discuss the
possibility of phase separation in the case of finite electron doping.Comment: 5 pages, 3 figure
Star Unfolding Convex Polyhedra via Quasigeodesic Loops
We extend the notion of star unfolding to be based on a quasigeodesic loop Q
rather than on a point. This gives a new general method to unfold the surface
of any convex polyhedron P to a simple (non-overlapping), planar polygon: cut
along one shortest path from each vertex of P to Q, and cut all but one segment
of Q.Comment: 10 pages, 7 figures. v2 improves the description of cut locus, and
adds references. v3 improves two figures and their captions. New version v4
offers a completely different proof of non-overlap in the quasigeodesic loop
case, and contains several other substantive improvements. This version is 23
pages long, with 15 figure
S-duality in Twistor Space
In type IIB string compactifications on a Calabi-Yau threefold, the
hypermultiplet moduli space must carry an isometric action of the modular
group SL(2,Z), inherited from the S-duality symmetry of type IIB string theory
in ten dimensions. We investigate how this modular symmetry is realized at the
level of the twistor space of , and construct a general class of
SL(2,Z)-invariant quaternion-Kahler metrics with two commuting isometries,
parametrized by a suitably covariant family of holomorphic transition
functions. This family should include corrected by D3-D1-D(-1)-instantons
(with fivebrane corrections ignored) and, after taking a suitable rigid limit,
the Coulomb branch of five-dimensional N=2 gauge theories compactified on a
torus, including monopole string instantons. These results allow us to
considerably simplify the derivation of the mirror map between type IIA and IIB
fields in the sector where only D1-D(-1)-instantons are retained.Comment: 29 pages, 1 figur
Giant enhancement of anisotropy by electron-phonon interaction
Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as . This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.Comment: 5 pages, 4 figure
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