2,603 research outputs found
Normal state diamagnetism of charged bosons in cuprate superconductors
Normal state orbital diamagnetism of charged bosons quantitatively accounts
for recent high-resolution magnetometery results near and above the resistive
critical temperature Tc of superconducting cuprates. Our parameter-free
descriptions of normal state diamagnetism, Tc, upper critical fields and
specific heat anomalies unambiguously support the 3D Bose-Einstein condensation
at Tc of preformed real-space pairs with zero off-diagonal order parameter
above Tc, at variance with phase fluctuation (or vortex) scenarios of the
"normal" state of cuprates.Comment: 10 pages, 4 figure
Gap and subgap tunnelling in cuprates
We describe strongly attractive carriers in cuprates in the framework of a
simple quasi-one dimensional Hamiltonian with a local attraction. In contrast
with the conventional BCS theory there are two energy scales, a temperature
independent incoherent gap and a temperature dependent coherent gap
combining into one temperature dependent global gap
. The temperature dependence of the gap
and single particle (Giaver) tunnelling spectra in cuprates are quantitatively
described. A framework for understanding of two distinct energy scales observed
in Giaver tunnelling and electron-hole reflection experiments is provided.Comment: 9 pages (RevTex), 4 postscript figures, typos correcte
Unconventional superconducting pairing by conventional phonons
The common wisdom that the phonon mechanism of electron pairing in the
weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors leads to
conventional s-wave Cooper pairs is revised. An inevitable anisotropy of sound
velocity in crystals makes the phonon-mediated attraction of electrons
non-local in space providing unconventional Cooper pairs with a nonzero orbital
momentum in a wide range of electron densities. As a result of this anisotropy
quasi-two dimensional charge carriers undergo a quantum phase transition from
an unconventional d-wave superconducting state to a conventional s-wave
superconductor with more carriers per unit cell. In the opposite
strong-coupling regime rotational symmetry breaking appears as a result of a
reduced Coulomb repulsion between unconventional bipolarons dismissing thereby
some constraints on unconventional pairing in the Bose-Einstein condensation
(BEC) limit. The conventional phonons, and not superexchange, are shown to be
responsible for the d-wave symmetry of cuprate superconductors, where the
on-site Coulomb repulsion is large.Comment: 4 pages, 4 figures, more references adde
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
On choice of connection in loop quantum gravity
We investigate the quantum area operator in the loop approach based on the
Lorentz covariant hamiltonian formulation of general relativity. We show that
there exists a two-parameter family of Lorentz connections giving rise to
Wilson lines which are eigenstates of the area operator. For each connection
the area spectrum is evaluated. In particular, the results of the su(2)
approach turn out to be included in the formalism. However, only one connection
from the family is a spacetime connection ensuring that the 4d diffeomorphism
invariance is preserved under quantization. It leads to the area spectrum
independent of the Immirzi parameter. As a consequence, we conclude that the
su(2) approach must be modified accordingly to the results obtained since it
breaks one of the classical symmetries.Comment: 11 pages, RevTEX; minor changes; a sign mistake correcte
Polaron and bipolaron transport in a charge segregated state of doped strongly correlated 2D semiconductor
The 2D lattice gas model with competing short and long range interactions is
appliedused for calculation of the incoherent charge transport in the classical
strongly-correlated charge segregated polaronic state. We show, by means of
Monte-Carlo simulations, that at high temperature the transport is dominated by
hopping of the dissociated correlated polarons, where with thetheir mobility is
inversely proportional to the temperature. At the temperatures below the
clustering transition temperature the bipolaron transport becomes dominant. The
energy barrier for the bipolaron hopping is determined by the Coulomb effects
and is found to be lower than the barrier for the single-polaron hopping. This
leads to drastically different temperature dependencies of mobilities for
polarons and bipolarons at low temperatures
Hilbert space structure of covariant loop quantum gravity
We investigate the Hilbert space in the Lorentz covariant approach to loop
quantum gravity. We restrict ourselves to the space where all area operators
are simultaneously diagonalizable, assuming that it exists. In this sector
quantum states are realized by a generalization of spin network states based on
Lorentz Wilson lines projected on irreducible representations of an SO(3)
subgroup. The problem of infinite dimensionality of the unitary Lorentz
representations is absent due to this projection. Nevertheless, the projection
preserves the Lorentz covariance of the Wilson lines so that the symmetry is
not broken. Under certain conditions the states can be thought as functions on
a homogeneous space. We define the inner product as an integral over this
space. With respect to this inner product the spin networks form an orthonormal
basis in the investigated sector. We argue that it is the only relevant part of
a larger state space arising in the approach. The problem of the
noncommutativity of the Lorentz connection is solved by restriction to the
simple representations. The resulting structure shows similarities with the
spin foam approach.Comment: 20 pages, RevTE
Diamagnetism of real-space pairs above Tc in hole doped cuprates
The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.
B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper
pairing and vortex liquid above the resistive critical temperature. Instead it
is perfectly compatible with the normal state Landau diamagnetism of real-space
composed bosons, which describes the nonlinear magnetization curves in less
anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in
strongly anisotropic bismuth-based cuprates in the whole range of available
magnetic fields.Comment: 4 pages, 4 figure
Theory of High Temperature Superconductivity in Doped Polar Insulator
In the last two decades there have been tremendous attempts to built an
adequate theory of high-temperature superconductivity. Most studies (including
our efforts) used some model Hamiltonians with input parameters not directly
related to the material. The dielectric response function of electrons in
strongly correlated high-temperature superconductors is apriori unknown. Hence
one has to start with the generic Hamiltonian including unscreened Coulomb and
Froehlich electron-phonon interactions operating on the same scale since any
ad-hoc assumption on their range and relative magnitude might fail. Using such
a generic Hamiltonian I have built the analytical theory of high-temperature
superconductivity in doped polar insulators predicting the critical temperature
in excess of a hundred Kelvin without any adjustable parameters. The
many-particle electron system is described by an analytically solvable
polaronic "t-Jp" Hamiltonian with reduced hopping integral, t, allowed double
on-site occupancy, large phonon-induced antiferromagnetic exchange, Jp >> t,
and a high-temperature superconducting state of small superlight bipolarons
protected from clustering.Comment: 6 pages, 2 figures, some citations are update
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
- …