14,621 research outputs found
Microscopic Model of Cuprate Superconductivity
We present a model for cuprate superconductivity based on the identification
of an experimentally detected "local superconductor" as a charge 2 fermion
pairing in a circular, stationary density wave. This wave acts like a highly
correlated local "boson" satisfying a modified Cooper problem with additional
correlation stabilization relative to the separate right- and left-handed
density waves composing it. This local "boson" could be formed in a two-bound
roton-like manner; it has Fermion statistics. Delocalized superconductive
pairing (superconductivity) is achieved by a Feshbach resonance of two unpaired
holes (electrons) resonating with a virtual energy level of the bound pair
state of the local "boson" as described by the Boson-Fermion-Gossamer (BFG)
model. The spin-charge order interaction offers an explanation for the overall
shape of the superconducting dome as well a microscopic basis for the cuprate
superconducting transition temperatures. An explanation of the correlation of
superconducting transition temperature with experimental inelastic neutron and
electron Raman scattering is proposed, based on the energy of the virtual bound
pair. These and other modifications discussed suggest a microscopic explanation
for the entire cuprate superconductivity dome shape.Comment: 27 pages, 7 figures, presented at the 50th Sanibel Symposiu
Kinetic Mixing and the Supersymmetric Gauge Hierarchy
The most general Lagrangian for a model with two U(1) gauge symmetries
contains a renormalizable operator which mixes their gauge kinetic terms. Such
kinetic mixing can be generated at arbitrarily high scales but will not be
suppressed by large masses. In models whose supersymmetry (SUSY)-breaking
hidden sectors contain U(1) gauge factors, we show that such terms will
generically arise and communicate SUSY-breaking to the visible sector through
mixing with hypercharge. In the context of the usual supergravity- or
gauge-mediated communication scenarios with D-terms of order the fundamental
scale of SUSY-breaking, this effect can destabilize the gauge hierarchy. Even
in models for which kinetic mixing is suppressed or the D-terms are arranged to
be small, this effect is a potentially large correction to the soft scalar
masses and therefore introduces a new measurable low-energy parameter. We
calculate the size of kinetic mixing both in field theory and in string theory,
and argue that appreciable kinetic mixing is a generic feature of string
models. We conclude that the possibility of kinetic mixing effects cannot be
ignored in model-building and in phenomenological studies of the low-energy
SUSY spectra.Comment: 16 pages, LaTeX, 1 figure. Revised to match published versio
Proposed lower bound for the shear viscosity to entropy density ratio in some dense liquids
Starting from relativistic quantum field theories, Kovtun et al. (2005) have
quite recently proposed a lower bound eta/s >= hbar /(4 pi kB), where eta is
the shear viscosity and s the volume density of entropy for dense liquids. If
their proposal can eventually be proved, then this would provide key
theoretical underpinning to earlier semiempirical proposals on the relation
between a transport coefficient eta and a thermodynamic quantity s. Here, we
examine largely experimental data on some dense liquids, the insulators
nitrogen, water, and ammonia, plus the alkali metals, where the shear viscosity
eta(T) for the four heaviest alkalis is known to scale onto an `almost
universal' curve, following the work of Tankeshwar and March a decade ago. So
far, all known results for both insulating and metallic dense liquids correctly
exceed the lower bound prediction of Kovtun et al.Comment: to appear in Phys. Lett.
Superconducting transition temperatures of the elements related to elastic constants
For a given crystal structure, say body-centred-cubic, the many-body
Hamiltonian in which nuclear and electron motions are to be treated from the
outset on the same footing, has parameters, for the elements, which can be
classified as (i) atomic mass M, (ii) atomic number Z, characterizing the
external potential in which electrons move, and (iii) bcc lattice spacing, or
equivalently one can utilize atomic volume, Omega. Since the thermodynamic
quantities can be determined from H, we conclude that Tc, the superconducting
transition temperature, when it is non-zero, may be formally expressed as Tc =
Tc^(M) (Z, Omega). One piece of evidence in support is that, in an atomic
number vs atomic volume graph, the superconducting elements lie in a well
defined region. Two other relevant points are that (a) Tc is related by BCS
theory, though not simply, to the Debye temperature, which in turn is
calculable from the elastic constants C_{11}, C_{12}, and C_{44}, the atomic
weight and the atomic volume, and (b) Tc for five bcc transition metals is
linear in the Cauchy deviation C* = (C_{12} - C_{44})/(C_{12} + C_{44}).
Finally, via elastic constants, mass density and atomic volume, a correlation
between C* and the Debye temperature is established for the five bcc transition
elements.Comment: EPJB, accepte
Integral equation for inhomogeneous condensed bosons generalizing the Gross-Pitaevskii differential equation
We give here the derivation of a Gross-Pitaevskii--type equation for
inhomogeneous condensed bosons. Instead of the original Gross-Pitaevskii
differential equation, we obtain an integral equation that implies less
restrictive assumptions than are made in the very recent study of Pieri and
Strinati [Phys. Rev. Lett. 91 (2003) 030401]. In particular, the Thomas-Fermi
approximation and the restriction to small spatial variations of the order
parameter invoked in their study are avoided.Comment: Phys. Rev. A (accepted
Scaling of the superconducting transition temperature in underdoped high-Tc cuprates with a pseudogap energy: Does this support the anyon model of their superfluidity?
In earlier work, we have been concerned with the scaling properties of some
classes of superconductors, specifically with heavy Fermion materials and with
five bcc transition metals of BCS character. Both of these classes of
superconductors were three-dimensional but here we are concerned solely with
quasi-two-dimensional high-Tc cuprates in the underdoped region of their phase
diagram. A characteristic feature of this part of the phase diagram is the
existence of a pseudogap (pg). We therefore build our approach around the
assumption that kB Tc / E_pg is the basic dimensionless ratio on which to
focus, where the energy E_pg introduced above is a measure of the pseudogap.
Since anyon fractional statistics apply to two-dimensional assemblies, we
expect the fractional statistics parameter allowing `interpolation' between
Fermi-Dirac and Bose-Einstein statistical distribution functions as limiting
cases to play a significant role in determining kB Tc / E_pg and experimental
data are analyzed with this in mind.Comment: Phys. Chem. Liquids, to be publishe
- …