15,613 research outputs found
Pairing in Inhomogeneous Superconductors
Starting from a t-J model, we introduce inhomogeneous terms to mimic stripes.
We find that if the inhomogeneous terms break the SU(2) spin symmetry the
binding between holes is tremendously enhanced in the thermodynamic limit. In
any other model (including homogeneous models) the binding in the thermodynamic
limit is small or neglible. By including these inhomogeneous terms we can
reproduce experimental neutron scattering data. We also discuss the connection
of the resulting inhomogeneity-induced superconductivity to recent experimental
evidence for a linear relation between magnetic incommensurability and the
superconducting transition temperature, as a function of doping.Comment: 4 pages, 2 figure
BCS-to-BEC crossover from the exact BCS solution
The BCS-to-BEC crossover, as well as the nature of Cooper pairs, in a
superconducting and Fermi superfluid medium is studied from the exact ground
state wavefunction of the reduced BCS Hamiltonian. As the strength of the
interaction increases, the ground state continuously evolves from a
mixed-system of quasifree fermions and pair resonances (BCS), to pair
resonances and quasibound molecules (pseudogap), and finally to a system of
quasibound molecules (BEC). A single unified scenario arises where the
Cooper-pair wavefunction has a unique functional form. Several exact analytic
expressions, such as the binding energy and condensate fraction, are derived.
We compare our results with recent experiments in ultracold atomic Fermi gases.Comment: 5 pages, 4 figures. Revised version with one figure adde
Inhomogeneity-Induced Superconductivity?
A t-J-like model for inhomogeneous superconductivity of cuprate oxides is
presented, in which local anisotropic magnetic terms are essential. We show
that this model predicts pairing, consistent with experiments, and argue how
the macroscopic phase-coherent state gradually grows upon lowering of the
temperature. We show that appropriate inhomogeneities are essential in order to
have significant pair binding in the thermodynamic limit. Particularly, {\it
local} breaking of SU(2) spin symmetry is an efficient mechanism for inducing
pairing of two holes, as well as explaining the magnetic scattering properties.
We also discuss the connection of the resulting inhomogeneity-induced
superconductivity to recent experimental evidence for a linear relation between
magnetic incommensurability and the superconducting transition temperature, as
a function of doping.Comment: 4 pages, REVTEX, 4 jpeg figures. To appear in Europhys. Let
Latitudinal variation of the solar photospheric intensity
We have examined images from the Precision Solar Photometric Telescope (PSPT)
at the Mauna Loa Solar Observatory (MLSO) in search of latitudinal variation in
the solar photospheric intensity. Along with the expected brightening of the
solar activity belts, we have found a weak enhancement of the mean continuum
intensity at polar latitudes (continuum intensity enhancement
corresponding to a brightness temperature enhancement of ).
This appears to be thermal in origin and not due to a polar accumulation of
weak magnetic elements, with both the continuum and CaIIK intensity
distributions shifted towards higher values with little change in shape from
their mid-latitude distributions. Since the enhancement is of low spatial
frequency and of very small amplitude it is difficult to separate from
systematic instrumental and processing errors. We provide a thorough discussion
of these and conclude that the measurement captures real solar latitudinal
intensity variations.Comment: 24 pages, 8 figs, accepted in Ap
Geometry of Discrete Quantum Computing
Conventional quantum computing entails a geometry based on the description of
an n-qubit state using 2^{n} infinite precision complex numbers denoting a
vector in a Hilbert space. Such numbers are in general uncomputable using any
real-world resources, and, if we have the idea of physical law as some kind of
computational algorithm of the universe, we would be compelled to alter our
descriptions of physics to be consistent with computable numbers. Our purpose
here is to examine the geometric implications of using finite fields Fp and
finite complexified fields Fp^2 (based on primes p congruent to 3 mod{4}) as
the basis for computations in a theory of discrete quantum computing, which
would therefore become a computable theory. Because the states of a discrete
n-qubit system are in principle enumerable, we are able to determine the
proportions of entangled and unentangled states. In particular, we extend the
Hopf fibration that defines the irreducible state space of conventional
continuous n-qubit theories (which is the complex projective space CP{2^{n}-1})
to an analogous discrete geometry in which the Hopf circle for any n is found
to be a discrete set of p+1 points. The tally of unit-length n-qubit states is
given, and reduced via the generalized Hopf fibration to DCP{2^{n}-1}, the
discrete analog of the complex projective space, which has p^{2^{n}-1}
(p-1)\prod_{k=1}^{n-1} (p^{2^{k}}+1) irreducible states. Using a measure of
entanglement, the purity, we explore the entanglement features of discrete
quantum states and find that the n-qubit states based on the complexified field
Fp^2 have p^{n} (p-1)^{n} unentangled states (the product of the tally for a
single qubit) with purity 1, and they have p^{n+1}(p-1)(p+1)^{n-1} maximally
entangled states with purity zero.Comment: 24 page
Quantum-Mechanical Position Operator and Localization in Extended Systems
We introduce a fundamental complex quantity, , which allows us to
discriminate between a conducting and non-conducting thermodynamic phase in
extended quantum systems. Its phase can be related to the expectation value of
the position operator, while its modulus provides an appropriate definition of
a localization length. The expressions are valid for {\it any} fractional
particle filling. As an illustration we use to characterize insulator
to ``superconducting'' and Mott transitions in one-dimensional lattice models
with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure
The dimerized phase of ionic Hubbard models
We derive an effective Hamiltonian for the ionic Hubbard model at half
filling, extended to include nearest-neighbor repulsion. Using a spin-particle
transformation, the effective model is mapped onto simple spin-1 models in two
particular cases. Using another spin-particle transformation, a slightly
modified model is mapped into an SU(3) antiferromagnetic Heisenberg model whose
exact ground state is known to be spontaneously dimerized. From the effective
models several properties of the dimerized phase are discussed, like
ferroelectricity and fractional charge excitations. Using bosonization and
recent developments in the theory of macroscopic polarization, we show that the
polarization is proportional to the charge of the elementary excitations
Striped superconductors in the extended Hubbard model
We present a minimal model of a doped Mott insulator that simultaneously
supports antiferromagnetic stripes and d-wave superconductivity. We explore the
implications for the global phase diagram of the superconducting cuprates. At
the unrestricted mean-field level, the various phases of the cuprates,
including weak and strong pseudogap phases, and two different types of
superconductivity in the underdoped and the overdoped regimes, find a natural
interpretation. We argue that on the underdoped side, the superconductor is
intrinsically inhomogeneous -- striped coexistence of of superconductivity and
magnetism -- and global phase coherence is achieved through Josephson-like
coupling of the superconducting stripes. On the overdoped side, the state is
overall homogeneous and the superconductivity is of the classical BCS type.Comment: 5 pages, 3 eps figures. Effect of t' on stripe filling + new
references are adde
Identifying Ultra-Cool Dwarfs at Low Galactic Latitudes: A Southern Candidate Catalogue
We present an Ultra-Cool Dwarf (UCD) catalogue compiled from low southern
Galactic latitudes and mid-plane, from a cross-correlation of the 2MASS and
SuperCOSMOS surveys. The catalogue contains 246 members identified from 5042
sq. deg. within 220 deg. <= l <= 360 deg. and 0 deg. < l <= 30 deg., for |b| <=
15 deg. Sixteen candidates are spectroscopically confirmed in the near-IR as
UCDs with spectral types from M7.5V to L9. Our catalogue selection method is
presented enabling UCDs from ~M8V to the L-T transition to be selected down to
a 2MASS limiting magnitude of Ks ~= 14.5 mag. This method does not require
candidates to have optical detections for catalogue inclusion. An optimal set
of optical/near-IR and reduced proper-motion selection criteria have been
defined that includes: an Rf and Ivn photometric surface gravity test, a dual
Rf-band variability check, and an additional photometric classification scheme
to selectively limit contaminants. We identify four candidates as possible
companions to nearby Hipparcos stars -- observations are needed to identify
these as potential benchmark UCD companions. We also identify twelve UCDs
within a possible distance 20 pc, three are previously unknown of which two are
estimated within 10 pc, complimenting the nearby volume-limited census of UCDs.
An analysis of the catalogue spatial completeness provides estimates for
distance completeness over three UCD MJ ranges, while Monte-Carlo simulations
provide an estimate of catalogue areal completeness at the 75 per cent level.
We estimate a UCD space density of Rho (total) = (6.41+-3.01)x10^3/pc^3 over
the range of 10.5 <= MJ ~< 14.9, similar to values measured at higher Galactic
latitudes (|b| ~> 10 deg.) in the field population and obtained from more
robust spectroscopically confirmed UCD samples.Comment: MNRAS accepted April 2012. Contains 30 figures and 11 tables. Tables
2 and 6 to be published in full and on-line only. The on-line tables can also
be obtained by contacting the author
- …