11,438 research outputs found
Improved design of electrophoretic equipment for rapid sickle-cell-anemia screening
Effective mass screening may be accomplished by modifying existing electrophoretic equipment in conjunction with multisample applicator used with cellulose-acetate-matrix test paper. Using this method, approximately 20 to 25 samples can undergo electrophoresis in 5 to 6 minutes
Effect of Electron-Electron Interactions on Rashba-like and Spin-Split Systems
The role of electron-electron interactions is analyzed for Rashba-like and
spin-split systems within a tight-binding single-band Hubbard model with
on-site and all nearest-neighbor matrix elements of the Coulomb interaction. By
Rashba-like systems we refer to the Dresselhaus and Rashba spin-orbit coupled
phases; spin-split systems have spin-up and spin-down Fermi surfaces shifted
relative to each other. Both systems break parity but preserve time-reversal
symmetry. They belong to a class of symmetry-breaking ground states that
satisfy: (i) electron crystal momentum is a good quantum number (ii) these
states have no net magnetic moment and (iii) their distribution of `polarized
spin' in momentum space breaks the lattice symmetry. In this class, the
relevant Coulomb matrix elements are found to be nearest-neighbor exchange ,
pair-hopping and nearest-neighbor repulsion . These ground states lower
their energy most effectively through , hence we name them Class states.
The competing effects of on the direct and exchange energies determine
the relative stability of Class states. We show that the spin-split and
Rashba-like phases are the most favored ground states within Class because
they have the minimum anisotropy in `polarized spin'. On a square lattice we
find that the spin-split phase is always favored for near-empty bands; above a
critical filling, we predict a transition from the paramagnetic to the
Rashba-like phase at and a second transition to the spin-split state
at . An energetic comparison with ferromagnetism highlights the
importance of the role of in the stability of Class states. We discuss
the relevance of our results to (i) the and phases proposed by
Wu and Zhang in the Fermi Liquid formalism and (ii) experimental observations
of spin-orbit splitting in \emph{Au}(111) surface states
Quasiparticle undressing in a dynamic Hubbard model: exact diagonalization study
Dynamic Hubbard models have been proposed as extensions of the conventional
Hubbard model to describe the orbital relaxation that occurs upon double
occupancy of an atomic orbital. These models give rise to pairing of holes and
superconductivity in certain parameter ranges. Here we explore the changes in
carrier effective mass and quasiparticle weight and in one- and two-particle
spectral functions that occur in a dynamic Hubbard model upon pairing, by exact
diagonalization of small systems. It is found that pairing is associated with
lowering of effective mass and increase of quasiparticle weight, manifested in
transfer of spectral weight from high to low frequencies in one- and
two-particle spectral functions. This 'undressing' phenomenology resembles
observations in transport, photoemission and optical experiments in high T_c
cuprates. This behavior is contrasted with that of a conventional electron-hole
symmetric Holstein-like model with attractive on-site interaction, where
pairing is associated with 'dressing' instead of 'undressing'
Electron-hole asymmetry is the key to superconductivity
In a solid, transport of electricity can occur via negative electrons or via
positive holes. In the normal state of superconducting materials experiments
show that transport is usually dominated by
. Instead, in the superconducting state experiments show that the
supercurrent is always carried by .
These experimental facts indicate that electron-hole asymmetry plays a
fundamental role in superconductivity, as proposed by the theory of hole
superconductivity.Comment: Presented at the New3SC-4 meeting, San Diego, Jan. 16-21 2003; to be
published in Int. J. Mod. Phys.
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Traffic flow densities in large transport networks
We consider transport networks with nodes scattered at random in a large
domain. At certain local rates, the nodes generate traffic flowing according to
some navigation scheme in a given direction. In the thermodynamic limit of a
growing domain, we present an asymptotic formula expressing the local traffic
flow density at any given location in the domain in terms of three fundamental
characteristics of the underlying network: the spatial intensity of the nodes
together with their traffic generation rates, and of the links induced by the
navigation. This formula holds for a general class of navigations satisfying a
link-density and a sub-ballisticity condition. As a specific example, we verify
these conditions for navigations arising from a directed spanning tree on a
Poisson point process with inhomogeneous intensity function.Comment: 20 pages, 7 figure
Large deviations in relay-augmented wireless networks
We analyze a model of relay-augmented cellular wireless networks. The network
users, who move according to a general mobility model based on a Poisson point
process of continuous trajectories in a bounded domain, try to communicate with
a base station located at the origin. Messages can be sent either directly or
indirectly by relaying over a second user. We show that in a scenario of an
increasing number of users, the probability that an atypically high number of
users experiences bad quality of service over a certain amount of time, decays
at an exponential speed. This speed is characterized via a constrained entropy
minimization problem. Further, we provide simulation results indicating that
solutions of this problem are potentially non-unique due to symmetry breaking.
Also two general sources for bad quality of service can be detected, which we
refer to as isolation and screening.Comment: 28 pages, 5 figures; corrected several misprint
- ā¦