15,932 research outputs found
Local physics of magnetization plateaux in the Shastry-Sutherland model
We address the physical mechanism responsible for the emergence of
magnetization plateaux in the Shastry-Sutherland model. By using a hierarchical
mean-field approach we demonstrate that a plateau is stabilized in a certain
{\it spin pattern}, satisfying {\it local} commensurability conditions derived
from our formalism. Our results provide evidence in favor of a robust local
physics nature of the plateaux states, and are in agreement with recent NMR
experiments on \scbo.Comment: 4 pages, LaTeX 2
Stripes, topological order, and deconfinement in a planar t-Jz model
We determine the quantum phase diagram of a two-dimensional bosonic t-Jz
model as a function of the lattice anisotropy gamma, using a quantum Monte
Carlo loop algorithm. We show analytically that the low-energy sectors of the
bosonic and the fermionic t-Jz models become equivalent in the limit of small
gamma. In this limit, the ground state represents a static stripe phase
characterized by a non-zero value of a topological order parameter. This phase
remains up to intermediate values of gamma, where there is a quantum phase
transition to a phase-segregated state or a homogeneous superfluid with dynamic
stripe fluctuations depending on the ratio Jz/t.Comment: 4 pages, 5 figures (2 in color). Final versio
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
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
Microscopic Scenario for Striped Superconductors
We argue that the superconducting state found in high- cuprates is
inhomogeneous with a corresponding inhomogeneous superfluid density. We
introduce two classes of microscopic models which capture the magnetic and
superconducting properties of these strongly correlated materials. We start
from a generalized t-J model, in which appropriate inhomogeneous terms mimic
stripes. We find that inhomogeneous interactions that break magnetic symmetries
are essential to induce substantial pair binding of holes in the thermodynamic
limit. We argue that this type of model reproduces the ARPES and neutron
scattering data seen experimentally.Comment: 4 pages, 2 psfigures. To appear in Physica
Zero Temperature Phases of the Electron Gas
The stability of different phases of the three-dimensional non-relativistic
electron gas is analyzed using stochastic methods. With decreasing density, we
observe a {\it continuous} transition from the paramagnetic to the
ferromagnetic fluid, with an intermediate stability range () for the {\it partially} spin-polarized liquid. The freezing
transition into a ferromagnetic Wigner crystal occurs at . We
discuss the relative stability of different magnetic structures in the solid
phase, as well as the possibility of disordered phases.Comment: 4 pages, REVTEX, 3 ps figure
A photometric search for active Main Belt asteroids
It is well known that some Main Belt asteroids show comet-like features. A
representative example is the first known Main Belt comet 133P/(7968)
Elst-Pizarro. If the mechanisms causing this activity are too weak to develop
visually evident comae or tails, the objects stay unnoticed. We are presenting
a novel way to search for active asteroids, based on looking for objects with
deviations from their expected brightnesses in a database. Just by using the
MPCAT-OBS Observation Archive we have found five new candidate objects that
possibly show a type of comet-like activity, and the already known Main Belt
comet 133P/(7968) Elst-Pizarro. Four of the new candidates, (315) Constantia,
(1026) Ingrid, (3646) Aduatiques, and (24684) 1990 EU4, show brightness
deviations independent of the object's heliocentric distance, while (35101)
1991 PL16 shows deviations dependent on its heliocentric distance, which could
be an indication of a thermal triggered mechanism. The method could be
implemented in future sky survey programmes to detect outbursts on Main Belt
objects almost simultaneously with their occurrence.Comment: 8 pages, 10 figures. Accepted for publication in A&A on December 20,
201
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
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