290 research outputs found
A Nearby Supernovae Search: Eros2
Type Ia supernovae (SNIa) have been used as approximate standard candles to
measure cosmological parameters such as the Hubble constant and the
deceleration parameter. These measurements rely on empirical correlations
between peak luminosities and other features that can be observed in the
supernovae spectra and their light curves. Such correlations deserve further
study since they have been established from small samples of nearby SNIa. Two
years ago, the EROS2 collaboration launched an automated search for supernovae
with the 1m Marly telescope operating at La Silla. In all, 57 SNe have been
discovered in this EROS2 search and spectra have been obtained for 26 of them.
We found that 75% were of type Ia and 25% of type II. Using this sample, a
preliminary SN explosion rate has been obtained. Our most recent observation
campaign took place in February and March 99. It was performed in the framework
of a large consortium led by the {\em Supernova Cosmology Project}. The aim of
this intensive campaign was to provide an independent set of high quality light
curves and spectra to study systematic effects in the measurement of
cosmological parameters. We will briefly describe our search procedure and
present the status of our ongoing analysis.Comment: 5 page
Quantum Hall fractions in ultracold fermionic vapors
We study the quantum Hall states that appear in the dilute limit of rotating
ultracold fermionic gases when a single hyperfine species is present. We show
that the p-wave scattering translates into a pure hard-core interaction in the
lowest Landau level. The Laughlin wavefunction is then the exact ground state
at filling fraction nu=1/3. We give estimates of some of the gaps of the
incompressible liquids for nu = p/(2p+-1). We estimate the mass of the
composite fermions at nu =1/2. The width of the quantum Hall plateaus is
discussed by considering the equation of state of the system.Comment: RevTex, 4 pages, 3 fig
A simple anisotropic three-dimensional quantum spin liquid with fracton topological order
We present a three-dimensional cubic lattice spin model, anisotropic in the
direction, that exhibits fracton topological order. The latter is a
novel type of topological order characterized by the presence of immobile
pointlike excitations, named fractons, residing at the corners of an operator
with two-dimensional support. As other recent fracton models, ours exhibits a
subextensive ground state degeneracy: On an
three-torus, it has a topological degeneracy, and an additional
non-topological degeneracy equal to . The fractons can be
combined into composite excitations that move either in a straight line along
the direction, or freely in the plane at a given height .
While our model draws inspiration from the toric code, we demonstrate that it
cannot be adiabatically connected to a layered toric code construction.
Additionally, we investigate the effects of imposing open boundary conditions
on our system. We find zero energy modes on the surfaces perpendicular to
either the or directions, and their absence on the surfaces
normal to . This result can be explained using the properties of the
two kinds of composite two-fracton mobile excitations.Comment: 8 pages, 9 figure
Characterization of quasiholes in two-component fractional quantum Hall states and fractional Chern insulators in flat bands
We perform an exact-diagonalization study of quasihole excitations for the
two-component Halperin state in the lowest Landau level and for several
bosonic fractional Chern insulators in topological flat bands with
Chern number . Properties including the quasihole size, charge, and
braiding statistics are evaluated. For the Halperin model state, we
observe isotropic quasiholes with a clear internal structure, and obtain the
quasihole charge and statistics matching the theoretical values. Interestingly,
we also extract the same quasihole size, charge, and braiding statistics for
the continuum model states of fractional Chern insulators, although the
latter possess a "color-entangled" nature that does not exist in ordinary
two-component Halperin states. We also consider two real lattice models with a
band having . There, we find that a quasihole can exhibit much stronger
oscillations of the density profile, while having the same charge and
statistics as those in the continuum models.Comment: 11 pages, 10 figures, small changes in the text related to the review
process (mostly improved presentation of the color-entangled BC), added
bibliographical detail
Entanglement signatures of quantum Hall phase transitions
We study quantum phase transitions involving fractional quantum Hall states,
using numerical calculations of entanglements and related quantities. We tune
finite-size wavefunctions on spherical geometries, by varying the interaction
potential away from the Coulomb interaction. We uncover signatures of quantum
phase transitions contained in the scaling behavior of the entropy of
entanglement between two parts of the sphere. In addition to the entanglement
entropy, we show that signatures of quantum phase transitions also appear in
other aspects of the reduced density matrix of one part of the sphere.Comment: 8 pages, 7 figure
Probing many-body localization with neural networks
We show that a simple artificial neural network trained on entanglement
spectra of individual states of a many-body quantum system can be used to
determine the transition between a many-body localized and a thermalizing
regime. Specifically, we study the Heisenberg spin-1/2 chain in a random
external field. We employ a multilayer perceptron with a single hidden layer,
which is trained on labeled entanglement spectra pertaining to the fully
localized and fully thermal regimes. We then apply this network to classify
spectra belonging to states in the transition region. For training, we use a
cost function that contains, in addition to the usual error and regularization
parts, a term that favors a confident classification of the transition region
states. The resulting phase diagram is in good agreement with the one obtained
by more conventional methods and can be computed for small systems. In
particular, the neural network outperforms conventional methods in classifying
individual eigenstates pertaining to a single disorder realization. It allows
us to map out the structure of these eigenstates across the transition with
spatial resolution. Furthermore, we analyze the network operation using the
dreaming technique to show that the neural network correctly learns by itself
the power-law structure of the entanglement spectra in the many-body localized
regime.Comment: 12 pages, 10 figure
Variational Ansatz for an Abelian to non-Abelian Topological Phase Transition in Bilayers
We propose a one-parameter variational ansatz to describe the
tunneling-driven Abelian to non-Abelian transition in bosonic
fractional quantum Hall bilayers. This ansatz, based on exact matrix product
states, captures the low-energy physics all along the transition and allows to
probe its characteristic features. The transition is continuous, characterized
by the decoupling of antisymmetric degrees of freedom. We futhermore determine
the tunneling strength above which non-Abelian statistics should be observed
experimentally. Finally, we propose to engineer the inter-layer tunneling to
create an interface trapping a neutral chiral Majorana. We microscopically
characterize such an interface using a slightly modified model wavefunction.Comment: 5 pages, 4 Figures and Supplementary Materials. Comments are welcome
Defining a bulk-edge correspondence for non-Hermitian Hamiltonians via singular-value decomposition
We address the breakdown of the bulk-boundary correspondence observed in
non-Hermitian systems, where open and periodic systems can have distinct phase
diagrams. The correspondence can be completely restored by considering the
Hamiltonian's singular value decomposition instead of its eigendecomposition.
This leads to a natural topological description in terms of a flattened
singular decomposition. This description is equivalent to the usual approach
for Hermitian systems and coincides with a recent proposal for the
classification of non-Hermitian systems. We generalize the notion of the
entanglement spectrum to non-Hermitian systems, and show that the edge physics
is indeed completely captured by the periodic bulk Hamiltonian. We exemplify
our approach by considering the chiral non-Hermitian Su-Schrieffer-Heger and
Chern insulator models. Our work advocates a different perspective on
topological non-Hermitian Hamiltonians, paving the way to a better
understanding of their entanglement structure.Comment: 6+5 pages, 8 figure
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