30 research outputs found
Composite symmetry protected topological order and effective models
Strongly correlated quantum many-body systems at low dimension exhibit a
wealth of phenomena, ranging from features of geometric frustration to
signatures of symmetry-protected topological order. In suitable descriptions of
such systems, it can be helpful to resort to effective models which focus on
the essential degrees of freedom of the given model. In this work, we analyze
how to determine the validity of an effective model by demanding it to be in
the same phase as the original model. We focus our study on one-dimensional
spin-1/2 systems and explain how non-trivial symmetry protected topologically
ordered (SPT) phases of an effective spin 1 model can arise depending on the
couplings in the original Hamiltonian. In this analysis, tensor network methods
feature in two ways: On the one hand, we make use of recent techniques for the
classification of SPT phases using matrix product states in order to identify
the phases in the effective model with those in the underlying physical system,
employing Kuenneth's theorem for cohomology. As an intuitive paradigmatic model
we exemplify the developed methodology by investigating the bi-layered
delta-chain. For strong ferromagnetic inter-layer couplings, we find the system
to transit into exactly the same phase as an effective spin 1 model. However,
for weak but finite coupling strength, we identify a symmetry broken phase
differing from this effective spin-1 description. On the other hand, we
underpin our argument with a numerical analysis making use of matrix product
states.Comment: 13 pages, 6 figure
A route towards engineering many-body localization in real materials
The interplay of interactions and disorder in a quantum many body system may
lead to the elusive phenomenon of many body localization (MBL). It has been
observed under precisely controlled conditions in synthetic quantum many-body
systems, but to detect it in actual quantum materials seems challenging. In
this work, we present a path to synthesize real materials that show signatures
of many body localization by mixing different species of materials in the
laboratory. To provide evidence for the functioning of our approach, we perform
a detailed tensor-network based numerical analysis to study the effects of
various doping ratios of the constituting materials. Moreover, in order to
provide guidance to experiments, we investigate different choices of actual
candidate materials. To address the challenge of how to achieve stability under
heating, we study the effect of the electron-phonon coupling, focusing on
effectively one dimensional materials embedded in one, two and three
dimensional lattices. We analyze how this coupling affects the MBL and provide
an intuitive microscopic description of the interplay between the electronic
degrees of freedom and the lattice vibrations. Our work provides a guideline
for the necessary conditions on the properties of the ingredient materials and,
as such, serves as a road map to experimentally synthesizing real quantum
materials exhibiting signatures of MBL.Comment: 12 pages, 7 figure
Ginzburg-Landau Theory for the Jaynes-Cummings-Hubbard Model
We develop a Ginzburg-Landau theory for the Jaynes-Cummings-Hubbard model
which effectively describes both static and dynamic properties of photons
evolving in a cubic lattice of cavities, each filled with a two-level atom. To
this end we calculate the effective action to first-order in the hopping
parameter. Within a Landau description of a spatially and temporally constant
order parameter we calculate the finite-temperature mean-field quantum phase
boundary between a Mott insulating and a superfluid phase of polaritons.
Furthermore, within the Ginzburg-Landau description of a spatio-temporal
varying order parameter we determine the excitation spectra in both phases and,
in particular, the sound velocity of light in the superfluid phase
Efficient variational contraction of two dimensional tensor networks with a non trivial unit cell
Tensor network states provide an efficient class of states that faithfully
capture strongly correlated quantum models and systems in classical statistical
mechanics. While tensor networks can now be seen as becoming standard tools in
the description of such complex many-body systems, close to optimal variational
principles based on such states are less obvious to come by. In this work, we
generalize a recently proposed variational uniform matrix product state
algorithm for capturing one-dimensional quantum lattices in the thermodynamic
limit, to the study of regular two-dimensional tensor networks with a
non-trivial unit cell. A key property of the algorithm is a computational
effort that scales linearly rather than exponentially in the size of the unit
cell. We demonstrate the performance of our approach on the computation of the
classical partition functions of the antiferromagnetic Ising model and
interacting dimers on the square lattice, as well as of a quantum doped
resonating valence bond state.Comment: 23 pages, 8 Figure
The High Time Resolution Universe Survey VI: An Artificial Neural Network and Timing of 75 Pulsars
We present 75 pulsars discovered in the mid-latitude portion of the High Time
Resolution Universe survey, 54 of which have full timing solutions. All the
pulsars have spin periods greater than 100 ms, and none of those with timing
solutions are in binaries. Two display particularly interesting behaviour; PSR
J1054-5944 is found to be an intermittent pulsar, and PSR J1809-0119 has
glitched twice since its discovery.
In the second half of the paper we discuss the development and application of
an artificial neural network in the data-processing pipeline for the survey. We
discuss the tests that were used to generate scores and find that our neural
network was able to reject over 99% of the candidates produced in the data
processing, and able to blindly detect 85% of pulsars. We suggest that
improvements to the accuracy should be possible if further care is taken when
training an artificial neural network; for example ensuring that a
representative sample of the pulsar population is used during the training
process, or the use of different artificial neural networks for the detection
of different types of pulsars.Comment: 15 pages, 8 figure
VizieR Online Data Catalog: HTRU survey. Timing of 54 pulsars (Bates+, 2012)
All the pulsars presented here were discovered in the HTRU mid-latitude survey, which has now been fully processed. The survey observed the Galactic plane in the region -120°-35° were regularly observed using the 76-m Lovell Telescope and those below this declination were observed as part of the HTRU timing programme at Parkes. (3 data files)
Entomological papers, being chiefly descriptions of new Ceylon Coleoptera with such observations on their habits etc., as appear in any way interesting
Volume: 25Start Page: 381End Page: 39