2,078 research outputs found
Apparent slow dynamics in the ergodic phase of a driven many-body localized system without extensive conserved quantities
We numerically study the dynamics on the ergodic side of the many-body
localization transition in a periodically driven Floquet model with no global
conservation laws. We describe and employ a numerical technique based on the
fast Walsh-Hadamard transform that allows us to perform an exact time evolution
for large systems and long times. As in models with conserved quantities (e.g.,
energy and/or particle number) we observe a slowing down of the dynamics as the
transition into the many-body localized phase is approached. More specifically,
our data is consistent with a subballistic spread of entanglement and a
stretched-exponential decay of an autocorrelation function, with their
associated exponents reflecting slow dynamics near the transition for a fixed
system size. However, with access to larger system sizes, we observe a clear
flow of the exponents towards faster dynamics and can not rule out that the
slow dynamics is a finite-size effect. Furthermore, we observe examples of
non-monotonic dependence of the exponents with time, with dynamics initially
slowing down but accelerating again at even larger times, consistent with the
slow dynamics being a crossover phenomena with a localized critical point.Comment: 9 pages, 8 figures; added details on the level statistics and the
energy absorptio
Quantum Mutual Information as a Probe for Many-Body Localization
We demonstrate that the quantum mutual information (QMI) is a useful probe to
study many-body localization (MBL). First, we focus on the detection of a
metal--insulator transition for two different models, the noninteracting
Aubry-Andr\'e-Harper model and the spinless fermionic disordered Hubbard chain.
We find that the QMI in the localized phase decays exponentially with the
distance between the regions traced out, allowing us to define a correlation
length, which converges to the localization length in the case of one particle.
Second, we show how the QMI can be used as a dynamical indicator to distinguish
an Anderson insulator phase from an MBL phase. By studying the spread of the
QMI after a global quench from a random product state, we show that the QMI
does not spread in the Anderson insulator phase but grows logarithmically in
time in the MBL phase.Comment: 4+2 pages, 5+5 figure
Many-body localization characterized from a one-particle perspective
We show that the one-particle density matrix can be used to
characterize the interaction-driven many-body localization transition in closed
fermionic systems. The natural orbitals (the eigenstates of ) are
localized in the many-body localized phase and spread out when one enters the
delocalized phase, while the occupation spectrum (the set of eigenvalues of
) reveals the distinctive Fock-space structure of the many-body
eigenstates, exhibiting a step-like discontinuity in the localized phase. The
associated one-particle occupation entropy is small in the localized phase and
large in the delocalized phase, with diverging fluctuations at the transition.
We analyze the inverse participation ratio of the natural orbitals and find
that it is independent of system size in the localized phase.Comment: 5 pages, 3 figures; v2: added two appendices and a new figure panel
in main text; v3: updated figur
Sharp entanglement thresholds in the logarithmic negativity of disjoint blocks in the transverse-field Ising chain
Entanglement has developed into an essential concept for the characterization
of phases and phase transitions in ground states of quantum many-body systems.
In this work, we use the logarithmic negativity to study the spatial
entanglement structure in the transverse-field Ising chain both in the ground
state and at nonzero temperatures. Specifically, we investigate the
entanglement between two disjoint blocks as a function of their separation,
which can be viewed as the entanglement analog of a spatial correlation
function. We find sharp entanglement thresholds at a critical distance beyond
which the logarithmic negativity vanishes exactly and thus the two blocks
become unentangled, which holds even in the presence of long-ranged quantum
correlations, i.e., at the system's quantum critical point. Using Time-Evolving
Block Decimation (TEBD), we explore this feature as a function of temperature
and size of the two blocks and present a simple model to describe our numerical
observations.Comment: 12 pages, 7 figure
XPS Characterization and Microhardness of Heat treated Co-W Coatings Electrodeposited with Gluconate Bath
Thermal stability and effect of heat treatment on electronic structure and microhardness of electrodeposited CoW alloy coatings using gluconate bath was characterized by DSC and XPS. XPS studies demonstrate that as-deposited alloy coating has significant amount of Co and W metals as well as Co2+ and W6+ species. There is a decrease in Co metal concentration in the alloy heated at 600 C and Co is in fully oxidized form when it is heat treated at 800 C. Marginal decrease in W metal concentration and presence of both W6+ and W5+ species are observed when the coating is heated at 600 C, whereas mostly W6+ species along with a little amount of W5+ could be seen in the coating heated at 800 C. Microhardness values of 1017 and 1336 HK are observed when Co−W coatings are heated at 500 and 600 C, respectively and they are comparable with as-deposited hard ch
Tactile sensors for robot handling
First and second generation robots have been used cost effectively in high‐volume ‘fixed’ or ‘hard’ automated manufacturing/assembly systems. They are ‘limited‐ability’ devices using simple logic elements or primitive sensory feedback. However, in the unstructured environment of most manufacturing plants it is often necessary to locate, identify, orientate and position randomly presented components.
Visual systems have been researched and developed to provide a coarse resolution outline of objects. More detailed and precise definition of parts is usually obtained by high resolution tactile sensing arrays. This paper reviews and discusses the current state of the art in tactile sensing
Perturbative and non-perturbative studies with the delta function potential
We show that the delta function potential can be exploited along with
perturbation theory to yield the result of certain infinite series. The idea is
that any exactly soluble potential if coupled with a delta function potential
remains exactly soluble. We use the strength of the delta function as an
expansion parameter and express the second-order energy shift as an infinite
sum in perturbation theory. The analytical solution is used to determine the
second-order energy shift and hence the sum of an infinite series. By an
appropriate choice of the unperturbed system, we can show the importance of the
continuum in the energy shift of bound states.Comment: 19 pages, 2 table
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been
investigated by single crystal inelastic neutron scattering and density
functional theory (DFT) calculations on the model compound SrNiVO.
Significant effects on low energy excitation spectra are found where the
Haldane gap (; where is the intrachain exchange
interaction) is replaced by three energy minima at different antiferromagnetic
zone centers due to the complex interchain couplings. Further, the triplet
states are split into two branches by single-ion anisotropy. Quantitative
information on the intrachain and interchain interactions as well as on the
single-ion anisotropy are obtained from the analyses of the neutron scattering
spectra by the random phase approximation (RPA) method. The presence of
multiple competing interchain interactions is found from the analysis of the
experimental spectra and is also confirmed by the DFT calculations. The
interchain interactions are two orders of magnitude weaker than the
nearest-neighbour intrachain interaction = 8.7~meV. The DFT calculations
reveal that the dominant intrachain nearest-neighbor interaction occurs via
nontrivial extended superexchange pathways Ni--O--V--O--Ni involving the empty
orbital of V ions. The present single crystal study also allows us to
correctly position SrNiVO in the theoretical - phase
diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)] showing where
it lies within the spin-liquid phase.Comment: 12 pages, 12 figures, 3 tables PRB (accepted). in Phys. Rev. B (2015
Collaborative Epistemic Discourse in Classroom Information Seeking Tasks
We discuss the relationship between information seeking, and epistemic beliefs – beliefs about the source, structure, complexity, and stability of knowledge – in the context of collaborative information seeking discourses. We further suggest that both information seeking, and epistemic cognition research agendas have suffered from a lack of attention to how information seeking as a collaborative activity is mediated by talk between partners – an area we seek to address in this paper. A small-scale observational study using sociocultural discourse analysis was conducted with eight eleven year old pupils who carried out search engine tasks in small groups. Qualitative and quantitative analysis were performed on their discussions using sociocultural discourse analytic techniques. Extracts of the dialogue are reported, informed by concordance analysis and quantitative coding of dialogue duration. We find that 1) discourse which could be characterised as ‘epistemic’ is identifiable in student talk, 2) that it is possible to identify talk which is more or less productive, and 3) that epistemic talk is associated with positive learning outcomes
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