405 research outputs found
Online Search Tool for Graphical Patterns in Electronic Band Structures
We present an online graphical pattern search tool for electronic band
structure data contained within the Organic Materials Database (OMDB) available
at https://omdb.diracmaterials.org/search/pattern. The tool is capable of
finding user-specified graphical patterns in the collection of thousands of
band structures from high-throughput ab initio calculations in the online
regime. Using this tool, it only takes a few seconds to find an arbitrary
graphical pattern within the ten electronic bands near the Fermi level for
26,739 organic crystals. The tool can be used to find realizations of
functional materials characterized by a specific pattern in their electronic
structure, for example, Dirac materials, characterized by a linear crossing of
bands; topological insulators, characterized by a "Mexican hat" pattern or an
effectively free electron gas, characterized by a parabolic dispersion. The
source code of the developed tool is freely available at
https://github.com/OrganicMaterialsDatabase/EBS-search and can be transferred
to any other electronic band structure database. The approach allows for an
automatic online analysis of a large collection of band structures where the
amount of data makes its manual inspection impracticable.Comment: 8 pages, 8 figure
Abrupt transition in quasiparticle dynamics at optimal doping in a cuprate superconductor system
We report time-resolved measurements of the photoinduced change in
reflectivity, Delta R, in the Bi2Sr2Ca(1-y)Dy(y)Cu2O(8+delta) (BSCCO) system of
cuprate superconductors as a function of hole concentration. We find that the
kinetics of quasiparticle decay and the sign of Delta R both change abruptly
where the superconducting transition temperature Tc is maximal. These
coincident changes suggest that a sharp transition in quasiparticle dynamics
takes place precisely at optimal doping in the BSCCO system.Comment: 10 pages, 4 figure
Ultrafast dynamics in the presence of antiferromagnetic correlations in electron-doped cuprate LaCeCuO
We used femtosecond optical pump-probe spectroscopy to study the photoinduced
change in reflectivity of thin films of the electron-doped cuprate
LaCeCuO (LCCO) with dopings of x0.08 (underdoped) and
x0.11 (optimally doped). Above T, we observe fluence-dependent
relaxation rates which onset at a similar temperature that transport
measurements first see signatures of antiferromagnetic correlations. Upon
suppressing superconductivity with a magnetic field, it is found that the
fluence and temperature dependence of relaxation rates is consistent with
bimolecular recombination of electrons and holes across a gap (2)
originating from antiferromagnetic correlations which comprise the pseudogap in
electron-doped cuprates. This can be used to learn about coupling between
electrons and high-energy () excitations in these
compounds and set limits on the timescales on which antiferromagnetic
correlations are static
Nonlinear optical probe of tunable surface electrons on a topological insulator
We use ultrafast laser pulses to experimentally demonstrate that the
second-order optical response of bulk single crystals of the topological
insulator BiSe is sensitive to its surface electrons. By performing
surface doping dependence measurements as a function of photon polarization and
sample orientation we show that second harmonic generation can simultaneously
probe both the surface crystalline structure and the surface charge of
BiSe. Furthermore, we find that second harmonic generation using
circularly polarized photons reveals the time-reversal symmetry properties of
the system and is surprisingly robust against surface charging, which makes it
a promising tool for spectroscopic studies of topological surfaces and buried
interfaces
Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO
The combination of ferromagnets with topological superconductors or
insulators allows for new phases of matter that support excitations such as
chiral edge modes and Majorana fermions. EuS, a wide-band-gap ferromagnetic
insulator with a Curie temperature around 16 K, and SrTiO (STO), an
important substrate for engineering heterostructures, may support these phases.
We present scanning superconducting quantum interference device (SQUID)
measurements of EuS grown epitaxially on STO that reveal micron-scale
variations in ferromagnetism and paramagnetism. These variations are oriented
along the STO crystal axes and only change their configuration upon thermal
cycling above the STO cubic-to-tetragonal structural transition temperature at
105 K, indicating that the observed magnetic features are due to coupling
between EuS and the STO tetragonal structure. We speculate that the STO
tetragonal distortions may strain the EuS, altering the magnetic anisotropy on
a micron-scale. This result demonstrates that local variation in the induced
magnetic order from EuS grown on STO needs to be considered when engineering
new phases of matter that require spatially homogeneous exchange
A catalog of stream processing optimizations
Cataloged from PDF version of article.Various research communities have independently arrived at stream processing as a programming model for efficient and parallel computing. These communities include digital signal processing, databases, operating systems, and complex event processing. Since each community faces applications with challenging performance requirements, each of them has developed some of the same optimizations, but often with conflicting terminology and unstated assumptions. This article presents a survey of optimizations for stream processing. It is aimed both at users who need to understand and guide the system's optimizer and at implementers who need to make engineering tradeoffs. To consolidate terminology, this article is organized as a catalog, in a style similar to catalogs of design patterns or refactorings. To make assumptions explicit and help understand tradeoffs, each optimization is presented with its safety constraints (when does it preserve correctness?) and a profitability experiment (when does it improve performance?). We hope that this survey will help future streaming system builders to stand on the shoulders of giants from not just their own community. © 2014 ACM
RailwayDB: adaptive storage of interaction graphs
We are living in an ever more connected world, where data recording the interactions between people, software systems, and the physical world is becoming increasingly prevalent. These data often take the form of a temporally evolving graph, where entities are the vertices and the interactions between them are the edges. We call such graphs interaction graphs. Various domains, including telecommunications, transportation, and social media, depend on analytics performed on interaction graphs. The ability to efficiently support historical analysis over interaction graphs requires effective solutions for the problem of data layout on disk. This paper presents an adaptive disk layout called the railway layout for optimizing disk block storage for interaction graphs. The key idea is to divide blocks into one or more sub-blocks. Each sub-block contains the entire graph structure, but only a subset of the attributes. This improves query I/O, at the cost of increased storage overhead. We introduce optimal integer linear program (ILP) formulations for partitioning disk blocks into sub-blocks with overlapping and nonoverlapping attributes. Additionally, we present greedy heuristics that can scale better compared to the ILP alternatives, yet achieve close to optimal query I/O. We provide an implementation of the railway layout as part of RailwayDB—an open-source graph database we have developed. To demonstrate the benefits of the railway layout, we provide an extensive experimental evaluation, including model-based as well as empirical results comparing our approach to baseline alternatives. © 2015, Springer-Verlag Berlin Heidelberg
River: an intermediate language for stream processing
Summary This paper presents both a calculus for stream processing, named Brooklet, and its realization as an intermediate language, named River. Because River is based on Brooklet, it has a formal semantics that enables reasoning about the correctness of source translations and optimizations. River builds on Brooklet by addressing the real-world details that the calculus elides. We evaluated our system by implementing front-ends for three streaming languages, and three important optimizations, and a back-end for the System S distributed streaming runtime. Overall, we significantly lower the barrier to entry for new stream-processing languages and thus grow the ecosystem of this crucial style of programming. Copyright © 2015 John Wiley & Sons, Ltd
Quantum correlations in a few-atom spin-1 Bose-Hubbard model
We study the thermal quantum correlations and entanglement in spin-1 Bose-Hubbard model with two and three particles. While we use negativity to calculate entanglement, more general non-classical correlations are quantified using a new measure based on a necessary and sufficient condition for zero-discord state. We demonstrate that the energy level crossings in the ground state of the system are signalled by both the behavior of thermal quantum correlations and entanglement
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