65 research outputs found
Comprehensive characterization of an open source document search engine
This work performs a thorough characterization and analysis of the open source Lucene search library. The article describes in detail the architecture, functionality, and micro-architectural behavior of the search engine, and investigates prominent online document search research issues. In particular, we study how intra-server index partitioning affects the response time and throughput, explore the potential use of low power servers for document search, and examine the sources of performance degradation ands the causes of tail latencies. Some of our main conclusions are the following: (a) intra-server index partitioning can reduce tail latencies but with diminishing benefits as incoming query traffic increases, (b) low power servers given enough partitioning can provide same average and tail response times as conventional high performance servers, (c) index search is a CPU-intensive cache-friendly application, and (d) C-states are the main culprits for performance degradation in document search.Web of Science162art. no. 1
Anharmonic lattice dynamics via the special displacement method
On the basis of the self-consistent phonon theory and the special
displacement method, we develop a new approach for the treatment of
anharmonicity in solids. We show that this approach enables the efficient
calculation of temperature-dependent anharmonic phonon dispersions, requiring
very few steps to achieve minimization of the system's free energy. We
demonstrate this methodology in the regime of strongly anharmonic materials
which exhibit a multi-well potential energy surface, like cubic SrTiO,
CsPbBr, CsPbI, CsSnI, and Zr. Our results are in good agreement
with experiments and previous first-principles studies relying on perturbative,
stochastic nonperturbative, and molecular dynamics simulations. We achieve a
very robust workflow by using harmonic phonons of the polymorphous ground state
as the starting point and an iterative mixing scheme of the dynamical matrix.
Given the simplicity, efficiency, and stability of the present treatment to
anharmonicity, it is especially suitable for use with any electronic structure
code and for investigating electron-phonon couplings in strongly anharmonic
systems.Comment: 10 figure
Economic impact and policy implications from urban shared transportation: The case of Pittsburgh’s shared bike system
During the last years the number of cities that have installed and started operating shared bike systems has significantly increased. These systems provide an alternative and sustainable mean of transportation to the city dwellers. Apart from the energy sustainability benefits, shared bike systems can have a positive effect on residents' health, air quality and the overall condition of the currently crumbling road network infrastructure. Anecdotal stories and survey studies have also identified that bike lanes have a positive impact on local businesses. In this study, driven by the rapid adoption of shared bike systems by city governments and their potential positive effects on a number of urban life facets we opt to study and quantify the value of these systems. We focus on a specific aspect of this value and use evidence from the real estate market in the city of Pittsburgh to analyze the effect on dwellers' properties of the shared bike system installed in the city in June 2015. We use quasi-experimental techniques and find that the shared bike system led to an increase in the housing prices (both sales and rental prices) in the zip codes where shared bike stations were installed. We further bring into the light potential negative consequences of this impact (i.e., gentrification) and discuss/propose two public policies that can exploit the impact of the system for the benefit of both the local government as well as the city dwellers
A machine learning route between band mapping and band structure
The electronic band structure (BS) of solid state materials imprints the
multidimensional and multi-valued functional relations between energy and
momenta of periodically confined electrons. Photoemission spectroscopy is a
powerful tool for its comprehensive characterization. A common task in
photoemission band mapping is to recover the underlying quasiparticle
dispersion, which we call band structure reconstruction. Traditional methods
often focus on specific regions of interests yet require extensive human
oversight. To cope with the growing size and scale of photoemission data, we
develop a generic machine-learning approach leveraging the information within
electronic structure calculations for this task. We demonstrate its capability
by reconstructing all fourteen valence bands of tungsten diselenide and
validate the accuracy on various synthetic data. The reconstruction uncovers
previously inaccessible momentum-space structural information on both global
and local scales in conjunction with theory, while realizing a path towards
integrating band mapping data into materials science databases
Electron-phonon physics from first principles using the EPW code
EPW is an open-source software for calculations of
electron-phonon interactions and related materials properties. The code
combines density functional perturbation theory and maximally-localized Wannier
functions to efficiently compute electron-phonon coupling matrix elements on
ultra-fine Brillouin zone grids. This data is employed for predictive
calculations of temperature-dependent properties and phonon-assisted quantum
processes in bulk solids and low-dimensional materials. Here, we report on
significant new developments in the code that occurred during the period
2016-2022, namely: a transport module for the calculation of charge carrier
mobility and conductivity under electric and magnetic fields within the
Boltzmann transport equation; a superconductivity module
for the calculation of critical temperature and gap structure in
phonon-mediated superconductors within the anisotropic
multi-band Eliashberg theory; an optics module for calculations of
phonon-assisted indirect transitions; a module for the calculation of small and
large polarons without supercells using the polaron
equations; and a module for calculating electron-phonon couplings, band
structure renormalization, and temperature-dependent optical spectra using the
special displacement method. For each capability, we outline the methodology
and implementation, and provide example calculations. We describe recent code
refactoring to prepare EPW for exascale architectures, we discuss efficient
parallelization strategies, and report on extreme parallel scaling tests.Comment: 61 pages, 9 figure
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