4,040 research outputs found
Piezoviscous effects in nonconformal contacts lubricated hydrodynamically
The analysis is concerned with the piezoviscous-rigid regime of lubrication for the general case of elliptical contacts. In this regime several formulas of the lubricant film thickness have been proposed by Hamrock and Dowson, by Dowson et al., and more recently by Houpert. However, either they do not include the load parameter W, which has a strong effect on film thickness, or they overestimate the film thickness by using the Barus formula for pressure-viscosity characteristics. The Roelands formula was used for the pressure-viscosity relationship. The effects of the dimensionless load, speed, and materials parameters, the radius ratio, and the lubricant entrainment direction were investigated. The dimensionless load parameter was varied over a range of one order of magnitude. The dimensionless speed parameter was varied by 5.6 times the lowest value. Conditions corresponding to the use of solid materials of steel, bronze, and silicon nitride and lubricants of paraffinic and naphthenic mineral oil were considered in obtaining the exponent in the dimensionless materials parameter. The radius ratio was varied from 0.2 to 64 (a configuration approaching a line contact). Forty-one cases were used in obtaining a minimum film thickness formula. Contour plots indicate in detail the pressure developed between the contacting solids
Social science data repositories in data deluge A case study of ICPSR's workflow and practices
Purpose - Owing to the recent surge of interest in the age of the data deluge, the importance of researching data infrastructures is increasing. The open archival information system (OAIS) model has been widely adopted as a framework for creating and maintaining digital repositories. Considering that OAIS is a reference model that requires customization for actual practice, this paper aims to examine how the current practices in a data repository map to the OAIS environment and functional components. Design/methodology/approach - The authors conducted two focus-group sessions and one individual interview with eight employees at the world's largest social science data repository, the Interuniversity Consortium for Political and Social Research (ICPSR). By examining their current actions (activities regarding their work responsibilities) and IT practices, they studied the barriers and challenges of archiving and curating qualitative data at ICPSR. Findings - The authors observed that the OAIS model is robust and reliable in actual service processes for data curation and data archives. In addition, a data repository's workflow resembles digital archives or even digital libraries. On the other hand, they find that the cost of preventing disclosure risk and a lack of agreement on the standards of text data files are the most apparent obstacles for data curation professionals to handle qualitative data; the maturation of data metrics seems to be a promising solution to several challenges in social science data sharing. Originality/value - The authors evaluated the gap between a research data repository's current practices and the adoption of the OAIS model. They also identified answers to questions such as how current technological infrastructure in a leading data repository such as ICPSR supports their daily operations, what the ideal technologies in those data repositories would be and the associated challenges that accompany these ideal technologies. Most importantly, they helped to prioritize challenges and barriers from the data curator's perspective and to contribute implications of data sharing and reuse in social sciences
Orbital-Free Density Functional Theory: Kinetic Potentials and Ab-Initio Local Pseudopotentials
In the density functional (DF) theory of Kohn and Sham, the kinetic energy of
the ground state of a system of noninteracting electrons in a general external
field is calculated using a set of orbitals. Orbital free methods attempt to
calculate this directly from the electron density by approximating the
universal but unknown kinetic energy density functional. However simple local
approximations are inaccurate and it has proved very difficult to devise
generally accurate nonlocal approximations. We focus instead on the kinetic
potential, the functional derivative of the kinetic energy DF, which appears in
the Euler equation for the electron density. We argue that the kinetic
potential is more local and more amenable to simple physically motivated
approximations in many relevant cases, and describe two pathways by which the
value of the kinetic energy can be efficiently calculated. We propose two
nonlocal orbital free kinetic potentials that reduce to known exact forms for
both slowly varying and rapidly varying perturbations and also reproduce exact
results for the linear response of the density of the homogeneous system to
small perturbations. A simple and systematic approach for generating accurate
and weak ab-initio local pseudopotentials which produce a smooth slowly varying
valence component of the electron density is proposed for use in orbital free
DF calculations of molecules and solids. The use of these local
pseudopotentials further minimizes the possible errors from the kinetic
potentials. Our theory yields results for the total energies and ionization
energies of atoms, and for the shell structure in the atomic radial density
profiles that are in very good agreement with calculations using the full
Kohn-Sham theory.Comment: To be published in Phys. Rev.
Selective interlayer ferromagnetic coupling between the Cu spins in YBa Cu O grown on top of La Ca MnO
Studies to date on ferromagnet/d-wave superconductor heterostructures focus
mainly on the effects at or near the interfaces while the response of bulk
properties to heterostructuring is overlooked. Here we use resonant soft x-ray
scattering spectroscopy to reveal a novel c-axis ferromagnetic coupling between
the in-plane Cu spins in YBa Cu O (YBCO) superconductor when it
is grown on top of ferromagnetic La Ca MnO (LCMO) manganite
layer. This coupling, present in both normal and superconducting states of
YBCO, is sensitive to the interfacial termination such that it is only observed
in bilayers with MnO_2but not with La Ca interfacial
termination. Such contrasting behaviors, we propose, are due to distinct
energetic of CuO chain and CuO plane at the La Ca and
MnO terminated interfaces respectively, therefore influencing the transfer
of spin-polarized electrons from manganite to cuprate differently. Our findings
suggest that the superconducting/ferromagnetic bilayers with proper interfacial
engineering can be good candidates for searching the theorized
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state in cuprates and studying the
competing quantum orders in highly correlated electron systems.Comment: Please note the change of the title. Text might be slightly different
from the published versio
Laboratory experimental study of ocean waves propagating over a partially buried pipeline in a trench layer
YesSeabed instability around a pipeline is one of the primary concerns in offshore pipeline projects. To date, most studies focus on investigating the wave/current-induced response within a porous seabed around either a fully buried pipeline or a thoroughly exposed one. In this study, unlike previous investigations, a series of comprehensive laboratory experiments are carried out in a wave flume to investigate the wave-induced pore pressures around a partially embedded pipeline in a trench layer. Measurements show that the presence of the partially buried pipeline can significantly affect the excess pore pressure in a partially backfilled trench layer, which deviates considerably from that predicted by the theoretical approach. The morphology of the trench layer accompanied with the backfill sediments, especially the deeper trench and thicker backfill (i.e.,b≥1D,e≥0.5D), provides a certain degree of resistance to seabed instability. The amplitude of excess pore pressure around the trench layer roughly exhibits a left-right asymmetric distribution along the periphery of the pipeline, and decays sharply from the upper layer of the trench to the lower region. Deeper trench depth and thicker buried layer significantly weaken the pore-water pressures in the whole trench area, thus sheltering and protecting the submarine pipeline against the transient seabed liquefaction.The National Key research and development program of China (2017YFC1404200), the research grants of Jiangsu (BK20150804), the marine renewable energy research project of State Oceanic Administration (GHME2015GC01), Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (Project No: 2016491011), the Royal Academy of Engineering the Distinguished Visiting Fellowship (DVF1718-8-7
Aharonov-Bohm Oscillations with Spin: Evidence for Berry's Phase
We report a study of the Aharonov-Bohm effect, the oscillations of the
resistance of a mesoscopic ring as a function of a perpendicular magnetic
field, in a GaAs two-dimensional hole system with a strong spin-orbit
interaction. The Fourier spectra of the oscillations reveal extra structure
near the main peak whose frequency corresponds to the magnetic flux enclosed by
the ring. A comparison of the experimental data with results of simulations
demonstrates that the origin of the extra structure is the geometric (Berry)
phase acquired by the carrier spin as it travels around the ring.Comment: To be published in Physical Review Letter
Explicit characterization of the identity configuration in an Abelian Sandpile Model
Since the work of Creutz, identifying the group identities for the Abelian
Sandpile Model (ASM) on a given lattice is a puzzling issue: on rectangular
portions of Z^2 complex quasi-self-similar structures arise. We study the ASM
on the square lattice, in different geometries, and a variant with directed
edges. Cylinders, through their extra symmetry, allow an easy determination of
the identity, which is a homogeneous function. The directed variant on square
geometry shows a remarkable exact structure, asymptotically self-similar.Comment: 11 pages, 8 figure
Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES
The Kondo insulator SmB6 has long been known to exhibit low temperature (T <
10K) transport anomaly and has recently attracted attention as a new
topological insulator candidate. By combining low-temperature and high
energy-momentum resolution of the laser-based ARPES technique, for the first
time, we probe the surface electronic structure of the anomalous conductivity
regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and
identify in-gap low-lying states within a 4 meV window of the Fermi level on
the (001)-surface of this material. The low-lying states are found to form
electron-like Fermi surface pockets that enclose the X and the Gamma points of
the surface Brillouin zone. These states disappear as temperature is raised
above 15K in correspondence with the complete disappearance of the 2D
conductivity channels in SmB6. While the topological nature of the in-gap
metallic states cannot be ascertained without spin (spin-texture) measurements
our bulk and surface measurements carried out in the
transport-anomaly-temperature regime (T < 10K) are consistent with the
first-principle predicted Fermi surface behavior of a topological Kondo
insulator phase in this material.Comment: 4 Figures, 6 Page
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