581 research outputs found
Energy spectrum for two-dimensional potentials in very high magnetic fields
A method, analogous to supersymmetry transformation in quantum mechanics, is
developed for a particle in the lowest Landau level moving in an arbitrary
potential. The method is applied to two-dimensional potentials formed by Dirac
delta scattering centers. In the periodic case, the problem is solved exactly
for rational values of the magnetic flux (in units of flux quantum) per unit
cell. The spectrum is found to be self-similar, resembling the Hofstadter
butterfly.Comment: 9 pages, 3 figures, REVTEX, to appear in Phys. Rev. B, Sep. 1
Suppression of superconductivity in high- cuprates due to nonmagnetic impurities: Implications for the order parameter symmetry
We studied the effects of nonmagnetic impurities on high-temperature
superconductors by solving the Bogoliubov-de Gennes equations on a
two-dimensional lattice via exact diagonalization technique in a fully
self-consistent way. We found that s-wave order parameter is almost unaffected
by impurities at low concentrations while -wave order parameter
exhibits a strong linear decrease with impurity concentration. We evaluated the
critical impurity concentration at which superconductivity ceases to be
0.1 which is in good agreement with experimental values. We also investigated
how the orthorhombic nature of the crystal structure affects the suppression of
superconductivity and found that anisotropy induces an additional s-wave
component. Our results support -wave symmetry for tetragonal and
-wave symmetry for orthorhombic structure.Comment: LaTeX, 5 pages, 4 figures, uses grafik.sty (included
Disorder and localization in the lowest Landau level in the presence of dilute point scatterers
Cataloged from PDF version of article.We study the localization properties of a two-dimensional noninteracting electron gas in the presence of randomly distributed short-range scatterers in very high magnetic fields. We evaluate the participation number of the eigenstates obtained by exact diagonalization technique. At low impurity concentrations we obtain self-averaged values showing that all states, except those exactly at the Landau level, are localized with finite localization length. We conclude that in this dilute regime the localization length does not diverge. We also find that the maximum localization length increases exponentially with impurity concentration. Our calculations suggest that scaling behavior may be absent even for higher concentrations of scatterers. (C) 1999 Elsevier Science Ltd. All rights reserved
Metal nanoring and tube formation on carbon nanotubes
The structural and electronic properties of aluminum covered single wall
carbon nanotubes (SWNT) are studied from first-principles for a large number of
coverage. Aluminum-aluminum interaction that is stronger than aluminum-tube
interaction, prevents uniform metal coverage, and hence gives rise to the
clustering. However, a stable aluminum ring and aluminum nanotube with well
defined patterns can also form around the semiconducting SWNT and lead to
metallization. The persistent current in the Al nanoring is discussed to show
that a high magnetic field can be induced at the center of SWNT.Comment: Submitted to Physical Review
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
Elastic scaling for data stream processing
Cataloged from PDF version of article.This article addresses the profitability problem associated with auto-parallelization of general-purpose distributed data stream processing applications. Auto-parallelization involves locating regions in the application's data flow graph that can be replicated at run-time to apply data partitioning, in order to achieve scale. In order to make auto-parallelization effective in practice, the profitability question needs to be answered: How many parallel channels provide the best throughput? The answer to this question changes depending on the workload dynamics and resource availability at run-time. In this article, we propose an elastic auto-parallelization solution that can dynamically adjust the number of channels used to achieve high throughput without unnecessarily wasting resources. Most importantly, our solution can handle partitioned stateful operators via run-time state migration, which is fully transparent to the application developers. We provide an implementation and evaluation of the system on an industrial-strength data stream processing platform to validate our solution
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