8,996 research outputs found
Parallel Algorithms for Generating Random Networks with Given Degree Sequences
Random networks are widely used for modeling and analyzing complex processes.
Many mathematical models have been proposed to capture diverse real-world
networks. One of the most important aspects of these models is degree
distribution. Chung--Lu (CL) model is a random network model, which can produce
networks with any given arbitrary degree distribution. The complex systems we
deal with nowadays are growing larger and more diverse than ever. Generating
random networks with any given degree distribution consisting of billions of
nodes and edges or more has become a necessity, which requires efficient and
parallel algorithms. We present an MPI-based distributed memory parallel
algorithm for generating massive random networks using CL model, which takes
time with high probability and space per processor,
where , , and are the number of nodes, edges and processors,
respectively. The time efficiency is achieved by using a novel load-balancing
algorithm. Our algorithms scale very well to a large number of processors and
can generate massive power--law networks with one billion nodes and
billion edges in one minute using processors.Comment: Accepted in NPC 201
Fermi surfaces and Phase Stability of Ba(FeM)As (M=Co, Ni, Cu, Zn)
BaFeAs with transition-metal doping exhibits a variety of rich
phenomenon from coupling of structure, magnetism, and superconductivity. Using
density functional theory, we systematically compare the Fermi surfaces (FS),
formation energies (), and density of states (DOS) of
electron-doped Ba(FeM)As with M={Co, Ni, Cu, Zn} in
tetragonal (I) and orthorhombic (F) structures in nonmagnetic (NM),
antiferromagnetic (AFM), and paramagnetic (PM, disordered local moment) states.
We explain changes to phase stability () and Fermi surfaces (and
nesting) due to chemical and magnetic disorder, and compare to
observed/assessed properties and contrast alloy theory with that expected from
rigid-band model. With alloying, the DOS changes from common-band (Co,Ni) to
split-band (Cu,Zn), which dictates and can overwhelm FS-nesting
instabilities, as for Cu,Zn cases
Fundamentals of PV Efficiency Interpreted by a Two-Level Model
Elementary physics of photovoltaic energy conversion in a two-level atomic PV
is considered. We explain the conditions for which the Carnot efficiency is
reached and how it can be exceeded! The loss mechanisms - thermalization, angle
entropy, and below-bandgap transmission - explain the gap between Carnot
efficiency and the Shockley-Queisser limit. Wide varieties of techniques
developed to reduce these losses (e.g., solar concentrators, solar-thermal,
tandem cells, etc.) are reinterpreted by using a two level model. Remarkably,
the simple model appears to capture the essence of PV operation and reproduce
the key results and important insights that are known to the experts through
complex derivations.Comment: 7 pages, 6 figure
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