1,136 research outputs found
Compressible streaming instabilities in rotating thermal viscous objects
We study electromagnetic streaming instabilities in thermal viscous regions
of rotating astrophysical objects, such as, protostellar and protoplanetary
magnetized accretion disks, molecular clouds, their cores, and elephant trunks.
The obtained results can also be applied to any regions of interstellar medium,
where different equilibrium velocities between charged species can arise. We
consider a weakly and highly ionized three-component plasma consisting of
neutrals and magnetized electrons and ions. The vertical perturbations along
the background magnetic field are investigated. The effect of perturbation of
collisional frequencies due to density perturbations of species is taken into
account. The growth rates of perturbations are found in a wide region of wave
number spectrum for media, where the thermal pressure is larger than the
magnetic pressure. It is shown that in cases of strong collisional coupling of
neutrals with ions the contribution of the viscosity is negligible.Comment: Accepted for publication in "Astrophysical Journal
Electronic Structure of New LiFeAs High-Tc Superconductor
We present results of it ab initio LDA calculations of electronic structure
of "next generation" layered ironpnictide High-Tc superconductor LiFeAs
(Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently
studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely
close to the Fermi level its electronic properties are also determined ma inly
by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs
are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of
the Fe-3d density o f states and Fermi surface.Comment: 4 pages, 5 figures; Electronic structure improved with respect to new
experimental crystal structure dat
Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors
This work is provoked by recent discovery of new class prototype systems
AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors
(Tc=36K). Here we report ab initio LDA results for electronic structure of the
AFFeAs systems. We provide detailed comparison between electronic properties of
both new systems and reference LaOFeAs (La111) compound. In the vicinity of the
Fermi level all three systems have essentially the same band dispersions.
However for iron fluoride systems F(2p) states were found to be separated in
energy from As(4p) ones in contrast to La111, where O(2p) states strongly
overlaps with As(4p). Thus it should be more plausible to include only Fe(3d)
and As(4p) orbitals into a realistic noninteracting model than for La111.
Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads
to a lattice contruction and stronger Fe(3d)-As(4p) hybridization resulting in
smaller value of the density of states at the Fermi level in the case of Ca
compound. So to some extend Ca system reminds RE111 with later Rare Earths.
However Fermi surface of new fluorides is found to be nearly perfect
two-dimensional. Also we do not expect strong dependence of superconducting
properties with respect to different types of A substitutes.Comment: 5 pages, 4 figure
Novel multiple-band superconductor SrPt2As2
We present LDA calculated electronic structure of recently discovered
superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and
crystal structure are somehow similar to FeAs-based high-temperature
superconductors, the electronic structure of SrPt2As2 is very much different.
Crystal structure is orthorhombic (or tetragonal if idealized) and has layered
nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr
ions. The Fermi level is crossed by Pt-5d states with rather strong admixture
of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional,
with complicated sheets corresponding to multiple bands. We compare SrPt2As2
with 1111 and 122 representatives of FeAs-class of superconductors, as well as
with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of
superconductivity in SrPt2As2 is also presented.Comment: 5 pages, 4 figure
High-precision molecular dynamics simulation of UO2-PuO2: pair potentials comparison in UO2
Our series of articles is devoted to high-precision molecular dynamics
simulation of mixed actinide-oxide (MOX) fuel in the approximation of rigid
ions and pair interactions (RIPI) using high-performance graphics processors
(GPU). In this first article 10 most recent and widely used interatomic sets of
pair potentials (SPP) are assessed by reproduction of solid phase properties of
uranium dioxide (UO2) - temperature dependences of the lattice constant, bulk
modulus, enthalpy and heat capacity. Measurements were performed with 1K
accuracy in a wide temperature range from 300K up to melting points. The best
results are demonstrated by two recent SPPs MOX-07 and Yakub-09, which both had
been fitted to the recommended thermal expansion in the range of temperatures
300-3100K. They reproduce the experimental data better than the widely used
SPPs Basak-03 and Morelon-03 at temperatures above 2500K.Comment: 11 pages, 9 figures, 4 table
Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P
We report LDA calculated band structure, densities of states and Fermi
surfaces for recently discovered Pt-pnictide superconductors APt3P
(A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is
essentially three dimensional, in contrast to Fe pnictides and chalcogenides.
LDA calculated Sommerfeld coefficient agrees rather well with experimental
data, leaving little space for very strong coupling superconductivity,
suggested by experimental data on specific heat of SrPt3P. Elementary estimates
show, that the values of critical temperature can be explained by rather weak
or moderately strong coupling, while the decrease of superconducting transition
temperature Tc from Sr to La compound can be explained by corresponding
decrease of total density of states at the Fermi level N(E_F). The shape of the
density of states near the Fermi level suggests that in SrPt3P electron doping
(such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g.
partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F)
and possibly Tc.Comment: 5 pages, 5 figure
Instanton Number of Noncommutative U(n) gauge theory
We show that the integral of the first Pontrjagin class is given by an
integer and it is identified with instanton number of the U(n) gauge theory on
noncommutative . Here the dimension of the vector space that
appear in the ADHM construction is called Instanton number. The calculation is
done in operator formalism and the first Pontrjagin class is defined by
converge series. The origin of the instanton number is investigated closely,
too.Comment: 6 color figures, 27 pages, some comments and references are
added,typos fixe
Quantum Foam and Topological Strings
We find an interpretation of the recent connection found between topological
strings on Calabi-Yau threefolds and crystal melting: Summing over statistical
mechanical configuration of melting crystal is equivalent to a quantum
gravitational path integral involving fluctuations of Kahler geometry and
topology. We show how the limit shape of the melting crystal emerges as the
average geometry and topology of the quantum foam at the string scale. The
geometry is classical at large length scales, modified to a smooth limit shape
dictated by mirror geometry at string scale and is a quantum foam at area
scales g_s \alpha'.Comment: 55 page
- …