53,526 research outputs found
Competitions in layered ruthenates: ferro- vs. antiferromagnetism and triplet vs. singlet pairing
Ru based perovskites demonstrate an amazing richness in their magnetic
properties, including 3D and quasi-2D ferromagnetism, antiferromagnetism, and
unconventional superconductivity. Tendency to ferromagnetism, stemming from the
unusually large involvement of O in magnetism in ruthenates, leads to
ferromagnetic spin fluctuations in Sr2RuO4 and eventually to p-wave
superconductivity. A related compound Ca2RuO4 was measured to be
antiferromagnetic, suggesting a possibility of antiferromagnetic fluctuations
in Sr2RuO4 as well. Here we report first principles calculations that
demonstrate that in both compounds the ferro- and antiferromagnetic
fluctuations coexist, leading to an actual instability in Ca2RuO4 and to a
close competition between p-wave and d-wave superconducting symmetries in
Sr2RuO4. The antiferromagnetism in this system appears to be mostly related
with the nesting, which is the strongest at Q=(2pi/3,2pi/3,0). Surprisingly,
for the Fermiology of Sr2RuO4 the p-wave state wins over the d-wave one
everywhere except in close vicinity of the antiferromagnetic instability. The
most stable state within the d-wave channel has vanishing order parameter at
one out of three Fermi surfaces in Sr2RuO4, while in the p channel its
amplitude is comparable at all three of them.Comment: 4 Revtex pages with 4 embedded postscript figure. Some figures are
color, but should look OK in B&W as wel
Nonlinear Dynamics of Nuclear-Electronic Spin Processes in Ferromagnets
Spin dynamics is considered in ferromagnets consisting of electron and
nuclear subsystems interacting with each other through hyperfine forces. In
addition, the ferromagnetic sample is coupled with a resonance electric
circuit. Under these conditions, spin relaxation from a strongly nonequilibrium
initial state displays several peculiarities absent for the standard set-up in
studying spin relaxation. The main feature of the nonlinear spin dynamics
considered in this communication is the appearance of ultrafast coherent
relaxation, with characteristic relaxation times several orders shorter than
the transverse relaxation time . This type of coherent spin relaxation can
be used for extracting additional information on the intrinsic properties of
ferromagnetic materials and also can be employed for different technical
applications.Comment: 1 file, 4 pages, RevTex, no figure
Electronic properties of graphene nano-flakes: Energy gap, permanent dipole, termination effect and Raman spectroscopy
The electronic properties of graphene nano-flakes (GNFs) with different edge
passivation is investigated by using density functional theory. Passivation
with F and H atoms are considered: C X (X=F or H). We studied
GNFs with and limit ourselves to the lowest energy configurations.
We found that: i) the energy difference between the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
decreases with , ii) topological defects (pentagon and heptagon) break the
symmetry of the GNFs and enhance the electric polarization, iii) the mutual
interaction of bilayer GNFs can be understood by dipole-dipole interaction
which were found sensitive to the relative orientation of the GNFs, iv) the
permanent dipoles depend on the edge terminated atom, while the energy gap is
independent of it, and v) the presence of heptagon and pentagon defects in the
GNFs results in the largest difference between the energy of the spin-up and
spin-down electrons which is larger for the H-passivated GNFs as compared to
F-passivated GNFs. Our study shows clearly the effect of geometry, size,
termination and bilayer on the electronic properties of small GNFs.This study
reveals important features of graphene nano-flakes which can be detected using
Raman spectroscopy.Comment: 23 pages, 14 figures, accepted in J. Chem. Phy
X-ray spectrum of the high polarization quasar PKS 1510-089
We present results on the X-ray spectra of the radio-loud, high-polarization
quasar, PKS 1510-089, based on new data obtained using ASCA, and from archival
ROSAT data. The X-ray spectrum obtained by ASCA is unusually hard, with the
photon index=1.30+-0.06, while the (non-simultaneous) ROSAT data indicate a
steeper spectrum (1.9+-0.3). The X-ray flux at 1 keV is within 10% during both
observations. A break in the underlying continuum at about 0.7 keV is
suggested. Flat X-ray spectra seem to be the characteristic of high
polarization quasars, and their spectra also appear to be harder than that of
the other radio-loud but low-polarization quasars. The multiwavelength spectrum
of PKS 1510-089 is similar to many other gamma-ray blazars, suggesting the
emission is dominated by that from a relativistic jet. A big blue-bump is also
seen in its multiwavelength spectrum, suggesting the presence of a strong
thermal component as well.Comment: 19 pages (Latex + 5 ps figures), Accpeted for publication in the
Astrophysical Journal, December 20, 199
Analysis of regulatory network involved in mechanical induction of embryonic stem cell differentiation
Embryonic stem cells are conventionally differentiated by modulating specific growth factors in the cell culture media. Recently the effect of cellular mechanical microenvironment in inducing phenotype specific differentiation has attracted considerable attention. We have shown the possibility of inducing endoderm differentiation by culturing the stem cells on fibrin substrates of specific stiffness [1]. Here, we analyze the regulatory network involved in such mechanically induced endoderm differentiation under two different experimental configurations of 2-dimensional and 3-dimensional culture, respectively. Mouse embryonic stem cells are differentiated on an array of substrates of varying mechanical properties and analyzed for relevant endoderm markers. The experimental data set is further analyzed for identification of co-regulated transcription factors across different substrate conditions using the technique of bi-clustering. Overlapped bi-clusters are identified following an optimization formulation, which is solved using an evolutionary algorithm. While typically such analysis is performed at the mean value of expression data across experimental repeats, the variability of stem cell systems reduces the confidence on such analysis of mean data. Bootstrapping technique is thus integrated with the bi-clustering algorithm to determine sets of robust bi-clusters, which is found to differ significantly from corresponding bi-clusters at the mean data value. Analysis of robust bi-clusters reveals an overall similar network interaction as has been reported for chemically induced endoderm or endodermal organs but with differences in patterning between 2-dimensional and 3-dimensional culture. Such analysis sheds light on the pathway of stem cell differentiation indicating the prospect of the two culture configurations for further maturation. © 2012 Zhang et al
Characteristics of alpha projectile fragments emission in interaction of nuclei with emulsion
The properties of the relativistic alpha fragments produced in interactions
of 84^Kr at around 1 A GeV in nuclear emulsion are investigated. The
experimental results are compared with the similar results obtained from
various projectiles with emulsion interactions at different energies. The
total, partial nuclear cross-sections and production rates of alpha
fragmentation channels in relativistic nucleus-nucleus collisions and their
dependence on the mass number and initial energy of the incident projectile
nucleus are investigated. The yields of multiple alpha fragments emitted from
the interactions of projectile nuclei with the nuclei of light, medium and
heavy target groups of emulsion-detector are discussed and they indicate that
the projectile-breakup mechanism seems to be free from the target mass number.
It is found that the multiplicity distributions of alpha fragments are well
described by the Koba-Nielsen-Olesen (KNO) scaling presentation. The mean
multiplicities of the freshly produced newly created charged secondary
particles, normally known as shower and secondary particles associated with
target in the events where the emission of alpha fragments were accompanied by
heavy projectile fragments having Z value larger than 4 seem to be constant as
the alpha fragments multiplicity increases, and exhibit a behavior independent
of the alpha fragments multiplicity.Comment: 33 pages, 8 figures and 3 tables (in press
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