1,213 research outputs found
Effect of quantum fluctuations on structural phase transitions in SrTiO_3 and BaTiO_3
Using path-integral Monte Carol simulations and an ab initio effective
Hamiltonian, we study the effects of quantum fluctuations on structural phase
transitions in the cubic perovskite compounds SrTiO3 and BaTiO3. We find
quantum fluctuations affect ferroelectric (FE) transitions more strongly than
antiferrodistortive (AFD) ones, even though the effective mass of a single FE
local mode is larger. For SrTiO3 we find that the quantum fluctuations suppress
the FE transition completely, and reduce the AFD transition temperature from
130K to 110K. For BaTiO3, quantum fluctuations do not affect the order of the
transition, but do reduce the transition temperature by 35-50 K. The
implications of the calculations are discussed.Comment: Revtex (preprint style, 14 pages) + 2 postscript figures. A version
in two-column article style with embedded figures is available at
http://electron.rutgers.edu/~dhv/preprints/index.html#wz_qs
Structural and dielectric properties of SrTiO from first principles
We have investigated the structural and dielectric properties of
SrTiO,the first member of the SrTiO
Ruddlesden-Popper series, within density functional theory. Motivated by recent
work in which thin films of SrTiO were grown by molecular beam
epitaxy (MBE) on SrTiO substrates, the in-plane lattice parameter was
fixed to the theoretically optimized lattice constant of cubic SrTiO
(n=), while the out-of-plane lattice parameter and the internal
structural parameters were relaxed. The fully relaxed structure was also
investigated. Density functional perturbation theory was used to calculate the
zone-center phonon frequencies, Born effective charges, and the electronic
dielectric permittivity tensor. A detailed study of the contribution of
individual infrared-active modes to the static dielectric permittivity tensor
was performed. The calculated Raman and infrared phonon frequencies were found
to be in agreement with experiment where available. Comparisons of the
calculated static dielectric permittivity with experiments on both ceramic
powders and epitaxial thin films are discussed.Comment: 11 pages, 1 figure, 8 tables, submitted to Phys. Rev.
Orbital Dependent Phase Control in Ca2-xSrxRuO4
We present first-principles studies on the orbital states of the layered
perovskites CaSrRuO. The crossover from antiferromagnetic (AF)
Mott insulator for to nearly ferromagnetic (FM) metal at is
characterized by the systematic change of the orbital occupation. For the
AF side (), we present firm evidence for the ferro-orbital
ordering. It is found that the degeneracy of (or ) states is
lifted robustly due to the two-dimensional (2D) crystal-structure, even without
the Jahn-Teller distortion of RuO. This effect dominates, and the
cooperative occupation of orbital is concluded. In contrast to recent
proposals, the resulting electronic structure explains well both the observed
X-ray absorption spectra and the double peak structure of optical conductivity.
For the FM side (), however, the orbital with half filling opens a
pseudo-gap in the FM state and contributes to the spin =1/2 moment (rather
than =1 for =0.0 case) dominantly, while states are itinerant
with very small spin polarization, explaining the recent neutron data
consistently.Comment: 17 pages, 5 figure
Three-Particle Correlations from Parton Cascades in Au+Au Collisions
We present a study of three-particle correlations among a trigger particle
and two associated particles in Au + Au collisions at = 200 GeV
using a multi-phase transport model (AMPT) with both partonic and hadronic
interactions. We found that three-particle correlation densities in different
angular directions with respect to the triggered particle (`center', `cone',
`deflected', `near' and `near-away') increase with the number of participants.
The ratio of `deflected' to `cone' density approaches to 1.0 with the
increasing of number of participants, which indicates that partonic Mach-like
shock waves can be produced by strong parton cascades in central Au+Au
collisions.Comment: 9 pages, 6 figures; Final version to appear in Physics Letters
Formation of Nanopits in Si Capping Layers on SiGe Quantum Dots
In-situ annealing at a high temperature of 640°C was performed for a low temperature grown Si capping layer, which was grown at 300°C on SiGe self-assembled quantum dots with a thickness of 50 nm. Square nanopits, with a depth of about 8 nm and boundaries along 〈110〉, are formed in the Si capping layer after annealing. Cross-sectional transmission electron microscopy observation shows that each nanopit is located right over one dot with one to one correspondence. The detailed migration of Si atoms for the nanopit formation is revealed by in-situ annealing at a low temperature of 540°C. The final well-defined profiles of the nanopits indicate that both strain energy and surface energy play roles during the nanopit formation, and the nanopits are stable at 640°C. A subsequent growth of Ge on the nanopit-patterned surface results in the formation of SiGe quantum dot molecules around the nanopits
Dual bio-responsive gene delivery via reducible poly(amido amine) and survivin-inducible plasmid DNA
A bioreducible poly(amido amine) (SS-PAA) gene carrier, known as poly (amido-butanol) (pABOL), was used to transfect a variety of cancer and non-cancer cell lines. To obtain cancer-specific transgene expression for therapeutic efficiency in cancer treatment, we constructed survivin-inducible plasmid DNA expressing the soluble VEGF receptor, sFlt-1, downstream of the survivin promoter (pSUR-sFlt-1). Cancer-specific expression of sFlt-1 was observed in the mouse renal carcinoma (RENCA) cell line. pABOL enhanced the efficiency of gene delivery compared to traditional carriers used in the past. Thus, a dual bio-responsive gene delivery system was developed by using bioreducible p(ABOL) for enhanced intracellular gene delivery and survivin-inducible gene expression system (pSUR-sFlt-1 or pSUR-Luc reporter gene) that demonstrates increased gene expression in cancer that has advantages over current gene delivery system
proBDNF inhibits the proliferation and migration of OLN-93 oligodendrocytes
In contrast with mature brain‑derived neurotrophic factor (mBDNF), proBDNF induces cell apoptosis. However, the function of proBDNF in oligodendrocytes remains unclear. In the present study, the OLN‑93 oligodendroglia cell line was utilized as an in vitro model to analyse the functions of proBDNF in oligodendroglia. p75NTR, sortilin and proBDNF were expressed in cultured OLN‑93 cells. It was indicated that proBDNF inhibited OLN‑93 cell proliferation in a dose‑dependent manner as determined using the MTT assay and BrdU staining. Furthermore, proBDNF suppressed the migration of OLN‑93 cells as demonstrated using the scratch assay. proBDNF also decreased cell viability and promoted apoptosis as indicated by activated cysteine‑aspartic acid protease‑3 (caspase‑3) immunocytochemistry. Notably, anti‑proBDNF treatment neutralized the effect of proBDNF and resulted in increased cell proliferation and migration and decreased apoptosis. However, these effects were not observed in the presence of recombinant p75NTR extracellular domain‑human FC fusion protein and p75NTR antibody, indicating that proBDNF imparts its inhibitory effects on oligodendrocytes through the p75NTR signal pathway.Shen Liu, Wei Guo, Hengxing Zhou, Liang Tang, Shiqing Feng, Jin-Hua Zhong, and Xi- Fu Zho
Independent Eigenstates of Angular Momentum in a Quantum N-body System
The global rotational degrees of freedom in the Schr\"{o}dinger equation for
an -body system are completely separated from the internal ones. After
removing the motion of center of mass, we find a complete set of
independent base functions with the angular momentum . These are
homogeneous polynomials in the components of the coordinate vectors and the
solutions of the Laplace equation, where the Euler angles do not appear
explicitly. Any function with given angular momentum and given parity in the
system can be expanded with respect to the base functions, where the
coefficients are the functions of the internal variables. With the right choice
of the base functions and the internal variables, we explicitly establish the
equations for those functions. Only (3N-6) internal variables are involved both
in the functions and in the equations. The permutation symmetry of the wave
functions for identical particles is discussed.Comment: 24 pages, no figure, one Table, RevTex, Will be published in Phys.
Rev. A 64, 0421xx (Oct. 2001
Longitudinal broadening of near side jets due to parton cascade
Longitudinal broadening along direction on near side in
two-dimensional () di-hadron correlation
distribution has been studied for central Au+Au collisions at =
200 GeV, within a dynamical multi-phase transport model. It was found that the
longitudinal broadening is generated by a longitudinal flow induced by strong
parton cascade in central Au+Au collisions, in comparison with p+p collisions
at = 200 GeV. The longitudinal broadening may shed light on the
information about strongly interacting partonic matter at RHIC.Comment: 5 pages, 4 figures; accepted by Eur. Phys. J.
System-size scan of dihadron azimuthal correlations in ultra-relativistic heavy ion collisions
System-size dependence of dihadron azimuthal correlations in
ultra-relativistic heavy ion collision is simulated by a multi-phase transport
model. The structure of correlation functions and yields of associated
particles show clear participant path-length dependences in collision systems
with a partonic phase. The splitting parameter and root-mean-square width of
away-side correlation functions increase with collision system size from
N+N to Au+Au collisions. The double-peak
structure of away-side correlation functions can only be formed in sufficient
"large" collision systems under partonic phase. The contrast between the
results with partonic phase and with hadron gas could suggest some hints to
study onset of deconfinment.Comment: 8 pages, 4 figures, 1 table; Nucl. Phys. A (accepted
- …