Ferroelectricity in strained Ca0.5_{0.5}Sr0.5_{0.5}TiO3_3 from first principles

We present a density functional theory investigation of strained Ca$_{0.5}$Sr$_{0.5}$TiO$_3$. We have determined the structure and polarization for a number of arrangements of Ca and Sr in a 2$\times$2$\times$2 supercell. The a and b lattice vectors are strained to match the lattice constants of the rotated Si(001) face. To set the context for the CSTO study, we also include simulations of the Si(001) constrained structures for CaTiO$_3$ and SrTiO$_3$. Our primary findings are that all Ca$_{0.5}$Sr$_{0.5}$TiO$_3$ structures examined except one are ferroelectric, exhibiting polarizations ranging from 0.08 C/m$^2$ for the lowest energy configuration to about 0.26 C/m$^2$ for the higher energy configurations. We find that the configurations with larger polarizations have lower c/a ratios. The net polarization of the cell is the result of Ti-O ferroelectric displacements regulated by A-site cations.Comment: 13 pages, 4 figure

Experimental Studies of NaCs

We present experimental studies of excited electronic states of the NaCs molecule that are currently underway in our laboratory. The optical-optical double resonance method is used to obtain Doppler-free excitation spectra for several excited states. These data are being used to obtain RydbergKlein-Rees (RKR) or Inverse Perturbation Approach (IPA) potential curves for these states. We are also trying to map the bound portion of the 1(a) 3Σ + potential using resolved laser-induced fluorescence and Fourier transform spectroscopy to record transitions into the shallow well. Bound-free spectra from single ro-vibrational levels of electronically excited states to the repulsive wall of the 1(a) 3Σ + state are also being recorded. Using the previously determined excited state potentials, we can fit the repulsive wall of the 1(a) 3Σ + state to reproduce the experimental spectra using LeRoy’s BCONT program. A slightly modified version of BCONT will also be used to fit the relative transition dipole moments, µe(R), as a function of internuclear separation R, for the various bound-free electronic transitions

Experimental Studies of the NaCs 53Π0 and a3Σ+ States

We report high resolution measurements of 372 NaCs 53Π0(v, J) ro-vibrational level energies in the range 0 ≤ v ≤ 22. The data have been used to construct NaCs 53Π0 potential energy curves using the Rydberg–Klein-Rees and inverted perturbation approximation methods. Bound-free 53Π0(v, J) → 1(a)3Σ+ emission has also been measured, and is used to determine the repulsive wall of the 1(a)3Σ+ state and the 53Π0 → 1(a)3Σ+ relative transition dipole moment function. Hyperfine structure in the 53Π0 state has not been observed in this experiment. This null result is explained using a simple vector coupling model

Collisional Transfer of Population and Orientation in NaK

We report current work to study transfer of population and orientation in collisions of NaK molecules with argon and potassium atoms using polarization labeling (PL) and laser- induced fluorescence (LIF) spectroscopy. In the PL experiment, a circularly polarized pump laser excites a specific NaK A1Σ +(v 0=16, J 0 ) ← X1Σ +(v 00=0, J 0 ± 1) transition, creating an orientation (non-uniform MJ0 level distribution) in both levels. The linearly polarized probe laser is scanned over various 31Π(v, J 0±1) ← A1Σ +(v 0=16, J 0 ) transitions. The probe laser passes through a crossed linear polarizer before detection, and signal is recorded if the probe laser polarization has been modified by the vapor (which occurs when it comes into resonance with an oriented level). Using both spectroscopic methods, analysis of weak collisional satellite lines adjacent to these directly populated lines, as a function of argon buffer gas pressure and cell temperature, allows us to discern separately the effects collisions with argon atoms and potassium atoms have on the population and orientation of the molecule. In addition, code has been written which provides a theoretical analysis of the process, through a solution of the density matrix equations of motion for the system

Polarization Spectroscopy and Collisions in NaK

We report current work to study transfer of population and orientation in collisions of NaK molecules with argon and potassium atoms using polarization labeling (PL) and laser-induced fluorescence (LIF) spectroscopy. In the PL experiment, a circularly polarized pump laser excites a specific NaK A1Σ +(v=16, J) ← X1Σ +(v=0, J ± 1) transition, creating an orientation (non-uniform MJ level distribution) in both levels. The linear polarized probe laser is scanned over various 3 1Π(v=8, J 0 ± 1) ← A1Σ +(v=16, J 0 ) transitions. The probe laser passes through a crossed linear polarizer before detection, and signal is recorded if the probe laser polarization has been modified by the vapor (which occurs when it comes into resonance with an oriented level). In addition to strong direct transitions (J 0 = J), we also observe weak collisional satellite lines (J 0 = J ±n with n = 1, 2, 3, ...) indicating that orientation is transferred to adjacent rotational levels during a collision. An LIF experiment (with linear polarized pump and probe beams) gives information on the collisional transfer of population. From these data, cross sections for both processes can be determined. We experimentally distinguish collisions of NaK with argon atoms from collisions with alkali atoms

DIRECT DEMONSTRATION OF THETA-POSITIVE ANTIGEN-BINDING CELLS, WITH ANTIGEN-INDUCED MOVEMENT OF THYMUS-DEPENDENT CELL RECEPTORS

Anti-θAKR antibody conjugated to fluorescein has been used in direct immunofluorescence tests to identify spleen θ+ (T) sheep erythrocyte rosette-forming cells in AKR mice. Specificity studies involving A and cogenic A/θAKR mice clearly demonstrated that the cell surface fluorescence and cytotoxicity produced by the antiserum is directed solely toward the θAKR alloantigen. Approximately ⅜ of rosette-forming and non-rosette-forming spleen cells were found to be θ+. The tendency for T cells to bind less antigen and the tendency for antigen-binding T cells to bear less θ than other spleen T cells, first suggested by other studies involving rosette-elimination by anti-θC3H plus complement, were confirmed by direct immunofluorescence. All AKR rosettes are specifically inhibitable by anti-immunoglobulin, including T rosettes. Antigen-induced redistribution of T cell receptors, analogous to that previously described for B cell receptors (16), occurs as readily in θ+RFC as in θ- RFC, without altering the symmetrical ring distribution of θAKR antigen

Isolates of Candida albicans that differ in virulence for mice elicit strain-specific antibody-mediated protective responses

Three distinct isolates of Candida albicans were used to establish systemic and oral infections in inbred mice that are genetically resistant or susceptible to tissue damage. Patterns of infection differed significantly between both yeasts and mouse strains. Systemic infection conferred significant protection against re-challenge with the homologous, but not the heterologous yeast; however, the protective effect was more evident in the tissue-susceptible CBA/CaH mice than in the resistant BALB/c strain. In contrast, oral infection induced protection against both homologous and heterologous oral challenge, although this was significant only in the CBA/CaH mice. CBA/CaH mice produced antibodies of both IgG1 and IgG2a subclasses, whereas BALB/c mice produced predominantly IgG1. Western blotting demonstrated considerable differences between epitopes recognised by serum antibodies from mice of both strains after immunisation with each of the three yeasts. Thus, different strains of yeast show considerable specificity in antibody responses elicited by either systemic or oral infection. (c) 2005 Elsevier SAS. All rights reserved

The Neutron Spin Structure Function from the Deuteron Data in the Resonance Region

Nuclear effects in the spin-dependent structure function $g_1$ of the deuteron are studied in the kinematics of future experiments at CEBAF, ($\nu \leq 3~GeV, ~Q^2 \leq 2~GeV^2$). The magnitude of nuclear effects is found to be significantly larger than the one occurring in deep inelastic scattering ($\nu\to \infty, ~Q^2\to \infty$). A possibility to measure the neutron structure functions in the CEBAF experiments with deuterium is analysed. It is found that disregarding or improperly treating nuclear effects in the region of nucleon resonances would lead to the ``extraction" of an unreliable function. A procedure aimed at correctly extracting the neutron structure function from the deuterium data is illustrated and conclusions about the experimental study of the $Q^2$ dependence of the Gerasimov-Drell-Hearn Sum Rule for the neutron are drawn.Comment: 11 pages, 4 Postscript figure

Nuclear parton distribution functions and their uncertainties

We analyze experimental data of nuclear structure-function ratios F_2^A/F_2^{A'} and Drell-Yan cross section ratios for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Valence-quark distributions are determined by the F_2 data at large x; however, the small-x part is not obvious from the data. On the other hand, the antiquark distributions are determined well at x~0.01 from the F_2 data and at x~0.1 by the Drell-Yan data; however, the large-x behavior is not clear. Gluon distributions cannot be fixed by the present data and they have large uncertainties in the whole x region. Parametrization results are shown in comparison with the data. We provide a useful code for calculating nuclear PDFs at given x and Q^2.Comment: 9 pages, REVTeX, 23 eps files, Phys. Rev. C in press. Nuclear PDF library is available at http://hs.phys.saga-u.ac.jp/nuclp.htm