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

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

On the Extraction of the Neutron Spin Structure Functions and the Gerasimov -- Drell -- Hearn Integral from 3He⃗(e⃗,e′)X^3\vec {He}(\vec e, e')X data

Nuclear effects in polarized inelastic electron scattering off polarized $^3He$ are discussed; in the kinematics of future experiments at CEBAF, Fermi motion effects are found to be much larger than in deep inelastic scattering. It is shown that improperly describing nuclear dynamics would lead to the extraction of unreliable neutron spin structure functions. On the other hand side, a simple and workable equation relating the Gerasimov -- Drell -- Hearn Integral for the neutron to the corresponding quantity for $^3He$ is proposed.Comment: 13 pages Latex including 3 postscript figure

Magnetic moment and anisotropy in FenCom clusters

Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of alloying on the magnetic moment and magnetic anisotropies. Our density-functional studies show that many mixed clusters have moments comparable to or higher than the pure clusters. The mixed clusters, however, have very low anisotropies and could be ideal as soft magnetic materials. It is shown that shape, composition, and compositional ordering must be considered for optimization of anisotropy energies