Spin-dependent Optical Potential Research Articles

Atoms in a spin dependent optical potential: ground state topology and magnetization

We investigate a Bose–Einstein condensate of F = 187Rb atoms in a 2D spin-dependent optical lattice generated by intersecting laser beams with a superposition of polarizations. For 87Rb the effective interaction of an atom with the electromagnetic field contains scalar and vector (called a fictitious magnetic field, B fic) potentials. The Rb atoms behave as a quantum rotor (QR) with angular momentum given by the sum of the atomic rotational motion angular momentum and the hyperfine spin. The ground state of the QR is affected upon applying an external magnetic field, B ext, perpendicular to the plane of QR motion and a sudden change of its topology occurs as the ratio B ext/B fic exceeds a critical value. It is shown that the change of topology of the QR ground state is a result of combined action of Zeeman and Einstein–de Haas effects. The first transfers atoms to the largest hyperfine component to polarize the sample along the field as the external magnetic field is increased. The second sweeps spin to rotational angular momentum, modifying the kinetic energy of the atoms.

Microwave control of atomic motional states in a spin-dependent optical lattice

Spin-dependent optical potentials allow us to use microwave radiation to manipulate the motional state of trapped neutral atoms (Förster et al 2009 Phys. Rev. Lett. 103 233001). Here, we discuss this method in greater detail, comparing it to the widely employed Raman sideband coupling method. We provide a simplified model for sideband cooling in a spin-dependent potential, and we discuss it in terms of the generalized Lamb–Dicke parameter. Using a master equation formalism, we present a quantitative analysis of the cooling performance for our experiment, which can be generalized to other experimental settings. We additionally use microwave sideband transitions to engineer motional Fock states and coherent states, and we devise a technique for measuring the population distribution of the prepared states.

Exclusive (e,e′pp) knockout reactions

Exclusive cross sections of the 16O(e,e′pp) 14C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.

Elastic scattering of 200 MeV polarized protons fromC12,13and the optical potential

The differential cross sections and analyzing powers for 200 MeV protons elastically scattered from $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ have been measured in the angular region 6-115\ifmmode^\circ\else\textdegree\fi{}. The $^{12}\mathrm{C}$ data were analyzed in terms of a local, complex, spindependent optical potential. The potentials were defined phenomenologically (simple or double Woods-Saxon forms) or derived from the impulse approximation in the Kerman-McManus-Thaler approach with and without a semiempirical correction for Pauli blocking. Acceptable fits to the data over the whole angular range were obtained only for a real central potential which exhibits a pronounced depression inside the nuclear surface, a feature also suggested by some theoretical models for medium-energy optical potentials.NUCLEAR REACTIONS $^{12,13}\mathrm{C}(\stackrel{\ensuremath{\rightarrow}}{p},p)$, ${E}_{p}=200$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$ and $A(\ensuremath{\theta})$; phenomenological optical-model analysis, deduced best-fit potentials for $^{12}\mathrm{C}$, comparison with impulse-approximation calculations.

“Spin-orbit deformation” in inelastic scattering of protons at medium energies

Distorted-wave Born-approximation calculations for medium-energy proton inelastic scattering to collective states, which use microscopic, spin-dependent optical potentials that properly describe proton elastic analyzing powers, show that inclusion of the deformation or collectivity of the spin-orbit term in the inelastic transition matrix element has an important effect on predictions for the available inelastic angular distributions and analyzing powers.

Determination of Deformation Parameters ofNe20andNe22by Inelastic Scattering of Polarized Protons

Measurements of cross sections and analyzing powers have been made for the inelastic scattering of 24.5-MeV polarized protons leading to the lowest ${0}^{+}$, ${2}^{+}$, and ${4}^{+}$ states in $^{20}\mathrm{Ne}$ and $^{22}\mathrm{Ne}$. The results for both $^{20}\mathrm{Ne}$ and $^{22}\mathrm{Ne}$ are best reproduced when the full Thomas form of the deformed, spin-dependent optical potential is employed in a coupled-channel calculation. In contrast to $^{20}\mathrm{Ne}$, the results for $^{22}\mathrm{Ne}$ indicate that only a small hexadecapole deformation is needed to reproduce both the cross sections and the analyzing powers for the $^{22}\mathrm{Ne}$ ground-state rotational band.

The spin dependent optical potential for deformed nuclei

We use Born approximation to compare the full spin dependent potential for a deformed nucleus with the simple spin-orbit term. We show that for large momentum transfers the latter is a good approximation but not otherwise.

Thomas form of the spin-dependent optical potential and asymmetry of 20 MeV polarized protons

Recently measured asymmetries produced in the inelastic scattering of 20.3 MeV polarized protons are analyzed in the DWBA collective model approximation. Substantial improvements are observed when the deformed spin-dependent optical potential is of the full Thomas form.

Inelastic proton scattering and the deformed spin-dependent optical potential

Inelastic proton asymmetries and spin flip probabilities, as well as cross sections, are calculated in the DWBA approximation with a deformed spin-dependent potential of the full Thomas form. Consideration of the full term improves substantially the fit to observed asymmetries when E p ⩾ 29 MeV.

Calculations of Inelastic Scattering of Neutrons by Heavy Nuclei

Neutron-scattering cross-section data between 0.1 and 3.0 MeV for U, Th, Bi, Pb, Au, W, and Ta have been fitted using a local, spherical, spin-dependent optical potential with a minimum number of parameters. Total cross sections and differential elastic angular distributions, corrected to include compound elastic scattering, are used. With the "entrance-channel" parameters thus determined, the Hauser-Feshbach statistical model gives reasonable predictions of the cross sections for inelastic scattering of neutrons by ${\mathrm{U}}^{238}$, ${\mathrm{Th}}^{232}$, ${\mathrm{Bi}}^{209}$, ${\mathrm{Pb}}^{208}$, ${\mathrm{Au}}^{197}$, ${\mathrm{W}}^{184}$, and ${\mathrm{Ta}}^{181}$. The latitude in fitting entrance-channel data allowed by the optical-model parameters is sufficient to mask effects arising from nonlocality, deformation, and width distributions. To a very high degree, this relatively simple model is shown to be adequate for the presently available data.

Spin-dependent Quasi-elastic Scattering of Fast Nucleons

The distortion produced by a spin-dependent optical potential in the scattering and polarization of fast nucleons by light nuclei is discussed for the case of excitation of the target nucleus to a low-lying state. The distorted Born approximation is used, with the distortion calculated according to the WKB approximation. The results are applied to the scattering of protons at around 185 Mev from C/sup 12/,O/sup 16/, and Ca/sup 40/. The approximate equality of polarization and asymmetry is discussed in relation to the surface nature of the spin-orbit potential. (auth)