Spin-dependent Contributions Research Articles

High-precision calculation of the parity-nonconserving amplitude in francium

A high-precision calculation of the $7s\ensuremath{-}8s$ parity-nonconserving (PNC) transition amplitude in francium, based on a relativistic all-order method, is presented. Our values for the PNC amplitudes in ${}^{223}\mathrm{Fr}$ and ${}^{210}\mathrm{Fr}$ are ${E}_{\mathrm{PNC}}=15.41(17)$ and $14.02(15),$ respectively, in units ${10}^{\ensuremath{-}11}i|e|{a}_{0}(\ensuremath{-}{Q}_{W}/N),$ where ${Q}_{W}$ is the weak charge and N is the neutron number. Spin-dependent contributions to the PNC amplitude are calculated for Fr isotopes with nucleon numbers $A=207,$ 209, 211, and 213. To assess the accuracy of our calculations, we apply the present all-order method to the $6s\ensuremath{-}7s$ PNC amplitude in cesium and obtain a result in close agreement with previous high-precision calculations.

Optical effects in spin-polarized scanning tunneling microscopy with GaAs probes

Experiments on spin-polarized tunneling with GaAs probes were performed in a scanning tunneling microscope. The central issue is the detection of a spin-dependent contribution to the tunnel current between a ferromagnetic sample and a GaAs probe tip, in which a spin polarization is optically induced. Measurements in ultrahigh vacuum on Co surfaces show a marked dependence on applied voltage and lateral variations on the scale of the magnetic domain size, as expected for spin-polarized tunneling. However, using Au test samples it is shown that purely optical effects complicate the data interpretation, and prevent an unambiguous demonstration of spin sensitivity with this method in its present form.

Anomalous electron transport in magnetic multilayers tuned by electric and magnetic fields

We report on anomalous quantum electron transport in low dimensional systems, which may be tuned by varying electric and magnetic fields. In magnetic trilayers Co/M/Co (Mnonmagnetic metallic spacer), which have been used to reveal ground state of disordered two-dimensional metal, at low temperatures a crossover from the logarithmic to the anomalous non-Fermi liquid behavior (Δρ~β*√T) is observed. This crossover is probably due to scattering of electrons by symmetrical two-level systems. We also demonstrate that magnetic multilayers, with enhanced spin dependent scattering effects, may be used to study quantum magnetic phase (QPT) transition phenomena. We observed that through phase transition from antiferromagnet to the ferromagnet induced by magnetic field the spindependent contribution to the resistivity varies as Δρs ~T α with α=f(H). In the vicinity of QPT, the spin dependent contribution to the resistivity varies linearly with temperature.

Spin holography

The interference of the directly emitted photoelectron wave and the wave scattered coherently by neighboring atoms gives holographic fringes in the photoelectron emission intensity $I(\mathbf{R}^)$. In the electron-emission holography technique in surface physics, $I(\mathbf{R}^)$ is inverted holographically to give a three-dimensional image of the environment of the source atom. Earlier [E. M. E. Timmermans, G. T. Trammell, and J. P. Hannon, Phys. Rev. Lett. 72, 832 (1994); J. Appl. Phys. 73, 6138 (1993)], we pointed out that the polarization pattern $\mathbf{P}(\mathbf{R}^)$ similarly can be viewed as a hologram of the spin environment of the source atom by virtue of the exchange scattering of the photoelectron by the neighboring atoms. In this paper, we point out that spin-orbit correlations in the photoelectron initial state are responsible for holographic spin-dependent contributions to the intensity hologram $I(\mathbf{R}^)$, even if the directly emitted photoelectrons are unpolarized. This remarkable result implies that the emission intensity contains spin information just as the polarization pattern $\mathbf{P}(\mathbf{R}^)$. Although the spin-dependent signal in the hologram is rather small $(\ensuremath{\sim}5%$ in most cases of interest), we show how spin information can be extracted from the intensity hologram, making use of the point symmetry of the environment of the source atom. This way of analyzing photoelectron intensity holograms to extract short-range spin information opens up a new avenue for surface magnetism studies.

Exciton saturation in room temperature GaAs/AlGaAs multiple quantum wells

We report time resolved studies of optically induced circular dichroism in room temperature GaAs/AlGaAs multiple quantum wells to resolve spin-dependent and spin-independent contributions to exciton saturation. Phase-space filling and Coulomb contributions were separated by spin dependence while the effects of broadening and screening were distinguished using different pulse bandwidths. Bound and free carrier contributions were compared by observing the temporal dependence of the circular dichroism on femtosecond timescales. The spin independent contribution to exciton saturation was found to be independent of whether the carriers were bound or free.

Measurement of Parity Nonconservation and an Anapole Moment in Cesium

The amplitude of the parity-nonconserving transition between the 6S and 7S states of cesium was precisely measured with the use of a spin-polarized atomic beam. This measurement gives Im(E1pnc)/beta = -1.5935(56) millivolts per centimeter and provides an improved test of the standard model at low energy, including a value for the S parameter of -1.3(3)exp (11)theory. The nuclear spin-dependent contribution was 0.077(11) millivolts per centimeter; this contribution is a manifestation of parity violation in atomic nuclei and is a measurement of the long-sought anapole moment.

A measurement of parity non-conserving optical rotation in atomic lead

We report a measurement of parity non-conserving (PNC) optical rotation in the vicinity of the 1.279 magnetic dipole transition in atomic lead. We obtain a value for the conventional parameter, , with limits on the nuclear spin-dependent contribution set by the anapole constant . The experimental results, when combined with the relevant atomic calculations, lead to a value for the mass of the boson or, alternatively, place a limit on physics beyond the standard model through the isospin-conserving parameter, S.

Parity nonconservation under spin rotation of an atom (molecule) travelling in a laser wave

It is shown that as an atom (molecule) passes through an area occupied by a laser wave, P- and T-noninvariant effects of spin rotation arise. These effects allow one to measure the spin dependent contribution to the P- and T-noninvariant interaction of the electrons and nucleus.

Positron-annihilation study of the half-metallic ferromagnet NiMnSb: Experiment.

Spin-polarized measurements of the two-dimensional angular correlation of annihilation radiation in NiMnSb are presented. By making use of the inherent partial polarization of the positron beam and the alignment of the magnetic domains with the aid of an external magnetic field, the sum and difference of the spin-dependent contributions to the angular correlation have been obtained for integration directions 〈100〉, 〈110〉, and 〈111〉. The results are compared with calculated distributions. A least-squares analysis of the data yields a value of (-8.4\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ for the three-photon difference effect in NiMnSb and establishes the half-metallic character of the band structure with an accuracy of ${\mathrm{\ensuremath{\sim}}}_{\mathrm{\ensuremath{-}}0.02}^{+0.01}$ electrons per formula unit.

BREIT-FERMI INTERACTIONS AND FINE-HYPERFINE SPLITTING IN HEAVY QUARKONIA

The adequacy of using the Breit-Fermi type of interactions for describing the fine hyperfine splittings of heavy quarkonia has been examined critically. A potential model which just about accommodates the trend of the recent data on 1P1 states has been studied. The possibility of including the spin-dependent contributions of a pseudoscalar exchange potential is also considered. It is shown that the choice of the Breit-Fermi form for the spin-dependent interactions severely constrains the generally accepted non-relativistic potential and do not allow enough freedom to fit the recent data on 1P1 levels.

An observation of the spin dependence of synchrotron radiation intensity

An experiment dealing with the observation of a change in the synchrotron radiation (SR) intensity during beam depolarization at the VEPP-4 storage ring is described. With a minute of measurement time, the magnitude of the effect exceeded ten statistical errors. The dependence of the sign of the effect on the sign of the magnetic field at the radiation point is demonstrated. The measured magnitude of the jump in the intensity is in agreement with the calculated spin-dependent contribution within a statistical error (equal to 7%). The suggested method is shown to be effectively applied to the observation of the transverse beam polarization in electron-positron storage rings.

On the possibility of using synchrotron radiation for measuring the electron beam polarization in a storage ring

The possibility of measuring the transverse polarization of electrons in a storage ring by detection of the spin-dependent contribution to the synchrotron radiation intensity is discussed. A Cherenkov counter is proposed as the detector. Numerical estimates are presented for electron energies of 5 and 70 GeV.

Theory of spin-polarized low-energy electron diffraction from ferromagnetics

A theory of elastic scattering of low-energy electrons is outlined, in which the electron-solid interaction contains two spin-dependent contributions: an elastic exchange-correlation part in the real ion-core potential and an inelastic imaginary part describing electron-hole excitation. Numerical calculations for the Fe(110) surface predict, for an unpolarized primary beam, large spin polarization effects (up to 60%), which are highly sensitive to the magnetization of the topmost stomic layer.

Quantum resonances in the valence bands of germanium. I. Theoretical considerations

The structure of the ${\ensuremath{\Gamma}}_{25}^{+}$ degenerate valence band of Ge has been investigated by a comprehensive study of the complex resonance spectra from electric-dipole transitions in the system of anomalously spaced low-lying Landau levels produced by an applied magnetic field. This paper, the first of a series of four, is devoted to the development of a systematic theoretical spectroscopy to serve as a framework for the analysis of our experiments which will be discussed in succeeding papers. Using group-theoretical techniques, following Luttinger, we construct a generalized effective-mass Hamiltonian for holes, including the effects of elastic strain, in the full six-dimensional space of ${\ensuremath{\Gamma}}_{25}^{+}$ to encompass the spin-orbit-split-off band. The formulation in the extended space sheds considerable light on two important consequences of spin-orbit coupling, the anisotropy of the hole $g$ factor (the Luttinger $q$ term) and a new effect, the spin-dependent contribution to the valence-band deformation potentials. From the Hamiltonian for ${\ensuremath{\Gamma}}_{25}^{+}$ we project, making the split appropriate to large spin-orbit interaction, the Hamiltonian belonging to the subspace of the band edge ${\ensuremath{\Gamma}}_{8}^{+}$. We examine the nature of its eigenstates and develop a systematic scheme based on group theory for classifying the magnetic eigenstates, in terms of which selection rules for quantum transitions can be expressed in unusually clear and compact form. A formalism is presented for generating complete, synthesized quantum-resonance spectra starting with the eigenvalues and eigenfunctions of the effective-mass Hamiltonian. A further projection, representing the decoupling of the ${\ensuremath{\Gamma}}_{8}^{+}$ band by large uniaxial stress, is expanded to second order to evaluate the corrections to the hole effective masses and $g$ factors at finite stress. Finally, we consider the interaction between the projected spaces of the stress-decoupled band-edge states and the spin-orbit-split-off states which contributes two important shifts to the quantum-resonance effective masses: the second-order magnetic interaction and the interaction from the cross terms of strain and magnetic field.

Concentration-dependence of resistivity of the kondo alloys

The spin dependent contribution to the resistivity of Kondo alloys in many impurities approximation is calculated using the Green function method.