Nonexistence of intrinsic spin currents

We describe the electron spin dynamics in the presence of Rashba spin-orbit interaction and disorder using the spin-density matrix method. We show that in the Born approximation in the scattering amplitude the spin current is zero for an arbitrary ratio of the spin-orbit splitting and the scattering rate. Various types of the disorder potential are studied. We argue that the bulk spin current always depends explicitly on scattering by impurities. In this sense universal intrinsic spin current does not exist.     

Effects of spin-orbit interaction on superconductor-ferromagnet heterostructures: Spontaneous electric and spin surface currents

We find proximity-induced spontaneous spin and electric surface currents at all temperatures below the super-conducting T _c in an isotropic s-wave superconductor deposited with a thin ferromagnetic metal layer with spin-orbit interaction. The currents are carried by Andreev surface states and generated as a joint effect of the spin-orbit interaction and the exchange field. The background spin current arises in the thin layer due to different local spin polarizations of electrons and holes, which have almost opposite velocities in each of the surface states. The spontaneous surface electric current in the superconductor originates in the asymmetry of Andreev states with respect to sign reversal of the momentum component parallel to the surface. The conditions for electric and spin currents to show up in the system significantly differ from each other.     

Full counting statistics of spin currents

We discuss how to detect fluctuating spin currents and derive full counting statistics of electron spin transfers. We consider several detectors in series monitoring different components of the spins. We have found that in general the statistics of the measurement outcomes cannot be explained with the projection postulate and essentially depends on the quantum dynamics of the detectors.     

Neutron spin rotation and P-violation

A new apparatus to measure the neutron spin rotation due to interaction with matter has been developed at KEK. The present apparatus enables us to measure the parity violating neutron spin rotation in the p-wave resonance in the energy region of eV.On leave from Tohoku University.     

Muon spin rotation in superconductors

By means of the muon spin rotation technique (⁺SR), the temperature dependence of the magnetic field inside the normal-conducting domains of high-purity tantalum crystals in the intermediate state has been measured in the temperature range 2.36K+SR. Possible applications of these findings to the study of long-range diffusion of positive muons at low temperatures are indicated.     

Electrodynamics of spin currents in superconductors

In recent work we formulated a new set of electrodynamic equations for superconductors as an alternative to the conventional London equations, compatible with the prediction of the theory of hole superconductivity that superconductors expel negative charge from the interior towards the surface. Charge expulsion results in a macroscopically inhomogeneous charge distribution and an electric field in the interior, and because of this a spin current is expected to exist. Furthermore, we have recently shown that a dynamical explanation of the Meissner effect in superconductors leads to the prediction that a spontaneous spin current exists near the surface of superconductors (spin Meissner effect). In this paper we extend the electrodynamic equations proposed earlier for the charge density and charge current to describe also the space and time dependence of the spin density and spin current. This allows us to determine the magnitude of the expelled negative charge and interior electric field as well as of the spin current in terms of other measurable properties of superconductors. We also provide a `geometric' interpretation of the difference between type I and type II superconductors, discuss how superconductors manage to conserve angular momentum, discuss the relationship between our model and Slater's seminal work on superconductivity, and discuss the magnitude of the expected novel effects for elemental and other superconductors.     

Spin-to-charge conversion of mesoscopic spin currents

Recent theoretical investigations have shown that spin currents can be generated by passing electric currents through spin-orbit coupled mesoscopic systems. Measuring these spin currents has, however, not been achieved to date. We show how mesoscopic spin currents in lateral heterostructures can be measured with a single-channel voltage probe. In the presence of a spin current, the charge current I(qpc) through the quantum point contact connecting the probe is odd in an externally applied Zeeman field B, while it is even in the absence of spin current. Furthermore, the zero-field derivative ?(B)I(qpc) is proportional to the magnitude of the spin current, with a proportionality coefficient that can be determined in an independent measurement. We confirm these findings numerically.     

Chemical aspects of muon spin rotation

High energy muons induce radiation damage when they interact with molecules or ions. The chemical reactions expected to occur are summarised, and attention is then given to events likely to give rise to radicals exhibiting muon-electron hyperfine coupling. Addition of muonium atoms to double or treble bonds, which is one example of the way this can occur is considered, and, in particular, the resulting proton-muon hyperfine isotope effect is discussed. Finally, possible chemical models for paramagnetic muon centres in carbon and silicon are outlined.I thank Dr. S. Cox and Dr. A. Hill for helpful discussions.     

Optically injected spin currents in semiconductors

We show that quantum interference of one and two photon absorption from a two color field allows one to optically inject ballistic spin currents in unbiased semiconductors. The spin currents can be generated with or without an accompanying electrical current and can be controlled using the relative phase of the two colors. We characterize the injected spin currents using symmetry arguments and an eight-band Kane model.     

Generating spin currents in semiconductors with the spin Hall effect

We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.     

Axial exchange currents and nucleon spin

We calculate the axial couplings g_A⁸(0) and g_A⁰(0) related to the spin of the nucleon in a constituent quark model. In addition to the standard one-body axial currents, the model includes two-body axial exchange currents. The latter are necessary to satisfy the Partial Conservation of Axial Current (PCAC) condition. For both axial couplings we find significant corrections to the standard quark model prediction. Exchange currents reduce the valence quark contribution to the nucleon spin and afford an interpretation of the missing nucleon spin as orbital angular momentum carried by nonvalence quark degrees of freedom.     

Scattering induced spin orientation and spin currents in gyrotropic structures

It is shown that additional contributions both to current-induced spin orientation and to the spin Hall effect arise in quantum wells due to the gyrotropy of the structures. Microscopically, they are related to the basic properties of gyrotropic systems, namely, to linearity in the wave vector terms in the matrix element of electron scattering and in the energy spectrum. Calculation shows that, in high-mobility structures, the contribution to the spin Hall current considered here can exceed the term originating from the Mott skew scattering. The text was submitted by the author in English.     

Dynamics of helimagnets with spin polarised currents

We study analytically, spin polarised current induced dynamics of Dzyaloshinskii-Moriya helimagnets within the phenomenological Landau-Lifshitz framework. Similarities and differences between two popular models of dissipative structures (Gilbert and Landau-Lifshitz dissipation) are explored. Analytical results are obtained and discussed for the magnetisation, the wave number and the velocity of the helical magnetisation structures which are analogous to the behaviour of domain walls under spin polarised current in ferromagnets.     

Generation of spin currents via Raman scattering

We show theoretically that stimulated spin-flip Raman scattering can be used to inject spin currents in doped semiconductors with spin-split bands. A pure spin current, where oppositely oriented spins move in opposite directions, can be injected in zinc blende crystals and structures. The calculated spin current should be detectable by pump-probe optical spectroscopy and anomalous Hall effect measurement.     

Muon spin rotation studies at SIN

Recently the accelerator and the muon channel at SIN became operational. We report here on some of the first positive muon spin rotation experiments at SIN. The first experiment we discuss concerns the chemistry of muonium. We have observed for the first time a muonium or radical signal in pure water. Next, we discuss the application of the positive muon for the study of ferromagnetic metals and alloys. First measurements on a single crystal of iron around liquid helium temperature seem to indicate that at low temperature the muon does not diffuse. We further report on the first stroboscopic observation of the muon spin rotation which will allow one to take full advantage of the high stopping density at SIN.     

Some examples from muon spin rotation

The method of μSR is briefly described and its potential in the field of solidstate physics (especially metal physics) is discussed. Some examples from the fields of magnetism, spin relaxation and particle diffusion in metals are shown and comparisons with conventional hyperfine interaction methods are made for some areas of application.     

Negative muon spin rotation in solids

Experimental advances in solid state physics research using polarized negative muons (in the ground state of muonic atoms) are reviewed. The main subject is studies of ^– hyperfine interactions in magnetic materials. Basic principles and distinctive features of the ^–SR method are also presented, and possible future developments are briefly discussed.     

Birefringence (spin rotation and spin dichroism) of high-energy deuterons

The phenomenon of birefringence (spin rotation and spin dichroism) of high-energy deuterons is a macroscopic quantum effect similar to optical birefringence. Spin dichroism has already been observed experimentally. The contribution to spin dichroism produced by the interaction of the deuteron electric quadrupole moment with the electric field of a nucleus is considered. It is found that the effect is sensitive to the behavior of wave functions of the deuteron ground state at short distances.