On the feasibility of using positive muons as probes of defect structure

A self-consistent calculation based on the density functional formalism yields the spin density at a muon in a vacant lattice site, which is significantly different from that at a muon in an interstitial site. It is argued that a precision μ⁺ spin rotation experiment should be able to detect the difference in spin density.     

Transverse spin current in the s-wave/p-wave Josephson junction

We report a theoretical study on spin transport in the hybrid Josephson junction composed of singlet s-wave and triplet p-wave superconductor. The node of the triplet pair potential is considered perpendicular to the interface of the junction. Based on a symmetry analysis, we predict that there is no net spin density at the interface of the junction but instead a transverse mode-resolved spin density can exist and a nonzero spin current can flow transversely along the interface of the junction. The predictions are numerically demonstrated by means of the lattice Matsubara Green's function method. It is also shown that, when a normal metal is sandwiched in between two superconductors, both spin current and transverse mode-resolved spin density are only residing at two interfaces due to the smearing effect of the multimode transport. Our findings are useful for identifying the pairing symmetry of the p-wave superconductor and generating spin current.     

Correlation between spin density and Vth instability of IGZO thin-film transistors

The electron spin resonance (ESR) detects point defect of the In-Ga-Zn oxide (IGZO) like singly ionized oxygen vacancies and excess oxygen, and get spin density as a parameter of defect state. So, we demonstrated the spin density measurement of the IGZO film with various deposition conditions and it has linear relationship. Moreover, we matched the spin density with the total BTS and the threshold voltage (V_th) distribution of the IGZO thin film transistors. The total BTS ΔV_th and the V_th distribution were degraded due to the spin density increases. The spin density is the useful indicator to predict V_th instability of IGZO TFTs.     

A virtual intersubband spin–flip spin–orbit coupling induced spin relaxation in GaAs (110) quantum wells

A spin relaxation mechanism is proposed based on a second-order spin–flip intersubband spin–orbit coupling together with the spin-conserving scattering. The corresponding spin relaxation time is calculated via the Fermi golden rule. It is shown that this mechanism is important in symmetric GaAs (110) quantum wells with high impurity density. The dependencies of the spin relaxation time on electron density, temperature and well width are studied with the underlying physics analyzed.     

Exact-exchange spin-current density-functional theory

A spin-current density-functional theory (SCDFT) is introduced, which takes into account the currents of the spin density and thus currents of the magnetization in addition to the electron density, the noncollinear spin density, and the density current, which are considered in standard current-spin-density-functional theory. An exact-exchange Kohn-Sham formalism based on SCDFT is presented, which represents a general framework for the treatment of magnetic and spin properties. As an illustration, an oxygen atom in a magnetic field is treated with the new approach.     

Spin and torsion in the very early universe

In the very early universe with temperature T between 10²⁴ K and 10³² K the gravitational effect of torsion is dominant if particles with spin are sufficiently polarized. The source of the torsion is the spin density and the latter is usually described by a classical theory of Weyssenhoff and Raabe. In this article the spinning particles are described quantum mechanically, i.e. with a Dirac field and the spin density is defined as the source of the torsion. The macroscopic average of the spin density is obtained by the relativistic Wigner function formalism. The expression of the spin density, as derived in this article, is different from the classical one, except when both are zero.     

Decay of correlations in spin systems

We investigate the decay of initial correlations in a spin system where each spin relaxes independently by an intramolecular mechanism. The equation of motion for the spin density matrix is assumed to be the Redfield equation, which is of the form of a quantum mechanical master equation. Our analysis of this problem is based on the techniques of Shuler, Oppenheim, and coworkers, who have studied the decay of correlations in systems which can be described by classical stochastic master equations. We find that the off-diagonal elements of the reduced spin density matrices approach their equilibrium values faster than the diagonal elements. The Ursell functions, which are a measure of the correlations in the system, decay to their zero equilibrium values faster than the spin density matrix except for the furthest off-diagonal elements. Far off-diagonal matrix elements of the spin density matrix approach equilibrium at the same rate as the Ursell functions, which is the important difference between the quantum mechanical model studied here and the classical models studied earlier.Supported in part by the National Science Foundation.     

Spinor dipolar bose-einstein condensates: classical spin approach

Bose-Einstein condensates which are dominated by magnetic dipole-dipole interaction are discussed under spinful situations. We treat the spin degrees of freedom as a classical spin vector, approaching from the large spin limit to obtain an effective minimal Hamiltonian. This is a version extended from a nonlinear sigma model. By solving the Gross-Pitaevskii equation, we find several novel spin textures where the mass density and spin density are strongly coupled, depending upon trap geometries due to the long-range and anisotropic natures of the dipole-dipole interaction.     

Magnetic phase diagram in a quasi-two-dimensional electron gas

Using spin density functional theory within the framework of the local spin density approximation with Perdew-Zunger type exchange-correlation energy, ferromagnetism in a quasi-two-dimensional electron gas (Q-2DEG) is studied. The electronic and magnetic structures of a thin film are calculated as a function of film thickness and electron density. Ferromagnetism in the Q-2DEG is found to appear at a higher electron density than in the three-dimensional electron gas. Unless a film is very thin, with decreasing electron density, a magnetic phase transition occurs from a spin-unpolarized fluid to a Wigner film with surface magnetism, in which the spin polarization localizes only in the neighborhood of surfaces. Further decreasing density induces another transition to a fully spin-polarized ferromagnetic Wigner film.     

The spin density waves in chromium detected by hyperfine field measurements

The magnetic hyperfine field of tantalum nuclei in a high purity chromium matrix has been measured using the Time Differential Perturbed Angular Correlation technique. The spectra show that the hyperfine field is proportional to the amplitude of the spin density wave of chromium and that the tantalum probe nuclei do not clamp the phase of the spin density wave. The incommensurate antiferromagnetic first order phase transition as well as the spin flip transition have been observed. The temperature dependence of the hyperfine field is shown to deviate from the temperature dependence of the maximum magnetization of the spin density wave.     

Current driven magnetization dynamics in helical spin density waves

A mechanism is proposed for manipulating the magnetic state of a helical spin density wave using a current. It is shown that a current through a bulk metal with a helical spin density wave induces a spin transfer torque, which gives rise to a rotation of the order parameter. The use of spin transfer torque to manipulate the magnetization in bulk systems does not suffer from the obstacles seen for magnetization reversal using interface spin transfer torque in multilayered systems. The effect is demonstrated by a quantitative calculation of the current induced magnetization dynamics of a rare earth metal, Er. Finally, we propose a setup for experimental verification.     

Gauge theory approach for diffusive and precessional spin dynamics in a two-dimensional electron gas

We develop a gauge theory for diffusive and precessional spin dynamics in a two-dimensional electron gas. Our approach reveals a direct connection between the absence of the equilibrium spin current and a strong anisotropy in the spin relaxation: both effects arise if spin-orbit coupling is reduced to a pure gauge SU(2) field. In this case, the spin-orbit coupling can be removed by a gauge transformation in the form of a local SU(2) spin rotation. The resulting spin dynamics is exactly described in terms of two kinetic coefficients: the spin diffusion and electron mobility. After the inverse transformation, full diffusive and precessional spin density dynamics, including the anisotropic spin relaxation, formation of stable spin structures, and spin precession induced by a macroscopic current are restored. Explicit solutions of the spin evolution equations are found for the initially uniform spin density and for stable, nonuniform structures. Our analysis demonstrates a universal relation between the spin relaxation rate and spin-diffusion coefficient.     

New properties of magnon density in uniaxial anisotropic ferromagnet on the background of spin wave

The N-soliton solutions of magnetization in uniaxial anisotropic ferromagnet on the background of spin wave are presented by using the effective Darboux transformation method. With the analytical solutions new properties of magnon density is studied in detail. On the ground state background the magnon density is constant for the spin wave solution and the magnetic soliton, respectively. However, on the spin wave background the magnon density possesses of temporal or spatial periodic oscillation. Moreover, the soliton solution possess the breather character in its propagation along the ferromagnet. These results show that during soliton propagation a periodic magnon exchange occurs between the magnetic soliton and the spin wave background.     

Spin torque transfer structure with new spin switching configurations

Spin torque transfer structures with new spin switching configurations are proposed, fabricated and investigated in this paper. The non-uniform current-induced magnetization switching is implemented based on both GMR and MTJ nano devices. The proposed new spin transfer structure has a hybrid free layer that consists of a layer with conductive channels (magnetic) and non-conductive matrix (non-magnetic) and traditional free layer(s). Two mechanisms, a higher local current density by nano-current-channels and a non-uniform magnetization switching (reversal domain nucleation and growth) by a magnetic nanocomposite structure, contribute in reducing the switching current density. The critical switching current density for the new spin transfer structure is reduced to one third of the typical value for the normal structure. It can be expected to have one order of magnitude or more reduction for the critical current density if the optimization of materials and fabrication processes could be done further. Meanwhile, the thermal stability of this new spin transfer structure is not degraded, which may solve the long-standing scaling problem for magnetic random access memory (MRAM). This spin transfer structure, with the proposed and demonstrated new spin switching configurations, not only provides a solid approach for the practical application of spin transfer devices but also forms a unique platform for researchers to explore the non-uniform current-induced switching process.     

On the calculation of spin densities

Methods are presented for calculating the expectation value of the spin density for the state resulting from spin-projection of an anti-symmetrized spin-orbital product. It is found that the spin densities for projected and unprojected states differ in the coefficients of the various spatial terms, and that these coefficients can be determined from the properties of the spin algebra. It is shown how the coefficients needed for the spin density are related to those previously derived by other workers for spin-free operators. Illustrative cases of the formalism are examined in detail to show how the calculated z component of the spin density depends upon the total spin as well as upon its z component.     

Electron spin torque in atoms

The spin torque and zeta force, which govern spin dynamics, are studied by using monoatoms in their steady states. We find nonzero local spin torque in transition metal atoms, which is in balance with the counter torque, the zeta force. We show that d-orbital electrons have a crucial effect on these torques. Nonzero local chirality density in transition metal atoms is also found, though the electron mass has the effect to wash out nonzero chirality density. Distribution patterns of the chirality density are the same for Sc–Ni atoms, though the electron density distributions are different.     

Density matrix renormalization group studies on one-dimensional -conjugated organic ferromagnet with side radicals

By using the density matrix renormalization group (DMRG) and the self-consistent numerical method, we obtain a high spin ground state with localized spin density describing spin localization and the soliton describing the distortion of the lattice configurations along the main chain. Different electron-phonon interactions result in different configurations of solitons. When the electron-phonon coupling along the main chain is larger than a critical value , a transition from a single soliton-like distortion to a pair of soliton-like distortions along the main chain takes place. Such critical value depends mainly on the intersite Coulomb interactions. The spin density wave along the main chain is always localized around the center of soliton-like distortions. Received 2 July 2001 and Received in final form 25 September 2001     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 1998     

Master Equation in Phase Space for a Uniaxial Spin System

A master equation, for the time evolution of the quasi-probability density function of spin orientations in the phase space representation of the polar and azimuthal angles is derived for a uniaxial spin system subject to a magnetic field parallel to the axis of symmetry. This equation is obtained from the reduced density matrix evolution equation (assuming that the spin-bath coupling is weak and that the correlation time of the bath is so short that the stochastic process resulting from it is Markovian) by expressing it in terms of the inverse Wigner-Stratonovich transformation and evaluating the various commutators via the properties of polarization operators and spherical harmonics. The properties of this phase space master equation, resembling the Fokker-Planck equation, are investigated, leading to a finite series (in terms of the spherical harmonics) for its stationary solution, which is the equilibrium quasi-probability density function of spin “orientations” corresponding to the canonical density matrix and which may be expressed in closed form for a given spin number. Moreover, in the large spin limit, the master equation transforms to the classical Fokker-Planck equation describing the magnetization dynamics of a uniaxial paramagnet.