Spin-flip Mechanism Research Articles

One-step replica symmetry breaking solution for a highly asymmetric two-sublattice fermionic Ising spin-glass model in a transverse field

The one-step replica symmetry breaking (RSB) is used to study a two-sublattice fermionic infinite-range Ising spin glass (SG) model in a transverse field $\Gamma$. The problem is formulated in a Grassmann path integral formalism within the static approximation. In this model, a parallel magnetic field $H$ breaks the symmetry of the sublattices. It destroys the antiferromagnetic (AF) order, but it can favor the nonergodic mixed phase (SG+AF) characterizing an asymmetric RSB region. In this region, intra-sublattice disordered interactions $V$ increase the difference between the RSB solutions of each sublattice. The freezing temperature shows a higher increase with $H$ when $V$ enhances. A discontinue phase transition from the replica symmetry (RS) solution to the RSB solution can appear with the presence of an intra-sublattice ferromagnetic average coupling. The $\Gamma$ field introduces a quantum spin flip mechanism that suppresses the magnetic orders leading them to quantum critical points. Results suggest that the quantum effects are not able to restore the RS solution. However, in the asymmetric RSB region, $\Gamma$ can produce a stable RS solution at any finite temperature for a particular sublattice while the other sublattice still presents RSB solution for the special case in which only the intra-sublattice spins couple with disordered interactions.

Electric field effect in the spin dynamics of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics in self-organized InAs/GaAs quantum dots (QDs) subject to vertical electric fields by photocurrent investigations. We propose a spin–flip mechanism involving a spin exchange between neighboring QDs. The spin–flip process is revealed in the photocurrent dynamics when the exciton population increases unexpectedly with reverse bias.

Pauli blockade of the electron spin flip in bulk GaAs

By means of time-resolved optical orientation under strong optical pumping, the k-dependence of the electron spin-flip time (t_sf) in undoped GaAs is experimentally determined. t_sf monotonically decreases by more than one order of magnitude when the electron kinetic energy varies from 2 to 30 meV. At the high excitation densities and low temperatures of the reported experiments the main spin-flip mechanism of the conduction band electrons is the Bir-Aronov-Pikus. By means of Monte-Carlo simulations we evidence that phase-space filling effects result in the blocking of the spin flip, yielding an increase of t_sf with excitation density. These effects obtain values of t_sf up to 30 ns at k=0, the longest reported spin-relaxation time in undoped GaAs in the absence of a magnetic field.

Fast Initialization of the Spin State of an Electron in a Quantum Dot in the Voigt Configuration

We consider the initialization of the spin state of a single electron trapped in a self-assembled quantum dot via optical pumping of a trion level. We show that with a magnetic field applied perpendicular to the growth direction of the dot, a near-unity fidelity can be obtained in a time equal to a few times the inverse of the spin-conserving trion relaxation rate. This method is several orders of magnitude faster than with the field aligned parallel, since this configuration must rely on a slow hole spin-flip mechanism. This increase in speed does result in a limit on the maximum obtainable fidelity, but we show that for InAs dots, the error is very small.

Spin injection into a short DNA chain

Quantun spin transport through a short DNA chain connected to ferromagnetic electrodes has been investigated by the transfer matrix method. We describe the system by a tight-binding model where the parameters are extracted from the experimental data and realistic metal energy bands. For ferromagnetic iron electrodes, the magnetoresistance of a 30-basepair Poly(G)-Poly(C) DNA is found to be lower than 10% at a bias of < 4 V, but can rach up to 20% at a bias of 5 V. In the presence of the spin-flip mechanism, the magnetoresistance is significantly enhanced when the spin-flip coupling is weak but as the coupling becomes stronger the decreasing magnetoresistance develops an oscillatory behavior.

Spin-Flip Processes and Ultrafast Magnetization Dynamics in Co: Unifying the Microscopic and Macroscopic View of Femtosecond Magnetism

The femtosecond magnetization dynamics of a thin cobalt film excited with ultrashort laser pulses has been studied using two complementary pump-probe techniques, namely, spin-, energy-, and time-resolved photoemission and the time-resolved magneto-optical Kerr effect. Combining the two methods, it is possible to identify the microscopic electron spin-flip mechanisms responsible for the ultrafast macroscopic magnetization dynamics of the cobalt film. In particular, we show that electron-magnon excitation does not affect the overall magnetization even though it is an efficient spin-flip channel on the sub-200 fs time scale. Instead, we find experimental evidence for the relevance of Elliott-Yafet-type spin-flip processes for the ultrafast demagnetization taking place on a time scale of 300 fs.

Neutron scattering study of magnetic structure and metamagnetic transition between low- and high-moment states ofNpNiGa5

Systematic neutron scattering experiments have been carried out in order to reveal the magnetic properties of $\mathrm{Np}\mathrm{Ni}{\mathrm{Ga}}_{5}$. We have observed a ferromagnetic transition with the magnetic moment parallel to the tetragonal $c$ axis at ${T}_{\mathrm{C}}=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have also observed antiferromagnetic reflections with the propagation vector $\mathbit{q}=(1∕2\phantom{\rule{0.3em}{0ex}}1∕2\phantom{\rule{0.3em}{0ex}}1∕2)$, coexisting with the ferromagnetic component below ${T}_{\mathrm{N}}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The direction of the antiferromagnetic component is perpendicular to the $c$ axis, most likely ${\ensuremath{\mu}}^{\mathrm{AF}}\ensuremath{\Vert}⟨1\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}0⟩$. Spin-polarized neutron scattering experiments have revealed that only Np carries a magnetic moment. The polarization of Ni atom is less than $0.03\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Ni}$, judging from our experimental accuracy. Thus, we conclude that the magnetic moments of Np atoms exhibit the simple ferromagnetic ordering, ${T}_{\mathrm{N}}&lt;T&lt;{T}_{\mathrm{C}}$, while a canted antiferromagnetic structure appears where $T&lt;{T}_{\mathrm{N}}$. A cant angle $\ensuremath{\theta}$ of $\ensuremath{\sim}90\ifmmode^\circ\else\textdegree\fi{}$ between two adjacent Np moments and a magnetic moment $\ensuremath{\mu}$ of $\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Np}$ at $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been estimated. The temperature and the magnetic field dependences of these two components have also been quantitatively analyzed. (i) It is quite unusual that the Np total moment increases with the transition from the ferromagnetic to canted antiferromagnetic structure. (ii) The metamagnetic transition cannot be explained in terms of a simple spin-flip mechanism with competing interactions and anisotropy. (iii) Furthermore, an unusual reduction of the Np magnetic moment is accompanied with the metamagnetic transition. From these results, we concluded that a remarkable change in the $5f$ electronic state between the ferromagnetic low-moment state and the canted antiferromagnetic high-moment state takes place at ${T}_{\mathrm{N}}$ and metamagnetic transition.

Weak localization — Precursor of unconventional superconductivity in nanocrystalline boron-doped diamond

Analyzing the magnetotransport data obtained on nanocrystalline variety of heavily boron-doped diamond, we have proposed a new model of unconventional superconductivity in this material based on the very experimental fact that the symptoms of localization are precursors of superconducting transition. It is shown by quantitative arguments that the system treated as disordered Mott's metal in which the transport is controlled by weak localization of carriers should reveal, in the Kelvin range, superconducting transition just when the holes occupying neighbouring weakly localized orbits are paired by means of a spin-flip mechanism.

On unconventional superconductivity in boron-doped diamond

Considering the magnetotransport data obtained on nanocrystalline variety of heavily boron-doped diamond films prepared by plasma-enhanced chemical vapour deposition technique, we have proposed a new model of unconventional superconductivity observed in this material ( T C = 1.66 K) which is based on the very experimental fact that symptoms of localization are precursors to the on-set of superconductivity. It is shown that the system treated as disordered Mott's metal in which the low-temperature transport is controlled by weak localization of carriers should reveal, in the Kelvin range, superconducting transition just when the holes occupying neighbouring weakly localized orbits are paired by means of spin-flip mechanism. Besides, close analogy between properties of a hole on weakly localized orbit and vortex in a classical type-II superconductor is suggested.

Energy-resolved electron spin dynamics at surfaces ofp-doped GaAs

Electron-spin relaxation at different surfaces of $p$-doped GaAs is investigated by means of spin, time, and energy-resolved two-photon photoemission. These results are contrasted with bulk results obtained by time-resolved Faraday rotation measurements as well as calculations of the Bir-Aronov-Pikus spin-flip mechanism. Due to the reduced hole density in the band bending region at the (100) surface the spin-relaxation time increases over two orders of magnitude towards lower energies. At the flat-band (011) surface a constant spin relaxation time in agreement with our measurements and calculations for bulk GaAs is obtained.

Interplay of direct and spin-flip contributions in Gamow-Teller decay to odd-odd N = Z nuclei

Strengths of Gamow-Teller decays of Tz = ±1 nuclei to Tz = 0 odd-odd nuclei have been calculated by using spherical shell model and deformed Nilsson wave functions. The role and competition of the microscopic direct and spin-flip mechanisms generating Gamow-Teller transitions are analyzed. Analytical expressions derived for the B(GT) values give useful insight into the regularities of B(GT) values along the N = Z line. The crucial role of configuration mixing is discussed.

Spin relaxation in diluted magnetic semiconductor quantum dots

Electron spin relaxation induced by phonon-mediated s-d exchange interaction in a II-VI diluted magnetic semiconductor quantum dot is investigated theoretically. The electron-acoustic phonon interaction due to piezoelectric coupling and deformation potential is included. The resulting spin lifetime is typically on the order of microseconds. The effectiveness of the phonon-mediated spin-flip mechanism increases with increasing Mn concentration, electron spin splitting, vertical confining strength and lateral diameter, while it shows non-monotonic dependence on the magnetic field and temperature. An interesting finding is that the spin relaxation in a small quantum dot is suppressed for strong magnetic field and low Mn concentration at low temperature.

Voltage Control of the Spin Dynamics of an Exciton in a Semiconductor Quantum Dot

We report the observation of a spin-flip process in a quantum dot whereby a dark exciton with total angular momentum L = 2 becomes a bright exciton with L = 1. The spin-flip process is revealed in the decay dynamics following nongeminate excitation. We are able to control the spin-flip rate by more than an order of magnitude simply with a dc voltage. The spin-flip mechanism involves a spin exchange with the Fermi sea in the back contact of our device and corresponds to the high temperature Kondo regime. We use the Anderson Hamiltonian to calculate a spin-flip rate, and we find excellent agreement with the experimental results.

Spin pump in the presence of a superconducting lead

We present a theoretical analysis of a spin pump in the presence of a superconducting lead. The spin pump is facilitated by a rotating magnetic field which provides a spin flip mechanism and hence can generate a spin current without an accompanying charge current. Using a nonequilibrium Green's function method, we obtain a general solution of the pumped charge current and spin current in both the adiabatic and non-adiabatic regimes. The numerical results for the charge current and spin current are presented as we vary different system parameters such as the gate voltage, the external magnetic field, and the pumping frequency. We find that for a quantum dot with a single resonant level in line with the Fermi energy of the left normal lead, a pure spin current is generated by a rotating magnetic field at any frequency. We have identified two kinds of photon-assisted processes which dominate at low pumping frequencies and high pumping frequencies, respectively.

Quantum spin field effect transistor

We propose, theoretically, a type of quantum field effect transistor that operates purely on the flow of spin current in the absence of charge current. This spin field effect transistor (SFET) is constructed without magnetic material, but with the help of a spin flip mechanism provided by a rotating external magnetic field. The SFET generates a constant instantaneous spin current that is sensitively controllable by a gate voltage as well as by the frequency and strength of the rotating field. The characteristics of a carbon nanotube based SFET is provided as an example.

Spin-flip Raman scattering in semi-magnetic quantum wells with in-plane anisotropy: Analysis of the intermediate states

Spin-flip Raman scattering has been studied in $\mathrm{CdTe}/{\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ quantum well structures which have previously been demonstrated to possess a reduced symmetry, leading to a hole g tensor which is highly anisotropic in the plane. This unusual behavior has been exploited to investigate in detail the spin-flip Raman-scattering mechanisms. Distinct signals arising from localized excitons and from excitons bound to neutral donors are observed. These signals are found to vary in intensity depending on the angle between the crystal axes and the in-plane magnetic field. The intensities of the signals as a function of laser energy have also been determined. From these observations, a model involving the nature of the intermediate states in the scattering process has been developed which allows the determination of the components of hole g tensor. This model is further applied to explain the observed angular dependence of the spin-flip Raman scattering of the ${3d}^{5}$ electrons of the ${\mathrm{Mn}}^{2+}$ ions. In addition, the model predicts a combined scattering process involving both a ${\mathrm{Mn}}^{2+}$ electron and a donor-bound electron; this process is indeed observed.

Annealing effect on the recovery of training effect in FeO/sub x/-CoFe polycrystalline systems

Recovery, by low-temperature (70/spl deg/C) annealing, of switching fields and training effect of a trained FeO/sub x/(/spl alpha/-Fe/sub 2/O/sub 3/ with a fraction of Fe/sub 0.98/O)(70 nm)-CoFe(5 nm) bilayer sample has been investigated. The recovery curve of the switching field in the direction opposite to the pinned direction was well fitted to a curve of a single activation energy response curve for about 1 eV. The training effect was also found to recover by annealing to some extent, but not as much as the switching field. Those results have been explained by a different mechanism from the thermal spin-flip mechanism contributing to the training effect itself.

Room temperature spin polarized injection in organic semiconductor

Spintronics is a new branch of electronics based on purely quantum effects. Instead of carrier charge transfer, as in usual electronics, it evokes carrier's spin transfer. The search of new materials suitable for injecting and transferring carriers with a preferential spin orientation is of paramount importance for the development of spintronics. Here we report a first experimental evidence of room temperature direct spin polarized injection in sexithienyl (T 6), a prototypical organic semiconductor, from colossal magnetoresistance manganite La 0.7Sr 0.3MnO 3 (LSMO). A strong magnetoresistance (up to 30%) was measured on nanostructured planar hybrid junctions LSMO/T 6/LSMO. The spin diffusion length in T 6 is about 200 nm at room temperature. The results are discussed taking into account possible spin-flip mechanisms in organic material and interface effects.

Spin relaxation in semiconductor quantum dots

We have studied the physical processes responsible for the spin -flip in GaAs quantum dots. We have calculated the rates for different mechanisms which are related to spin-orbit coupling and cause a spin-flip during the inelastic relaxation of the electron in the dot both with and without a magnetic field. We have shown that the zero-dimensional character of the problem when electron wave functions are localized in all directions leads to freezing out of the most effective spin-flip mechanisms related to the absence of the inversion centers in the elementary crystal cell and at the heterointerface and, as a result, to unusually low spin-flip rates.

Tunnelling current in semimagnetic semiconductor heterostructures

We have investigated theoretically the spin-polarised tunnelling current through a triple-barrier structure based on semimagnetic semiconductors. In this device the resonant tunnelling for each spin state is achieved at different bias, yielding well defined peaks in the I– V curve that have high degree of spin polarisation. Spin-flip scattering gives rise to additional peaks in the I– V curve when the alignments of the levels with opposite spins occur. We have used perturbation theory to treat the spin-dependent scattering mechanism, which in our model is driven by the thermal fluctuation of the ion magnetic moments. A double-barrier-tunnelling device has also been investigated, in which we found current with high degree of spin polarisation, even when the aforementioned spin-flip mechanism is considered. This device has potential applications as a spin filter.