Subsecond spin relaxation times in quantum dots at zero applied magnetic field due to a strong electron-nuclear interaction

A key to ultralong electron spin memory in quantum dots (QDs) at zero magnetic field is the polarization of the nuclei, such that the electron spin is stabilized along the average nuclear magnetic field. We demonstrate that spin-polarized electrons in n-doped (In,Ga)As/GaAs QDs align the nuclear field via the hyperfine interaction. A feedback onto the electrons occurs, leading to stabilization of their polarization due to formation of a nuclear spin polaron [I. A. Merkulov, Phys. Solid State 40, 930 (1998)]. Spin depolarization of both systems is consequently greatly reduced, and spin memory of the coupled electron-nuclear spin system is retained over 0.3 sec at temperature of 2 K.     

Spin-polarized nuclei as probes of electromagnetic field distributions on solid surfaces

Recent experiments using thermal beams of nuclear-spin-polarized alkali atoms adsorbed on hot metal surfaces show that polarized nuclei are sensitive probes of surface electromagnetic field distributions. The high polarization of the probe beams, when coupled with the efficiency of atomic physics techniques used for monitoring the polarization of desorbed particles, makes possible a variety of interesting spinrelaxation experiments on single-crystal surfaces, including nuclear magnetic resonance. Extension of the current experimental method to semiconductor and insulator surfaces at arbitrary temperatures appears to be straightforward. The information from spin-polarized nuclear surface spectroscopy (SPNSS) will allow detailed tests of charge-density profiles now available in self-consistent surface structure calculations. Moreover, the variety of presently available polarized nuclear species suggests that the chemistry of many interesting adsorbate-surface systems could also be profitably investigated by this technique. The use of spin-polarized hydrogen nuclei in particular offers enticing prospects for fundamental studies in catalysis, surface structure and basic two-dimensional physics.     

Nuclear spin polarization in silicon nanostructures with charge carrier injection

We theoretically examine injection polarization of nuclear spins in silicon nanostructures with hyperfine interaction of nuclei with excited triplet states. We predict the possibility of the appearance of self-sustaining nuclear spin polarization, initiated by an external field. We show that if the external magnetic field is varied, we observe up to a 600-fold jump in the number of spin-polarized nuclei. A similar up to 40-fold jump also appears as the charge carrier injection rate increases. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 5, pp. 647–653, September–October, 2006.     

Strongly dipolar Bose-Einstein condensate of dysprosium

We report the Bose-Einstein condensation (BEC) of the most magnetic element, dysprosium. The Dy BEC is the first for an open f-shell lanthanide (rare-earth) element and is produced via forced evaporation in a crossed optical dipole trap loaded by an unusual, blue-detuned and spin-polarized narrowline magneto-optical trap. Nearly pure condensates of 1.5 × 10(4) (164)Dy atoms form below T = 30 nK. We observe that stable BEC formation depends on the relative angle of a small polarizing magnetic field to the axis of the oblate trap, a property of trapped condensates only expected in the strongly dipolar regime. This regime was heretofore only attainable in Cr BECs via a Feshbach resonance accessed at a high-magnetic field.     

Dynamic nuclear spin resonance in n-GaAs

The dynamics of optically detected nuclear magnetic resonance is studied in n-GaAs via time-resolved Kerr rotation using an on-chip microcoil for rf field generation. Both optically allowed and optically forbidden NMR are observed with a dynamics controlled by the interplay between dynamic nuclear polarization via hyperfine interaction with optically generated spin-polarized electrons and nuclear spin depolarization due to magnetic resonance absorption. Comparing the characteristic nuclear spin relaxation rate obtained in experiment with master equation simulations, the underlying nuclear spin depolarization mechanism for each resonance is extracted.     

Theory for p-wave Feshbach molecules

We determine the physical properties of p-wave Feshbach molecules in doubly spin-polarized 40K and find excellent agreement with recent experiments. We show that these molecules have a large probability Z to be in the closed channel or bare molecular state responsible for the Feshbach resonance. In the superfluid state this allows for observation of Rabi oscillations between the molecular and atomic components of the Bose-Einstein condensed pairs, which contains a characteristic signature of the quantum phase transition that occurs as a function of applied magnetic field.     

Dynamics of two overlapping spin ensembles interacting by spin exchange

We describe linear and nonlinear dynamics of spin-polarized K and 3He ensembles interacting by spin exchange. The interactions are dominated by the imaginary part of the spin-exchange cross section and each spin species is primarily affected by the average magnetization of the other. Operating in a very low magnetic field we demonstrate novel dynamics when the electron and nuclear spin precession frequencies are nearly matched. We observe transverse damping as well as a dynamic instability of the 3He spins interacting with polarized K vapor. We also demonstrate operation as a self-compensating comagnetometer, useful for tests of CPT violation and other precision measurements.     

Unconventional proximity effect and inverse spin-switch behavior in a model manganite-cuprate-manganite trilayer system

The proximity effect in a model manganite-cuprate system is investigated theoretically. We consider a situation in which spin-polarized electrons in manganite layers antiferromagnetically couple with electrons in cuprate layers as observed experimentally. The effect of the interfacial magnetic coupling is found to be much stronger than the injection of spin-polarized electrons into the cuprate region. As a result, the superconducting transition temperature depends on the thickness of the cuprate layer significantly. Since the magnetic coupling creates negative polarization, an applied magnetic field and the negative polarization compete, resulting in the inverse spin-switch behavior where the superconducting transition temperature is increased by applying a magnetic field.     

Electrical detection of spin accumulation at a ferromagnet-semiconductor interface

We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and -GaAs can be detected electrically. The spin accumulation leads to an additional voltage drop across the barrier that is suppressed by a small transverse magnetic field, which depolarizes the spins in the semiconductor. The dependence of the electrical accumulation signal on magnetic field, bias current, and temperature is in good agreement with the predictions of a drift-diffusion model for spin-polarized transport.     

Spin-polarized to valley-polarized transition in graphene bilayers at ν=0 in high magnetic fields

We investigate the transverse electric field (E) dependence of the ν=0 quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic fields. The longitudinal resistivity ρ(xx) measured at ν=0 shows an insulating behavior which is strongest in the vicinity of E=0, as well as at large E fields. At a fixed perpendicular magnetic field (B), the ν=0 QHS undergoes a transition as a function of the applied E, marked by a minimum, temperature-independent ρ(xx). This observation is explained by a transition from a spin-polarized ν=0 QHS at small E fields to a valley- (layer-)polarized ν=0 QHS at large E fields. The E field value at which the transition occurs follows a linear dependence on B.     

Spin filtering in a nonuniform quantum wire with Rashba spin-orbit interaction

We investigate theoretically the spin-polarized electron transport for a wide-narrow-wide (WNW) quantum wire under the modulation of Rashba spin-orbit interaction (SOI). The influence of both the structure of the quantum wire and the interference between different pairs of subbands on the spin-polarized electron transport is taken into account simultaneously via the spin-resolved lattice Green function method. It is found that a very large vertical spin-polarized current can be generated by the SOI-induced effective magnetic field at the structure-induced Fano resonance even in the presence of strong disorder. Furthermore, the magnitude of the spin polarization can be tuned by the Rashba SOI strength and structural parameters. Those results may provide an effective way to design a spin filter device without containing any magnetic materials or applying a magnetic field.     

Dynamics of magnetization in ferromagnet with spin-transfer torque

We review our recent works on dynamics of magnetization in ferromagnet with spin-transfer torque. Driven by constant spin-polarized current, the spin-transfer torque counteracts both the precession driven by the effective field and the Gilbert damping term different from the common understanding. When the spin current exceeds the critical value, the conjunctive action of Gilbert damping and spin-transfer torque leads naturally the novel screw-pitch effect characterized by the temporal oscillation of domain wall velocity and width. Driven by space- and time-dependent spin-polarized current and magnetic field, we expatiate the formation of domain wall velocity in ferromagnetic nanowire. We discuss the properties of dynamic magnetic soliton in uniaxial anisotropic ferromagnetic nanowire driven by spin-transfer torque, and analyze the modulation instability and dark soliton on the spin wave background, which shows the characteristic breather behavior of the soliton as it propagates along the ferromagnetic nanowire. With stronger breather character, we get the novel magnetic rogue wave and clarify its formation mechanism. The generation of magnetic rogue wave mainly arises from the accumulation of energy and magnons toward to its central part. We also observe that the spin-polarized current can control the exchange rate of magnons between the envelope soliton and the background, and the critical current condition is obtained analytically. At last, we have theoretically investigated the current-excited and frequency-adjusted ferromagnetic resonance in magnetic trilayers. A particular case of the perpendicular analyzer reveals that the ferromagnetic resonance curves, including the resonant location and the resonant linewidth, can be adjusted by changing the pinned magnetization direction and the direct current. Under the control of the current and external magnetic field, several magnetic states, such as quasi-parallel and quasi-antiparallel stable states, out-of-plane precession, and bistable states can be realized. Th     

Magnetization of High Density Hadronic Fluid

In the present paper the magnetization of a high density relativistic fluid of elementary particles is studied. At very high densities, such as may be found in the interior of a neutron star, when the external magnetic field is gradually increased, the energy of the normal phase of the fluid remains practically constant before extremely high magnetic fields are reached. However, if pion condensation occurs, the energy decreases linearly while the magnetic field strength increases, so that a non vanishing magnetization, independent of the magnetic field, is present. The expression of the magnetization is derived by first considering and solving the Dirac equation of a fermion in interaction with a magnetic field and with a chiral sigma-pion pair. The solution provides the energies of single-particle states. The energy of the system is found by summing up contributions from all particles in the particle fluid. For nuclear densities above 2 to 3ρ ₀, where ρ ₀ is the equilibrium nuclear density, the resulting magnetic field turns out to be rather huge, of the order of 10¹⁷ Gauss.     

Mn基钙钛矿氧化物中的自旋极化隧道输运

借助自旋极化隧道模型，对具有不同居里温度的Mn基钙铁矿氧化物的电阻率随温度和磁场的变化行为进行了计算。结果表明，模型给出的结果和献上普遍报道的实验结果在行为上有非常好的一致性，表明这类材料中所观察到的电子输运和磁性质可以基于这一模型而得以理解。     

Spin-polarized transport across sharp antiferromagnetic boundaries

We report spin-polarized transport experiments across antiphase domain boundaries which act as atomically sharp magnetic interfaces. The antiphase boundaries are prepared by growing Fe(3)O(4) epitaxially on MgO, the magnetic coupling over a large fraction of these boundaries being antiferromagnetic. Magnetoresistance measurements yield linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane, respectively. This behavior can be explained by a hopping model in which spin-polarized electrons traverse an antiferromagnetic interface between two ferromagnetic chains.     

Production of Fe clusters by collisions of metal vapour with supersonic argon beams

For a spin-polarized plane wave passing through a spin-rotator containing uniform magnetic field, we provide a detailed analysis for solving the appropriate Schrödinger equation. A modified expression for spin precession is obtained which reduces to the standard Larmor precession relation when kinetic energy is very large compared to the spin-magnetic field interaction. We show that there are experimentally verifiable regimes of departure from the standard Larmor precession formula. The treatment is then extended to the case of a spin-polarized wave packet passing through a uniform magnetic field. The results based on the standard expression for Larmor precession and that obtained from the modified formula are compared in various regimes of the experimental parameters.     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

A simple model for maser action obtained through a magnetic tunnel junction placed inside a resonant cavity

Magnetic tunnel junctions are currently being used in magnetoresistive reading heads, magnetic field sensors and MRAMs, due to its giant magnetoresistance effect whose roots are linked to strong spin-dependent scattering mechanisms. The existence of spin-polarized currents in such devices posed us the question over the possibility to generate coherent microwave radiation in a spin inverted population medium, maintained through a spin-polarized current. In this paper we investigate the possibility of obtaining a maser effect considering a magnetic tunnel junction placed inside a resonant cavity. We put forward a simple model based on phenomenological rate equations, being the spin-polarized currents determined by the physics of the magnetic tunnel junction.     

Effective Hamiltonian for striped and paired states at the half-filled Landau level

We study a pairing mechanism for the quantum Hall system using a mean field theory with a basis on the von Neumann lattice, on which the magnetic translations commute. In the Hartree–Fock–Bogoliubov approximation, we solve the gap equation for spin-polarized electrons at the half-filled Landau levels. We obtain an effective Hamiltonian which shows a continuous transition from the compressible striped state to the paired state. Furthermore, a crossover occurs in the pairing phase. The energy spectrum and energy gap of the quasiparticle in the paired state is calculated numerically at the half-filled second Landau level.     

Electron spin precession in two-dimensional electron gas with Rashba spin-orbit coupling

Using the time-dependent Schrödinger equation, we present the analytical result of the expectation value of spin injected into a two-dimensional electron gas with respect to an arbitrarily spin-polarized electron state and monitor the spin time-evolution. We demonstrate that the expectation value of spin operator Sx is the time-independent, and only the expectation values in the Sy-Sz plane are time-dependent. A detailed study of spin precession in the spin-valve and spin-transistor geometry is presented, in which the initial spin-polarized electron state point perpendicular and parallel to the current direction, respectively. We put forward the possible reason that the resistance change is independent of gate voltage in the spin-valve geometry. Furthermore, it has been shown that the effective magnetic field generated by the spin-orbit interaction is not same with the truly magnetic field. The main effect of the truly magnetic field is to align the spin along the field direction, but the effective magnetic field generated by the spin-orbit interaction does not.