Spin waves in layered conductors

The spectrum of spin waves propagating in layered conductors with a quasi-bidimensional law of charge carrier dispersion was determined for the case of an arbitrary correlation function and an external magnetic field perpendicular to the conducting layers.     

Spin polarization of two-dimensional electron gas in hybrid ferromagnetic/semiconductor nanostructures

We present a theoretical study on the spin-dependent transport of electrons in hybrid ferromagnetic/semiconductor nanosystem under an applied bias voltage. Experimentally, this kind of nanosystem can be realized by depositing a magnetized ferromagnetic stripe with arbitrary magnetization direction on the surface of a semiconductor heterostructure. It is shown that large spin-polarized current can be achieved in such a nanosystem. It is also shown that the spin polarity of the electron transport can be switched by adjusting the applied bias voltage. These interesting properties may provide an alternative scheme to realize spin injection into semiconductors, and such a nanosystem may be used as a tunable spin-filter by bias voltage.     

Spin dynamics in a one-dimensional ferromagnetic bose gas

We investigate the propagation of spin excitations in a one-dimensional ferromagnetic Bose gas. While the spectrum of longitudinal spin waves in this system is soundlike, the dispersion of transverse spin excitations is quadratic, making a direct application of the Luttinger liquid theory impossible. By using a combination of different analytic methods we derive the large time asymptotic behavior of the spin-spin dynamical correlation function for strong interparticle repulsion. The result has an unusual structure associated with a crossover from the regime of trapped spin wave to an open regime and does not have analogues in known low-energy universality classes of quantum 1D systems.     

Spin waves in dilute ferromagnetic alloys of Pd-Fe type

The concentration dependence of spin waves stiffness coefficient in Pd-Fe alloys is calculated. Our results are in a good agreement with experimental results.     

Spin waves in a one-dimensional spinor bose gas

We study a one-dimensional (iso)spin 1/2 Bose gas with repulsive delta-function interaction by the Bethe Ansatz method and discuss the excitations above the polarized ground state. In addition to phonons the system features spin waves with a quadratic dispersion. We compute analytically and numerically the effective mass of the spin wave and show that the spin transport is greatly suppressed in the strong coupling regime, where the isospin-density (or "spin charge") separation is maximal. Using a hydrodynamic approach, we study spin excitations in a harmonically trapped system and discuss prospects for future studies of two-component ultracold atomic gases.     

Spin waves in degenerate ferromagnetic semiconductors at low temperatures

Thes-d(f) exchange model of a ferromagnetic semiconductor is considered. With the use of the perturbation expansion in the formal parameter (2S)^–1,S being the magnitude of magnetic ion spin, the magnon Green function is calculated. The spectrum and damping of spin waves in the presence of conduction electrons is investigated. It is demonstrated that the change of sign of the temperature dependent correction to the spin-wave stiffness is possible provided that the Fermi energy exceeds some critical value. This may lead to the increase of the spin-wave stiffness with temperature unlike the case of the Heisenberg ferromagnet. The estimations of this effect for doped EuO and EuS are carried out.     

Plasma frequency in a layered electron gas

The equation-of-motion method is applied in the RPA for an electron-hole pair operator in the layered electron gas of Visscher and Falicov. The band of plasma frequencies is obtained at zero temperature.     

Friedell oscillations in a layered electron gas

The oscillations appearing in the potential of a layered electron gas in the random phase approximation are investigated for arbitrary plane distances. Due to the two-dimensional Fermi sphere, they turn out to be always of two-dimensional character, in contrast to the three-dimensional screening behaviour of the system.     

External forces outside of a layered electron gas

We calculate the external forces outside of the surface of a layered electron gas (LEG). The LEG is a model of a metal where the electrical current is carried in parallel layers, and there is no current between layers. It describes the high-temperature cuprate superconductors and many other layered solids. We calculate the image potential from an external charge, the van der Waals potential from a neutral atom and the Casimir force between the parallel surfaces of two LEGs. Our theory does not use dielectric functions. We write down the quantum mechanical Hamiltonian, calculate the exact ground state energy and deduce the forces from the energy. We also show that the LEG has no surface plasmon.     

Moment sum rules for a layered electron gas

Summary We have evaluated the first few frequency moments of the dissipation spectra for longitudinal and transverse excitations in a layered electron gas model, consisting of a periodic stack of two-dimensional electron gases. Short-range interplanar correlations yield a specific contribution to the first moment of the current-current dissipation spectra whenever the wave vector has a finite perpendicular component. The result is interpreted as evidence for additional excitations lying above the well-known acoustic plasmon in this model system.

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Riassunto Il calcolo dei momenti spettrali per eccitazioni longitudinali e trasversali in un gas elettronico stratificato mostra un contributo specifico al primo momento degli spettri di dissipazione corrente-corrente, dovuto a correlazioni interplanari a corto raggio nel caso che il vettore d'onda abbia una componente perpendicolare agli strati. Si interpreta questo risultato come evidenza per l'esistenza di eccitazioni sovrastanti il plasmone acustico del sistema.

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Реэюме Мы оцениваем первые несколько частотных моментов диссипативных спектров для продольных и поперечных воэбуждений в модели слоистого электронного газа, состоящего из периодической стопки двумерных электронных гаэов. Короткодействующие межплоскостные корреляции дают специфический вклад в первый момент ток-токовых диссипативных спектров, когда волновой вектор имеет конечную перпендикулярную компоненту. Этот результат интерпретируется, как подтверждение дополнительных возбуждений, лежащих выше хорошо известного акустического плазмона в этой модельной системе.

     

Spin flip inelastic scattering in electron energy loss spectroscopy of a ferromagnetic metal

A model ferromagnetic metal is used to calculate the spin-polarization which occurs during inelastic electron-metal scattering with the production of an electron-hole pair. The polarization is found to have contributions from unequal spin-flip as well as non-flip energy loss rates. Our results indicate an asymmetry of the order of a few percent with parameters roughly modelling iron.     

Dispersion of a layered electron gas with nearest neighbour-tunneling

The dispersion of the first plasmon band is calculated within the Random Phase Approximation for a superlattice of two-dimensional electrongases, mutually interacting, and with nearest neighbour hopping between the planes. It is further shown that the deviations of this dispersion from the one in systems with zero interplane motion are very small in commonly realized experimental situations and that they are expected to be observable only in samples with plane distances of 150 Å and less.     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

The plasmon density of states of a layered electron gas

The plasmon density of states (DOS)p() of a layered electron gas (LEG) is studied theoretically. It is shown thatp() is a linear function of frequency as goes to 0, and then increases more rapidly as tends to the screened, three dimensional plasma frequency _p. We also study the partial densities of state for constantK andq _z , whereK andq _z are the magnitude of the transverse and perpendicular momentum transfers. A possible experimental probe for the plasmon DOS is discussed.     

Spin polarization of two-dimensional electron gas in a hybrid magnetic-electric barrier

We report on a theoretical study of spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a stripe of ferromagnetic metal under an applied voltage. A general formula of transmission probability for electronic tunneling through this system is obtained. Based on this formula, it is shown that large spin-polarized current can be achieved in such a device. It is also shown that the degree of electron-spin polarization is strongly dependent upon the applied voltage to the stripe in the device. These interesting properties may provide an alternative scheme to spin-polarize electrons into semiconductors, and this device may be used as a voltage-tunable spin-filter.     

Collective excitations of a layered electron gas in a strong magnetic field

We determine the excitations of a layered electron gas in a strong magnetic field calculating the density-density response function, the transverse dynamical spin susceptibility and the phonon propagator by means of a Green's function technique. Electron tunneling processes between adjacent planes are taken into and found to be important for our results. In the limita/d1 (wherea is the magnetic length andd the distance between neighbouring planes) a very simple expression for the dynamical local field correction is found allowing us the evaluation of dispersion relations for magneto-plasmon, magneto-magnon and magneto-phonon in a straightforward manner. The transition temperature for a Magneto-Peierls transition is determined in dependence on electron-phonon coupling matrix element, tunneling probability, magnetic length and interplane distance.