X-ray resonant magnetic scattering by Fe/Cr superlattices

We have studied the structural and magnetic properties of an antiferromagnetically (AF) coupled Fe/Cr superlattice by means of soft x-ray resonant magnetic scattering. Strong and purely magnetic Bragg peaks are observed at the half-order positions in reciprocal space parallel to the [001] growth direction and in between the structural Bragg reflections from the superlattice periodicity. The magnetic hysteresis loops measured at the first-order and at the half-order Bragg peaks clearly demonstrate the strong AF coupling of the Fe/Cr multilayer. Transverse scans and off-specular reflectivity measurements confirm an AF domain structure of the superlattice in remanence with large perpendicular correlation. In addition, the transverse scan of the half-order Bragg peak exhibits a Lorentzian line shape at zero field, which diminishes in higher fields, indicative of a remanent multidomain state approaching a single-domain state towards saturation.     

Theory of parametric amplification in superlattices

We consider a high-frequency response of electrons in a single miniband of superlattice subject to dc and ac electric fields. We show that Bragg reflections in miniband result in a parametric resonance which is detectable using ac probe field. We establish theoretical feasibility of phase-sensitive THz amplification at the resonance. The parametric amplification does not require operation in conditions of negative differential conductance. This prevents a formation of destructive domains of high electric field inside the superlattice.     

Effect of inter-miniband tunneling on current resonances due to the formation of stochastic conduction networks in superlattices

We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances.     

Complete reorganization of the mode structure of the wave function of a channeled electron in a crystal with a superlattice

The dynamics of relativistic electrons subjected to planar channeling in distorted crystals is investigated using step-by-step numerical integration of the Schrödinger equation. It is shown that periodic distortion of a planar channel (the formation of a “superlattice”) can cause complete reorganization of the mode structure of the wave function. In the limit of weak perturbations of the crystal lattice, the simulation results are consistent with the analytical results from perturbation theory. The position of each line in the spontaneous emission spectrum in the case of resonant distortions depends on the length of the superlattice period.     

Electron transport across a quantum wire embedding a saw-tooth superlattice

By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant peak splitting of this kind of structure is different from that of ‘magnetic' and ‘electric' superlattices in two-dimensional electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.     

Ultrafast Fiske effect in semiconductor superlattices

The current flowing across a semiconductor superlattice in tilted electric and magnetic fields is known to exhibit resonant enhancement, when Landau states of neighboring wells align at certain ratios of the field strengths. We show that the ultrafast version of this effect, in which coherent electron wave packets are involved, has a profound analogy to the Fiske effect in superconductor Josephson junctions and superfluid weak links, in that the coupling of the tunneling-induced charge oscillations (magneto-Bloch versus Josephson oscillations) to another oscillator (in-plane cyclotron oscillations versus external oscillator modes) opens an elastic rectifying transport channel. We explore the superlattice effect both theoretically and experimentally, and find that the transient self-induced current can be adequately modeled if the damping of both types of coupled electron oscillations is properly taken into account.     

用低杂波驱动电流的等离子体电导率

本文用Liu的低杂波驱动电流模型,导出了弱场近似下低杂波驱动电流的等离子体电导率。结果表明,电导率对磁场B_0,电子密度n_0,以及电子温度T_e非常敏感。而这些参量决定着共振电子的数目和速度。     

Triple-q(-->) octupolar ordering in NpO2

We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.     

Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

Proposal of a Microwave-Driven Semiconductor Superlattice Oscillator for Generation of THz Radiation

We present the proposal of a microwave-driven semiconductor superlattice oscillator. We show that the interplay of a microwave pump field with a synchronous harmonic field can make a semiconductor superlattice to a gain medium for the harmonic field. Placing the superlattice in a resonator for the harmonic field allows the operation of an oscillator. The gain mechanism is based on Bloch oscillations of miniband electrons. The gain is mediated either by the interaction of the high-frequency field with the single electrons or with space charge domains or with both. The microwave-driven superlattice oscillator should be suitable for generation of coherent radiation up to several THz.     

Dynamic conductivity of ac–dc-driven graphene superlattice

The dynamic conductivity of graphene superlattice in the presence of ac electric field and dc electric field with longitudinal and transversal components with respect to superlattice axis was calculated. In the case of strong transversal component of dc field conductivity of graphene superlattice was shown to be such as if the electrons had got the effective mass. In the case of weak transversal component of dc field conductivity was shown to change its sign if the frequency of ac field was an integer multiple of half of Bloch frequency.     

弱磁场下三阱光学超晶格中自旋为1的超冷原子特性研究

双阱光学超晶格中的超冷原子是近期冷原子物理领域的研究热点. 本文推广提出了实现三阱光学超晶格的方案, 并采用精确对角化的方法分别研究了弱磁场下对称三阱 光学超晶格中铁磁性和反铁磁性的自旋为1的原子系统的基态, 发现二者的相图很不相同: 反铁磁性原子对应的相图中没有沿磁场方向总自旋磁量子数为±2的基态, 而铁磁性原子对应的相图中可能有. 在负的二次塞曼能量区域, 铁磁性原子的相图中只有完全极化态. 分析了可控参数影响基态的物理本质. 由于这些量子自旋态可以通过调节外磁场和光势垒的高度非常简便而精确地控制, 适合用来研究自旋纠缠. 关键词： 三阱光学超晶格 自旋为1的原子 弱磁场     

Multistability,absolute negative conductivity and spontaneous current generation in semiconductor superlattices in large magnetic fields

We discuss electron transport through a semiconductor superlattice subject to an electric field parallel to, and a magnetic field perpendicular to, the growth axis using a semiclassical balance equation model. We find that the current–voltage characteristic becomes multistable in a large magnetic field; furthermore, hot electrons display novel features in their current–voltage characteristic, including absolute negative conductivity and a spontaneously generated dc current at zero bias.     

Mixing of bound states with electron transport by a radiation field in waveguides

Electron transmission in the two-, three-, and four-terminal nanostructures is considered under the influence of a radiation field. The frequency of the radiation field is tuned to the transition between the energy of a bound state and the Fermi energy of the incident electrons. The radiation induced resonant peaks and dips of the electron transport are exhibited for zero and low magnetic fields. It is shown that rotation of the radiation field polarization can effectively control the electron transport into different electrodes attached to the structures because of the symmetry of the structures. The resonant anomalies of the Hall resistance are found in a weak magnetic field. Zh. éksp. Teor. Fiz. 114, 1954–1970 (December 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Magnetic surface levels

As is well known, the energy spectrum of conduction electrons in a metal in a magnetic field is split into the Landau levels. These levels give rise to several phenomena whose essence is in the oscillatory dependence of some property of the metal characteristics on the strength of the magnetic field, in the range of strong and medium fields. Of these the most famous are the de Haas-van Alphen effect, and the Shubnikov-de Haas effect. Electronic transitions between the Landau levels give rise to the cyclotron resonance.

In a surface layer of a metal placed in a weak magnetic field another system of levels appears for electrons moving along shallow arcs, with their ends resting upon the surface of the metal. These levels originate from the quantized periodic motion of electrons along such ‘skipping’ trajectories due to a specular reflection at the metal surface. The spectrum of the system of magnetic surface levels manifests itself in an oscillatory dependence of the surface impedance on a weak magnetic field. The oscillations are due to a resonant absorption of microwave radiation in transitions of electrons between the magnetic levels occurring at discrete values of the magnetic field.

This new quantum effect discovered in several metals in both the normal and superconducting states should, in principle, be common to all the conductors. Studies of the effect are being extended rapidly, and one foresees a discovery of some new phenomena due to the surface bound states of charged quasiparticles, arising in a conductor when exposed to a weak magnetic field.     

斜向静磁场中的薄膜磁光效应和静磁波传播特性

理论计算表明，与垂直静磁场情形相比，斜向静磁场可以明显增强磁性薄膜波导中的法拉第效应，同时会引起静磁波共振频率和带宽等许多静磁波特性的变化.合理利用这些变化特性，可望大幅度提高导波光的衍射效率，改善磁光波导器件的特性. 关键词：     

Galvanomagnetic phenomena in semiconductor with a superlattice in the case of inelastic electron scattering

Galvanomagnetic phenomena in semiconductor with a superlattice in static electrical and magnetic fields in the case of strongly inelastic scattering of electrons by Optical phonons are considered.The existance of additional threshold perculiarities arising in magnetic field—current characterstics due to the finite width of conduction band is predicted.     

Frequency multiplication using induced dipole domains in a semiconductor superlattice

We theoretically studied the possibility of frequency multiplication using propagating dipole domains which are induced in a semiconductor superlattice by microwave radiation. We have investigated the dynamics of electrons in a superlattice submitted to both a static voltage and a microwave field by performing a simulation based on a drift-diffusion model and incorporating current-limiting boundary conditions. The motion of electrons in the superlattice was governed by an Esaki–Tsu drift velocity field characteristic with a negative differential mobility above a critical electrical field. The simulation delivered, for a static voltage larger than a critical voltage, the periodic formation and annihilation of propagating dipole domains and, as a consequence, a reduction of the direct current through the superlattice. Our simulation showed that an additional microwave field can periodically induce and subsequently quench domains giving rise to a strongly anharmonic current. The anharmonicity of the current is the origin for the generation of higher harmonics of the microwave field. Both the formation and annihilation of a domain can take place within a time of about 1 ps suggesting that the mechanism of domain induction and quenching can be used for generation of radiation up to almost 1 THz.     

On the possibility of natural formation of a transverse wave with a phase velocity lower than the velocity of light

The formation of a transverse wave with a phase velocity lower than the velocity of light, which can exist in an equilibrium plasma without a slow-wave structure in zero magnetic field, is described. It involves the transformation of a transverse wave with trapped electrons, traveling along the magnetic field, into a slow transverse wave after the removal of the magnetic field. During the evolution of the wave with trapped electrons, the magnetic induction decreases very slowly in the direction of the wave propagation. As a result, the velocity at which electrons are in resonant interaction with the wave increases; therefore, the electrons fall to the bottom of potential wells. Under the influence of the trapped electrons, the phase velocity of the wave decreases and becomes lower than the velocity of light. It becomes equal to the velocity at which the electrons are in resonance interaction with the wave at the instant when the magnetic field vanishes. It is demonstrated that a transverse wave with a velocity lower than the velocity of light can exist in an equilibrium plasma even after the magnetic field vanishes; in this case, the flow of trapped electrons serves as a slow-wave structure.     

磁场下非对称T型量子波导的电子输运行为

运用模匹配方法和求解单电子薛定谔方程,来演示非对称T型磁量子结构的电子输运性质.结果表明,结构因子和磁势垒都能改变电子散射模数,电子输运谱因此变得复杂而丰富,散射区域出现了完全局域态和磁边缘态.在特定的结构参数和磁场强度下,能观测到宽谷、尖峰、共振透射和共振反射等电子输运现象,即可以通过调节磁场大小和结构参数来实现波矢过滤. 关键词： 介观体系 电子输运 磁效应