Proposed nonmagnetic Stern-Gerlach experiment using electron diffraction

We demonstrate that a transverse spin current can be generated simply by diffraction through a single slit in the spin-orbit coupling system of the two-dimensional electron gas. In the regime of spin-orbit coupling ~10(-13) eV·m, an out-of-plane component of the electron spin of up to 0.42? can be generated. Based on this effect, a novel device consisting of a grating to distill spin is designed. Two first diffraction peaks of electron carry different spins, providing a nonmagnetic version of the Stern-Gerlach experiment. The direction of the spin current can be controlled by the gate voltage with low energy cost.     

Free electron laser based on the Smith–Purcell radiation

A Smith–Purcell (S–P) free electron laser (FEL) composed of a metallic diffraction flat grating, an open cylindrical mirror cavity and a relativistic sheet electron beam with moderate energy, is presented. The characteristics of this device are studied by theoretical analysis, experimental measurements and particle-in-cell (PIC) simulation method. Results indicate that coherent radiation with output peak power up to 50 MW at millimeter wavelengths can be generated by using relativistic electron beam of moderate energy.     

Tunneling between two-dimensional electron systems in a strong magnetic field

We calculate the tunnel current between two parallel two-dimensional electron systems in a strong perpendicular magnetic field. We model the strongly correlated electron systems by Wigner crystals, and describe their low-energy dynamics in terms of magnetophonons. The effects of the magnetophonons on the tunneling processes can be described by an exactly solvable independent-boson model. A tunneling electron shakes up magnetophonons, which results in a conductance peak that is displaced away from zero voltage and broadened compared with the case of no magnetic field. At low temperatures and low enough voltages the tunneling conductance is strongly suppressed, and the I–V characteristics exhibit a power-law behavior. The zero-voltage conductance is thermally activated with an activation temperature 10 K. The results are in very good agreement with experiment.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Tunable terahertz radiation from arbitrary profile dielectric grating coated with graphene excited by an electron beam

In this paper, the enhanced terahertz radiation transformed from surface plasmon polaritons, excited by a uniformly moving electron bunch, in a structure consisting of a monolayer graphene supported on a dielectric grating with arbitrary profile is investigated. The results show that the grating profile has significant influence on the dispersion curves and radiation characteristics including radiation frequency and intensity. The dependence of dispersion and radiation characteristics on the grating shape for both the symmetric and asymmetric gratings is studied in detail. Moreover, we find that, for an asymmetric grating with certain profile, there exist two different diffraction types, and one of the two types can provide higher radiation intensity comparing to the other one. These results will definitely facilitate the practical application in developing a room-temperature, tunable, coherent and miniature terahertz radiation source.     

The interplay of gate potential ande–escattering in surface superlattices

Transport measurements were performed on surface superlattices, formed by a grating gate on a two-dimensional electron gas. We find a suppression of the differential conductance with increasing electric fields, on a scale of a few V cm⁻¹. Even more remarkable is a strong suppression of conductance with increasing temperatureT, where theT-dependence is quadratic. We attribute these observations to electron–electron (e–e) scattering which, in the presence of an external modulated potential, can have a significant influence on the conductance. We also discuss the role of disorder in these systems.     

Compressing ultrafast electron pulse by radio frequency cavity

An ultrafast electron diffraction technique with both high temporal and spatial resolution has been shown to be a powerful tool to observe the material transient structural change on an atomic scale.The space charge forces in a multi-electron bunch will greatly broaden the electron pulse width,and therefore limit the temporal resolution of the high brightness electron pulse.Here in this work,we design an ultrafast electron diffraction system,and utilize a radio frequency cavity to realize the ultrafast electron pulse compression.We experimentally demonstrate that the stretched electron pulse width of14.98 ps with an electron energy of 40 keV and the electron number of 1.0 ×10~5 can be maximally compressed to about0.61 ps for single-pulse measurement and 2.48 ps for multi-pulse measurement by using a 3.2-GHz radiofrequency cavity.We also theoretically and experimentally analyze the parameters influencing the electron pulse compression efficiency for single-and multi-pulse measurements by considering radiofrequency field time jitter,electron pulse time jitter and their relative time jitter.We suggest that increasing the electron energy or shortening the distance between the compression cavity and the streak cavity can further improve the electron pulse compression efficiency.These experimental and theoretical results are very helpful for designing the ultrafast electron diffraction experiment equipment and compressing the ultrafast electron pulse width in a future study.     

飞秒电子衍射系统的静态特性研究

超快电子衍射系统是认识超快物理、化学及生物过程的重要工具之一.介绍了自主研制的一套飞秒电子衍射系统，调试并测量了该系统的电子束斑特性、X-Y偏转板的偏转灵敏度等.在该系统上进行了金膜的静态电子衍射图像的测量. 关键词： 飞秒电子衍射 偏转灵敏度 时间分辨 空间分辨     

Kapitza-Dirac diffraction without standing waves: diffraction without a grating?

We discuss electron diffraction from two counterpropagating light waves with two different frequencies. We show that, even though these waves do not form a standing wave, electron diffraction similar to the conventional Kapitza-Dirac effect, i.e., scattering on a standing wave, is still possible. The nonlinear response of the electron to the laser fields creates a stationary diffraction grating from which the same electron scatters.     

Single-shot femtosecond electron diffraction with laser-accelerated electrons: experimental demonstration of electron pulse compression

We report the first experimental demonstration of longitudinal compression of laser-accelerated electron pulses. Accelerated by a femtosecond laser pulse with an intensity of 101? W/cm2, an electron pulse with an energy of around 350 keV and a relative momentum spread of about 10?2 was compressed to a 500-fs pulse at a distance of about 50 cm from the electron source by using a magnetic pulse compressor. This pulse was used to generate a clear diffraction pattern of a gold crystal in a single shot. This method solves the space-charge problem in ultrafast electron diffraction.     

Reconstructing the electron in a fractionalized quantum fluid

The low-energy physics of the fractional Hall liquid is described in terms of quasiparticles that are qualitatively distinct from electrons. We show, however, that a long-lived electronlike quasiparticle also exists in the excitation spectrum: the state obtained by the application of an electron creation operator to a fractional quantum Hall ground state has a nonzero overlap with a complex, high energy bound state containing an odd number of composite-fermion quasiparticles. The electron annihilation operator similarly couples to a bound complex of composite-fermion holes. We predict that these bound states can be observed through a conductance resonance in experiments involving a tunneling of an external electron into the fractional quantum Hall liquid. A comment is made on the origin of the breakdown of the Fermi liquid paradigm in the fractional Hall liquid.     

Single electron charging effects in semiconductor quantum dots

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e ²/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.     

Diffraction efficiency and electron microscopy of large amplitude holographic gratings

The diffraction efficiency of Ag coated holographic gratings was measured as a function of the grating amplitude, H, for a He-Ne laser incident at 30° and 70° in the plane perpendicular to the grating grooves. The grating amplitude was determined using a scanning electron microscope. It was found that the diffraction efficiency of s polarized light increased monotonically with increasing H for H < 250 nm. In contrast, for the same amplitude range, the diffraction efficiency of p polarized light oscillated through two maxima at H = 90 nmand 220 nm. These results were compared to the theoretical calculations of Heitmann and found to agree within 10% for H < 60 nm in the s polarization case and for H < 15 nm in the p polarization case. For larger H, the data deviated significantly, presumably due to a breakdown of the small amplitude assumption on which the calculations were based.     

A bias-tunable electron-spin filter based on a two-dimensional electron gas modulated by ferromagnetic-Schottky metal stripes

We investigate the effect of the bias in an electron-spin filter based on a two-dimensional electron gas modulated by ferromagnetic-Schottky metal stripes. The numerical results show that the electron transmission and the conductance as well as the spin polarization are strongly dependent on the bias applied to the device.     

Valley-dependent electron transport in ferromagnetic/normal/ferromagnetic silicene junction

The electron transport through ferromagnetic/normal/ferromagnetic silicene junction with an induced energy gap is investigated in this work. The energy gap can be tuned by applying electric field or exchange fields due to the buckled structure of silicene. We analyze the local electric field, exchange field, length of normal region-dependence transmission probabilities of four groups and valley conductance. These transmission probabilities and valley conductance can be turned on or off by adjusting the local electric field and exchange field. In particular, a fully valley polarized conductance with 80% transmission is found in this junction, which can be caused by the interplay of valley-dependent massive Dirac electron, the exchange potential and the on-site potential difference of sublattices. Our findings will benefit applications in silicene-based high performance nano-electronics.     

电子脉冲在飞秒电子衍射系统中的传输特性

优化设计了一套超快电子衍射系统.用轨迹追踪法讨论了光电子在阴极上有150 fs的时间弥散量，及其静态、动态特性(电子束斑、时间弥散、方位角、倾角)在超快电子衍射系统中的传输情况.用Monte Carlo方法对光电子的初始状态进行抽样、用有限差分法计算二维、三维空间电场分布，用有限元法计算磁场分布.     

Optimization of organic light-emitting diode containing hydrochlorosilole electron transport layer through impedance spectroscopy analysis

Impedance spectroscopy measurements provide resistance, conductance and quality factor which play key roles to find optimum thickness of electron transport layer (ETL) in organic devices. The optimized thickness of hydrochlorosilole in an electron only device was determined to be 7 nm by the minimum contact and bulk resistances along with the maximum conductance and quality factor. We confirm that the optimized thickness from the electron only device works as the optimum condition for optical emission for multi-layer light emitting diodes which contain hydrochlorosilole.     

Photo-triggered pulsed cavity compressor for bright electron bunches in ultrafast electron diffraction

Temporally resolved observation of microscopic structural dynamics of solids with ultrafast electron diffraction (UED) requires extremely short pulsed, highly charged, monoenergetic electron beams with sufficient transverse coherence length of several unit cells of the investigated samples. However, Coulomb repulsion defeats these parameters in free propagation of an electron pulse initially bright on the photo cathode. We demonstrate a new electron pulse compressor design based on a simple and compact RF structure incorporating a pair of gallium arsenide photoconductive semiconductor switches that are triggered by femtosecond laser pulses, thereby providing a longitudinal voltage gradient of up to 20?V/ps. Our proof of principle experiment achieved compression of bunches containing 26,000 electrons to a duration of below 750?fs and a beam diameter of 300???m in the temporal and spatial focus of the device while maintaining the good beam collimation required for time resolved electron diffraction experiments. The simplicity of the compressor provides a strong incentive for its further development toward practical implementation in sub-relativistic UED experiments requiring the highest possible source brightness.     

Extraordinary electron transmission through a periodic array of quantum dots

We study electron transmission through a periodic array of quantum dots (QD) sandwiched between doped semiconductor leads. When the Fermi wavelength of tunneling electron exceeds the array lattice constant, the off-resonant per QD conductance is enhanced by several orders of magnitude relative to the single-QD conductance. The physical mechanism of the enhancement is delocalization of a small fraction of system eigenstates caused by coherent coupling of QDs via the electron continuum in the leads.     

A theoretical calculation of tunneling spectroscopy of an electron waveguide with or without a scatterer

A theoretical study on the tunneling spectroscopy of an electron waveguide recently observed by Eugster and del Alamo is presented. A narrow electron waveguide coupled with another much wider one by a thin barrier between them is taken as a theoretical model for the leaky electron waveguide implemented by Eugster et al., and the transport properties of electrons are studied comprehensively through the wavefunction of the system. The results demonstrate that the conductance for the current tunneling out the barrier oscillates strongly with the width of the narrow electron waveguide, in line with its conductance steps. The theory is in good agreement with the experiments and confirm that the oscillations of the tunneling current can be considered as a spectroscopy of the 1D DOS (one dimensional electron density of states) in the electron waveguide as proposed by Eugster et al. In order to study the effects of scatterers on the transport properties of the leaky electron waveguide, a δ-function is used to simulate the scattering potential The results show that the presence of even a single scatterer located in the waveguide will lead to obvious distortion of the shape of conductance steps, and will greatly influence the oscillations of the tunneling current observed in clean waveguides. However the effects of scatterers located outside the tunneling barrier on either the conductance steps or the oscillations of the tunneling current are negligible.