Spin-polarized electron scattering at single oxygen adsorbates on a magnetic surface

Scanning tunneling spectroscopy (STS) on the system of isolated oxygen atoms adsorbed on the double layer of Fe on W(110) shows highly anisotropic spatial oscillations in the local density of states in the vicinity of the adsorbates. We explain this in terms of a single-particle model as electron waves being scattered by the potential induced by the presence of the oxygen atoms. Analysis of the wavelength of the standing electron waves and comparison with ab initio spin-resolved electronic structure calculations reveal that minority-spin bands of d-like symmetry are involved in the scattering process. By applying spin-polarized STS, we observe this standing wave pattern on one particular type of magnetic domain of Fe on W(110) only, thereby proving that the standing electron waves are highly spin polarized.     

Emission of Dislocation Loops from Nanovoids in an FCC Crystal Subjected to Shear Deformation under Post-Cascade Shock Waves

The method of molecular dynamics is applied to the study of the effect of post-cascade shock waves generated in a solid irradiated by high-energy particles on the heterogeneous formation of dislocation loops in a simulated gold crystal containing a spherical nanovoid, which is subjected to shear deformation. The interaction between atoms is described with the use of a potential calculated by the embedded atom method. Shock waves are created by assigning a velocity exceeding the speed of sound in the simulated material to the boundary atoms of the computational cell. It is shown that two regions of increased mechanical stress are formed under shear deformation near the surface of a nanovoid, which are the sources of emerging partial dislocations. The main mechanism for the formation of dislocations is the displacement of a group of atoms towards the inner surface of the void, which does not contradict modern ideas about the heterogeneous formation of dislocations. It is shown that, when the values of shear stress are insufficient for the formation of dislocations, loop emission can be initiated by a post-cascade shock wave generated in the computational cell. As temperature increases, the number of nucleated dislocation loops increases, and, in addition, the formation of Lomer–Cottrell dislocations is observed, which is attributed to the additional tangential stresses created by the unloading wave. In this case, the formation of a stable dislocation loop in which the linear tension is balanced by the Peach–Koehler force due to external stress requires that the shock wave front affect the regions of increased stress near the void surface while propagating through the simulated crystal.     

Laser cooling in an optical shaker

We propose a novel generic approach to laser cooling based on the nonresonant interactions of atoms and molecules with optical standing waves experiencing sudden phase jumps. The technique, termed "optical shaking," combines the elements of stochastic cooling and Sisyphus cooling. An optical signal that measures the instantaneous force applied by the standing wave on the ensemble of particles is used as feedback to determine the phase jumps. This guarantees a drift towards lower energies and higher phase-space density without the loss of particles typical of evaporative cooling.     

Finely striped structure at the edges of localized deformation bands

Under conditions of high-rate loading, plastic strain localization is a result of tension in the zone of interference of unloading waves rather than of thermal softening. At stresses close to the dynamic strength of the material, the microstructure of localized strain bands consists of strongly deformed material, with a large number of incipient microdiscontinuities. At stresses below the Hugoniot elastic limit, the microstructure looks as a set of barely visible stripes. The finely striped structure at the edges of the bands of a spall damage arises as a result of the stretching of initially rounded damage centers attached to the matrix material during dynamic deformation.     

Frequency multi-mode lens for atoms and molecules: Application to nanofabrication

We present a frequency multi-mode lens with corrected spherical aberration suitable for focusing of both atoms and molecules. The lens is formed by superposing of several harmonics of standing light waves in such a way that to create harmonic potential for all particles crossing the light field. The application to nanolithography is discussed emphasizing the possibility to improve the contrast of deposited structures.     

Chemical transformations in the zone of spall damageability

The results of experiments on studying the perlite–ferrite structure in steels under short-term negative pressures are described. It is shown that in the localized deformation bands formed in the zone of interference of unloading waves, where the tension stress is lower than the dynamic strength of the material, the cementite bands in perlite are crushed, their fragments are in part dissolved and enriched with carbon, and the cementite can pass into a steady spherical form on the boundary with ferrite. At relatively high shock-wave amplitudes, the perlite in its entirety acquires a spheroidal shape.     

Excitation of standing electron waves in fast-ion tracks

The specific case of slow-electron diffraction in fast-ion tracks is considered. The excitation condition for standing waves of δ electrons is a strong screening of the Coulomb interaction, for which most of the δ electrons are knocked out in radial directions relative to the track axis. In that case, an appreciable number of δ electrons with a wavelength of the order of the interatomic distance experience multiple backscatterings between the atomic chains located near the track axis. The lifetime of standing electron waves is estimated from diffractometric measurements of the decrease in atomic density on the track axis. The mobility of crystal atoms, their displacements, and the expenditure of energy on deformation are also estimated.     

Self-healing effect of spallation damageability

The self-healing effect has been found in a study of the microstructure of the bands of localized deformation. It has been shown that interstitial elements (O, C) and the particles of a doping phase migrate to the zone of growing spallation damageability from the matrix material. When considering the wave pattern of the process of localization, it has been ascertained that the formation of bands of localized deformation is accompanied by the process of reverberation which is characterized by the formation of periodically repeated compression–extension cycles. A weak attenuation of the reverberation has led to an increase in the duration of the deformation pulse of the sample by two to three orders of magnitude compared with the time of the initial compression pulse.     

Cooling of atoms below the recoil energy under multizone Raman excitation

Cooling of atoms below the temperature determined by the recoil energy is proposed on the basis of the Raman excitation of three-level atoms by the field of standing waves with a relative spatial shift. The advantage of this cooling mechanism is a weak sensitivity to the shape and duration of light pulses used for the transfer of population under Raman excitation. It is shown that the effectiveness of such cooling increases sharply when multizone excitation is used and already several interaction zones are enough for deep transverse cooling of an atomic beam to a temperature significantly lower than the recoil energy.     

Interference of atom,and atomic spatial lattices in light fields

A scheme is proposed to observe interference of atoms by using a weakly coherent atomic beam scattered at two standing light waves. It is shown that atoms can transfer spatial coherence over rather large distances.     

Strain localization and its connection with the deformed state of the material

The detection of the passage of a shear band over an undeformed material poses a new question about the causal connection between the strain and the formation of shear bands. The collapse of a thick-walled tube is considered from the standpoint of the spall mechanism, according to which localized strain bands under pulsed loading are the result of interference of unloading waves; the negative stresses in the extension zone in this case do not exceed the strength of the material. It is found that radial cracks and their continuations in the form of shear bands appear at the unloading stage after the strained state of the material has already formed as a result of collapse. In other words, damageability is superimposed on the deformed material, and both these processes are independent and accompany each other.     

Asymmetry of the scattering amplitude of atoms in the field of short counterpropagating light pulses

Rectification in bipotential scattering of a beam of atoms in the field of short pulses of traveling and standing waves is studied: As a result of the coherence induced by the traveling-wave pulse, the momentum transferred to the atomic beam during scattering by the standing wave is nonzero. The magnitude and sign of the asymmetry in the scattering amplitude are oscillatory functions of the duration of the traveling-wave pulse and the detuning of the frequency of the field from atomic resonance. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 920–923 (25 June 1996)     

粉末高速压制成形过程中的应力波分析

应用离散单元法,模拟了粉末高速压制成形过程中的压力传播过程.根据粉末高速压制成形的工艺特性,把一次压制过程分为弹性加载、塑性变形、弹性卸载三个阶段;基于离散单元法的基本理论,分别建立了三个阶段的控制方程;应用PFC2D软件对粉末高速压制过程模腔内部颗粒的运动状态进行了数值模拟,给出了压坯内部的压力分布,得出了实验中无法测量的压坯内部应力传播波形.数值模拟结果显示,压力作用曲线表现出明显的弛豫现象,形成了倾斜度不同的锯齿状加载波形和卸载波形,压坯底层的应力波与实验得到的应力波相符. 关键词： 高速压制成形 离散单元法 接触模型 应力波     

Particle transport by standing waves on an electric curtain

Discrete-element simulations are performed to study particle transport by standing waves on an electric curtain. An electric curtain consists of a series of parallel electrodes with oscillating potential field embedded in a dielectric surface. The study shows that particles can be transported in two different modes under excitation by standing waves. In the first mode, particles roll along the surface in a constant direction with average velocity equal to the wave speed. In the second mode, particles hop along the surface in a manner akin to a Brownian motion. Effect of particle collisions on these transport modes is evaluated.     

The recoil effect in a strong electromagnetic field

The recoil effect in a strong resonance field of a standing electromagnetic wave has been studied. In order to find the response to the external field (the induced dipole momentum) it is necessary to take into account the variations in the motion of the atoms in this field: the response and the energy spectrum of the atoms are determined from the system of wave equations for the multi-level atoms. The problem of a weak signal absorption in the presence of a strong standing wave which is in resonance with the adjacent transition, is solved. In this case the contour of the absorption line is shown to diverge into two wide bands, their width being proportional to the strong field amplitude. The contribution of continuous and discrete states of the atoms in a standing wave field has been found.     

Avoiding the opposite beams interaction in the He−Ne linear laser at 632.8 nm

Summary In the paper the role of the opposite beams in laser generations is discussed on the basis of experimental investigations performed on the He−Ne long linear laser. It is stated that standing waves do not interact with atoms in one-photon transitions. Laser oscillations always avoid generating standing waves inside the laser medium. The best way to avoid such standing waves is laser generation in the form of one short pulse travelling to and fro inside the resonator—the self-locking generation. Also discussed is the laser generation of two or more pulses inside the resonator. The work was supported by CPBR 8.14.     

Measurement of the Seperation between Atoms beyond Diffraction Limit

We propose a scheme to obtain the distance of two identical atoms placed inside the standing wave field by monitoring the collective resonance fluorescence spectrum emitted by the two particles. We find three different parameter ranges, depending on the distance of the atoms as compared to the transition wavelength. For large interparticle distances, dipole-dipole coupling is negligible, and the main system evolution arises from the interaction with the standing wave field. In the small-distance limit, the dynamics is dominated by the dipole-dipole interaction. Finally, in the intermediate region, a rich interplay of the various couplings arises, which however is lifted for strong driving laser fields. The present measurement procedure allows us to distinguish the three cases. In each of the cases, we show how to determine the distance of the two particles and their respective positions relative to the nodes of the standing wave field with fractional-wavelength precision.     

Continous coherent radiation in methane at λ=3.39 μm in spacially separated fields

Continuous coherent radiation in a gas observed in a spacially separated standing wave fields is reported for the first time. The radiation was observed in the center of lineF ₂ ⁽²⁾ in methane at the interaction of atoms with two standing waves separated by the distance of 3.5 cm.     

Doppler-free resonances of the second-order raman scattering

The possibility of the observation of Raman scattering resonances completely free from the influence of the Doppler effect has been examined for the first time. The phenomenon is based on the excitation of a Raman oscillation standing wave in a gas by two standing light waves, whose frequency difference is equal to half the Raman frequency. The complete compensation of Doppler shifts results from the simultaneous interactions between atomic particles and two pairs of counter-propagating waves. Doppler-free resonances of the second-order Raman light scattering appear in the number of particles excited to the upper Raman level and in the radiation at the Stokes and anti-Stokes frequencies. The amplitude estimate for the resonance in the number of particles is given for the example of neon.     

Particle separation in microfluidics using different modal ultrasonic standing waves

Microfluidic technology has great advantages in the precise manipulation of micro and nano particles, and the separation of micro and nano particles based on ultrasonic standing waves has attracted much attention for its high efficiency and simplicity of structure. This paper proposes a device that uses three modes of ultrasonic standing waves to continuously separate particles with positive acoustic contrast factor in microfluidics. Three modes of acoustic standing waves are used simultaneously in different parts of the microchannel. According to the different acoustic radiation force received by the particles, the particles are finally separated to the pressure node lines on both sides and the center of the microchannel. In this separation method, initial hydrodynamic focusing and satisfying various equilibrium constraints during the separation process are the key. Through numerical simulation, the resonance frequency of the interdigital transducer, the distribution of sound pressure in the liquid, and the relationship between the interdigital electrode voltage and the output sound pressure are obtained. Finally, the entire separation process in the microchannel was simulated, and the separation of the two particles was successfully achieved. This work has laid a certain theoretical foundation for the rapid diagnosis of diseases in practical applications.