New Experimental Limit on the Pauli Exclusion Principle Violation by Electrons��The VIP Experiment

We present an experimental test of the validity of the Pauli Exclusion Principle for electrons based on the concept put forward a few years ago by Ramberg and Snow (Ramberg and Snow in Phys. Lett. B 238:438, 1990). In this experiment we perform a very accurate search of X-rays from the Pauli-forbidden atomic transitions of electrons in the already filled 1S shells of copper atoms. Although the experiment has a simple structure, it poses deep conceptual and interpretational problems. Here we describe the experimental method and recent experimental results, which we interpret as an upper limit for the probability to violate the Pauli Exclusion Principle. We present also future plans to upgrade the experimental apparatus using Silicon Drift Detectors.     

Monte Carlo simulation of the coherent backscattering of electrons in a ballistic system

We study weak localization effects in the ballistic regime as induced by man-made scatterers. Specular reflection of the electrons off these scatterers causes backscattered trajectories to occur, which interfere with their time-reversed path resulting in weak localization corrections to the resistance. Using a semi-classical theory, we calculate the change in resistance due to these backscattered trajectories. We found that the inclusion of the exact shape of the scatterers is very important in order to explain the experimental results of Katine et al.[Superlattices and Microstructures 20 , 337 (1996)].     

Electron momentum density and Compton profile by a semi-empirical approach

Here we propose a semi-empirical approach to describe with good accuracy the electron momentum densities and Compton profiles for a wide range of pure crystalline metals. In the present approach, we use an experimental Compton profile to fit an analytical expression for the momentum densities of the valence electrons. This expression is similar to a Fermi–Dirac distribution function with two parameters, one of which coincides with the ground state kinetic energy of the free-electron gas and the other resembles the electron–electron interaction energy. In the proposed scheme conduction electrons are neither completely free nor completely bound to the atomic nucleus. This procedure allows us to include correlation effects.We tested the approach for all metals with Z=3–50 and showed the results for three representative elements: Li, Be and Al from high-resolution experiments.     

Efficient spin injection and extraction in modified reverse and forward biased ferromagnetic–semiconductor junctions and low-power ultrafast spin injection devices

This paper addresses recent theoretical and experimental advances in obtaining large spin polarization in semiconductors. In particular, we describe tunneling of electrons between nonmagnetic semiconductors (S) and ferromagnets (FM) through a Schottky barrier modified by very thin heavily doped interfacial layer. It is shown that in such reverse (forward) biased FM-S junctions electrons with a certain spin projection can be efficiently injected in (extracted from) S. This occurs due to spin filtering of electrons in a tunneling process. We find conditions for most efficient extraction and accumulation of spin and show that spin polarization of electrons near the interface can, at least in principle, be made close to 100% in nondegenerate S at room temperature and certain bias voltages. Extraction of spin can proceed in degenerate semiconductors at any (low) temperature. A new class of spin valve ultrafast devices with small dissipated power is described: a magnetic sensor, a spin transistor, an amplifier, a frequency multiplier, and a square-law detector. PACS 72.25.Hg; 72.25.Mk; 72.25.Rb     

Aluminum Hugoniot from model calculations including semicore electrons based on density functional theory

We present a method to obtain Hugoniot from model calculations based on density functional theory, and apply the method to aluminum Hugoniot. Technological advances have extended the experimental research of high energy density physics, and call for quantitative theoretical analysis. However, direct computation of Hugoniot from density functional theory is very difficult. We propose two step calculations of Hugoniot from density functional theory. The first step is molecular dynamics simulations with an ambient temperature for electrons. The second step is total energy calculations of a crystal with desired high temperatures for electrons and with the ambient temperature for electrons. We treated the semicore 2s and 2p electrons of aluminum as valence electrons only for the total energy calculations of the aluminum crystal. The aluminum Hugoniot from our model calculations is in excellent agreement with available experimental data and the previous density functional theory calculations in the literature.     

Ellipticity dependence of high-order above-threshold ionization from aligned diatomic molecules

We investigate high-order above-threshold ionization of the Ar₂ molecule by an elliptically polarized laser field. In order to describe this process we use the modified molecular strong-field approximation. We have found that the differential ionization rate is a very sensitive function of the ellipticity of the laser field and of the molecular orientation. For parallel orientation the rate has the maximum value for linear polarization, independently of the laser intensity. On the other hand, for perpendicular orientation the rate is maximal for a particular value of the ellipticity which is different from zero. Such a behavior can be related to the molecular symmetry. We consider the differential ionization rate of high-energy electrons as a function of the ellipticity for different internuclear distances. We also present, in the electron energy-emission angle plane, our new results for the focal-averaged spectra of electrons ionized by an elliptically polarized laser field. We have found that, in this case, the two-source two-rescattering-centers interference minima are blurred.     

Electrons in a cryogenic planar Penning trap and experimental challenges for quantum processing

We report on trapping of clouds of electrons in a cryogenic planar Penning trap at T ≤100 mK. We describe the experimental conditions to load, cool and detect electrons. Perspectives for the trapping of a single electron and for quantum information processing are given.     

高能带电粒子直接产生电子对

本文叙述利用场强为6200高斯的磁云室,对级联簇射中的能量大于30兆电子伏的电子在铝中直接产生电子对的平均总截面进行了测量。实验上总共找到真正由电子产生的电子三岔8.1个,相应于三岔产生的平均自由路程为672±585厘米或平均总截面为(0.25±0.22)×10^-25厘米²。在数据处理中,对假三岔数目的除去作了应有的适当考虑。此外对Bhabha及Murota等的总截面公式也作了适合于于我们实验具体情况的修正,并算出总截面值,最后将它同实验值作了比较。结果我们认为实验上测得的值和Murota等的总截面值较为接近,但也不与Bhabha的截面值矛盾。     

Dynamical mean-field approach to materials with strong electronic correlations

We review recent results on the properties of materials with correlated electrons obtained within the LDA+DMFT approach, a combination of a conventional band structure approach based on the local density approximation (LDA) and the dynamical mean-field theory (DMFT). The application to four outstanding problems in this field is discussed: (i) we compute the full valence band structure of the charge-transfer insulator NiO by explicitly including the p-d hybridization, (ii) we explain the origin for the simultaneously occuring metal-insulator transition and collapse of the magnetic moment in MnO and Fe₂O₃, (iii) we describe a novel GGA+DMFT scheme in terms of plane-wave pseudopotentials which allows us to compute the orbital order and cooperative Jahn-Teller distortion in KCuF₃ and LaMnO₃, and (iv) we provide a general explanation for the appearance of kinks in the effective dispersion of correlated electrons in systems with a pronounced three-peak spectral function without having to resort to the coupling of electrons to bosonic excitations. These results provide a considerable progress in the fully microscopic investigations of correlated electron materials.     

Theorie des stimulierten Raman-Effekts

We consider stimulated Raman emission in solids, placed in a plane laser beam external to the cavity. The Hamiltonian of the system of phonons, electrons and electromagnetic fields is derived within the framework of a generalized adiabatic approximation for electrons and nuclei. It contains terms due to nonlinear interactions between electrons and phonons. Because the usual time-dependent perturbation theory cannot describe coherence effects properly we turn toHeisenberg's equations of motion for the operators of photons, phonons and electron excitations. In order to solve these equations in the steady state we apply an iteration procedure. We start with the light waves which give rise to electron transitions. The electrons such excited create phonons which then react on the electrons. Finally the electrons are coupled again to the lightfield. This procedure yields besides the usual wellknown Raman process two main processes occurring in stimulated Raman emission: a coupled two step Raman process and a parametric process. In the first one two phonons are involved. If the linewidth of phonons is comparable to the phonon frequencies the non-resonant parts of the above processes also become important. In solving the set of coupled equations for the light amplitudes, obtained from the iteration procedure, we only consider terms due to the first Stokes, the first anti-Stokes and the laser line. We then find frequency shifts of these lines due to the stimulated emission which are of the order of the linewidth of photons if this linewidth is very much smaller than that of phonons as it is the case in solids. This means that the coupled two step Raman process is dominant, in good agreement with measurements ofChiao andStoicheff in calcite.     

10-25 keV电子致厚W,Au靶韧致辐射谱的测量

测量了10-25 keV电子碰撞厚W, Au靶产生的韧致辐射谱, 并与Monte Carlo程序PENELOPE模拟的X射线谱进行了比较, 除在3 keV前实验谱略低于理论谱外, 整体上两者符合得很好. 在模拟电子与靶材料相互作用产生韧致辐射时, PENELOPE程序中只包含有普通韧制辐射的截面数据. 我们的实验结果表明, 在电子与固体靶相互作用时, 没有明显的极化韧致辐射产生, PENELOPE程序能够可靠地描述电子与固体厚靶相互作用产生的韧致辐射.     

Influence of fast heating of electrons on the optical properties of metals

We describe quantitatively how fast inhomogeneous heating of electrons influences the complex reflection coefficient of the s-polarized light wave. When the collision frequency of heated electrons is of the order of the field frequency, we observe a significant difference in values of the absorption coefficient and the phase shift of the reflected wave and the ones given by previous theoretical considerations.     

Electron beam effects on oxygen exposed aluminum surfaces

The damaging effects of electron beams during the acquisition of electron spectra have long been an obstacle in surface analysis. In order to understand the physico-chemical processes which take place under electron irradiation in an AlO system, we have carried out an experiment in which artifices, such as heating, charging, and gas contamination, were absent. We have observed with Auger Electron Spectra increases of the oxidation extent and the oxygen concentration on an oxygen exposed (111) textured polycrystalline surface under electron irradiation (5 keV, 9 × 10^?5 A/cm²). These increases were not observed on a clean surface, and were very feeble on a (100) single crystal surface. The increase of oxygen concentration was independent of residual gas pressure (3 × 10^?9 to 6 × 10^?10 Torr) and its composition; and therefore cannot be explained by gas contamination during the experimental period (about 70 min). We attribute the increase of oxidation degree to the transition of chemisorbed oxygen atoms into oxide through direct momentum transfer from the incident electrons. We suggest that the increase of oxygen within the irradiated area is due to the surface diffusion of chemisorbed oxygen atoms from outside the irradiated area. These oxygen atoms are excited by the electrons scattered from the vacuum chamber walls and gain energy through Franck-Condon type mechanism. The absence of chemisorbed oxygen atoms on (100) surface explains the near absence of these increases on this surface.     

Electron bound states on liquid helium

Using an exactly soluble model of image potential which has the form -Ze²/ (z+β), we have calculated the energy levels of electrons trapped on the surface of liquid helium as a function of the parameter β. Our results show that the experimental values of Grimes et al can be fit very well by locating the effective liquid helium-vacuum interface at β = 1.01 Å.     

Quantum ratchets and quantum heat pumps

Quantum ratchets are Brownian motors in which the quantum dynamics of particles induces qualitatively new behavior. We review a series of experiments in which asymmetric semiconductor devices of sub-micron dimensions are used to study quantum ratchets for electrons. In rocked quantum-dot ratchets electron-wave interference is used to create a non-linear voltage response, leading to a ratchet effect. The direction of the net ratchet current in this type of device can be sensitively controlled by changing one of the following experimental variables: a small external magnetic field, the amplitude of the rocking force, or the Fermi energy. We also describe a tunneling ratchet in which the current direction depends on temperature. In our discussion of the tunneling ratchet we distinguish between three contributions to the non-linear current–voltage characteristics that lead to the ratchet effect: thermal excitation over energy barriers, tunneling through barriers, and wave reflection from barriers. Finally, we discuss the operation of adiabatically rocked tunneling ratchets as heat pumps. Received: 8 February 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Energy spectra of electrons reflected from layered targets

Here we consider backscattering of electrons with an initial energy from 10 keV to several MeV by plane-parallel sandwich targets. Using invariance principles, we obtain a formula describing the changes in the energy spectra of electrons reflected into a given solid angle. These changes occur with increasing thickness of films of different materials on substrates of finite and infinite thickness. We present methods for calculating the reflection and transmission function used in the above calculation. Repeating this method for each layer gives the spectra of electrons backscattered by multilayer targets. Comparison with experimental data shows that the theory developed adequately describes the process of electron backscattering. Our results indicate that the spectroscopy of nonelastically reflected electrons could be very useful forin situ nondestructive express analysis of depth profiles of multicomponent materials.     

Theory of nonlinear optical response of gauge invariant currents to linear and nonlinear couplings

When a gauge field interacts with a quantum condensed matter system, at first order of the gauge field it couples to the current operator of the electrons. Higher orders of the gauge field couple to electrons through other operators such as the stress tensor, etc. On the other hand, when one performs a measurement on a quantum system, not only the current operator, but also stress tensor operator of the electrons, etc. are hidden in the measurement, as they contribute to the gauge invariant current. We formulate a general problem of nonlinear optical response of the gauge invariant currents in presence of nonlinear couplings. We show that the new couplings along with new responses arising from field current have a very simple structure which can be formulated as time ordered multi-particle correlation functions. We also obtain their Lehman representation and thereby show that one need not use non-equilibrium formulations to deal with them. These new correlation functions suggest that in nonlinear optical response many new processes are possible. The experimental detection of the new terms in the current operator, and application corresponding multi-photon processes needs further theoretical and experimental investigations.     

Far-infrared spectra of imperfect YBaCuO films on Si substrates

We have measured the far-infrared (FIR) transmittance of low conductivity, imperfect YBCO films deposited on Si, at both 300 K and 20 K. Test reflectance measurements were also performed at room temperature. Our aim was both to study YBCO phonons by transmission measurements and to check if FIR spectra of imperfect YBCO films can be described in the framework of the BCS theory. In order to describe our measurements, we provide exact expressions for both transmittance and reflectance, averaged with respect to the interference fringes of the substrate. By modelling the complex dielectric constant of the film, we then study the YBCO phonons which are evident in the measured spectra. The comparison between spectra of normal and superconducting phases shows that the superconducting transition affects our spectra only at very low frequencies. By considering a two fluid model in which only a small fraction of electrons becomes superconducting and by assuming an energy gap of the order of 130 cm⁻¹, we find that our data can be explained in terms of the Mattis-Bardeen theory.     

Instability of layered quantum liquids: 3. New phase in fermion superlattices

Starting with a layered interacting Fermi liquid we describe a possible origin of high-T _c superconductivity. We argue that for a electron densityN smaller than a critical densityN _c many-body effects are very large in layered electron systems leading, to an attraction of electrons in neighboring planes. This attraction leads to pairing of two electrons. The paired electrons are bosons, which undergo a Bose-Einstein condensation and form a Schafroth superconductor (local pairing) with a boson densityN _B expressed asT _c N _B N[1-(N/N _c )^1/2]. This superconductor is characterized by a short coherence length. We compare our theoretical results for the critical temperatureT _c , the pressure effect coefficient and the isotope effect coefficient with experimental results and find good agreement.     

Solutions of the Dirac–Fock Equations for Atoms¶and Molecules

The Dirac-Fock equations are the relativistic analogue of the well-known Hartree-Fock equations. They are used in computational chemistry, and yield results on the inner-shell electrons of heavy atoms that are in very good agreement with experimental data. By a variational method, we prove the existence of infinitely many solutions of the Dirac-Fock equations "without projector", for Coulomb systems of electrons in atoms, ions or molecules, with Z h 124, N h 41, N h Z. Here, Z is the sum of the nuclear charges in the molecule, N is the number of electrons.