The tube character of electron drift in condensed inert gases

The behavior of an excess electron in condensed inert gases in an external electric field is considered at densities and temperatures at which the mobility of a slow electron is relatively high. On the basis of experimental data and a model of a pair electron interaction with atoms, an effective potential energy surface is constructed for an excess electron inside a dense inert gas. The region available for a slow electron consists of many intersecting channels that form a Delaunay network located between atoms. A drifting electron, as a quantum object, propagates along these channels (tubes), and electron transition between intersecting potential energy tubes of different directions provides an effective electron scattering. This mechanism of electron drift and scattering differs from that in gases and crystals. Peculiarities of electron drift inside dense inert gases are analyzed within the framework of this mechanism of electron scattering, leading to a moderate change of the electron mobility upon melting.     

Free electron in compressed inert gases

The behavior of excess and intrinsic free electrons inside compressed inert gases is described as a function of pressure by using a pairwise approximation for the electron interaction with atomic surroundings. The change of sign from negative to positive for the xenon atom electric potential inside condensed xenon is predicted to occur at a pressure around 3 GPa, preventing slow electron embedding into solid xenon from the gas phase at higher pressure. To overcome this difficulty, the electrons should be injected into a solid sample just before its pulsed shock loading. The ionization of xenon by pressure and its further metallization are described by decreasing the forbidden gap at the expense of increasing the xenon ground electronic term and simultaneous splitting of the upper ionized electronic state. A good coincidence between the calculated and measured pressure of the dielectric-metal transition in xenon is demonstrated. The text was submitted by the authors in English.     

Electron clusters in inert gases

This Letter addresses the counterintuitive behavior of electrons injected into dense cryogenic media with negative scattering length L. Instead of strongly reduced mobility at all but the lowest densities due to the polaronic effect involving the formation of density enhancement clusters (expected in the theory with a simple gas-electron interaction successfully applied earlier to electrons in helium where L>0) which should substantially decrease the electron mobility, an opposite picture is observed: with increasing |L| (the trend taking place for inert gases with the growth of atomic number) and the gas density, the electrons remain practically free. An explanation of this behavior is provided based on consistent accounting for the nonlinearity of the electron interaction with the gaseous medium in the gas atom number density.     

Excitation of helium atoms in collisions with plasma electrons in an electric field

The rate constants are evaluated for excitation of helium atoms in metastable states by electron impact if ionized helium is located in an external electric field and is supported by it, such that a typical electron energy is small compared to the atomic excitation energy. Under these conditions, atomic excitation is determined both by the electron traveling in the space of electron energies toward the excitation threshold and by the subsequent atomic excitation, which is a self-consistent process because it leads to a sharp decrease in the energy distribution function of electrons, which in turn determines the excitation rate. The excitation rate constant is calculated for the regimes of low and high electron densities, and in the last case, it is small compared to the equilibrium rate constant where the Maxwell distribution function is realized including its tail. Quenching of metastable atomic states by electron impact results in excitation of higher excited states, rather than transition to the ground electron state for the electric field strengths under consideration. Therefore, at restricted electron number densities, the rate of emission of resonant photons of the wavelength 58 nm, which results from the transition from the 2¹ P state of the helium atom to the ground state, is close to the excitation rate of metastable atomic states. The efficiency of atomic excitation in ionized helium, i.e., the part of energy of an electric field injected in ionized helium that is spent on atomic excitation, is evaluated. The results exhibit the importance of electron kinetics for an ionized gas located in an electric field.     

Short range attraction between two similarly charged silica surfaces

Using an atomic force microscope we measure the interaction between two identically charged silica surfaces in the presence of a saline solution. For pure NaCl the interaction is always repulsive. Upon addition of cobalt hexamine ions, Co(NH(3))(6)(+3), the repulsion is gradually suppressed and a pronounced attraction develops at distances much shorter than the screening length. Higher concentrations of cobalt hexamine turn the attraction back into repulsion. Measurements of surface charge renormalization by the trivalent cations provide their surface density and their association constant to the negatively charged silica surface. These estimates tend to exclude interaction between two condensed Wigner crystals as an explanation for the attraction.     

Townsend coefficient,escape curve,and fraction of runaway electrons in copper vapor

Ionization and drift characteristics of electrons in copper vapor in the presence of an external electric field are analyzed. In contrast to normal gases, in copper vapor, the excitation energy of lower states is significantly lower than the ionization potential and the excitation cross section is several times greater than the ionization cross section at the incident-electron energy on the order of the ionization energy. This can affect the characteristics of electron bunching in gas. It is demonstrated that, as in previously studied gases, the notion of the Townsend coefficient remains meaningful even in the presence of strong fields at which the electric force exceeds the electron drag force acting in gas. The dependences of the main ionization and drift characteristics on the reduced field strength, the escape curve (which separates the region of effective electron multiplication and the region where electrons leave the discharge gap without multiplication), and the curves of equal efficiency for the formation of runaway electrons are obtained. It is demonstrated that a relatively high excitation cross section of copper levels leads to a sharper peak on the dependence of the Townsend coefficient on the field strength and a narrower region of the effective electron multiplication in comparison with previously studied gases.     

On the force constants of graphene

It has been shown that the inclusion of an additional short-range potential in the repulsion energy of neighboring atoms allows one to satisfactorily describe the frequency of the phonon doublet at the Γ point for atomic vibrations in the plane of the graphene sheet and to calculate the corresponding Grüneisen coefficient. The results obtained are compared with those derived by other authors.     

High-temperature atomic superfluidity in lattice Bose-Fermi mixtures

We consider atomic Bose-Fermi mixtures in optical lattices and study the superfluidity of fermionic atoms due to s-wave pairing induced by boson-fermion interactions. We prove that the induced fermion-fermion coupling is always attractive if the boson-boson on-site interaction is repulsive, and predict the existence of an enhanced BEC-BCS crossover as the strength of the lattice potential is varied. We show that for direct on-site fermion-fermion repulsion, the induced attraction can give rise to superfluidity via s-wave pairing at striking variance with the case of pure systems of fermionic atoms with direct repulsive interactions.     

Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.     

玻色-爱因斯坦凝聚领域Feshbach共振现象研究进展

Feshbach共振现象是当前玻色一爱因斯坦凝聚领域中的一个研究热点．目前在大多数低温碱金属原子气体里都已观测到Feshbach共振现象．在实验里利用Feshbach共振可以任意改变这些系统中原子之间的相互作用强度，从强相互排斥作用到强相互吸引作用都可以实现．文章详细介绍Feshbach共振现象以及目前它在原子气体系统里的最重要的两个应用，研究费米子气体里的超流态和有强相互作用的玻色子气体．     

Statistics of internal excitations of atomic systems

The character of internal excitations is compared for phase transitions and chemical transitions in atomic systems. Although the temperature dependences of some physical parameters of atomic systems have resonance-like structures with maxima in both cases, the dependences of the partition functions on the number of elementary excitations or the excitation energy differ because of the difference in the numbers of interactions that govern the transitions. The phase changes of condensed rare gases are considered in the case where the external pressure is small and the differences between phases are predominantly associated with differences in configurations. Important energy parameters of rare gases are determined by the attractive part of the pairwise interaction potential between atoms. The statistical analysis shows the existence of a “freezing limit” temperature for these systems, below which the liquid state becomes unstable. The kinetics of decay of such unstable states is analyzed in terms of the diffusion of voids.     

A molecular dynamics study on the role of attractive and repulsive forces in excess heat capacity at constant volume of dense fluids

The curves of experimental heat capacity against density show a minimum around and below the critical temperature (Tc), but at higher temperatures, this minimum is not observed. In this study, the role of attractive and repulsive forces on excess heat capacity of Lennard–Jones (LJ) dense fluids has been investigated using a molecular dynamics simulation technique. LJ potential is divided into attractive and repulsive parts. From the molecular dynamics calculations, potential energy and heat capacities have been obtained for Argon at temperatures of 100–500?K. The repulsive forces play the main role in causing the heat capacities at temperatures greater than critical point. Around and below the critical temperature, the role of repulsion is dominant at high densities, but attraction has the main role at low densities, consequently at middle densities, a minimum is formed.     

Electron mobility limited by surface and interface roughness scattering in AlxGa1-xN/GaN quantum wells

The electron mobility limited by the interface and surface roughness scatterings of the two-dimensional electron gas in AlxGa1-xN/GaN quantum wells is studied. The newly proposed surface roughness scattering in the AlGaN/GaN quantum wells becomes effective when an electric field exists in the AlxGa1-xN barrier. For the AlGaN/GaN potential well, the ground subband energy is governed by the spontaneous and the piezoelectric polarization fields which are determined by the barrier and the well thicknesses. The thickness fluctuation of the AlGaN barrier and the GaN well due to the roughnesses cause the local fluctuation of the ground subband energy, which will reduce the 2DEG mobility.     

An experimental approach for investigating many-body phenomena in Rydberg-interacting quantum systems

Recent developments in the study of ultracold Rydberg gases demand an adwanced level of experimental sophistication, in which high atomic and optical densities must be combined with excellent control of external fields and sensitive Rydberg atom detection. We describe a tailored experimental system used to produce and study Rydberg-interacting atoms excited from dense ultracold atomic gases. The experiment has been optimized for fast duty cycles using a high flux cold atom source and a three beam optical dipole trap. The latter enables tuning of the atomic density and temperature over several orders of magnitude, all the way to the Bose--Einstein condensation transition. An elec- trode structure surrounding the atoms allows for precise control over electric fields and single-particle sensitive field ionization detection of Rydberg atoms. We review two experiments which highlight the influence of strong Rydberg---Rydberg interactions on different many-body systems. First, the Rydberg blockade effect is used to pre-structure an atomic gas prior to its spontaneous evolution into an ultracold plasma. Second, hybrid states of photons and atoms called dark-state polaritons are studied. By looking at the statistical distribution of Rydberg excited atoms we reveal correlations between dark-state polaritons. These experiments will ultimately provide a deeper understanding of many-body phenomena in strongly-interacting regimes, including the study of strongly-coupled plasmas and interfaces between atoms and light at the quantum level.     

Hund's rules and the interpretation and common misinterpretation of energy differences

With a Z expansion formalism which is exact in non-relativistic quantum mechanics it has been shown that for multiplets of neutral atoms and of many positive ions the state of the highest energy has the lowest expectation value for the interelectronic repulsion energy. This reversed order of the repulsion energy occurs for cases which obey Hund's first rule as well as for cases which obey the second of Hund's rules. It can be shown that the energy differences are in all cases dominated by the difference in electron nuclear attraction energy and not by the difference in electron repulsion energy.

The lowest ¹Π _u and ³Π _u states of the H₂ molecule have similar features. At many internuclear distances, including the equilibrium ones, the ¹Π _u state has the highest energy but the lower kinetic energy and electron repulsion energy but again the higher electron nuclear attraction energy. These results contradict clearly the usual theories for energy differences between spectroscopic states in atoms and molecules.     

High-field transport in semiconductor superlattices for interacting Wannier–Stark levels

We developed a microscopic theory of electron transport in superlattices within the Wannier–Stark approach by including the interaction associated with Zener tunneling between the energy levels pertaining to adjacent quantum wells. By using a Monte Carlo technique we have simulated the hopping motion associated with absorption and emission of polar optical phonons and determined the main transport parameters for the case of a GaAs/GaAlAs structure at room temperature. Interaction between the levels is found to be responsible for a systematic increase of the level energy with respect to the bottom of the quantum well at electric fields above about 20 kV/cm. When compared with the non-interacting case, at the highest fields the average carrier energy evidences a consistent increase, which leads to a significant softening of the negative slope of both the drift velocity and diffusivity versus electric field behavior.     

The heat of solution and interaction effects for noble gas atoms in metals

A simple model is presented from which the enthalpy of solution of noble gases in arbitrary metals can be estimated quantitatively.The heat of solution is derived as the interfacial energy at the contact interface between the metal and a noble gas atomic cell in solution.The model picture offers an explanation for the strong attraction between noble gas atoms, in particular helium, and vacancies, other noble gas atoms or metallic impurities with a low surface energy.     

Interaction of two dielectric macroparticles

The electrostatic interaction of two charged dielectric spherical particles with a nonuniform freecharge distribution over their surfaces in an external homogeneous electric field is considered. An exact solution for the electric field potential is obtained, and an analytical expression for the interaction force between these two particles is found. The case of a uniform free-charge distribution is considered in detail, and the region of parameters in the plane “the ratio of the radii vs. the ratio of the charges,” where repulsion between two like-charged particles turns into attraction as the interparticle distance decreases is established.     

磁光阱中冷原子的实验特性

冷原子的获得对于原子物理的研究具有重要的意义,文章综述了激光冷却与囚禁原子技术发展以来对磁光阱中中性冷原子特性的研究进展,包括冷原子对光子的吸收与散射,冷原子之间的吸引与排斥导致的超冷碰撞与长程分子状态,朱子的非线性特性等。