Valence and core excitations in rare gas mono- and multilayers: Production,decay, and desorption of neutrals and ions

The aim of this survey is the understanding of the dynamics of medium to high energy excitations in simple condensed systems on very short time scales. For this purpose we examine the modifications of the electronic excitations and their evolution in rare gases (mainly Ar and Kr) due to a nearby metal surface (monolayer case) or by embedding into a rare gas condensate (multilayers). Ionic excitations are shifted to lower energies compared to the gas phase by polarization of the surroundings, while neutral excitations stay constant or are shifted to somewhat higher energies. This decreases the spacing between excitonic and ionic states from both sides. Deexcitation events can be analysed by linewidths for valence excitations, and by comparison of autoionisation and Auger spectra for core excitations. For monolayers, we conclude that excitonic states are unstable relative to ionic states but nevertheless are quite long-lived. For multilayers, only minor modifications relative to the gas phase are usually found. All electronic excitations in Ar and Kr mono- or multilayers lead to desorption of neutrals; core excitations in multilayers also lead to ions and cluster ions. The probable mechanisms in all cases are discussed, and open questions are pointed out.     

Temperature-dependent plasmon excitations in the surface layer of ordered Cu-22.5 at % Mn alloy

Temperature and energy dependences of the characteristic electron energy losses on plasmon excitations are studied in the surface layer of ordered polycrystalline Cu-22.5 at % Mn alloy. Features in the near-surface distribution of Mn are found from an analysis of plasmon excitations and the data of Auger spectroscopy. The observed temperature dependences of the electron energy loss spectra have features in the range of 650–750 K that include the temperature of atomic disordering (T _k = 675 K) in the bulk of the alloy.     

Electron coherent and incoherent pairing instabilities in inhomogeneous bipartite and nonbipartite nanoclusters

Exact calculations of collective excitations and charge/spin (pseudo) gaps in an ensemble of bipartite and nonbipartite clusters yield level crossing degeneracies, spin-charge separation, condensation and recombination of electron charge and spin driven by interaction strength, inter-site couplings and temperature. Near crossing degeneracies, the electron configurations of the lowest energies control the physics of electronic pairing, phase separation and magnetic transitions. Rigorous conditions are found for the smooth and dramatic phase transitions with competing stable and unstable inhomogeneities. Condensation of electron charge and spin degrees at various temperatures offers a new mechanism of pairing and a possible route to superconductivity in inhomogeneous systems, different from the BCS scenario. Small bipartite and frustrated clusters exhibit charge and spin inhomogeneities in many respects typical for nano and heterostructured materials. The calculated phase diagrams in various geometries may be linked to atomic scale experiments in high Tc cuprates, manganites and other concentrated transition metal oxides.     

Two-Electron-Impact Excitation of Calcium, Strontium, and Barium Atoms

We have measured the energy dependences that govern the cross sections for Minsk, electron-impact excitation of spectral transitions from the levels of alkali-earth atoms that correspond to a simultaneous excitation of s ²-valent electrons. Measurements were performed on a setup with intersecting beams of atoms and electrons. The energy dependences for two- and one-electron excitations are compared. It is found that the energy dependences for the spectral transitions from one- and two-electron excitation levels that have the same orbital quantum numbers are similar.     

Processes in condensed inert gases involving excess electrons

Drift of an excess electron in dense and condensed inert gases in external electric field and excitation of atoms by electron impact in these systems are analyzed. The effective potential energy surface for an excess electron at a given electric field strength consists of wells and hills, and the actions of neighboring atoms are therefore separated by saddles of the potential energy. At such atomic densities that the difference of interaction potentials for an excess electron between neighboring wells and hills of the potential energy surface becomes small, the electron mobility is large. This is realized for heavy inert gases (Ar, Kr, Xe) with a negative scattering length of an electron on individual atoms. In these cases, the average potential energy of the electron interaction with atoms corresponds to attraction at low atomic densities and to repulsion at high densities. The transition from attraction to repulsion at moderate atomic densities leads to a maximum of the electron mobility. A gas model for electron drift in condensed inert gases is constructed on the basis of this character of interaction. Due to high electron mobility, condensed inert gases provide high efficiency of transformation of the electric field energy into the energy of emitting photons through drifting electrons. It is shown that, although the role of formation of autodetaching states in the course of electron drift is more important for condensed inert gases than for rare gases, this effect acts weakly on exciton production at optimal atomic densities. The parameters of a self-maintained electric discharge in condensed inert gases as a source of ultraviolet radiation are discussed from the standpoint of electron drift processes.     

Corpuscular Diagnosis of the Plasma of Penning Ion Sources

The results of the studies of the energy distribution and atomic-molecular composition of the ions emitted from two Penning ion sources are presented. The transitions between different discharge modes, when the operating pressure and voltage on the anode are changed, are investigated. The energy spectra and mass-charge spectra are measured and the dependences of the fraction of atomic ions on the discharge parameters are determined.     

X-ray scattering and theα-α′ phase transition in coherent metal-hydrogen systems

Metallic singlet ground state systems have often anomalous elastic properties which result from the coupling of the phonons to the crystal field energy levels of the rare earth ions. It is shown that structural and magnetic phase transitions should occur in the same way in those systems. They are caused by a sufficiently strong electron-ion interaction. While the orbital part of this interaction is responsible for possible structural phase transitions, the spin dependent part is responsible for magnetic phase transitions. This is demonstrated in detail by considering as an example the aspherical Coulomb charge scattering and the isotropic exchange interaction. We calculate the sound velocity, the sound attenuation and the excitations in the presence of the aspherical Coulomb charge scattering. Furthermore we discuss the mutual coupling of structural and magnetic phase transitions. This includes a consideration of other types of coupling than aspherical Coulomb scattering and isotropic exchange.     

Observation of intersubband excitations in a multilayer two dimensional electron gas

We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.     

Quantum phase transitions and vortex dynamics in superconducting networks

Josephson-junction arrays are ideal model systems to study a variety of phenomena such as phase transitions, frustration effects, vortex dynamics and chaos. In this review, we focus on the quantum dynamical properties of low-capacitance Josephson-junction arrays. The two characteristic energy scales in these systems are the Josephson energy, associated with the tunneling of Cooper pairs between neighboring islands, and the charging energy, which is the energy needed to add an extra electron charge to a neutral island. The phenomena described in this review stem from the competition between single-electron effects with the Josephson effect. They give rise to (quantum) superconductor–insulator phase transitions that occur when the ratio between the coupling constants is varied or when the external fields are varied. We describe the dependence of the various control parameters on the phase diagram and the transport properties close to the quantum critical points. On the superconducting side of the transition, vortices are the topological excitations. In low-capacitance junction arrays these vortices behave as massive particles that exhibit quantum behavior. We review the various quantum–vortex experiments and theoretical treatments of their quantum dynamics.     

Amplitude-mode dynamics of polariton condensates

We study the stability of collective amplitude excitations in nonequilibrium polariton condensates. These excitations correspond to renormalized upper polaritons and to the collective amplitude modes of atomic gases and superconductors. They would be present following a quantum quench or could be created directly by resonant excitation. We show that uniform amplitude excitations are unstable to the production of excitations at finite wave vectors, leading to the formation of density-modulated phases. The physical processes causing the instabilities can be understood by analogy to optical parametric oscillators and the atomic Bose supernova.     

Electronic absorption spectrum of Cs<Subscript>2</Subscript>ZnI<Subscript>4</Subscript> thin films

The absorption spectrum of Cs₂ZnI₄ thin films in the energy range 3–6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI₄ structural elements of the lattice. Phase transitions at 280 K (paraelectric phase ? incommensurate phase), 135 K (incommensurate phase ? monoclinic ferroelastic phase), and 96 K (monoclinic phase ? triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.     

Phase transitions in two-dimensional uniformly frustratedXY models. II. General scheme

For two-dimensional uniformly frustratedXY models the group of symmetry spontaneously broken in the ground state is a cross product of the group of two-dimensional rotations by some discrete group of finite order. Different possibilities of phase transitions in such systems are investigated. The transition to the Coulomb gas with noninteger charges is widely used when analyzing the properties of relevant topological excitations. The number of these excitations includes not only domain walls and traditional (integer) vortices, but also vortices with a fractional number of circulation quanta which are to be localized at bends and intersections of domain walls. The types of possible phase transitions prove to be dependent on their relative sequence: in the case the vanishing of domain wall free energy occurs earlier (at increasing temperature) than the dissociation of pairs of ordinary vortices, the second phase transition is to be associated with dissociation of pairs of fractional vortices. The general statements are illustrated with a number of examples.     

Low-energy monopole modes of a trapped atomic Fermi gas

We consider the low energy collective monopole modes of a trapped weakly interacting atomic Fermi gas in the collisionless regime. The spectrum is calculated for varying coupling strength and chemical potential. Using an effective Hamiltonian, we derive analytical results that agree well with numerical calculations in various regimes. The onset of superfluidity is shown to lead to effects such as the vanishing of the energy required to create a Cooper molecule at a critical coupling strength and to the emergence of pair vibration excitations. Our analysis suggests ways to experimentally detect the presence of the superfluid phase in trapped atomic Fermi gases.     

Quantum phase transitions in electronic systems

Quantum phase transitions occur at zero temperature when some non‐thermal control‐parameter like pressure or chemical composition is changed. They are driven by quantum rather than thermal fluctuations. In this review we first give a pedagogical introduction to quantum phase transitions and quantum critical behavior emphasizing similarities with and differences to classical thermal phase transitions. We then illustrate the general concepts by discussing a few examples of quantum phase transitions occurring in electronic systems. The ferromagnetic transition of itinerant electrons shows a very rich behavior since the magnetization couples to additional electronic soft modes which generates an effective long‐range interaction between the spin fluctuations. We then consider the influence of rare regions on quantum phase transitions in systems with quenched disorder, taking the antiferromagnetic transitions of itinerant electrons as a primary example. Finally we discuss some aspects of the metal‐insulator transition in the presence of quenched disorder and interactions.     

The thermodynamic properties and special features of spectra of elementary excitations of unstable valence Sm-and Ce-based compounds

The experimental data on the spectra of elementary excitations measured by inelastic neutron scattering and on the heat capacity and the coefficient of thermal expansion are used to analyze the correlation between the spectral characteristics of the electron and phonon subsystems and the special features of the temperature dependence of the thermodynamic properties of a number of unstable valence Sm-and Ce-based compounds. The anomalous behavior of the thermodynamic properties of these compounds is defined by the special features of their phonon and electron (4f and conduction electrons) spectra. The rearrangement of the 4f-electron spectrum as a result of temperature variation plays a decisive part in the formation of temperature dependences of the heat capacity and the coefficient of thermal expansion of unstable valence systems.     

Electronic transitions of Ar,Xe, N2, CO physisorbed on Ag(111) and Al(111)

High Resolution Electron Energy Loss Spectroscopy has been extended to study also the excitonic (low lying electronic) transitions of physisorbed rare gas atoms (Ar, Xe) and diatomic molecules (N₂, CO) on Ag(111) and Al(111) surfaces at ～20K. Electron Loss Spectra were performed using a pair of hemispherical analyzers mounted at a fixed scattering angle (90°). This spectrometer allowed high transmission in the range of 0–15eV loss energies and incident beam energies up to 2OeV. AES, LEED and UV Photoemission (HeI) were also used in situ to characterize these surfaces and to identify the adsorbed gases and delineate their absolute coverage regimes.In contrast to optical absorption experiments, we observe both, optical (dipole) forbidden and allowed electronic transitions which show vibrational line structure for condensed multilayers. By comparison to gas phase data we find only weak perturbations in the condensed state. The observed electronic excitations show changes in intensity and FWHM depending on the coverage of the adsorbed gases.The FWHM of the electronic excitations of CO and N₂ adsorbed in the monolayer regime is larger than in multilayers. Nitrogen, on both surfaces exhibits an increase from 60meV to 120meV (FWHM) whereas for CO the vibronic features are broadened out leaving peaks with FWHM of ～1eV.The intensities of the electronic losses for all gases are smaller in the first monolayer than in the second or in multilayers. At submonolayer coverage the loss intensifies due to electronic excitations are strongly reduced and no longer observable although vibrational bands and photoelectron spectra show the presence of physisorbed adsorbates.Our results will be compared to optical absorption experiments (ref.1) on similar systems and to atom-on-jellium calculations (ref.2).     

Brillouin scattering studies of rare gas solids

Abstract

The scattering of light by elementar excitations in the matter is results in two phenomena, discriminated by the zero wavevector frequency of the excitation: if this frequency is zero, one deals with Brillouin scattering, and with Raman scattering in the other case. Brillouin scattering results from the interaction of light with thermal excitations (acoustic phonons in a crystal) of a material, or, from a classical point of view, with density waves. Contrary to Raman scattering, the selection rules allow always the observation of at least one mode. It is a powerful technic in the study of rare gases under pressure: at ambient temperature, rare gases crystallize in the face centered cubic structure (except helium which structure was recently found to be hexagonal) and are therefore Raman and infrared inactive.

Experimental results will be reviewed on rare gases and rare gas mixtures in the fluid phase, like He-Ne and He-H₂. These results will be discussed in relation with recent measurements of the frequency of global oscillations of Jupiter.     

A phenomenological model of phase transitions in lawsonite

A phenomenological model that describes the sequence of phase transitions in lawsonite is considered. A phase diagram of the model is constructed and theoretical temperature dependences of order parameters and inverse permittivity in low-symmetry phases are calculated. The comparison of theoretical curves with experimental results showed satisfactory qualitative agreement.     

Phase transitions in simple clusters

Formation of the liquid state of clusters with pairwise interactions between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters. From that standpoint, phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during a solid-liquid phase transition into two parts: one corresponds to configurational excitation at zero temperature and the other arises from thermal vibrations of atoms. The latter part contributes approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms, increasing to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though the distinctive mechanism of this phase transition results from configurational excitations. It is shown that the void concept allows analyzing various aspects of the liquid state of clusters including the existence of a limiting freezing temperature below which no metastable liquid state exists, as well as the existence and properties of glassy states that may exist below the freezing limit.     

Quantum phase transitions in mesoscopic systems

Quantum phase transitions in mesoscopic systems are studied. It is shown that the main features of phase transitions, defined for infinite number of particles, N--> infinity, persist even for moderate N approximately 10. A Landau analysis of first order transitions is done and a "critical" exponent at the spinodal point is defined. Two order parameters are introduced to distinguish first from second order transitions. Applications to atomic nuclei, molecules, atomic clusters, and finite polymers are mentioned. Experimental evidence in atomic nuclei is presented.