The melting of crystals has resisted efforts to understand the microscopic process for more than a century. The course of the struggle has stimulated the development of quantum mechanics, concepts of long-range order, the role of dimensionality in condensed matter, surface physics, and more sensitive experimental techniques to test the theories. After years of probing the mechanism within the bulk material, we learn that the answer has been lying on the surface.

This paper presents simple models useful in analyzing the growth of nanostructures obtained by cluster deposition. After a brief survey of applications and experimental methods, the author describes the Monte Carlo techniques for simulating nanostructure growth. Simulations of the first stages, the submonolayer regime, are reported for a wide variety of experimental situations: complete condensation, growth with reevaporation, nucleation on defects, and total or null cluster-cluster coalescence. [Note: Software for all these simulation programs, which are also useful for analyzing growth from atomic beams, is available on request from the author.] The aim of the paper is to help experimentalists, in analyzing their data, to determine which processes are important and to quantify them. Experiments on growth from cluster beams are discussed, as is the measurement of cluster mobility on the surface. Surprisingly high mobility values are found. An important issue for future technological applications of cluster deposition is the relation between the size of the incident clusters and the size of the islands obtained on the substrate, which is described by an approximate formula depending on the melting temperature of the deposited material. Finally, the author examines the atomic mechanisms that can explain the diffusion of clusters on a substrate and their mutual interaction, to aggregate keeping their integrity or to coalesce.

Understanding the microscopic processes by which the electrons within conductors are scattered at surfaces and interfaces is important for both fundamental physics and technology. The authors review what has been learned so far about scattering of electrons at a variety of surfaces and interfaces using a technique, transverse electron focusing (TEF), that involves two point contacts in a uniform magnetic field. Transverse electron focusing is a sort of β-ray spectrometer in a metal, except that, whereas the β-ray spectrometer requires a narrow beam because the energy and momentum of a free electron can be arbitrary, in TEF the electrons of interest all have the Fermi energy and momentum, so focusing occurs even for electrons injected isotropically in angle. Transverse electron focusing is unique in its ability to probe localized and selectable portions of the interface from inside the conductor, using conduction electrons on only small parts of the Fermi surface. The authors first briefly review the essential features of TEF and of ideal and rough surfaces and describe the three techniques now used for injecting and detecting electrons: needle contacts, lithographically fabricated contacts, and light-induced injection. They then turn to measurements in metals and semimetals of the probability of specular reflection q from a given interface for electrons at the Fermi energy impinging at perpendicular incidence. They examine how q varies over different crystal faces for different electron orbits on the Fermi surfaces of a variety of conductors and how it is affected by changes in the de Broglie wavelength ${\lambda }_{\mathrm{dB}},$ by chemical etching, ion etching, or physical damage, by a molecular overlayer condensed from the surrounding atmosphere, and, for semimetals, by surface band bending (surface charge). The authors also explain how to measure the dependence of q upon the angle of incidence $q\left(\theta \right),$ which gives information about surface structure. Transverse electron focusing studies of a variety of quasiparticle effects arising because the electrons are in a solid are described. These include (a) scattering of excitations moving on “holelike” orbits—q can depend upon the sign of the particle charge; (b) scattering involving a surface reciprocal-lattice vector ${\mathbf{G}}_{\tau },$ including surface resonances induced by an artificial grating etched onto a Bi surface; and (c) scattering between different parts of the Fermi surface—intervalley scattering (IVS)—including scattering in which the sign of the quasiparticle charge changes. The authors review studies of scattering of electrons from a normal-metal (or semimetal)/superconductor interface, which involves an unusual phenomenon called Andreev reflection, in which the signs of both the charge and mass change. Also described are TEF studies of scattering of light-excited electrons from an intercrystalline boundary and recent TEF measurements of q for scattering from the boundary of the two-dimensional electron gas. The authors conclude with a list of future TEF studies of conduction-electron/interface interactions that they believe to be interesting and important.

The authors review the nonlinear optical properties of semiconductor quantum wells that are grown inside high-Q Bragg-mirror microcavities. Light-matter coupling in this system is particularly pronounced, leading in the linear regime to a polaritonic mixing of the excitonic quantum well resonance and the single longitudinal cavity mode. The resulting normal-mode splitting of the optical resonance is observed in reflection, transmission, and luminescence experiments. In the nonlinear regime the strong light-matter coupling influences the excitation-dependent bleaching of the normal-mode resonances for nonresonant excitation, leads to transient saturation and normal-mode oscillations for resonant pulsed excitation and is responsible for the density-dependent signatures in the luminescence characteristics. These and many more experimental observations are summarized and explained in this review using a microscopic theory for the Coulomb interacting electron-hole system in the quantum well that is nonperturbatively coupled to the cavity light field.

Nonclassical effects such as squeezing, antibunching, and sub-Poissonian statistics of photons have been attracting attention in quantum optics over the last decade. Up to now most theoretical and experimental investigations have been carried out exclusively in the time domain while neglecting the spatial aspects by considering only one spatial mode of the electromagnetic field. In many situations such an approximation is well justified. There are, however, problems that do not allow in principle a single-mode consideration. This is the case when one wants to investigate the quantum fluctuations of light at different spatial points in the plane perpendicular to the direction of propagation of the light beam. Such an investigation requires a complete description of quantum fluctuations of light in both time and space and cannot be done within a single-mode theory. This space-time description brings about a natural generalization into the spatial domain of such notions as the standard quantum limit, squeezing, antibunching, etc. It predicts, for example, the possibility of generating a light beam with sub-Poissonian statistics of photons not only in time but also in the beam’s transverse plane. Of particular relevance to the applications is a situation in which the cross section of the light beam contains several nonoverlapping areas with sub-Poissonian statistics of photons in each. Photodetection of such a beam produces several sub-shot-noise photocurrents depending on the number of independent areas with sub-Poissonian statistics. This is in marked contrast to the case of a single-mode sub-Poissonian light beam in which any attempt to collect light from only a part of the beam deteriorates the degree of shot-noise reduction. This property of multimode squeezed light opens a range of interesting new applications in optical imaging, optical parallel processing of information, parallel computing, and many other areas in which it is desirable to have a light beam with regular photon statistics across its transverse area. The aim of this review is to describe the recent development in this branch of quantum optics.

The environment in which bremsstrahlung and pair creation occurs can strongly affect cross sections for these processes. Because ultrarelativistic electromagnetic interactions involve very small longitudinal momentum transfers, the reactions occur gradually, spread over long distances. During this time, even relatively weak factors can accumulate enough to disrupt the interaction. In the Landau-Pomeranchuk-Migdal effect, multiple scattering reduces the bremsstrahlung and pair production cross section. This review will discuss this and a variety of other factors that can suppress bremsstrahlung and pair production, as well as related effects involving beamstrahlung and QCD processes. After surveying different theoretical approaches, experimental measurements will be covered. Recent accurate measurements by the SLAC E-146 Collaboration will be highlighted, along with several recent theoretical works relating to the experiment.

Contrary to naive cosmological expectations, all evidence suggests that the universe contains an abundance of matter over antimatter. This article reviews the currently popular scenario in which testable physics, present in the standard model of electroweak interactions and its modest extensions, is responsible for this fundamental cosmological datum. A pedagogical explanation of the motivations and physics behind electroweak baryogenesis is provided, and analytical approaches, numerical studies, up to date developments, and open questions in the field are also discussed.

The authors survey the current status of light-meson spectroscopy, beginning with a general introduction to meson spectroscopy and its importance in understanding the physical states of quantum chromodynamics. Phenomenological models of hadron spectroscopy are described with particular emphasis on the constituent-quark model and the qualitative features it predicts for the meson spectrum. The authors next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids and glueballs, as well as multiquark states. The established meson states are compared to the quark-model predictions and most meson states are found to be well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. The review ends with a brief description of future directions in meson spectroscopy.

HERA, the first electron-proton collider, has been delivering luminosity since 1992. It is the natural extension of an impressive series of fixed-target lepton-nucleon scattering experiments. The increase of a factor of 10 in center-of-mass energy over that available for fixed-target experiments has allowed the discovery of several important results, such as the large number of slow partons in the proton and the sizable diffractive cross section at large $Q^2.$ Recent data point to a possible deviation from standard-model expectations at very high $Q^2,$ highlighting the physics potential of HERA for new effects. The HERA program is currently in a transition period. The first six years of data taking have primarily elucidated the structure of the proton, have allowed detailed QCD studies, and have had a strong impact on the understanding of QCD dynamics. The coming years will bring the era of electroweak studies and high $Q^2$ measurements. This is therefore an appropriate juncture at which to review HERA results.

The authors review some aspects of the interplay between the dynamics of branes in string theory and the classical and quantum physics of gauge theories with different numbers of supersymmetries in various dimensions.

The authors review the status of searches for supersymmetric particles at the Tevatron collider. After discussing the theoretical aspects relevant to the production and decay of supersymmetric particles at the Tevatron, the authors present the current results for runs Ia and Ib as of the summer of 1997.

Progress in high-energy gamma-ray astronomy has depended upon the development of sophisticated detectors and analysis techniques. Observations in this decade using space-based and ground-based detectors have observed gamma-ray emission from a variety of sources. For the first time a consistent picture of the γ-ray sky has emerged. This article describes the detection techniques in γ-ray astronomy, the nature of the astrophysical objects studied, and the present state of the observations. Several possible new directions in the field are also described.

The static Casimir effect describes an attractive force between two conducting plates, due to quantum fluctuations of the electromagnetic (EM) field in the intervening space. Thermal fluctuations of correlated fluids (such as critical mixtures, super-fluids, liquid crystals, or electrolytes) are also modified by the boundaries, resulting in finite-size corrections at criticality, and additional forces that affect wetting and layering phenomena. Modified fluctuations of the EM field can also account for the “van der Waals” interaction between conducting spheres, and have analogs in the fluctuation-induced interactions between inclusions on a membrane. We employ a path integral formalism to study these phenomena for boundaries of arbitrary shape. This allows us to examine the many unexpected phenomena of the dynamic Casimir effect due to moving boundaries. With the inclusion of quantum fluctuations, the EM vacuum behaves essentially as a complex fluid, and modifies the motion of objects through it. In particular, from the mechanical response function of the EM vacuum, we extract a plethora of interesting results, the most notable being: (i) The effective mass of a plate depends on its shape, and becomes anisotropic. (ii) There is dissipation and damping of the motion, again dependent upon shape and direction of motion, due to emission of photons. (iii) There is a continuous spectrum of resonant cavity modes that can be excited by the motion of the (neutral) boundaries.