Ronald Fuchs

Optical Modes of Vibration in an Ionic Crystal Sphere

The long-wave optical modes of vibration in an ionic crystal sphere have been determined, including retardation of the Coulomb forces. These modes, which correspond to coupled excitations of phonons and photons, are also known as polaritons. Their frequencies are complex, the imaginary parts arising from both anharmonic and radiative damping; hence they are virtual modes. It is found that the mode frequencies depend on the radius of the sphere only if retardation is included. The absorption and extinction cross sections for spheres of various sizes are calculated as a function of the frequency of the incident light, and it is shown how the structure in the cross sections is related to the properties of the virtual modes. The theory is used to explain the position and width of an optical absorption peak measured in a polyethylene film containing UO2 particles.

Electronic Properties of the Tungsten Bronzes

The magnetic susceptibility and specific heat of the metallic sodium tungsten bronzes NaxWO3 are explained by a model in which the Fermi energy does not depend on the sodium concentration x. The disappearance of metallic conductivity at small x is associated with finite cluster sizes of sodium atoms. Expressions are derived for the dependence of the conductivity on x in the metallic region.

Observation of field-dependent magnetic parameters in the magnetic molecule {Ni4Mo12}

We investigate the bulk magnetic, electron paramagnetic resonance, and magneto-optical properties of {Ni4Mo12}, a magnetic molecule with antiferromagnetically coupled tetrahedral {Ni4Mo12} in a diamagnetic molybdenum matrix. The low-temperature magnetization exhibits steps at irregular field intervals, a result that cannot be explained using a Heisenberg model even if it is augmented by magnetic anisotropy and biquadratic terms. Allowing the exchange and anisotropy parameters to depend on the magnetic field provides the best fit to our data, suggesting that the molecular structure (and thus the interactions between spins) may be changing with applied magnetic field.

Optical properties of small-particle composites

A new theory for the effective dielectric constant of a small‐particle composite system includes broadening of single‐particle resonances by dipolar interactions and percolation at high particle density. Comparisons are made with effective medium theory, Maxwell‐Garnett theory, resistor network models, and with optical measurements on metallic and dielectric composites.

Electron energy-loss spectroscopy of inhomogeneous systems

After beginning with a brief review of the theory of electron energy loss by an unbounded random system of spherical inclusions characterized by a local dielectric function, we examine several extensions of this theory. We first treat an inhomogeneous system of spherical particles confined to a half-space. A surface response function, which can be used tocalculate the energy-loss spectrum for charged particles moving outside the system, is defined, and this response function is written in a spectral representation. We discuss different approaches to this problem: the semiclassical infinite barrier model, an exact formal theory, and a continuous effective medium theory. Finally, we develop a theory of electron energy loss for a mixture of two components with arbitrary geometry, unbounded in three dimensions.

Optical and dielectric properties of self-similar structures

The effective dielectric constant and spectral function of a composite system prepared by recursively introducing inclusions of a given component are calculated. Connections are made with differential effective medium theory, and dynamical critical behavior is studied.

Optical excitations in small particles and thin films

The optical absorption in small cubic particles and thin films, composed of an ionic crystal or a free-electron metal, is calculated by using both a local dielectric constant and more-exact microscopic methods. It is found that a local theory gives a qualitatively correct description of the absorption in cubes but not in thin films. The electric field is calculated in thin metallic films, and the results are applied to the theory of surface-enhanced Raman scattering.

Local-Field Effects at Crystalline Surfaces: Electron Energy Loss from Ordered Arrays of Spheres

Local-field effects at crystalline surfaces are analyzed on a classical system consistent of ordered arrays of polarizable spheres. A theory for the energy loss of fast electrons traveling parallel to these arrays is presented. A spectral representation of the surface response function is used to calculate this energy loss. The poles and weights in this representation are determined through the eigenvalues and eigenvectors of an interaction matrix which takes into account the quasi-static electromagnetic fields to an arbitrary multipolar order. We apply the theory to calculate the energy-loss spectra for cubic arrays of aluminum spheres embedded in vacuum and compare the results with those obtained using a dielectric continuum model.

The effect of dissipation on the torque and force experienced by nanoparticles in an AC field

We discuss the force and torque acting on spherical particles in an ensemble in the presence of a uniform AC electric field. We show that for a torque causing particle rotation to appear the particle must be absorptive. Our proof includes all electromagnetic excitations, which in the case of two or more particles gives rise to one or more resonances in the spectrum of force and torque depending on interparticle distance. Several peaks are found in the force and torque between two spheres at small interparticle distances, which coalesce to just one as the separation grows beyond three particle radii. We also show that in the presence of dissipation the force on each particle is non conservative and may not be derived from the classical interaction potential energy as has been done in the past.