I am a condensed matter theorist, and my general areas of research are the optical and dielectric properties of composite media. Most recently I have worked on the theory of forces between polarizable nanoparticles in a time-dependent electric field, together with Francisco Claro, from the Universidad Catolica, Santiago, Chile. A general theory of forces between an arbitrary number of spheres, including all multipoles, has been published in Physical Review B39, 3875 (1989). If the electric field is originates from a beam of light, we assume that the distances between spheres are much smaller than the wavelength of light. The dominant force changes from attraction to repulsion as the frequency is increased, an effect that may be useful for manipulating the particles. A short account of the force between two spheres, calculated using the dipole approximation, has been published in Applied Physics Letters 85, 3280 (2004). If the applied electric field has a frequency that can excite surface modes in a many-particle system, the forces between the particles are nonconservative and greatly enhaned, and the force lines have a bizarre appearance, spiraling into a fixed point where the force is zero.
We also have found general expressions for the torques on the individual spheres, including all multipoles: Forces and torques on spherical particles in a time-dependent electric field. Our original goal in this research was to find how the applied electric field causes the spheres to move in a fluid medium. However, the rotation produced by the torque makes the paths of the spheres quite difficult to determine.
I also have been working on a collaborative experimental and theoretical effort to understand the magnetic properties of a complicated molecule in which the magnetic moment arises from four Ni ions in an approximately tetrahedral arrangement. These Ni ions have spins s=1 and are coupled by an antiferromagnetic superexchange interaction. A simple theory shows that if one applies a magnetic field B, the magnetic moment M of the molecule displays four steps before reaching saturation at about B = 20 T. These steps occur at equally spaced values of B, where the lowest Zeeman-split levels of adjacent S = 0, 1,2,3,4 multiplets cross. However, experiments show that the spacing between steps increases for higher values of B, and saturation of the magnetic moment does not occur until B ~ 50 or 60 T. Modifications of this model cannot eliminate the discrepancy between theory and experiment. The only way we have found to explain the experiment is to assume that the strength of the superexchange interaction parameter J increases with increasing B, an effect which might be explained by B-dependent distortion of the molecule. However, there is no basic physical understanding of why distortion should increase, rather than decrease, the strength of J. This work has been published in Physical Review B 73, 094401 (2006).
Previously I worked on electron energy-loss spectroscopy (EELS) of colloidal systems. This work was motivated by EELS measurements by A. Howie and C.A. Walsh on colloidal aluminum particles embedded in an aluminum fluoride matrix. The energy-loss spectrum has three peaks in the 0 to 40 eV energy range: a broad peak at 36 eV associated with bulk plasmon excitations in aluminum fluoride, a narrow peak at at 16 eV associated with bulk plasmon excitations of aluminum, and a peak of intermediate width at 8 eV associated with interfacial excitations.
Previous theoretical work in this area was mostly based on the concept of a fast electron on a classical trajectory passing near or through a single metallic particle (usually spherical), losing energy by exciting collective modes (bulk and surface plasmons). The theories were unable to treat a system containing many particles, as they could not take into account the interactions between the particles, average over all possible electron trajectories, or treat properly the random positions of the particles. In our new theory, we found the inverse nonlocal dielectric function of the system of spherical particles, from which the energy-loss spectrum can be calculated. The theory automatically includes an average over classical electron trajectories, and includes both the coulomb interaction between the spherical particles as a multipolar series carried out to arbitrary order and a configuation average over random sphere positions. An exact formal theory was constructed, and a spectral representation for the dielectric function was found. Numerical calculations were done using a mean-field approximation, which allowed us to express the interactions between spheres in terms of the two-particle correlation function. We were able to fit the data with an aluminum sphere radius of 2.6 nm and an aluminum volume fraction 0.25.
The reference for this work is
"Theory of electron energy loss in a random system of spheres",
R. G. Barrera and R. Fuchs, Phys. Rev. B 52, 3256 (1995).
More recent publications in the same area are listed below.
Ronald Fuchs, Ruben G. Barrera, Jose Luis Carrillo, "Spectral representations of the electron energy loss in composite media", Phys. Rev. B54, 12824 (1996).
Ronald Fuchs, Carlos I. Mendoza, Ruben G. Barrera, Jose Luis Carrillo, "Electron energy-loss spectroscopy of inhomogeneous systems", Physica A 241, 29 (1997).
Carlos I. Mendoza, Ruben G. Barrera, Ronald Fuchs, "Energy loss of electrons traveling parallel to the interface of a semi-infinite granular composite", Phys. Rev. B57, 11193 (1998).