Structural Aspects of Condensed Matter Systems
Structure plays an important role in understanding the physical properties of solid systems. The recognition that most solids have a well defined atomic structure provided a unification of a number of subfields in physics and played an important role in the emergence of the field that we now know as condensed matter physics. Professor Bridges works on a number of topics including the coupling between the crystal lattice and the electron system in correlated electron systems, the unusual properties of tunneling defect atoms, and the surface contact sticking between very cold frost-coated surfaces (dynamics of Saturn's rings).
One of Bridges' major areas of research is the study of local structure in the vicinity of specific atoms using X-ray Absorption Fine Structure spectroscopy (XAFS). This technique requires the very high x-ray intensity available at a Synchrotron source such as the nearby Stanford Synchrotron radiation Laboratory. Materials investigated include quasicrystals, fullerenes, superconductors and, recently, the colossal magnetoresistance (CMR) materials. In the Hg series of high Tc materials they have discovered the unusual phenomena of negatively correlated displacements for some pairs of atoms. In the CMR materials they find a large change in the O disorder (associated with polaron formation) about the Mn atoms that is correlated with the sample magnetization.
Another area is the study of atomic defects. Using high frequency microwaves he has studied the tunneling properties of small defect atoms in crystals. One of the reasons these systems are interesting is that they have a large number of local potential minima (typically 8-12) which results in a rich array of physical properties.
In a rather different set of experiments Bridges and collaborators have studied the dynamics of ice particle collisions. At very low speeds, ice particles are surprisingly elastic, comparable or better than super-balls (i.e., less than 10 % energy loss per collision). The energy loss increases with velocity and for compacted frost-covered surfaces can be greater than 65 % at only 1 mm/s. For freshly deposited water frost with a dendritic-like surface structure, the surfaces stick together in low speed contacts at low temperatures. Most recently they have found that surface layers of methanol or water/methanol mixtures remain plastic to very low temperatures and provide much larger sticking forces in the 100 to 150K range than water frost. These phenomena play an essential role in the dynamics in rings systems such as those on Saturn. They are also important for understanding the aggregation processes as macroscopic objects (including comets) formed in the early Solar system.
Bridges' research group involves a number of graduate and undergraduate students; a range of microwave, x-ray and cryogenic equipment are available.