Condensed matter physics is one of the major areas of emphasis at the USC Physics and Astronomy Department. Research topics under investigation by the experimental group include superfluidity of ³He, electron transport at low temperatures, two-dimensional inversion layers in semiconductors, semiconductor physics and semiconductor device properties at low temperatures, magnetism at ultralow temperatures, and electromagnetic properties of superconductors.
Individual theorists are currently working on strongly interacting electron systems, the theory of conducting polymers, and soliton transport in low-dimensional materials.
Included in the Condensed Matter group are Professors Gerd Bergmann, Gene Bickers, Hans Bozler, Christopher Gould, Vitaly Kresin, Anupam Madhukar, Kazumi Maki, and Richard Thompson.
Professor Gerd Bergmann's interest lies in the field of electronic quantum interference in two-dimensional electron systems. By applying a magnetic field perpendicular to a thin metal film, he studies the propagation of the conduction electrons as a function of time and determines their lifetime in an energy state, the rotation of the electron spin and magnetic scattering. Using an advanced evaporation technique, Professor Bergmann and his students and co-workers are studying the interactions of the electrons with defects of the film, phonons and other excitations.
Another field of interest are the non-equilibrium properties of thin narrow films (micro-structures). In the presence of high current densities the electron and the phonon system can be driven out of equilibrium. The non-equilibrium distribution of the electrons and the phonons and the electron-phonon relaxation processes can be studied by this method.
Professor Gene Bickers is currently investigating aspects of the theory of strongly interacting electron systems. In most conducting materials, the Coulomb interaction between electrons may be ignored in first approximation by describing properties in terms of weakly interacting "quasiparticles." In strongly interacting systems, local electronic correlations are so large that the quasiparticle picture breaks down, resulting in novel electronic ground states. Experimental examples of strongly interacting systems include the "heavy-electron" metals, the Bechgaard salts, and the technologically promising high-temperature oxide superconductors. Professor Bickers is using a variety of analytical and numerical techniques to study models for these systems. His other research interests include localization effects in low-dimensional conductors and the development of novel quantum simulation techniques.
Professor Hans Bozler and his students are carrying out experiments to investigate the properties of superfluid ³He using nuclear magnetic resonance (NMR) and ultrasound propagation. His group is also studying surface magnetism and surface heat conduction in ³He as well as transport properties of one- and two-dimensional systems. Considerable effort is also being directed toward the task of developing new ultralow temperature techniques.
Professor Chris Gould also studies the behavior of matter at ultralow temperatures. His principal interest lies in precision measurements of the several superfluid phases of liquid ³He which exist at temperatures below a few millikelvin. He is also concerned with investigating questions involving quantum transport in systems of reduced dimensionality at ultralow temperatures.
The ultralow temperature group of Professors Bozler and Gould performs all of their experiments on several cryostats built by them, their postdocs, and their graduate students. One dilution refrigerator cryostat cools to 4 mK, while two nuclear demagnetization cryostats cool well below 0.4 mK. Students in the ultralow temperature group deal with a wide variety of experimental equipment and techniques including NMR, rf electronics, ultrasonic propagation, and SQUIDs (superconducting quantum interference devices). Most experiments are interfaced directly to one of several laboratory microcomputers which in turn are part of the larger university network of computers.
Professor Vitaly Kresin is investigating clusters - agglomerates of a finite number of atoms (from a few to thousands) which are bigger than a molecule but smaller than a piece of bulk matter. By generating a free beam of cluster particles and passing them through a mass spectrometer, the evolution of their properties can be monitored literally atom-by-atom. In this way the gradual development of various bulk characteristics can be mapped out, and quantum effects unique to finite systems can be observed. Current research focuses on metal clusters and on the cage-like carbon fullerenes. Both types of particles contain systems of delocalized electrons. The dynamics of these electrons are probed by polarizability measurements, electron-beam scattering, laser spectroscopy, and other methods. Work in this field allows for considerable interaction with other branches of physics, and has practical implications for surface and nanostructure science, etc.
Professor Anupam Madhukar is currently involved in experimental and theoretical studies of condensed matter in reduced dimensions focusing on (1) molecular beam epitaxial growth of quantum wells, superlattices, quantum wires and boxes, including all ultra high vacuum processing and in situ studies of growth kinetics; (2) optical and structural properties of such structures employing techniques such as photoluminescence and excitation spectroscopy, photo and electroreflectance, spectroscopic ellipsometry, absorption and magneto-optics, and high resolution electron microscopy; (3) optical and electrical properties of confined electron and hole gases via above noted optical techniques and mobility, Shubnikov-de Haas effect, quantum Hall effect, resonant tunnelling and photo-current spectroscopy.
Professor Kazumi Maki has worked on theoretical problems in condensed matter physics: gapless superconductors, type II superconductors, superfluid ³He, and polyacetylene. His current interest is concentrated on the Frhlich conduction in organic conductors like Bechgaard salts (e.g., (TMTSF)2ClO4), which is a realization of the quasi-two-dimensional conductors. He and his collaborators are able to interpret a cascade of spin density waves (SDW) induced by a high magnetic field (H=5-10T) in terms of the quasi-two-dimensional electrons interacting via the short range Coulomb. Further they have shown that these SDWs exhibit the Frohlich conduction (the electronic conduction associated with the bodily motion [sliding] of the spin density wave).
Professor Richard S. Thompson is working on the theory of superconductivity, particularly as applied to dynamical problems. He has calculated the effects of thermodynamic fluctuations and of the motion of quantized magnetic flux lines on the electrical conductivity of superconductors. Currently the theory is being applied to the recently discovered materials with high transition temperatures.