Condensed Matter Physics/Biophysics
Condensed matter (CM) research at Notre Dame encompasses topics of research ranging from “hard” CM problems such as semiconductor or superconductor systems to “soft” CM problems such as studies of multicellular aggregates or the application of network theory to biological systems. The topics studied are described below:
Physics on the Nanoscale
Single-electron charging effects and related phenomena are explored to probe the basic physics of few-atom clusters, fullerenes and other exotic systems comprised of only a few atoms. The growth and self assembly of quantum dots, quantum wires, and heterostructures in semiconductor systems is also studied extensively. Work on heterostructures includes the development of blue-light semiconducting lasers. Self-organized quantum dots and other nanophase systems are grown and characterized using optical, magnetic, transport, and x-ray techniques. Facilities include a dual-chamber molecular beam epitaxy machine, extensive facilities for optical and magneto-optical studies of nanoscale systems with micrometer-scale and sub-micrometer-scale (near field) resolution, and instrumentation for the study of electrical transport and magnetic properties.
Semiconductor Physics and Magnetism
Thin-film II-VI, III-V and other semiconductor samples are prepared by molecular beam epitaxy. III-V semiconductors which incorporate Mn ions in the lattice are ferromagnets and are expected to play a key role in future “spintronic” devices. These, as well as other magnetic samples, are studied by a variety of experimental techniques including laser magneto-spectroscopy, x-ray and neutron scattering, and electron transport. Facilities include extensive capabilities for the study of electrical properties, magnetization, and state of the art apparatus for the study of magnetic resonance. In addition, magnetic properties of solids are studied by neutron scattering, carried out off campus at the National Institute for Standards and Technology and at the University of Missouri Research Reactor Center (MURR).
X-ray scattering and X-ray absorption fine structure (XAFS) are used to study the surfaces and internal interfaces of solids and liquids, phase transformations and ordering phenomena in condensed-matter systems. Examples of recent studies atomic-scale structure of “highly correlated” magnetic materials, interfaces and structure of magnetic semiconductors, the structure of complex nanophase materials, the structure of metalloproteins, and environmental systems on the molecular scale. Because of the unique advantages of synchrotron radiation, these experiments are conducted at national facilities located at the Advanced Photon Source, Argonne National Laboratory, where Notre Dame is a major participant.
Superconductivity and Vortices
High-temperature superconductors are studied from the perspective of microwave absorption and other techniques with a view to probing fundamental mechanisms. These include investigations of the response of high-temperature superconductor thin-film systems to ultrashort duration, far-infrared light to evaluate potential applications for and the intrinsic electronic properties of these novel materials. New materials are synthesized using the traveling solvent float zone (TSFZ) technique in a mirror furnace-based system.
In a separate effort, new superconducting systems based on dilute-doped elemental superconductors are being developed for micro-refrigerators and transition-edge x-ray sensors for space missions. Facilities include thermal evaporation and multi-source sputtering systems, a cold head for electro-optic studies down to 25K, a SQUID voltmeter, a 10 T superconducting magnet, low-temperature equipment for work to 1 K, and a clean room for contact lithography. A fiber optic link to the lab of a collaborating atomic physicist permits the piping of modulated laser light to these experiments. Collaborations with NIST, Boulder, provide access to an extensive class-100 clean-room, adiabatic refrigeration to 60 mK, and magneto-optic facilities.
Scanning tunneling microscopy and spectroscopy (STM/STS) are used to image vortices induced by an applied magnetic field and probe their spectroscopic properties. These measurements are complemented with studies of the vortex lattice structure using small-angle neutron scattering (SANS). Combined, the two techniques allows a study of how the superconducting gap and the vortex lattice symmetry and orientation evolves as a function of temperature and field. On-site facilities include a low-temperature, ultra-high vacuum STM (under construction) while the neutron scattering studies are largely conducted at the Institut Laue-Langevin, Grenoble, France.
Theoretical Condensed Matter Physics
Notre Dame theoretical condensed matter physicists study superconductors, semiconductors, soft matter, and properties of networks.
In one theoretical effort in superconductivity, finite temperature field-theory techniques are used to study two-dimensional antiferromagnets. Also studied are highly-correlated electronic systems, including disordered and frustrated ferromagnets, such as magnetic semiconductors, high temperature superconductors, the novel superconducting compound, MgB 2 , and mesoscopic superconductivity. In semiconductors, an active collaboration exists between theorists and experimentalists studying mesoscopic and nanoscopic physics. In particular, Zeeman-induced nanoscale localization of spin-polarized carriers in magnetic semiconductor-permalloy hybrids is studied. In another project, Monte Carlo simulations are used to study the microstructure of strained semiconductor alloys and compounds.
Finally, the tools of statistical mechanics are applied to understanding real networks, including metabolic and genetic networks, social networks, the Internet, and the World Wide Web. A special focus is towards understanding the implications of the scale-free characteristics of real networks, a concept developed at Notre Dame.
The department hosts an active program In biophysics, focusing on modeling the structure and development of various biological systems. A strong focus is on understanding the topological properties of cellular networks--the networks formed by the Interactions between metabolites, genes and proteins, modeling both their structure and dynamical behavior. Using techniques from statistical mechanics, models of “convergent extension” cell rearrangements have been developed as a way to understand one step in embryonic development. At a higher level, multicellular aggregates, such as embryonic and mature tissues, are modeled. These systems often share the properties of “excitable media” and “soft matter,” familiar to modern condensed matter physics and dynamical systems theory. Biological research is carried out in collaboration with other groups on the campus, involving faculty from biochemistry and biology, under the coordination of the Center for Biocomplexity.