Condensed Matter Research

Condensed matter physics, the single largest area of physics world-wide, encompasses a diverse range of sub-fields such as correlated systems, magnetism, structural properties, and nanoscience. The condensed matter group has 7 faculty members, both experimentalists and theorists, with a close collaborations with researchers at UWM and elsewhere. While most of the research, funded mainly by NSF and DOE, is carried out at UWM, faculty and students also carry out experiments at various national user facilities.  

Faculty: Daniel Agterberg, Marija Gajdardziska-Josifovska, Prasenjit Guptasarma, Lian Li, Abbas Ourmazd, Dilano Saldin, Bimal Sarma and Michael Weinert.


Two half-Vortices coupled to dislocations in a PDW superconductor

Daniel Agterberg is a theorist working in the area of superconductivity and strongly correlated electronic materials, focusing on topics such as the nature of high-temperature superconductors and the consequences of topological structures on electronic wave functions. His research is driven by close communications with experimentalists to identify relevant problems that lie at the forefront of materials science, and combines analytical many-body/symmetry-based techniques with numerical calculations. He has recently collaborated with Scientists at Cornell University, the Swiss Federal Institute of Technology (ETH-Zurich), Stanford University, and the University of Tokyo.

Floating zone crystal growth chamber

Prasenjit Guptasarma is interested in the materials science of systems with strongly correlated electrons. His work seeks to elucidate the fundamental physics of unusual electronic and magnetic properties of materials, such as those near a critical phase transition, or bordering an unconventional quantum physical ground state. Current activities in his group include studies in the areas of novel superconductivity and magnetism, ferroelectricity, and multiferroics. Guptasarma's research lab hosts equipment for floating-zone growth of high-purity bulk single crystals, growth of nanostructures using high-pressure and solution-based techniques, and measurement of properties such as magnetic, transport, dielectric, specific heat, and ultrasound velocity, at extreme temperatures (350mK - 800K) and in magnetic fields (up to 9 Tesla). In addition, his group performs experiments at synchrotron light sources, neutron sources, and high magnetic field facilities in North America and abroad.

Atomic-scale imaging of ridges on expitaxial graphene on 6H-SiC(0001)

Lian Li conducts research to unveil structure and property relationships of condensed matter at the atomic scale. His current focus is on diluted magnetic semiconductors (DMS). The research addresses two fundamental questions in condensed matter physics: 1) how are local magnetic moments created in semiconductors (e.g., graphene and GaN), and 2) how do these moments interact with each other to attain long-range ferromagnetic ordering. The studies involve material growth using molecular beam epitaxy (MBE), atomic-scale characterization using spin-polarized scanning tunneling microscopy (SP-STM) and spectroscopy, synchrotron-based x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD), and first-principles calculations. 

Structural and electrical properties of a nano-scale transistor

Abbas Ourmazd is an expert in high-resolution mapping of structure and internal fields in condensed matter systems in physics and biology. His work is currently focused on innovative approaches for determining the structure and conformations of biological systems ranging from single molecules to whole cells. The central hypothesis rests on new evidence that advanced algorithms stemming from Riemannian geometry, graph theory, and machine learning can be combined to forge a powerful new approach to biological structure determination in a way which circumvents the limits set by noise and radiation damage. These algorithms can be used with existing techniques, such as cryo-electron microscopy (cryo-EM), and emerging approaches exploiting the extreme brightness of X-ray Free Electron Lasers (XFELs). More generally, this work has important implications for tomography of faintly scattering, non-stationary objects at any length scale.

Direct method reconstruction of the GaAs(111) (2x2) surface

Dilano Saldin's research has focussed on the use of scattering methods for the elucidation of the structure of materials on the atomic scale. Work has included the theory of electron microscope images of lattice defects, optical holography and phase conjugation, including four-wave mixing, the theory of x-ray absorption (XAS) and electron energy loss spectroscopy (EELS) of materials such as metal oxides and perovskite high temperature superconductors. Current interests include the structure of liquid crystals, and the novel capabilities offered by the x-ray free electron lasers (XFELs), currently under construction. In particular a recent focus has been on the development of novel diffraction techniques for determining the structures of non-crystallized biomolecules. These include the determination of the structures of membrane proteins in situ, of protein molecules in solution, and the so-called "diffract and destroy" method whereby the structure of a biomolecule or virus is reconstructed from diffraction patterns formed by ultrashort. high-intensity x-ray pulses from an XFEL before the particle is destroyed by the x-ray pulse. One of the techniques used is the reconstruction of single-particle diffraction patterns from angular correlation functions. 

Ultrasonic properties of URu_2Si_2

Bimal Sarma's interest in condensed matter is the study of materials in extreme environments -- at extremely low temperatures and extremely high magnetic fields. Sarma, whose work has spanned a wide arena of experimental physics from neutron diffraction to low-temperature techinques, has been concerned with the use of sound waves to investigate phase transitions and the actual phases themselves in magnetic systems and superconducting/superfluid systems. Sarma's landmark study of heavy-fermion superconductors showed the existence of a metallic superconductor whose microscopic behavior does not conform to the standard BCS model of electron-pairing and led to the remarkable discovery of multiple superconducting phases in UPt3. Sarma's current studies involve high-Tc and heavy-fermion superconductivity and the phases of materials in some of the highest magnetic fields. 

Calculated spectral weight for interfacial graphene on 6H-SiC(0001) and for epitaxial graphene

Michael Weinert's research is focused on understanding the electronic, magnetic, and structural properties of complex materials at the atomic level, primarily through the use of first-principles electronic structure calculations. Much of his research is done in close collaboration with experimentalists, both at UWM and elsewhere. Research topics include the effects of external electric fields on the electronic and magnetic properties of surfaces, interfaces, and nanostructures; phase stability of alloys and the role of defects; the electronic structure of oxides and related systems; magnetic semiconductors; the interpretation of various electron spectroscopies (e.g., STM, APECS, photoemission); and the development of new computational approaches and high-performance (parallel) computing applied to materials physics.