Our current research centers on strongly correlated electron materials – often called quantum materials – such as superconductors, magnets, and charge density wave solids. We synthesize single crystals, fabricate nanoscale devices, and characterize their physical properties with a suite of tools that include electron and scanning-probe microscopies, ultrafast optical spectroscopy, and synchrotron x-ray techniques.
Because these systems are mesoscopic and sensitive to defects and dopants, high spatial and temporal resolution is essential for disentangling the collective behavior of correlated electrons.
Superconductivity and Magnetism
More than a century after its discovery in 1911, superconductivity remains a central topic in condensed-matter physics, driven both by the desire to understand its microscopic origin and by its applications in power cables, high-field magnets, single-photon sensing, and quantum-computing qubits.
Abrikosov vortices in superconductors play a key role: their motion dissipates energy in superconducting devices and cables. In our laboratory we study vortices in mesoscopic superconductors and in hybrid superconductor/ferromagnet systems.
We are also interested in complex materials that host competing order parameters – magnetism, superconductivity, and charge-density waves – simultaneously. We grow single crystals and thin films and examine how these orders evolve when we tune the system by electronic doping or controlled introduction of defects (disorder).
On the applications side, we work with materials such as NbN and TiN in superconducting nanowire single-photon detectors (SNSPDs) and microwave circuits, optimizing them for low loss and for retention of superconductivity in high magnetic fields (Ising superconductivity). Also we contribute to nuclear physics projects by developing superconducting transition-edge sensors for particle and photon detection.
Charge Density Wave Systems
Charge density wave (CDW) materials are metals, semimetals, or semiconductors that exhibit a periodic charge modulation on top of the underlying crystal lattice. This modulation can be commensurate or incommensurate with the crystal lattice. In the incommensurate case, the CDW can slide under an applied electric field, producing distinctive dynamic effects. Certain CDW patterns are chiral; the chirality can be controlled with polarized light, opening routes to optically manipulate the charge order.
Our studies of CDW dynamics and disorder near the phase transition are carried out in collaboration with the group of Prof. Anshul Kogar (UCLA) and Dr. Xiao Chen (Brookhaven National Laboratory). Using the bright x-ray beam at NSLS II we perform temperature-dependent x-ray photon correlation spectroscopy (XPCS) to track the dynamics of the CDW order from nanoseconds to minutes.
Energy Conversion
Over the past decade, substantial effort has gone into proton-exchange-membrane fuel cells that convert hydrogen and oxygen into electricity (for example, for automotive power). A central challenge is the design of inexpensive, stable catalysts for the oxygen-reduction reaction.
In collaboration with researchers at Drexel and Argonne National Laboratory, we use scanning probe microscopy to examine the activity of material surfaces and specific atomic sites in electrochemical processes.