Why does an electrical discharge, such as lightning, have a branching pattern? Why is not the pattern in a plasma ball spherically symmetric because of the spherically symmetric electric field inside?
Professor Cooper received a B.S. in Physics summa cum laude from the University of Virginia in 1982, and a Ph.D in Physics from the University of Illinois in 1988. After a two-year postdoctoral appointment at Bell Labs, Professor Cooper joined the UIUC faculty in 1990. From 1993-1995, he was a member of the Defense Science Study Group, which provides analysis for the DOD through the Institute for Defense Analysis.
Professor Cooper's group has pioneered techniques for using optical spectroscopy to reveal the structure and behavior of novel states of matter in strongly correlated materials. His group has developed particular expertise in light-scattering experiments on materials under extreme conditions of low temperature, high pressure, and high magnetic field. The Cooper group's Raman spectroscopy experiments have shed light on the behavior of matter through various pressure- and magnetic-field-tuned quantum (T~0 K) phase transitions.
The Cooper group's first accomplishment with its “extreme conditions” facility was a study of the evolution of the crystal lattice ("phonon") and atomic spin dynamics through the pressure-tuned destruction of the insulating state of layered ruthenate materials. More recently, his group has studied how high pressures "melt" charge- and orbital-ordered insulating states, even at T=0 K, creating novel metallic phases. His group has also shown that magnetic fields can be used both to control the elastic properties of materials (e.g., "magnetic field induced shape memory") and to thwart long-range order down to T=0 K.Cooper's group has also recently developed the capability to grow high quality single crystals using floating zone, vapor transport, evaporative, and other methods. His group has successfully grown high quality single crystals of spinel materials such as Mn3O4, orbital ordering materials such as KCuF3, layered chalcogenide materials such as TiSe2, and topological insulators like Bi2Se3.
Field- and pressure-tuned spectroscopy of magnetically frustrated and strong spin-lattice coupled materials
The development at low temperatures of some form of long-range order -- such as magnetism, orbital-order, charge-order, or superconductivity -- is ubiquitous in materials, and reflects the tendency of a material to lower its ground state degeneracy near T=0 K. We are interested in growing -- using float zone and other growth techniques -- and spectroscopically studying materials in which structural geometry and competing interactions conspire to frustrate the onset of long range magnetic and/or orbital order, even down to T=0 K. This interest is motivated by the novel low temperature phase behavior frustrated materials have been proposed to exhibit -- including orbital- and spin-liquid phases -- and by a desire to elucidate the connection between frustration and exotic properties such as colossal magnetoresistance, and multiferroic and magnetodielectric behavior. Our current efforts include using various single crystal growth methods to grow geometically frustrated materials, and then applying field- and pressure-dependent optical spectroscopy to study orbital- and spin-disordered phases in several classes of materials, including the layered ruthenates, spinels such as Mn3O4, iridates like Sr2IrO4, and vanadates such as Ni3V2O8. Our results have revealed interesting routes by which magnetic and orbital frustration can be tuned with field or pressure and show the connection between orbital/spin frustration and highly tunable properties of matter.
Field- and pressure-tuned melting of orbital order in correlated materials
We are also interested in creating and investigating novel orbital-liquid phases in various orbital-ordered systems such as Ca2RuO4, Ca3Ru2O7,and KCuF3. Our results have revealed pressure-induced transitions to novel quantum liquid-like phases in which structural elements fluctuate even at T=0 K, as well as pressure- and magnetic-field-tunable insulator-metal transitions governed by controllable changes induced in the orbital population.
Pressure-tuned quantum phase transitions and superconductivity in layered chalcogenide materials
We are interested in studying how charge ordered and charge density wave (CDW) states melt into disordered quantum phases at low temperatures and investigating the novel phases that are predicted to develop under these conditions. To study this, we use vapor transport growth methods to grow various layered chalcogenide single crystals, including TiSe2, TaSe2, TaS2, Bi2Se3, and Bi2Te3, and we study the quantum (T~0 K) phase transitions in these materials using pressure- and temperature-dependent inelastic light scattering. For example, our low temperature, pressure-dependent inelastic light scattering studies of the critical ('soft') mode in 1T-TiSe2 indicate that lattice compression leads to quantum melting of the CDW phase through a novel incommensurate phase that may have hexatic order, and our more recent light scattering studies of the soft mode in CuxTiSe2 provided evidence for x-dependent quantum mode softening and the coexistence of fluctuating CDW order and superconductivity in this system.
218 Seitz Materials Research Lab
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