Department of Physics at the University of Illinois at Urbana-Champaign

S. Lance Cooper

john Bardeen faculty scholar and Professor of Physics

Professor Cooper received his bachelor's degree in physics from the University of Virginia in 1982, and his Ph.D in physics from the University of Illinois. After a postdoctoral appointment at Bell Labs, Professor Cooper joined the faculty here in 1990. Since 1983, he has been a Member of the Defense Science Study Group, which provides analysis for DOD and DOE through the Institute for Defense Analysis.

Professor Cooper has developed pioneering techniques in the use of optical spectroscopy to reveal the structure and behavior of matter, including the locations, currents, and modes of excitation of the charged particles (electrons and nuclei) within bulk samples. His group has developed particular expertise in light-scattering experiments on samples under extreme conditions of temperature, pressure, and magnetic field. His definitive Raman and Brillouin spectroscopy experiments have shed light on some of the most exciting and important themes in contemporary research on the physics of systems comprising many strongly interacting particles, including quantum phase transitions.

Professor Cooper's first accomplishment with his “extreme conditions” facility was a study of the evolution of the crystal lattice dynamics and magnetism through the pressure-tuned destruction of the insulating state of layered ruthenate materials. In his most recent work, he has focused on Mott insulators and shown that high pressures can destroy their insulating character and create magnetism.

Research Area: experimental condensed matter physics, optical spectroscopy, strongly correlated systems, superconductivity

Description of Current Research

Spectroscopic Studies of Low Carrier Density Magnetic Systems
We are interested in a number of low-carrier-density magnetic systems that have rich phase diagrams as a function of doping, including insulating, ferromagnetic metal, and antiferromagnetic ground states. The diverse phase diagrams of these materials derive largely from the competition between strong Coulomb correlations, electron-phonon coupling, and spin interactions. We are using various optical techniques, including reflectance and light-scattering spectroscopies, to characterize the excitation spectra of these materials, and to elucidate the mechanisms driving the different phase transitions.

Spectroscopic Studies of the Magnetic Oxides
The magnetic oxides exhibit a wide variety of exotic phenomena, including paramagnetic insulating-to-ferromagnetic metal transitions and "colossal magnetoresistance" behavior at intermediate doping, as well as ordered charge and spin structures at high doping. We are attempting to elucidate the physics governing these interesting phase regions by using reflectance, Raman, and infrared spectroscopies to study the interactions between the lattice, charge, and spin degrees of freedom in these materials.

Selected Publications

H Rho, SL Cooper, S Nakatsuji, H Fukazawa, and Y Maeno, Lattice dynamics and the electron-phonon interaction in Ca₂RuO₄. Phys. Rev. B 71, 245121-1-6 (2005).

H Rho, H, SL Cooper, S Nakatsuji, H Fukazawa, and Y Maeno. Raman scattering studies of
Ca_2-xSr_xRuO₄. Proc. of the Intl. Conf. on Strongly Correlated Electron Systems. July 26-30, 2004) Physica B: 359-361, 1270-1272 (2005).

JF Karpus, R Gupta, H Barath, SL Cooper, and G Cao. Raman scattering studies of field-induced melting of the orbital-ordered state of Ca₃Ru₂O₇. Proc. of the Intl. Conf. on Strongly Correlated Electron Systems. July 26-30, 2004); Physica B 359-361, 1234-1236 (2005).

JK Freericks, TP Devereaux, M Moraghebi, and SL Cooper. Optical sum rules that relate to the potential energy of strongly correlated systems. Phys. Rev. Lett. 94, 216401-1-4 (2005).

A. Comment, et al. Magnetic properties of pure and Gd-doped EuO probed by NMR. Phys. Rev. B 72, 014428-1-12 (2005).

SG Choi, CJ Palmstrom, YD Kim, SL Cooper, and DE Aspnes. Dielectric functions of Al_xGa_1-xSb
(0.00 ≤ x ≤ 0.39) alloys from 1.5 to 6.0 eV. J. Appl. Phys. 98, 104108-1-5 (2005).

Honors and Awards

• Sony Faculty Scholar, College of Engineering, 2003–2006
• Fellow, American Physical Society, 2003
• Andersen Award for Excellence in Undergraduate Advising, 1992 and 1996
• IBM Pre-doctoral Fellowship, 1986–1987
• Phi Beta Kappa, 1982