Physics Department Seminar

Wednesday, 11^th October, 1.30pm, Rm 1311HN

SHINING A LIGHT ON LOW-DIMENSIONAL SYSTEMS

Jiufeng J. Tu

Department of Physics

The City College of New York

Whether a material is an insulator or a metal (maybe a superconductor) is one of the most fundamental questions in physics. While this question can be easily addressed in three-dimension, the situation in lower dimensions is much more complicated. For example, no metallic phase was predicted theoretically in two-dimension (2D) for many years. Experiments on quasi-two dimensional electron systems have continued to reveal some of the most unexpected and theoretically challenging behavior in condensed matter physics. These phenomena include the pseudogap behavior in underdoped high-Tc cuprates and an apparent metal-insulator transition in dilute 2D electron gases. Such anomalous behavior is widely believed to be rooted in the proximity of these systems to quantum phase transitions. Ultra-thin films have served as model quasi-2D systems for many years. In this talk, I will discuss our recent optical studies of these systems in the frequency domain. Our preliminary THz study has identified a percolation transition at sheet resistance () ~ 3 kW (in addition to the superconductor-to-insulator transition near = h/4e² or 6.44 kW), possibly associated with a metal-to-insulator transition. We have proposed a new phase diagram for ultra-thin films that is similar to the phase diagram for high-Tc cuprates.