P H Y S I C S
C O L L O Q U I U M

Yong P. Chen
Rice University

Quantum Coherence in Insulators

ABSTRACT
Quantum coherence in condensed matter systems is often associated with 
superfluids and superconductors. In this talk, I will describe two 
experiments from my recent research to illustrate that quantum coherence can 
be important even in a macroscopic insulator.

In the first example, a Bose-Einstein condensate (BEC) of 7Li atoms is 
subjected to a disordered optical potential made by laser speckle, where the 
disorder strength (Vd) is tunable by varying the laser intensity. Increasing 
Vd drives the system from a superfluid to an insulator, signaled by the 
suppression of transport. However, for a wide range of Vd even in the 
insulating regime, time of flight images upon releasing the atoms show 
striking interference-like fringes, interpreted as due to finite quantum 
coherence in the disordered BEC. These fringes disappear when Vd is further 
increased to break the BEC into multiple pieces with no phase coherence 
between them. I will also discuss tuning atomic interactions with Feshbach 
resonance to study phenomena in disordered systems ranging from Anderson 
localization to the interplay between interaction and disorder.

In the second example, a semiconductor double quantum well with low 
interlayer tunneling but strong interlayer Coulomb interaction is subjected 
to a transverse magnetic field. When there is one magnetic flux quantum per 
carrier, the system condenses into a bilayer excitonic condensate (BLEC) 
displaying counterflow superfluidity. Increasing the magnetic field drives 
the system into a bilayer Wigner crystal (BWC), which is an insulator due to 
pinning by disorder. Microwave spectroscopy of the pinning mode resonance 
has revealed evidence suggesting that long-range phase coherence (similar to 
that in the BLEC) may coexist with the long-range crystal order in the BWC. 
I will also discuss possible future experiments looking for an electronic 
supersolid phase in this and other systems (such as cuprate high Tc 
superconductors).

Tuesday, February 6, 2007
4:00 p.m.
ENPH 202T