Research and Papers
Date: March 2007
Abstract: Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral-weight transfers from the high-energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction—a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2O8+delta. Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond-centered electronic pattern without long-range order but with 4a0-wide unidirectional electronic domains dispersed throughout (a0: the Cu-O-Cu distance). The emerging picture is then of a partial hole localization within an intrinsic electronic glass evolving, at higher hole densities, into complete delocalization and highest-temperature superconductivity.
Date: September 2004
Here is the cover of Physics Today for September 2004. I created the image for the cover using POV-Ray and the raw data from our NaCCOC experiments. The image was rendered and slightly filtered. The top layer is a measure of the conductance of the sample, which is often theoretically proportional to the local denisty of electronic states (LDOS). The bottom layer is the topograph for the sample, which you can think of as being the atoms themselves. This cover image is associated with a story done by Physics Today corresponding to the publication of our Nature paper (below) which was released about the same time.
This is a paper reporting our discovery in Fall 2004, and it was published
in Nature as of 8/26/04. Okay, so I know my name doesn't appear among the
list of authors, but I did do some of the work! :-) (It was mostly data
analysis and some acquisition.) However, the author list was getting a
little bit long, and I was relatively new to the group compared to the
other veterans when this paper was submitted. The main point of this work
is the discovery of a 3/4 periodicity in the conductance of the
spatially-dependent local density of electronic states (LDOS) in the (at
the time) newly-growable cuprate superconductor
Date: Spring 2001
Final Paper for Electrical Engineering Senior Design at UIUC. This project won the area award in semiconductors among 54 other senior design projects in the Spring of 2001. The research was done with the MBE group at the Microelectronics Laboratory at UIUC. See the "official" URL here. See our award certificate here.
Date: August 2000