Tunneling Spectroscopy with Extraordinary Energy Resolution: The Discovery of a New Quasiparticle Raymond Ashoori, Professor of Physics, Massachusetts Institute of Technology

Tunneling spectroscopy is among the most direct probes of the quantum mechanical structure of electronic systems. Unfortunately, implementation of accurate tunneling spectroscopy has often been plagued by problems ranging from heating from tunneling currents to inability to study systems with low conductivity. We have developed a method of precision tunneling spectroscopy that solves the common problems (it even allows tunneling study of insulators!) and produces spectra with energy resolution limited only by sample temperature.Using the new method, we have made an intriguing discovery in the quantum Hall system. Nearly 20 years ago, theorists learned that the fractional quantum Hall effect can be described as the integer quantum Hall effect for quasiparticles called "composite fermions" that interact weakly with one another. These quasiparticles consist of an electron with two quanta of magnetic flux attached. A large body of experimental work now demonstrates the existence of these quasiparticles, and we see evidence of them in our tunneling spectra. Surprisingly, we find much more prominent features in our spectra from an unexpected composite particle with only one flux quantum attached.