Physics (Internal)

Precise Measurement of Electronic States Above and Below the Fermi Level

This event is part of the Biophysics/Condensed Matter Seminar Series.

Abstract: In many ways, electrons in a metal behave much like sand in a jar. The sand fills the jar up to a certain level, just as electrons fill energy levels in a metal up to an energy known as the Fermi-level. Any sand grains put in above this level will fall down to the sand-level, and plucking a sand grain from below the sand level will create a vacancy (or hole) that will work its way up to the sand-level as grains fall into the vacancy. The same process works for injecting an electron above the Fermi-level or pulling out an electron from below the Fermi level to leave a hole. This injection or ejection of electrons into and out of a system can be accomplished by means of quantum mechanical tunneling. We have developed a method to measure tunneling into a two-dimensional metal with extraordinary resolution and precision. We call our method “Time Domain Capacitance Spectroscopy” (TDCS), and it works by averaging results from using millions of very short (50 ns) pulses to inject electrons into the system of interest with negligible heating. We use TDCS to study the two-dimensional electron system, host to the integer and fractional quantum Hall effects. We can precisely measure energies of and tunneling strengths for quantum states above and below the Fermi energy. The measurements allow us to determine the lifetimes of electrons in these states (the time they spend in the states before they fall down to the Fermi energy) and measure the effects of magnetism and many-body physics on the spectra. They have also led us to discover new “quasiparticles” whose origin is at present a mystery.