Condensed-Matter Physics & Materials Science Seminar

Spectroscopic probes of the single-particle energy levels in materials are a uniquely powerful probe of structure and interactions in material systems. Two-dimensional electron systems (2DES), among the most interesting highly correlated systems, have proven difficult to probe spectroscopically. Typical energy scales of interest are less than an meV, requiring high resolution, while correlated states of interest, such as those found in the integer and fractional quantum hall effects are destroyed by excessive electron heating. Approaches based on tunneling spectroscopy have been hampered by problems ohmic heating and low in-plane conductivity. In this talk, I will present a highly accurate pulsed spectroscopic technique in which does not suffer from either of these effects. We call the method "time domain capacitance spectroscopy". We use it to measure the tunneling density of states of a 2DES with resolution limited only by the ambient sample temperature. We measure, for the first time, spin splittings in Landau levels well above and below the Fermi level over a wide range of Landau level occupancies. Our results appear to be inconsistent with any existing calculation. Observation of Landau level broadenings with excitation energy permits determination of electron lifetimes resulting from electron-electron interactions. We observe Landau level structure in the excitation spectrum of the empty 2D system. Consistent with Fermi liquid theory, the levels broaden as we raise the electron density up from zero, and levels far from the Fermi energy disappear entirely. These levels reappear at higher electron densities when they move close to the Fermi Energy. The method promises quantitative extraction of the single-particle density of states with resolution several orders of magnitude better than photoemission spectroscopy, and we hope to extend its use to a variety of correlated electronic systems.