QDev Seminar: Raymond Ashoori

Momentum, Energy, and Spin Resolved Tunneling of Two-Dimensional Electronic Systems

The single-particle spectral function measures the density of electronic states (DOS) in a material as a function of both momentum and energy, providing central insights into phenomena such as superconductivity and Mott insulators. While scanning tunneling microscopy (STM) and other tunneling methods have provided partial spectral information, until now only angle-resolved photoemission spectroscopy (ARPES) has permitted a comprehensive determination of the spectral function of materials in both momentum and energy. However, ARPES operates only on electronic systems at the material surface and cannot work in the presence of applied magnetic fields. We have developed an extremely high resolution and contactless pulsed tunneling method and applied it to study quantum Hall system in GaAs and in graphene.  In the GaAs measurements, we recently discovered a delicate resonance, appearing only at dilution refrigerator temperatures, that arises from coupling of tunneling electrons to phonons of an electronic Wigner Crystal. We have also extended this pulsed tunneling method to perform full momentum and energy resolved tunneling spectroscopy (MERTS)  to reveal the momentum resolved electronic spectral function of a 2DES embedded in a semiconductor. In contrast to ARPES, the technique remains operational in the presence of large externally applied magnetic fields and functions even for electronic systems with zero electrical conductivity or with zero electron density. MERTS provides a direct high-resolution and high-fidelity probe of the dispersion and dynamics of the interacting 2D electron system. Using this technique, we uncover signatures of many-body effects involving electron-phonon interactions, plasmons, polarons with unprecedented resolution. When a perpendicular magnetic field is applied, the spectra evolve into discrete Landau levels. The massively degenerate electronic states strongly interact with nearly dispersionless LO-phonons and give rise to a novel phonon analog of the vacuum Rabi splitting in atomic systems. We have also extended the pulsed tunneling method to probe the spin polarization of quantum Hall systems. Our data show an oscillating pattern of the spin-polarized currents. In particular, the drastic decrease of the spin-polarized current near ν = 1 is consistent with the formation of skyrmions. The new spin-resolved technique also allows us to determine the spin polarization of the excited state spectrum, and we directly determine the spin polarization of features in the tunneling spectrum arising from short range interactions between electrons.