Condensed Matter Seminar "Microwave spectroscopy of valley states in silicon"

The bulk conduction band of Si has six equivalent valleys. Strain in Si/SiGe heterostructures partially lifts the six-fold valley degeneracy by raising the energy of the four in-plane valleys. It is known that large electric fields can lift the degeneracy of the remaining two low-lying valleys. However, the measured valley splittings range from 10-300 micro-eV, suggesting that microscopic details such as interface roughness and disorder impact the valley splitting. In this lecture I will describe how microwave spectroscopy can be applied to probe valley states in silicon nanostructures. In the first experiment, a cavity coupled Si double quantum dot is probed using microwave frequency photons. The transmission of the photons through the microwave cavity displays signatures that are consistent with the valley degree of freedom and the data can be modeled using cavity input-output theory. We also use Landau-Zener interferometry to probe the low-lying energy level structure of a silicon double quantum dot. The observed Landau-Zener interference pattern persists down to low driving frequencies of 50 MHz, suggesting relatively long-lived charge coherence. Low-lying valley states result in a unique Landau-Zener interference pattern that is in contrast with measurements on conventional two-level charge qubits. These new probes of valley states have high energy resolution and may be applied to other low energy degrees of freedom.