Princeton University

School of Engineering & Applied Science

Tunable Fermi Contour Anisotrophy in GaAs Electron and Hole Systems

Dobromir Kamburov
Engineering Quadrangle J401
Thursday, June 26, 2014 - 11:00am to 12:30pm

This work explores the ballistic transport of electrons and holes confined to move in a plane in GaAs quantum wells and subjected to low temperatures in the presence of an applied magnetic field. The surface of the samples has a periodic modulation which triggers commensurability features in the sample’s resistance whenever the diameter of the carriers' orbits fit an integer number of modulation periods. We establish how the commensurability signatures depend on the sample parameters, including the carrier density, the modulation period, and the width of the quantum well to which they are confined. We then use the commensurability effects to directly probe the size of the particles’ trajectories at low magnetic field.
In the presence of a small perpendicular magnetic field, the trajectories of both electrons and holes are essentially circular. When an additional parallel component of the magnetic field is introduced, it couples to the carriers' out-of-plane motion and leads to a severe distortion of their orbits. The degree of anisotropy is typically stronger in the wider quantum wells but it also depends on the carrier type.
In addition to the electron and hole data at low perpendicular magnetic fields, we study the transport properties of composite fermions, exotic particles formed by the attachment of magnetic field flux to the ordinary carriers at strong magnetic fields. We observe commensurability features of composite fermions with unprecedented quality. Our data reveal an asymmetry of the features which we interpret as a fascinating manifestation of a subtle breaking of the particle-hole equivalence.
We also employ commensurability oscillations as a tool to probe and quantify the effect of an in-plane field on the trajectories of composite fermions. Our measurements indicate that, thanks to the finite layer thickness of the carriers and the coupling of their out-of-plane motion to the in-plane field, their orbits are significantly distorted. Comparing this distortion to our low-field data, we learn important details about the exotic physics of composite fermions.