Studies of Two Dimensional Electron Systems via Surface Acoustic Waves and Nuclear Magnetic Resonance Techniques

Citation

Tracy, Lisa A (2008) Studies of Two Dimensional Electron Systems via Surface Acoustic Waves and Nuclear Magnetic Resonance Techniques. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/E1X6-F676. https://resolver.caltech.edu/CaltechETD:etd-11032007-160944

Abstract

This thesis presents measurements investigating the spin degree of freedom in two dimensional electron systems (2DES’s). The measurements use nuclear magnetic resonance (NMR) techniques to study the role of spin in several 2DES states.

We first examine the spin transition that occurs in a half-filled Landau level in a single layer 2DES and compare our measurements to expectations from a composite fermion (CF) model. We show the temperature and density dependence of the nuclear T1 and resistively-detected NMR signal. The T₁ data can be roughly understood via a Korringa-like model of nuclear spin relaxation. However, the observed density dependence of both T₁ and the NMR signal is not explained by conventional CF theory.

We next consider a bilayer 2DES consisting of two closely spaced 2D electron layers, where each of the individual layers contains a half-filled Landau level. In this system, a transition occurs from a compressible single layer-like state to an incompressible correlated bilayer state as a function of the effective spacing between the two layers. When the effective spacing is made small enough, interactions between the two layers lead to the formation of a new state that can be viewed as a Bose condensate of excitons. Using NMR techniques we show that the spin degree of freedom is active during this transition.

In a single-layer 2DES with one completely filled Landau level (ν = 1), charged spin-texture excitations called skyrmions" are expected to exist. We probe the spin dynamics near this state using NMR. We find relatively fast nuclear relaxation rates that are consistent with a theory of spin excitations for a skyrmion solid. Our measurements also provide clues as to the origin of an "anomalous" NMR lineshape seen near ν=1.

We also present surface acoustic wave (SAW) measurements in a low density 2DES at zero magnetic field, under conditions where a 2D metal-insulator transition may occur. We find that our SAW data are consistent with a disorder-driven, percolation-type transition.

Thesis Files