Magnetotransp ort in quantum Hall systems at high Landau levels

Abstract

Ever since the discovery of integer quantum Hall (QH) state by von Klitzing in 1980, a two-dimensional electron gas (2DEG) subject to a perpendicular magnetic field has been a major playground of condensed matter physics, generating both exotic phases and intriguing concepts. In the quantum limit, or the lowest Landau level (LL), the most famous example is the fractional QH state, an incompressible quantum liquid manifesting quasiparticles of fractional charge. In this thesis, we present experimental magnetotransport studies in the high LL regime, exploiting state-of-art GaAs 2DEGs. For LL index N ≥ 2, instead of the fractional QH states, the ground state is replaced by charge density waves (CDW) - stripe and bubble phases. At very high LLs where the energy scales are comparable between cyclotron gap and disorder and radiation frequency, a distinct class of non-equilibrium oscillations emerge when the system is driven by microwaves. In the first part of the thesis, we provide a general introduction to the fundamentals of the magnetotransport of quantum Hall system, focusing on high Landau levels. In chapter 1, we discuss the vehicle of choice for our experiments, which is a GaAs 2DEG. We discuss how a clean 2DEG is achieved, what the residue disorders are, and the attempt to characterize the “quality” of a 2DEG. The development of the field of quantum Hall systems has been largely driven by experiments, especially on magnetotransport. Nevertheless, rather than going by the experimental phenomena, here we take an approach which separates the problem into two layers. In chapter 2, we discuss the ingredients in the problem, and focus on the question: what is the ground state of the 2DEG in the presence of the magnetic field? In chapter 3, we focus on the magnetotransport properties of a 2DEG in different phases. The second part of the thesis is devoted to experiments on the stripe and bubble phases. In chapter 4, we present a detailed study of the stripe orientation under in-plane magnetic fields. We establish a rich phase diagram demonstrating that the stripe orientation under the in-plane magnetic field is sensitive to a number of parameters, including the spin and LL index, partial filling factor, and the magnitude of in-plane field. Our findings highlight that the tripe orientation is a robust reflection of the electron-electron interactions especially screening properties in the high LL regime, and shed new light on the longstanding mystery of native stripe orientation. In chapter 5, we show that the stripes can be prepared into a metastable orientation and study the relaxation process from one orientation to another. We find sharp jumps of resistance in the relaxation process, similar to the Barkhausen noise in magnetic systems while of much larger amplitudes, as well as telegraph noise, in our macroscopic samples. Our results reveal unambiguously the existence of domain structures in the stripe phase, and suggests large correlation length of the stripe phase. Chapter 6 presents the study of the effect of alloy disorder in stripe and bubble phases. We find that with increasing alloy disorder concentration, the resistance anisotropy in the stripe phase decreases, and stripes decay faster as one moves to high LLs. On the other hand, the melting temperature of stripes and the in-plane field-induced reorientation of stripes, has very weak dependence on the concentration of alloy disorder, suggesting that these processes are dominated by the alignment of stripe domains (rather than local stripe order), which is insensitive to alloy disorder. We also find that the addition of alloy disorder stabilizes the bubble phases. The third part consists of experiments under microwave radiations at very high LLs. In chapter 7 we present the discover of fine structure in microwave-induced resistance oscillations (MIRO). The fine structures manifest the peculiar feature that the resistance oscillations as a function of the magnetic field have the extrema (in B) not only decided by the radiation frequency, but also the power. We identify the origin of the fine structures as due to multi-photon assisted scatterings with sharp disorder. In chapter 8, we present the detection of magneto-plasmons (MP) extending to order n = 25, in the Shubnikov-de Haas oscillation (SdHO) regime. The observed MP modes demonstrate alternating behavior at even and odd order, as well as retardation effects due to light-electron interactions at low orders . Our experiments demonstrate the technique exploiting SdHO as a surprisingly sensitive and elegant means to detect and investigate high-order MP modes.