Microwave spectroscopy of two-dimensional electrons in tilted magnetic field.

摘要

A two-dimensional electron system in a perpendicular magnetic field exhibits a large family of phases with crystalline or partial crystalline order. These include the Wigner solid (WS) at small filling factor nu of the lowest Landau level (LL), WS formed of dilute quasiparticles near integer nu, and the bubble and the stripe phases in the third or higher LL's.;These phases are pinned by sample disorder, and exhibit collective oscillation modes, or pinning modes, which give rise to resonances in microwave conductivity spectra, as found in previous studies. The resonances serve not only as evidence for the particular phases, but also as tools for probing charge ordering, dynamics, and disorder effects.;This thesis presents our microwave spectroscopic study of these phases in the presence of an in-plane magnetic field, Bip, which can affect the electron orbitals and spins.;We find that applying Bip expands the range of nu in which WS occurs in the lowest LL. In the second LL, near integer nu, applying Bip similarly expands the range of nu for the quasiparticle WS, accompanied by a significant increase in the pinning energy. The stripe phase shows a resonance only if measured with a microwave electric field polarized perpendicular to the stripes. The resonance polarization can switch in Bip, indicating reorientation of the stripes; the strength of the disorder pinning depends on B ip, and plays an important role in orienting the stripes. The bubble phase shows isotropic resonance at zero Bip; applying Bip isotropically shifts up the resonance frequency, and induces anisotropy in the resonance intensity. Because the experimental systems have finite layer thickness, we ascribe these effects to Bip altering the electron orbital wave functions, which changes electron-electron interaction and electron-disorder interaction.;Applying Bip also increases the Zeeman gap, and affects the spin degree of freedom. We find the pinning-mode resonance frequency of the solid near nu = 1 depends on the Zeeman gap, in a manner consistent with expectation for a solid formed of skyrmions, textures involving multiple flipped spins.;At the end of the thesis we also report pinning-mode evidence for WS of e/3 quasiparticles near nu = 1/3.