Fractal Magneto-conductance Fluctuations in Mesoscopic Semiconductor Billiards

Abstract

Negatively biased surface-gates allow electrostatic depletion of selected regions of a 2DEG, forming confined regions of specific geometry called billiards, in which ballistic transport occurs. At millikelvin temperatures, the electron phase coherence length is sufficient that quantum interference effects produce reproducible magneto-conductance fluctuations (MCF) that act as a 'magneto-fingerprint' of the scattering dynamics in the billiard. It has been predicted that billiard MCF are fractal. Fractal MCF in mesoscopic semiconductor billiards are investigated experimentally. The MCF of a Sinai billiard displayed exact self-similarity (ESS). A correlation function analysis is used to quantify the presence of ESS. A model for the Sinai billiard MCF based on a Weierstrass function is presented. Using a bridging interconnect, a continuous transition between the Sinai and an empty square geometry is achieved. The removal of the circle induces a transition from ESS to statistical self-similarity (SSS), suggesting that ESS is due to the presence of an obstacle at the center of the billiard. The physical dependencies of SSS are investigated and show variation in the fractal dimension, rather than the fractal scaling range. SSS obeys a unified picture where the fractal dimension depends only on the ratio between the average spacing and broadening of billiard energy levels, irrespective of other billiard parameters. The semiclassical origin of SSS is demonstrated and the suppression of SSS is observed in both the quantum and classical limits. The influence of soft-wall potential profile on fractal MCF is investigated using double-2DEG billiards. Detailed reviews of semiconductor billiard fabrication, low-temperature electrical measurements and fractal analysis are also presented.