Steven Klepper

“Universal” Conductance Fluctuations (UCF) are studied by measuring electron magnetoconductance in micron-size wires fabricated from GaAs/AlGaAs heterostructures. These aperiodic fluctuations in conductance arise from the complex quantum phase-coherent scattering of conduction electrons from the particular impurity configuration of a sample. Two experiments have been performed: one to study the sensitivity of UCF to individual elastic scatterers, and the second to examine the effect of electron spin-orbit scattering on UCF.

In the first experiment, elastic scatterers are introduced at a controlled rate into heterostructure devices by infrared photoionization of DX centers in the doped AlGaAs. Ionized DX centers are elastic scatterers of electrons in the two-dimensional electron gas. A novel technique of UCF difference traces allows the quantitative study of changes in the characteristic magnetoconductance fluctuations of a sample. These changes arise from adding a small number of scatterers to a device. The amplitude and temperature dependence of these conductance changes agree well with theory. For comparison, the effect of changing the electrochemical potential in mesoscopic gated devices was examined. The UCF decorrelation associated with changing gate voltage differs markedly from that caused by elastic scatterer addition, allowing changes in the electrochemical potential to be distinguished from those in the random scattering potential. One can also resolve switching events in the device conductance due to the addition of individual elastic scatterers.

A second experiment studied the temperature dependence of magnetoconductance, for fixed impurity configuration, from T = 60 mK to 7 K. Weak localization data show that the spin-orbit (SO) scattering rate exceeds the phase-breaking rate below T = 2 K. A significant reduction in the conductance fluctuation amplitude is observed, also below 2 K, as compared to that extrapolated from higher temperatures. This reduction is due to SO scattering. The amplitude and functional form of the UCF reduction factor for the transition from the weak (high-T) to the strong (low-T) SO scattering regimes agree well with theory. SO scattering also influences the magnetic correlation field for the UCF.