We report an experimental technique to measure and manipulate the arrival-time and energy distributions of electrons emitted from a semiconductor electron pump, operated as both a single-electron source and a two-electron source. Using an energy-selective detector whose transmission we control on picosecond time scales, we can measure directly the electron arrival-time distribution and we determine the upper bound to the distribution width to be 30 ps. We study the effects of modifying the shape of the voltage wave form that drives the electron pump, and show that our results can be explained by a tunneling model of the emission mechanism. This information was in turn used to control the emission-time difference and energy gap between a pair of electrons.

Recently, it has been possible to design independently contacted electron-hole bilayers (EHBLs) with carrier densities  cm2in each layer and a separation of 10–20 nm in a GaAs/AlGaAs system. In these EHBLs, the interlayer interaction can be stronger than the intralayer interactions. Theoretical works have indicated the possibility of a very rich phase diagram in EHBLs consisting of excitonic superfluid phases, charge density waves, and Wigner crystals. Experiments have revealed that the Coulomb drag on the hole layer shows strong nonmonotonic deviations from a behaviour expected for Fermi-liquids at low temperatures. Simultaneously, an unexpected insulating behaviour in the single-layer resistances (at a highly “metallic” regime with ) also appears in both layers despite electron mobilities of above and hole mobilities over . Experimental data also indicates that the point of equal densities () is not special.

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. The Coulomb drag resistivity is a direct measure of the strength of the interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter r s up to 14). Our ambipolar device design allows comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions, and also shows the effects of the different effective masses of electrons and holes in GaAs.

To cool a high mobility two-dimensional electron gas (2DEG) at a GaAs-AlGaAs heterojunction to milliKelvin temperatures, we have fabricated low resistance ohmic contacts based on alloys of Au, Ni and Ge. The ohmic contacts have a typical contact resistance of R C ≈ 0.8 Ω at 4.2 K, which drops to 0.2 Ω below 0.9 K. Scanning electron microscope images establish that the contacts have the same inhomogeneous microstructure that has been observed in previous studies. Measurements of the contact resistance R C , the four-terminal resistance along the top of a single contact, and the vertical resistance R V , all show that there is a superconductor in the ohmic contact which can be turned completely normal with a magnetic field of 0.15 T. We briefly discuss how this superconductivity may be affecting the electrical transport measurements of 2DEGs, especially how it may hinder the cooling of electrons in a 2DEG below 0.1 K.