A quantum point contact (QPC) is a very basic nano-electronic device: a short and narrow transport channel between two electron reservoirs. In clean channels electron transport is ballistic and the conductance $G$ is then quantised as a function of c
hannel width with plateaus at integer multiples of $2e^2/h$ ($e$ is the electron charge and $h$ Plancks constant). This can be understood in a picture where the electron states are propagating waves, without need to account for electron-electron interactions. Quantised conductance could thus be the signature of ultimate control over nanoscale electron transport. However, even studies with the cleanest QPCs generically show significant anomalies on the quantised conductance traces and there is consensus that these result from electron many-body effects. Despite extensive experimental and theoretical studies understanding of these anomalies is an open problem. We report evidence that the many-body effects have their origin in one or more spontaneously localised states that emerge from Friedel oscillations in the QPC channel. Kondo physics will then also contribute to the formation of the many-body state with Kondo signatures that reflect the parity of the number of localised states. Evidence comes from experiments with length-tunable QPCs that show a periodic modulation of the many-body physics with Kondo signatures of alternating parity. Our results are of importance for assessing the role of QPCs in more complex hybrid devices and proposals for spintronic and quantum information applications. In addition, our results show that tunable QPCs offer a rich platform for investigating many-body effects in nanoscale systems, with the ability to probe such physics at the level of a single site.
The parity nonconserving longitudinal analyzing power A_L is calculated in elastic pp scattering at the energies below the approximate inelastic region T_lab = 350 MeV. The short-ranged heavy meson rho and omega exchanges as well as the longer-ranged
two pion exchanges are considered as the mediators of the parity nonconserving interactions. The DDH best coupling values are used as the parity nonconserving meson-NN couplings. Also three different parity nonconserving two-pion exchange potentials by various authors are compared.
Ohmic contacts to a two-dimensional electron gas (2DEG) in GaAs/AlGaAs heterostructures are often realized by annealing of AuGe/Ni/Au that is deposited on its surface. We studied how the quality of this type of ohmic contact depends on the annealing
time and temperature, and how optimal parameters depend on the depth of the 2DEG below the surface. Combined with transmission electron microscopy and energy-dispersive X-ray spectrometry studies of the annealed contacts, our results allow for identifying the annealing mechanism and proposing a model that can predict optimal annealing parameters for a certain heterostructure.
