A. A. Kozikov

Interference of electrons in backscattering through a quantum point contact

Scanning gate microscopy is used to locally investigate electron transport in a high-mobility two-dimensional electron gas formed in a GaAs/AlGaAs heterostructure. Using quantum point contacts, we observe branches caused by electron backscattering decorated with interference fringes similar to previous observations by Topinka et al (2000 Science 289 2323). We investigate the branches at different points of a conductance plateau as well as between plateaus, and demonstrate that the most dramatic changes in branch pattern occur at the low-energy side of the conductance plateaus. The branches disappear at magnetic fields as low as 50 mT, demonstrating the importance of backscattering for the observation of the branching effect. The spacing between the interference fringes varies by more than 50% for different branches across scales of microns. Several scenarios are discussed to explain this observation.

Imaging magnetoelectric subbands in ballistic constrictions

We perform scanning gate experiments on ballistic constrictions in the presence of small perpendicular magnetic fields. The constrictions form the entrance and exit of a circular gate-defined ballistic stadium. Close to constrictions we observe sets of regular fringes creating a checker board pattern. Inside the stadium conductance fluctuations governed by chaotic dynamics of electrons are visible. The checker board pattern allows us to determine the number of transmitted modes in the constrictions forming between the tip-induced potential and gate-defined geometry. Spatial investigation of the fringe pattern in a perpendicular magnetic field shows a transition from electrostatic to magnetic depopulation of magnetoelectric subbands. Classical and quantum simulations agree well with different aspects of our observations.

Scanning-gate-induced effects and spatial mapping of a cavity

Tailored electrostatic potentials are the foundation of scanning gate microscopy. We present several aspects of the tip-induced potential on the two-dimensional electron gas. First, we give methods on how to estimate the size of the tip-induced potential. Then, a ballistic cavity is formed and studied as a function of the bias-voltage of the metallic top gates and probed with the tip-induced potential. It is shown how the potential of the cavity changes by tuning the system to a regime where conductance quantization in the constrictions formed by the tip and the top gates occurs. This conductance quantization leads to a unprecedented rich fringe pattern over the entire structure. Finally, the effect of electrostatic screening of the metallic top gates is discussed.

Mode Specific Backscattering in a Quantum Point Contact

We demonstrate a scanning gate grid measurement technique consisting in measuring the conductance of a quantum point contact (QPC) as a function of gate voltage at each tip position. Unlike conventional scanning gate experiments, it allows investigating QPC conductance plateaus affected by the tip at these positions. We compensate the capacitive coupling of the tip to the QPC and discover that interference fringes coexist with distorted QPC plateaus. We spatially resolve the mode structure for each plateau.

Classical origin of conductance oscillations in an integrable cavity

Scanning gate microscopy measurements in a circular ballistic cavity with a tip placed near its center yield a nonmonotonic dependence of the conductance on the tip voltage. Detailed numerical quantum calculations reproduce these conductance oscillations, and a classical scheme leads to its physical understanding. The large-amplitude conductance oscillations are shown to be of classical origin, and they are well described by the effect of a particular class of short trajectories.

Scanning gate imaging in confined geometries

This article reports on tunable electron backscattering investigated with the biased tip of a scanning force microscope. Using a channel defined by a pair of Schottky gates, the branched electron flow of ballistic electrons injected from a quantum point contact is guided by potentials of a tunable height well below the Fermi energy. The transition from injection into an open two-dimensional electron gas to a strongly confined channel exhibits three experimentally distinct regimes: one in which branches spread unrestrictedly, one in which branches are confined but the background conductance is affected very little, and one where the branches have disappeared and the conductance is strongly modified. Classical trajectory-based simulations explain these regimes at the microscopic level. These experiments allow us to understand under which conditions branches observed in scanning gate experiments do or do not reflect the flow of electrons.

Electron backscattering in a cavity: Ballistic and coherent effects

Numerous experimental and theoretical studies have focused on low-dimensional systems locally perturbed by the biased tip of a scanning force microscope. In all cases either open or closed weakly gate-tunable nanostructures have been investigated, such as quantum point contacts, open or closed quantum dots, etc. We study the behavior of the conductance of a quantum point contact with a gradually forming adjacent cavity in series under the influence of a scanning gate. Here, an initially open quantum point contact system gradually turns into a closed cavity system. We observe branches and interference fringes known from quantum point contacts coexisting with irregular conductance fluctuations. Unlike the branches, the fluctuations cover the entire area of the cavity. In contrast to previous studies, we observe and investigate branches under the influence of the confining stadium potential, which is gradually built up. We find that the branches exist only in the area surrounded by cavity top gates. As the stadium shrinks, regular fringes originate from tip-induced constrictions leading to quantized conduction. In addition, we observe arclike areas reminiscent of classical electron trajectories in a chaotic cavity. We also argue that electrons emanating from the quantum point contact spread out like a fan leaving branchlike regions of enhanced backscattering.

Preface to Special Topic: Selected Contributions to the 31st International Conference on the Physics of Semiconductors, Zurich, 2012

(Received 26 February 2013; accepted 6 March 2013; published online 29 March 2013)[http://dx.doi.org/10.1063/1.4795810]It is our great pleasure to present in this issue of theJournal of Applied Physics selected contributions to the 31stInternational Conference on the Physics of Semiconductors(ICPS-31), which took place in Zurich, Switzerland, fromJuly 29 to August 3rd, 2012. The contributions comprise theplenary talk by Professor Hideo Ohno on “Bridging semicon-ductor and magnetism” and ten invited talks by differentauthors. The articles in this collection have undergone thesame peer-review process as regular journal articles.The International Conference on the Physics ofSemiconductors has a long history and covers all the impor-tant subfields of this research area. The thirteen specifictopics presented at the Zurich conference in 2012 were1. Material Structure,2. Wide Bandgap Semiconductors,3. Narrow-gap Semiconductors,4. Carbon: Nanotubes and Graphene,5. Organic Semiconductors,6. Topological Insulators,7. Transport in Heterostructures,8. Quantum Hall Effects,9. Spintronics and Spin Phenomena,10. Electron Devices and Applications,11. Optical Properties of Heterostructures,12. Quantum Optics and Nanophotonics,13. Quantum Information.Since the first conference took place in Reading,England, in 1950, the field of Semiconductor Physics hascontinued to present exciting new and important results forphysicists and engineers. At ICPS-31, highlights also cov-ered with plenary talks were the progress in quantum infor-mation processing with single spins and spintronics, theexperimental and theoretical investigation of Majorana fer-mions, the physics of graphene, exciton-polariton conden-sates, and solid-state cavity QED with a quantum dotcoupled to a photonic crystal.The organizers of ICPS31 are particularly grateful forthe continued partnership with the American Institute ofPhysics that allows us to publish plenary and invited papersin the Journal of Applied Physics, thus making the valuablecontributions of the most committed invited and plenaryspeakers available to a broad audience in applied physicsand related fields. We wish to thank the authors who took theextra effort to condense their excellent talks into the pre-sented journal articles. We hope that the readers will find theresults interesting and scientifically stimulating.