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 behaviour 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 arc-like 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 branch-like regions of enhanced backscattering.Comment: 7 pages, 4 figure

Magneto-electrical subbands of freely suspended quantum point contacts

We present a versatile design of freely suspended quantum point contacts with particular large one-dimensional subband quantization energies of up to 10meV. The nanoscale bridges embedding a two-dimensional electron system are fabricated from AlGaAs/GaAs heterostructures by electron-beam lithography and etching techniques. Narrow constrictions define quantum point contacts that are capacitively controlled via local in-plane side gates. Employing transport spectroscopy, we investigate the transition from electrostatic subbands to Landau-quantization in a perpendicular magnetic field. The large subband quantization energies allow us to utilize a wide magnetic field range and thereby observe a large exchange splitted spin-gap of the two lowest Landau-levels

Nematic state of the FeSe superconductor

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real space and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives to temperatures above 90 K, where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 Å. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations

Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires

We report on a dynamic photoconductive gain effect in quantum wires which are lithographically fabricated in an AlGaAs/GaAs quantum well via a shallow-etch technique. The effect allows resolving the one-dimensional subbands of the quantum wires as maxima in the photoresponse across the quantum wires. We interpret the results by optically induced holes in the valence band of the quantum well which shift the chemical potential of the quantum wire. The non-linear current-voltage characteristics of the quantum wires also allow detecting the photoresponse effect of excess charge carriers in the conduction band of the quantum well. The dynamics of the photoconductive gain are limited by the recombination time of both electrons and holes

Laterally defined freely suspended quantum dots in GaAs/AlGaAs heterostructures

Free standing beams containing a two-dimensional electron system are shaped from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum dots are laterally defined using metal top gates. We investigate the electronic properties of these nanostructures by transport spectroscopy. Tunable localized electron states in freely suspended nanostructures are a promising tool to investigate the electron-phonon-interaction

An Evaluation of Body-grip Trap Trigger Configurations for Reducing River Otter Take Incidental to Beaver Trapping

River otter (Lontra canadensis) populations in North America have been the focus of significant restoration efforts. Wildlife management agencies, concerned about the unintentional take of river otters incidental to beaver (Castor canadensis) trapping, may recommend techniques to avoid capturing river otters. River otter avoidance techniques that are ineffective or diminish trap performance for beavers are undesirable. We conducted a field evaluation in 2015 and 2016 in Wisconsin to assess how two trigger configurations (offset and center) on body-grip traps would affect the incidental capture rate of river otters during beaver trapping. We also evaluated effects of each configuration on beaver capture rates, body lengths, and anatomical locations of trap-jaw strikes. We used size 330 body-grip traps equipped with identical triggers and alternated between trigger configurations during beaver damage management activities. We captured 8 river otters with each trap trigger configuration. Trap-jaw strikes on beavers differed between trigger configurations, with offset triggers resulting in more abdomen strikes and center triggers causing more cervical vertebrae strikes. We found that an offset trigger configuration did not reduce incidental take of otters and was less effective for trapping beavers

Optically induced transport properties of freely suspended semiconductor submicron channels

We report on optically induced transport phenomena in freely suspended channels containing a two-dimensional electron gas (2DEG). The submicron devices are fabricated in AlGaAs/GaAs heterostructures by etching techniques. The photoresponse of the devices can be understood in terms of the combination of photogating and a photodoping effect. The hereby enhanced electronic conductance exhibits a time constant in the range of one to ten milliseconds

Staircase baker's map generates flaring-type time series

The baker’s map, invented by Eberhard Hopf in 1937, is an intuitively accesible, two-dimensional chaos-generating discrete dynamical system. This map, which describes the transformation of an idealized two-dimensional dough by stretching, cutting and piling, is non-dissipative. Nevertheless the “x” variable is identical with the dissipative, one-dimensional Bernoulli-shift-generating map. The generalization proposed here takes up ideas of Yaacov Sinai in a modified form. It has a staircase-like shape, with every next step half as high as the preceding one. Each pair of neighboring elements exchanges an equal volume (area) during every iteration step in a scaled manner. Since the density of iterated points is constant, the thin tail (to the right, say) is visited only exponentially rarely. This observation already explains the map's main qualitative behavior: The “x” variable shows “flares”. The time series of this variable is closely analogous to that of a flaring-type dissipative dynamical system – like those recently described in an abstract economic model. An initial point starting its journey in the tale (or “antenna”, if we tilt the map upwards by 90 degrees) is predictably attracted by the broad left hand (bottom) part, in order to only very rarely venture out again to the tip. Yet whenever it does so, it thereby creates, with the top of a flare, a new “far-from-equilibrium” initial condition, in this reversible system. The system therefore qualifies as a discrete analogue to a far-from-equilibrium multiparticle Hamiltonian system. The height of the flare hereby corresponds to the momentary height of the H function of a gas. An observable which is even more closely related to the momentary negative entropy was recently described. Dependent on the numerical accuracy chosen, “Poincaré cycles” of two different types (periodic and nonperiodic) can be observed for the first time