Contact resistance dependence of crossed Andreev reflection

We show experimentally that in nanometer scaled superconductor/normal metal hybrid devices and in a small window of contact resistances, crossed Andreev reflection (CAR) can dominate the nonlocal transport for all energies below the superconducting gap. Besides CAR, elastic cotunneling (EC) and nonlocal charge imbalance (CI) can be identified as competing subgap transport mechanisms in temperature dependent four-terminal nonlocal measurements. We demonstrate a systematic change of the nonlocal resistance vs. bias characteristics with increasing contact resistances, which can be varied in the fabrication process. For samples with higher contact resistances, CAR is weakened relative to EC in the midgap regime, possibly due to dynamical Coulomb blockade. Gaining control of CAR is an important step towards the realization of a solid state entangler.Comment: 5 pages, 4 figures, submitted to PR

Scavenger 0.1: A Theorem Prover Based on Conflict Resolution

This paper introduces Scavenger, the first theorem prover for pure first-order logic without equality based on the new conflict resolution calculus. Conflict resolution has a restricted resolution inference rule that resembles (a first-order generalization of) unit propagation as well as a rule for assuming decision literals and a rule for deriving new clauses by (a first-order generalization of) conflict-driven clause learning.Comment: Published at CADE 201

Magnetoresistence engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates

We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25\% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find {\it two} sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transition between singlet and a triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin-based Bell inequalities.Comment: 7 pages, 6 figure

Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts

Advanced synthesis of semiconductor nanowires (NWs) enables their application in diverse fields, notably in chemical and electrical sensing, photovoltaics, or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an ideal platform for quantum devices, e.g. they may host topological Majorana states. While the synthesis has been continously perfected, only few techniques were developed to tailor individual NWs after growth. Here we present three wet chemical etch methods for the post-growth morphological engineering of InAs NWs on the sub-100 nm scale. The first two methods allow the formation of self-aligned electrical contacts to etched NWs, while the third method results in conical shaped NW profiles ideal for creating smooth electrical potential gradients and shallow barriers. Low temperature experiments show that NWs with etched segments have stable transport characteristics and can serve as building blocks of quantum electronic devices. As an example we report the formation of a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure

Carbon nanotube quantum dots on hexagonal boron nitride

We report the fabrication details and low-temperature characteristics of the first carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride (hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard chemical vapor deposition and that standard scanning electron microscopy imaging and lithography can be employed to fabricate nanoelectronic structures when using optimized parameters. This proof of concept paves the way to more complex devices on hBN, with more predictable and reproducible characteristics and electronic stability.Comment: 4 pages, 4 figure

Classication of Agricultural Fields in Satellite Images Using Two-Dimensional Hidden Markov Models

Image segmentation is a key competence for many real life applications such as precision agriculture. In this work we present an approach to classify agricultural fields in noisy satellite images. We start with the Markovian neighborhood hypothesis from where on we derive a general two-dimensional hidden Markov model (2D-HMM). To make the 2D-HMM feasible we apply the Path-Constrained Variable-State Viterbi Algorithm (PCVSVA) which allows us to approximate the optimal hidden state map. We evaluate the PCVSVA for a Landsat image of the province of C´ordoba, Argentina and a synthetic satellite image. In both cases we use Cohen’s κb coefficient to compare the PCVSVA and the solution obtained by maximum likelihood (ML) to show the effectiveness of 2D-HMM of solving image segmentation tasks.Fil: Baumgartner, J. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina.Fil: Giménez, J. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina.Fil: Pucheta, J. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina.Fil: Flesia, A.G. Universidad Nacional de Córdoba. Facultad de Matemática, Astronomía, Física y Computación; Argentina.Fil: Flesia, A. G. Universidad Tecnológica Nacional. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina.Sistemas de Automatización y Contro

Scaling of polymers in aligned rods

We study the behavior of self avoiding polymers in a background of vertically aligned rods that are either frozen into random positions or free to move horizontally. We find that in both cases the polymer chains are highly elongated, with vertical and horizontal size exponents that differ by a factor of 3. Though these results are different than previous predictions, our results are confirmed by detailed computer simulations.Comment: 4 pages, 4 figure

Contrasting motivation and learning strategies of ex-mathematics and ex-mathematical literacy students

Abstract: This inquiry contrasts motivation and learning strategies of ex-Mathematics (Maths) and ex-Mathematical Literacy (ML) students. ML ideally delivers candidates who can make sense of and actively participate in a world of numbers and numerical arguments, but ex-ML students are excluded from many undergraduate studies at most South African higher education institutions (HEIs). Institutions employ various strategies in enhancing student transition to higher education (HE), however, such options are rare for ex-ML students. A year-long foundation programme offered by a private HEI is one exception. This inquiry employed the Motivated Strategies for Learning Questionnaire and t-test, detecting significant differences in motivation and learning strategies between 111 ex-Maths and 81 ex-ML students. The intrinsic goal orientation, task value, self-efficacy, effort regulation and test anxiety-handling abilities of ex-Maths students were significantly superior. An integrated solution process addressing academic content and social-psychological attributes to improve the motivation of ex-ML students in support of their academic development is proffered

Local electrical tuning of the nonlocal signals in a Cooper pair splitter

A Cooper pair splitter consists of a central superconducting contact, S, from which electrons are injected into two parallel, spatially separated quantum dots (QDs). This geometry and electron interactions can lead to correlated electrical currents due to the spatial separation of spin-singlet Cooper pairs from S. We present experiments on such a device with a series of bottom gates, which allows for spatially resolved tuning of the tunnel couplings between the QDs and the electrical contacts and between the QDs. Our main findings are gate-induced transitions between positive conductance correlation in the QDs due to Cooper pair splitting and negative correlations due to QD dynamics. Using a semi-classical rate equation model we show that the experimental findings are consistent with in-situ electrical tuning of the local and nonlocal quantum transport processes. In particular, we illustrate how the competition between Cooper pair splitting and local processes can be optimized in such hybrid nanostructures.Comment: 9 pages, 6 figures, 2 table