Tunable effective g-factor in InAs nanowire quantum dots

We report tunneling spectroscopy measurements of the Zeeman spin splitting in InAs few-electron quantum dots. The dots are formed between two InP barriers in InAs nanowires with a wurtzite crystal structure grown by chemical beam epitaxy. The values of the electron g-factors of the first few electrons entering the dot are found to strongly depend on dot size and range from close to the InAs bulk value in large dots |g^*|=13 down to |g^*|=2.3 for the smallest dots. These findings are discussed in view of a simple model.Comment: 4 pages, 3 figure

Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts

The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in Applied Physics Letter

Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot

We demonstrate control of the electron number down to the last electron in tunable few-electron quantum dots defined in catalytically grown InAs nanowires. Using low temperature transport spectroscopy in the Coulomb blockade regime we propose a simple method to directly determine the magnitude of the spin-orbit interaction in a two-electron artificial atom with strong spin-orbit coupling. Due to a large effective g-factor |g*|=8+/-1 the transition from singlet S to triplet T+ groundstate with increasing magnetic field is dominated by the Zeeman energy rather than by orbital effects. We find that the spin-orbit coupling mixes the T+ and S states and thus induces an avoided crossing with magnitude $\Delta_{SO}$=0.25+/-0.05 meV. This allows us to calculate the spin-orbit length $\lambda_{SO}\approx$127 nm in such systems using a simple model.Comment: 21 pages, 7 figures, including supplementary note

Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire

Nanowire heterostructures define high-quality few-electron quantum dots for nanoelectronics, spintronics and quantum information processing. We use a cooled scanning probe microscope (SPM) to image and control an InAs quantum dot in an InAs/InP nanowire, using the tip as a movable gate. Images of dot conductance vs. tip position at T = 4.2 K show concentric rings as electrons are added, starting with the first electron. The SPM can locate a dot along a nanowire and individually tune its charge, abilities that will be very useful for the control of coupled nanowire dots

Power Utility Maximization in Discrete-Time and Continuous-Time Exponential Levy Models

Consider power utility maximization of terminal wealth in a 1-dimensional continuous-time exponential Levy model with finite time horizon. We discretize the model by restricting portfolio adjustments to an equidistant discrete time grid. Under minimal assumptions we prove convergence of the optimal discrete-time strategies to the continuous-time counterpart. In addition, we provide and compare qualitative properties of the discrete-time and continuous-time optimizers.Comment: 18 pages, to appear in Mathematical Methods of Operations Research. The final publication is available at springerlink.co

The Core of the Participatory Budgeting Problem

In participatory budgeting, communities collectively decide on the allocation of public tax dollars for local public projects. In this work, we consider the question of fairly aggregating the preferences of community members to determine an allocation of funds to projects. This problem is different from standard fair resource allocation because of public goods: The allocated goods benefit all users simultaneously. Fairness is crucial in participatory decision making, since generating equitable outcomes is an important goal of these processes. We argue that the classic game theoretic notion of core captures fairness in the setting. To compute the core, we first develop a novel characterization of a public goods market equilibrium called the Lindahl equilibrium, which is always a core solution. We then provide the first (to our knowledge) polynomial time algorithm for computing such an equilibrium for a broad set of utility functions; our algorithm also generalizes (in a non-trivial way) the well-known concept of proportional fairness. We use our theoretical insights to perform experiments on real participatory budgeting voting data. We empirically show that the core can be efficiently computed for utility functions that naturally model our practical setting, and examine the relation of the core with the familiar welfare objective. Finally, we address concerns of incentives and mechanism design by developing a randomized approximately dominant-strategy truthful mechanism building on the exponential mechanism from differential privacy

Adaptive immunity alters distinct host feeding pathways during nematode induced inflammation, a novel mechanism in parasite expulsion

Gastrointestinal infection is often associated with hypophagia and weight loss; however, the precise mechanisms governing these responses remain poorly defined. Furthermore, the possibility that alterations in feeding during infection may be beneficial to the host requires further study. We used the nematode Trichinella spiralis, which transiently inhabits the small intestine before migrating to skeletal muscle, as a biphasic model of infection to determine the cellular and molecular pathways controlling feeding during enteric and peripheral inflammation. Through the infection of genetically modified mice lacking cholecystokinin, Tumor necrosis factor α receptors and T and B-cells, we observed a biphasic hypophagic response to infection resulting from two separate immune-driven mechanisms. The enteroendocrine I-cell derived hormone cholecystokinin is an essential mediator of initial hypophagia and is induced by CD4+ T-cells during enteritis. In contrast, the second hypophagic response is extra-intestinal and due to the anorectic effects of TNFα during peripheral infection of the muscle. Moreover, via maintaining naive levels of the adipose secreted hormone leptin throughout infection we demonstrate a novel feedback loop in the immunoendocrine axis. Immune driven I-cell hyperplasia and resultant weight loss leads to a reduction in the inflammatory adipokine leptin, which in turn heightens protective immunity during infection. These results characterize specific immune mediated mechanisms which reduce feeding during intestinal or peripheral inflammation. Importantly, the molecular mediators of each phase are entirely separate. The data also introduce the first evidence that I-cell hyperplasia is an adaptively driven immune response that directly impinges on the outcome to infection

Electronic structure of strained InP/GaInP quantum dots

We calculate the electronic structure of nm scale InP islands embedded in $Ga_{0.51}In_{0.49}P$. The calculations are done in the envelope approximation and include the effects of strain, piezoelectric polarization, and mixing among 6 valence bands. The electrons are confined within the entire island, while the holes are confined to strain induced pockets. One pocket forms a ring at the bottom of the island near the substrate interface, while the other is above the island in the GaInP. The two sets of hole states are decoupled. Polarization dependent dipole matrix elements are calculated for both types of hole states.Comment: Typographical error corrected in strain Hamiltonia

Intersubband Quantum Disc-in-Nanowire Photodetectors with Normal-incidence Response in the Long-wavelength Infrared

Semiconductor nanowires offer great potential for realizing broadband photodetectors that are compatible with silicon technology. However, the spectral range of such detectors has so far been limited to selected regions in the ultraviolet, visible and near infrared. Here, we report on broadband nanowire heterostructure array photodetectors exhibiting a photoresponse from the visible to long-wavelength infrared. In particular, the infrared response from 3-20 um is enabled by normal incidence excitation of intersubband transitions in low-bandgap InAsP quantum discs synthesized axially within InP nanowires. The optical characteristics are explained by the excitation of the longitudinal component of optical modes in the photonic crystal formed by the nanostructured portion of the detectors, combined with a non-symmetric potential profile of the discs resulting from synthesis. Our results provide a generalizable insight into how broadband nanowire photodetectors may be designed, and how engineered nanowire heterostructures open up new fascinating opportunities for optoelectronics