Phase switching in a voltage-biased Aharonov-Bohm interferometer

Recent experiment [Sigrist et al., Phys. Rev. Lett. {\bf 98}, 036805 (2007)] reported switches between 0 and $\pi$ in the phase of Aharonov-Bohm oscillations of the two-terminal differential conductance through a two-dot ring with increasing voltage bias. Using a simple model, where one of the dots contains multiple interacting levels, these findings are explained as a result of transport through the interferometer being dominated at different biases by quantum dot levels of different "parity" (i.e. the sign of the overlap integral between the dot state and the states in the leads). The redistribution of electron population between different levels with bias leads to the fact that the number of switching events is not necessarily equal to the number of dot levels, in agreement with experiment. For the same reason switching does not always imply that the parity of levels is strictly alternating. Lastly, it is demonstrated that the correlation between the first switching of the phase and the onset of the inelastic cotunneling, as well as the sharp (rather than gradual) change of phase when switching occurs, give reason to think that the present interpretation of the experiment is preferable to the one based on electrostatic AB effect.Comment: 12 pages, 9 figure

Spin-Orbit Assisted Variable-Range Hopping in Strong Magnetic Fields

It is shown that in the presence of strong magnetic fields, spin-orbit scattering causes a sharp increase in the effective density of states in the variable-range hopping regime when temperature decreases. This effect leads to an exponential enhancement of the conductance above its value without spin-orbit scattering. Thus an experimental study of the hopping conductivity in a fixed, large magnetic field, is a sensitive tool to explore the spin-orbit scattering parameters in the strongly localized regime.Comment: 9 pages + 2 figures (enclosed), Revte

Evidence for localization and 0.7 anomaly in hole quantum point contacts

Quantum point contacts implemented in p-type GaAs/AlGaAs heterostructures are investigated by low-temperature electrical conductance spectroscopy measurements. Besides one-dimensional conductance quantization in units of $2e^{2}/h$ a pronounced extra plateau is found at about $0.7(2e^{2}/h)$ which possesses the characteristic properties of the so-called "0.7 anomaly" known from experiments with n-type samples. The evolution of the 0.7 plateau in high perpendicular magnetic field reveals the existence of a quasi-localized state and supports the explanation of the 0.7 anomaly based on self-consistent charge localization. These observations are robust when lateral electrical fields are applied which shift the relative position of the electron wavefunction in the quantum point contact, testifying to the intrinsic nature of the underlying physics.Comment: 4.2 pages, 3 figure

Geographic variation in flower color patterns within Calceolaria uniflora Lam. in Southern Patagonia

Infraspecific variation in flower colors was evaluated in 26 populations of Calceolaria uniflora Lam. in Southern Patagonia, Argentina. Computerized analysis of high-resolution photo-images was used to estimate the proportions of red, orange and yellow in surfaces of two corolla parts, “instep” and “throat”, in field samples of 20–35 flowers per population. The between-populations component accounted for 48% of variance for instep colors and 24% for throat colors. Geographic differentiation was found between populations with a uniform red instep in the Andes in the west, and populations with a maculate yellow-and-red instep in the Magellanic steppe to the east. Mixed populations occurred in a transition zone. Throat colors showed a different, north-south geographic trend. Based on color pattern and distribution, two subspecies may be differentiated within C. uniflora. Their overall geographic distribution is related to climate and vegetation, but their detailed distribution is better explained by isolation by distance and barriers to gene flow.Fil: Mascó, Mercedes. Universidad Nacional de la Patagonia Austral; ArgentinaFil: Noy-Meir, I.. Hebrew University of Jerusalem. Faculty of Agricultural Food and Environmental Quality Sciences. Institute of Plant Sciences; IsraelFil: Sersic, Alicia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; Argentin

A forward-backward splitting algorithm for the minimization of non-smooth convex functionals in Banach space

We consider the task of computing an approximate minimizer of the sum of a smooth and non-smooth convex functional, respectively, in Banach space. Motivated by the classical forward-backward splitting method for the subgradients in Hilbert space, we propose a generalization which involves the iterative solution of simpler subproblems. Descent and convergence properties of this new algorithm are studied. Furthermore, the results are applied to the minimization of Tikhonov-functionals associated with linear inverse problems and semi-norm penalization in Banach spaces. With the help of Bregman-Taylor-distance estimates, rates of convergence for the forward-backward splitting procedure are obtained. Examples which demonstrate the applicability are given, in particular, a generalization of the iterative soft-thresholding method by Daubechies, Defrise and De Mol to Banach spaces as well as total-variation based image restoration in higher dimensions are presented

Many Body Effects on Electron Tunneling through Quantum Dots in an Aharonov-Bohm Circuit

Tunneling conductance of an Aharonov-Bohm circuit including two quantum dots is calculated based on the general expression of the conductance in the linear response regime of the bias voltage. The calculation is performed in a wide temperature range by using numerical renormalization group method. Various types of AB oscillations appear depending on the temperature and the potential depth of the dots. Especially, AB oscillations have strong higher harmonics components as a function of the magnetic flux when the potential of the dots is deep. This is related to the crossover of the spin state due to the Kondo effect on quantum dots. When the temperature rises up, the amplitude of the AB oscillations becomes smaller reflecting the breaking of the coherency.Comment: 21 pages, 11 PostScript figures, LaTeX, uses jpsj.sty epsbox.st

Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot

Employing the Anderson impurity model, we study tunneling properties through an ideal quantum dot near the conductance minima. Considering the Coulomb blockade and the quantum confinement on an equal footing, we have obtained current contributions from various types of tunneling processes; inelastic cotunneling, elastic cotunneling, and resonant tunneling of thermally activated electrons. We have found that the inelastic cotunneling is suppressed in the quantum confinement limit, and thus the conductance near its minima is determined by the elastic cotunneling at low temperature ($k_BT \ll \Gamma$, $\Gamma$: dot-reservoir coupling constant), or by the resonant tunneling of single electrons at high temperature ($k_BT \gg \Gamma$).Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.

Fine structure in the off-resonance conductance of small Coulomb blockade systems

We show how a fine, multiple-peak structure can arise in the off-resonance, zero-bias conductance of Coulomb blockade systems. In order to understand how this effect comes about one must abandon the orthodox, mean-field understanding of the Coulomb blockade phenomenon and consider quantum fluctuations in the occupation of the single-particle electronic levels. We illustrate such an effect with a spinless Anderson-like model for multi-level systems and an equation-of-motion method for calculating Green's functions that combines two simple decoupling schemes.Comment: 5 pages, 3 figures, postscript file also available at http://www.pa.uky.edu/~palacios/papers/eom.ps One figure added. Discussion of results extende

Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential

The spatial distribution of electric current under magnetic field and the resultant orbital magnetism have been studied for two-dimensional electrons under a harmonic confining potential V(\vecvar{r})=m \omega_0^2 r^2/2 in various regimes of temperature and magnetic field, and the microscopic conditions for the validity of Landau diamagnetism are clarified. Under a weak magnetic field (\omega_c\lsim\omega_0, \omega_c being a cyclotron frequency) and at low temperature (T\lsim\hbar\omega_0), where the orbital magnetic moment fluctuates as a function of the field, the currents are irregularly distributed paramagnetically or diamagnetically inside the bulk region. As the temperature is raised under such a weak field, however, the currents in the bulk region are immediately reduced and finally there only remains the diamagnetic current flowing along the edge. At the same time, the usual Landau diamagnetism results for the total magnetic moment. The origin of this dramatic temperature dependence is seen to be in the multiple reflection of electron waves by the boundary confining potential, which becomes important once the coherence length of electrons gets longer than the system length. Under a stronger field (\omega_c\gsim\omega_0), on the other hand, the currents in the bulk region cause de Haas-van Alphen effect at low temperature as T\lsim\hbar\omega_c. As the temperature gets higher (T\gsim\hbar\omega_c) under such a strong field, the bulk currents are reduced and the Landau diamagnetism by the edge current is recovered.Comment: 15 pages, 11 figure

A multivariate approach to heavy flavour tagging with cascade training

This paper compares the performance of artificial neural networks and boosted decision trees, with and without cascade training, for tagging b-jets in a collider experiment. It is shown, using a Monte Carlo simulation of $WH \to l\nu q\bar{q}$ events, that for a b-tagging efficiency of 50%, the light jet rejection power given by boosted decision trees without cascade training is about 55% higher than that given by artificial neural networks. The cascade training technique can improve the performance of boosted decision trees and artificial neural networks at this b-tagging efficiency level by about 35% and 80% respectively. We conclude that the cascade trained boosted decision trees method is the most promising technique for tagging heavy flavours at collider experiments.Comment: 14 pages, 12 figures, revised versio