2,501 research outputs found
Josephson Junctions defined by a Nano-Plough
We define superconducting constrictions by ploughing a deposited Aluminum
film with a scanning probe microscope. The microscope tip is modified by
electron beam deposition to form a nano-plough of diamond-like hardness, what
allows the definition of highly transparent Josephson junctions. Additionally a
dc-SQUID is fabricated to verify appropriate functioning of the junctions. The
devices are easily integrated in mesoscopic devices as local radiation sources
and can be used as tunable on-chip millimeter wave sources
Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study
We use spin-density-functional theory to study the spacing between
conductance peaks and the ground-state spin of 2D model quantum dots with up to
200 electrons. Distributions for different ranges of electron number are
obtained in both symmetric and asymmetric potentials. The even/odd effect is
pronounced for small symmetric dots but vanishes for large asymmetric ones,
suggesting substantially stronger interaction effects than expected. The
fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure
The Metropolis and Evangelical Life: Coherence and Fragmentation in the âLost City of Londonâ
This article examines the interplay of different processes of cultural and subjective fragmentation experienced by conservative evangelical Anglicans, based on an ethnographic study of a congregation in central London. The author focuses on the evangelistic speaking practices of members of this church to explore how individuals negotiate contradictory norms of interaction as they move through different city spaces, and considers their response to tensions created by the demands of their workplace and their religious lives. Drawing on Georg Simmelâs âThe Metropolis and Mental Lifeâ, the author argues that their faith provides a sense of coherence and unity that responds to experiences of cultural fragmentation characteristic of everyday life in the city, while simultaneously leading to a specific consciousness of moral fragmentation that is inherent to conservative evangelicalism
Microwave spectroscopy on a double quantum dot with an on-chip Josephson oscillator
We present measurements on microwave spectroscopy on a double quantum dot
with an on-chip microwave source. The quantum dots are realized in the
two-dimensional electron gas of an AlGaAs/GaAs heterostructure and are weakly
coupled in series by a tunnelling barrier forming an 'ionic' molecular state.
We employ a Josephson oscillator formed by a long Nb/Al-AlO/Nb junction as
a microwave source. We find photon-assisted tunnelling sidebands induced by the
Josephson oscillator, and compare the results with those obtained using an
externally operated microwave source.Comment: 6 pages, 4 figure
Energy Level Statistics of Quantum Dots
We investigate the charging energy level statistics of disordered interacting
electrons in quantum dots by numerical calculations using the Hartree
approximation. The aim is to obtain a global picture of the statistics as a
function of disorder and interaction strengths. We find Poisson statistics at
very strong disorder, Wigner- Dyson statistics for weak disorder and
interactions, and a Gaussian intermediate regime. These regimes are as expected
from previous studies and fundamental considerations, but we also find
interesting and rather broad crossover regimes. In particular, intermediate
between the Gaussian and Poisson regimes we find a two-sided exponential
distribution for the energy level spacings. In comparing with experiment, we
find that this distribution may be realized in some quantum dots.Comment: 21 pages 10 figure
Addition Spectra of Chaotic Quantum Dots: Interplay between Interactions and Geometry
We investigate the influence of interactions and geometry on ground states of
clean chaotic quantum dots using the self-consistent Hartree-Fock method. We
find two distinct regimes of interaction strength: While capacitive energy
fluctuations follow approximately a random matrix prediction for
weak interactions, there is a crossover to a regime where is
strongly enhanced and scales roughly with interaction strength. This
enhancement is related to the rearrangement of charges into ordered states near
the dot edge. This effect is non-universal depending on dot shape and size. It
may provide additional insight into recent experiments on statistics of Coulomb
blockade peak spacings.Comment: 4 pages, final version to appear in Phys. Rev. Let
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A New Order of Things: Network Mechanisms of Field Evolution in the Aftermath of an Exogenous Shock
This study examines the role of a major environmental shock in triggering change in the social structure of an organizational field. Based on the longitudinal analysis of changing network configurations in the global airline industry, we explore how logics of attachment shift before, during and after an exogenous shock and how the rewiring of network ties in response to the shock may act as a countervailing force to the network dynamics that drive field stratification. Using the terrorist attacks of September 11, 2001 as a natural experiment, our work reveals how shocks may affect key mechanisms of network evolution thus altering tie distribution and access among members of the field. Overall this article contributes to emergent literature on field dynamics by exposing the evolution of interorganizational dynamics when external events produce unsettled times that render extant logics brittle and open prospects for change
Quantum Dots with Disorder and Interactions: A Solvable Large-g Limit
We show that problem of interacting electrons in a quantum dot with chaotic
boundary conditions is solvable in the large-g limit, where g is the
dimensionless conductance of the dot. The critical point of the
theory (whose location and exponent are known exactly) that separates strong
and weak-coupling phases also controls a wider fan-shaped region in the
coupling-1/g plane, just as a quantum critical point controls the fan in at
T>0. The weak-coupling phase is governed by the Universal Hamiltonian and the
strong-coupling phase is a disordered version of the Pomeranchuk transition in
a clean Fermi liquid. Predictions are made in the various regimes for the
Coulomb Blockade peak spacing distributions and Fock-space delocalization
(reflected in the quasiparticle width and ground state wavefunction).Comment: 4 pages, 2 figure
Statistics of Coulomb Blockade Peak Spacings within the Hartree-Fock Approximation
We study the effect of electronic interactions on the addition spectra and on
the energy level distributions of two-dimensional quantum dots with weak
disorder using the self-consistent Hartree-Fock approximation for spinless
electrons. We show that the distribution of the conductance peak spacings is
Gaussian with large fluctuations that exceed, in agreement with experiments,
the mean level spacing of the non-interacting system. We analyze this
distribution on the basis of Koopmans' theorem. We show furthermore that the
occupied and unoccupied Hartree-Fock levels exhibit Wigner-Dyson statistics.Comment: 5 pages, 2 figures, submitted for publicatio
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