2,441 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
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
Statistics of conductance oscillations of a quantum dot in the Coulomb-blockade regime
The fluctuations and the distribution of the conductance peak spacings of a
quantum dot in the Coulomb-blockade regime are studied and compared with the
predictions of random matrix theory (RMT). The experimental data were obtained
in transport measurements performed on a semiconductor quantum dot fabricated
in a GaAs-AlGaAs heterostructure. It is found that the fluctuations in the peak
spacings are considerably larger than the mean level spacing in the quantum
dot. The distribution of the spacings appears Gaussian both for zero and for
non-zero magnetic field and deviates strongly from the RMT-predictions.Comment: 7 pages, 4 figure
A Parameterized Centrality Metric for Network Analysis
A variety of metrics have been proposed to measure the relative importance of
nodes in a network. One of these, alpha-centrality [Bonacich, 2001], measures
the number of attenuated paths that exist between nodes. We introduce a
normalized version of this metric and use it to study network structure,
specifically, to rank nodes and find community structure of the network.
Specifically, we extend the modularity-maximization method [Newman and Girvan,
2004] for community detection to use this metric as the measure of node
connectivity. Normalized alpha-centrality is a powerful tool for network
analysis, since it contains a tunable parameter that sets the length scale of
interactions. By studying how rankings and discovered communities change when
this parameter is varied allows us to identify locally and globally important
nodes and structures. We apply the proposed method to several benchmark
networks and show that it leads to better insight into network structure than
alternative methods.Comment: 11 pages, submitted to Physical Review
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
Size scaling of the addition spectra in silicon quantum dots
We investigate small artificial quantum dots obtained by geometrically
controlled resistive confinement in low mobility silicon-on-insulator
nanowires. Addition spectra were recorded at low temperature for various dot
areas fixed by lithography. We compare the standard deviation of the addition
spectra with theory in the high electron concentration regime. We find that the
standard deviation scales as the inverse area of the dot and its absolute value
is comparable to the energy spacing of the one particle spectrum.Comment: 4 pages, 5 figure
Smoke gets in your eyes:what is sociological about cigarettes?
Contemporary public health approaches increasingly draw attention to the unequal social distribution of cigarette smoking. In contrast, critical accounts emphasize the importance of smokers’ situated agency, the relevance of embodiment and how public health measures against smoking potentially play upon and exacerbate social divisions and inequality. Nevertheless, if the social context of cigarettes is worthy of such attention, and sociology lays a distinct claim to understanding the social, we need to articulate a distinct, positive and systematic claim for smoking as an object of sociological enquiry. This article attempts to address this by situating smoking across three main dimensions of sociological thinking: history and social change; individual agency and experience; and social structures and power. It locates the emergence and development of cigarettes in everyday life within the project of modernity of the nineteenth and twentieth centuries. It goes on to assess the habituated, temporal and experiential aspects of individual smoking practices in everyday lifeworlds. Finally, it argues that smoking, while distributed in important ways by social class, also works relationally to render and inscribe it
Finite temperature effects in Coulomb blockade quantum dots and signatures of spectral scrambling
The conductance in Coulomb blockade quantum dots exhibits sharp peaks whose
spacings fluctuate with the number of electrons. We derive the
temperature-dependence of these fluctuations in the statistical regime and
compare with recent experimental results. The scrambling due to Coulomb
interactions of the single-particle spectrum with the addition of an electron
to the dot is shown to affect the temperature-dependence of the peak spacing
fluctuations. Spectral scrambling also leads to saturation in the temperature
dependence of the peak-to-peak correlator, in agreement with recent
experimental results. The signatures of scrambling are derived using discrete
Gaussian processes, which generalize the Gaussian ensembles of random matrices
to systems that depend on a discrete parameter -- in this case, the number of
electrons in the dot.Comment: 14 pages, 4 eps figures included, RevTe
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Aerosols-cloud microphysics-thermodynamics-turbulence: Evaluating supersaturation in a marine stratocumulus cloud
This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd = 464 cm−3. Furthermore, a 50% activation diameter of Dp50≈115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1%≤σS'≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S' with σS'=0.1% at cloud base and a decreasing σS' with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two
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
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