5,422 research outputs found
Lithographic band gap tuning in photonic band gap crystals
We describe the lithographic control over the spectral response of three-dimensional photonic crystals. By precise microfabrication of the geometry using a reproducible and reliable procedure consisting of electron beam lithography followed by dry etching, we have shifted the conduction band of crystals within the near-infrared. Such microfabrication has enabled us to reproducibly define photonic crystals with lattice parameters ranging from 650 to 730 nm. In GaAs semiconductor wafers, these can serve as high-reflectivity (> 95%) mirrors. Here, we show the procedure used to generate these photonic crystals and describe the geometry dependence of their spectral response
Novel fluorohydrocarbons
Novel fluorohydrocarbons include a fluoroalkyl unit terminating in a tertiary carbon atom which is directly linked to an aliphatic moiety of the compound. The compounds contain at least 9 carbon atoms and usually no more than 13 carbon atoms. The compounds are synthesized by addition of a fluoride atom to the tertiary carbon atom of a fluorocarbon material to form a carbanion followed by alkylation of the carbanion. The fluorohydrocarbons will find use as blood substitutes or as electronic fluids
Single-charge devices with ultrasmall Nb/AlOx/Nb trilayer Josephson junctions
Josephson junction transistors and 50-junction arrays with linear junction
dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb
trilayers. The fabrication process includes electron beam lithography, dry
etching, anodization, and planarization by chemical-mechanical polishing. The
samples were characterized at temperatures down to 25 mK. In general, all
junctions are of high quality and their I-U characteristics show low leakage
currents and high superconducting energy gap values of 1.35 meV. The
characteristics of the transistors and arrays exhibit some features in the
subgap area, associated with tunneling of Cooper pairs, quasiparticles and
their combinations due to the redistribution of the bias voltage between the
junctions. Total island capacitances of the transistor samples ranged from 1.5
fF to 4 fF, depending on the junction sizes. Devices made of junctions with
linear dimensions below 100 nm by 100 nm demonstrate a remarkable
single-electron behavior in both superconducting and normal state. We also
investigated the area dependence of the junction capacitances for transistor
and array samples.Comment: 19 pages incl. 2 tables and 11 figure
The polarizability of the pion: no conflict between dispersion theory and chiral perturbation theory
Recent attempts to determine the pion polarizability by dispersion relations
yield values that disagree with the predictions of chiral perturbation theory.
These dispersion relations are based on specific forms for the absorptive part
of the Compton amplitudes. The analytic properties of these forms are examined,
and the strong enhancement of intermediate-meson contributions is shown to be
connected with spurious singularities. If the basic requirements of dispersion
relations are taken into account, the results of dispersion theory and
effective field theory are not inconsistent.Comment: 30 pages, 8 figures, 6 table
Tests of Lorentz and CPT symmetry with hadrons and nuclei
We apply chiral-perturbation-theory techniques to the QCD sector of the Lorentz and CPT violating standard-model extension. We derive the effective Lagrangian in terms of pions and nucleons for a selected set of dimension-five operators involving quarks and gluons. This derivation is based on chiral-symmetry properties of the operators, as well as on their behaviour under C,P, and T transformations. We consider the power counting rules and apply the heavy-baryon approach to account for the large nucleon mass. Having obtained the relevant Lorentz-violating contributions to the pion-nucleon Lagrangian, we proceed to derive the particle and anti-particle Hamiltonian, from which we obtain the Lorentz-violating contribution to comagnetometer experiments. This allows us to place stringent limits on some of the parameters. For some other parameters we find that the best bounds will come from nucleon nucleon interactions, and we derive the relevant nucleon-nucleon potential. These considerations imply possible new opportunities for spin-precession experiments involving for example the deuteron.Portuguese Foundation for Science and Technology (FCT) [SFRH/BPD/101403/2014]; program POPH/FS
Spectral properties of molecular oligomers. A non-Markovian quantum state diffusion approach
Absorption spectra of small molecular aggregates (oligomers) are considered.
The dipole-dipole interaction between the monomers leads to shifts of the
oligomer spectra with respect to the monomer absorption. The line-shapes of
monomer as well as oligomer absorption depend strongly on the coupling to
vibrational modes. Using a recently developed approach [Roden et. al, PRL 103,
058301] we investigate the length dependence of spectra of one-dimensional
aggregates for various values of the interaction strength between the monomers.
It is demonstrated, that the present approach is well suited to describe the
occurrence of the J- and H-bands
Superconducting Electrometer Based on the Resistively Shunted Bloch Transistor
We have fabricated the Bloch transistor shunted on-chip by a small-sized Cr
resistor with Rs about 1 kOhm. The Bloch transistor normally consists of two
small Josephson junctions connected in series, which in our case have been
replaced by two superconducting interferometer loops, each with two junctions
in parallel. A capacitively coupled gate is supplied to control the induced
charge of the small intermediate electrode (island) of the transistor. The
measured I-V curves show no hysteresis and correspond to the operation of a
effective Josephson junction at the high-damping and strong-noise limits. The
critical current of the system was found to be close to its nominal value, that
is in accordance with the electromagnetic environment theory. The I-V curves
were modulated by the gate with a period of e and a maximum swing of about 2
/mu_V. Such rather moderate modulation results from the Josephson-to- charging
energies ratio, Ej/Ec about 9, in our sample being far from its optimum value
of 0.3 up to 1.Comment: To be published in IEEE Transactions on Applied Superconductivity,
June 199
Classical predictability and coarse-grained evolution of the quantum baker's map
We investigate how classical predictability of the coarse-grained evolution
of the quantum baker's map depends on the character of the coarse-graining. Our
analysis extends earlier work by Brun and Hartle [Phys. Rev. D 60, 123503
(1999)] to the case of a chaotic map. To quantify predictability, we compare
the rate of entropy increase for a family of coarse-grainings in the decoherent
histories formalism. We find that the rate of entropy increase is dominated by
the number of scales characterising the coarse-graining.Comment: 28 pages, 1 figur
Noise in Al single electron transistors of stacked design
We have fabricated and examined several Al single electron transistors whose
small islands were positioned on top of a counter electrode and hence did not
come into contact with a dielectric substrate. The equivalent charge noise
figure of all transistors turned out to be surprisingly low, (2.5 - 7)*10E-5
e/sqrt(Hz) at f = 10 Hz. Although the lowest detected noise originates mostly
from fluctuations of background charge, the noise contribution of the tunnel
junction conductances was, on occasion, found to be dominant.Comment: 4 pages of text with 1 table and 5 figure
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