949 research outputs found
Dynamical Kohn Anomaly in Surface Acoustic Wave Response in Quantum Hall Systems Near
The dynamical analog of the Kohn Anomaly image of the Fermi Surface is
demonstrated for the response functions to the surface acoustic waves in
Quantum Hall Systems near . Kinks appear in the velocity shift
and attenuation coefficient . The effect is considerably
enhanced under periodic modulation and should be observable.Comment: 5 pages, 2 figures, the published versio
Mapping the matrix: using compendium as a tool for recording the analytic group
This paper describes the application of Compendium, a knowledge cartography software tool, for the recording of group process. As a hypertext tool, it enables analysts to visualise connections between people, ideas and information, establishing an evidence base within and across contexts (such as group sessions). After customising its visual language, templates and keyword system, it has been piloted as a research tool for the measurement of group process. This would appear to hold the promise of providing a “digital substrate” for recording, discussing and analysing long term group dynamics in new ways. While the project is in its early stages, early indications are that it is a useful tool which can highlight group process and record change over time. In the longer term, it seems plausible that group processes such as multiple mirroring and identification, and such complex structures as the matrix, could be made visible and researchable through this methodology
Lyapunov exponents for products of complex Gaussian random matrices
The exact value of the Lyapunov exponents for the random matrix product with each , where
is a fixed positive definite matrix and a complex Gaussian matrix with entries standard complex normals, are
calculated. Also obtained is an exact expression for the sum of the Lyapunov
exponents in both the complex and real cases, and the Lyapunov exponents for
diffusing complex matrices.Comment: 15 page
Charge and Current in the Quantum Hall Matrix Model
We extend the quantum Hall matrix model to include couplings to external
electric and magnetic fields. The associated current suffers from matrix
ordering ambiguities even at the classical level. We calculate the linear
response at low momenta -- this is unambigously defined. In particular, we
obtain the correct fractional quantum Hall conductivity, and the expected
density modulations in response to a weak and slowly varying magnetic field.
These results show that the classical quantum Hall matrix models describe
important aspects of the dynamics of electrons in the lowest Landau level. In
the quantum theory the ordering ambiguities are more severe; we discuss
possible strategies, but we have not been able to construct a good density
operator, satisfying the pertinent lowest Landau level commutator algebra.Comment: 12 pages, no figures; a logical error below the proposed density
operator (46) in version 1 is corrected, and the claim that this density
operator satisfy the magnetic algebra (2) is withdrawn. Some formulations
have been changed and a few misprints correcte
Warp propagation in astrophysical discs
Astrophysical discs are often warped, that is, their orbital planes change
with radius. This occurs whenever there is a non-axisymmetric force acting on
the disc, for example the Lense-Thirring precession induced by a misaligned
spinning black hole, or the gravitational pull of a misaligned companion. Such
misalignments appear to be generic in astrophysics. The wide range of systems
that can harbour warped discs - protostars, X-ray binaries, tidal disruption
events, quasars and others - allows for a rich variety in the disc's response.
Here we review the basic physics of warped discs and its implications.Comment: To be published in Astrophysical Black Holes by Haardt et al.,
Lecture Notes in Physics, Springer 2015. 19 pages, 2 figure
Quasiparticle spectrum of d-wave superconductors in the mixed state: a large Fermi-velocity anisotropy study
The quasiparticle spectrum of a two-dimensional d-wave superconductor in the
mixed state, H_c1 << H << H_c2, is studied for large values of the ``anisotropy
ratio'' alpha_D = v_F/v_Delta. For a square vortex lattice rotated by 45
degrees from the quasiparticle anisotropy axes (and the usual choice of
Franz--Tesanovic singular gauge transformation) we determine essential features
of the band structure asymptotically for large alpha_D, using an effective
one-dimensional model, and compare them to numerical calculations. We find that
several features of the band structure decay to zero exponentially fast for
large alpha_D. Using a different choice of singular gauge transformation, we
obtain a different band structure, but still find qualitative agreement between
the 1D and full 2D calculations. Finally, we distort the square lattice into a
non-Bravais lattice. Both the one- and two-dimensional numerical calculations
of the energy spectra show a gap around zero-energy, with our gauge choice, and
the two excitation spectra agree reasonably well.Comment: 14 pages, 13 figures, revte
Narrow Line X-Ray Calibration Source for High Resolution Microcalorimeters
We are developing a narrow line calibration source for use with X-ray microcalorimeters. At energies below 300 electronvolts fluorescent lines are intrinsically broad, making calibration of high resolution detectors difficult. This source consists of a 405 nanometers (3 electronvolts) laser diode coupled to an optical fiber. The diode is pulsed to create approximately one hundred photons in a few microseconds. If the pulses are short compared to the rise time of the detector, they will be detected as single events with a total energy in the soft X-ray range. Poisson fluctuations in photon number per pulse create a comb of X-ray lines with 3 electronvolts spacing, so detectors with energy resolution better than 2 electronvolts are required to resolve the individual lines. Our currently unstabilized diode has a multimode width less than 1 nanometer, giving a 300 electronvolt event a Full width at half maximum (FWHM) less than 0.1 electronvolts. By varying the driving voltage, or pulse width, the source can produce a comb centered on a wide range of energies. The calibration events are produced at precisely known times. This allows continuous calibration of a flight mission without contaminating the observed spectrum and with minimal deadtime
Growing Scale-Free Networks with Small World Behavior
In the context of growing networks, we introduce a simple dynamical model
that unifies the generic features of real networks: scale-free distribution of
degree and the small world effect. While the average shortest path length
increases logartihmically as in random networks, the clustering coefficient
assumes a large value independent of system size. We derive expressions for the
clustering coefficient in two limiting cases: random (C ~ (ln N)^2 / N) and
highly clustered (C = 5/6) scale-free networks.Comment: 4 pages, 4 figure
Highly clustered scale-free networks
We propose a model for growing networks based on a finite memory of the
nodes. The model shows stylized features of real-world networks: power law
distribution of degree, linear preferential attachment of new links and a
negative correlation between the age of a node and its link attachment rate.
Notably, the degree distribution is conserved even though only the most
recently grown part of the network is considered. This feature is relevant
because real-world networks truncated in the same way exhibit a power-law
distribution in the degree. As the network grows, the clustering reaches an
asymptotic value larger than for regular lattices of the same average
connectivity. These high-clustering scale-free networks indicate that memory
effects could be crucial for a correct description of the dynamics of growing
networks.Comment: 6 pages, 4 figure
Integrating Al with NiO nano honeycomb to realize an energetic material on silicon substrate
Nano energetic materials offer improved performance in energy release, ignition, and mechanical properties compared to their bulk or micro counterparts. In this study, the authors propose an approach to synthesize an Al/NiO based nano energetic material which is fully compatible with a microsystem. A two-dimensional NiO nano honeycomb is first realized by thermal oxidation of a Ni thin film deposited onto a silicon substrate by thermal evaporation. Then the NiO nano honeycomb is integrated with an Al that is deposited by thermal evaporation to realize an Al/NiO based nano energetic material. This approach has several advantages over previous investigations, such as lower ignition temperature, enhanced interfacial contact area, reduced impurities and Al oxidation, tailored dimensions, and easier integration into a microsystem to realize functional devices. The synthesized Al/NiO based nano energetic material is characterized by scanning electron microscopy, X-ray diffraction, differential thermal analysis, and differential scanning calorimetry
- …