55,452 research outputs found
Detection of Striped Superconductors Using Magnetic Field Modulated Josephson Effect
In a very interesting recent Letter\cite{berg}, the authors suggested that a
novel form of superconducting state is realized in LaBaCuO with
close to 1/8. This suggestion was based on experiments\cite{li} on this
compound which found predominantly two-dimensional (2D) characters of the
superconducting state, with extremely weak interplane coupling. Later this
specific form of superconducting state was termed striped
superconductors\cite{berg08}. The purpose of this note is to point out that the
suggested form\cite{berg} of the superconducting order parameter can be
detected directly using magnetic field modulated Josephson effect.Comment: Expanded version as appeared in prin
Micromachined membrane particle filters
We report here several particle membrane filters (8 x 8 mm^2) with circular, hexagonal and rectangular through holes. By varying hole dimensions from 6 to 12 pm, opening factors from 4 to 45 % are achieved. In order to improve the filter robustness, a composite silicon nitride/Parylene membrane technology is developed. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical simulation using the Navier-Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified
Parallel Computing on a PC Cluster
The tremendous advance in computer technology in the past decade has made it
possible to achieve the performance of a supercomputer on a very small budget.
We have built a multi-CPU cluster of Pentium PC capable of parallel
computations using the Message Passing Interface (MPI). We will discuss the
configuration, performance, and application of the cluster to our work in
physics.Comment: 3 pages, uses Latex and aipproc.cl
Disorder-induced melting of the charge order in thin films of Pr0.5Ca0.5MnO3
We have studied the magnetic-field-induced melting of the charge order in
thin films of Pr0.5Ca0.5MnO3 (PCMO) films on SrTiO3 (STO) by X-ray diffraction,
magnetization and transport measurement. At small thickness (25 nm) the films
are under tensile strain and the low-temperature melting fields are of the
order of 20 T or more, comparable to the bulk value. With increasing film
thickness the strain relaxes, which leads to a strong decrease of the melting
fields. For a film of 150 nm, with in-plane and out-of-plane lattice parameters
closer to the bulk value, the melting field has reduced to 4 T at 50 K, with a
strong increase in the hysteretic behavior and also an increasing fraction of
ferromagnetic material. Strain relaxation by growth on a template of
YBa2Cu3O(7-delta) or by post-annealing yields similar results with an even
stronger reduction of the melting field. Apparently, strained films behave
bulk-like. Relaxation leads to increasing suppression of the CO state,
presumably due to atomic scale disorder produced by the relaxation process.Comment: 7 pages, 4 fig
Pattern formation of indirect excitons in coupled quantum wells
Using a nonlinear Schr\"odinger equation including short-range two-body
attraction and three-body repulsion, we investigate the spatial distribution of
indirect excitons in semiconductor coupled quantum wells. The results obtained
can interpret the experimental phenomenon that annular exciton cloud first
contracts then expands when the number of confined excitons is increased in
impurity potential well, as observed by Lai \emph{et al.} [Lai ,
Science \textbf{303}, 503 (2004)]. In particular, the model reconciles the
patterns of exciton rings reported by Butov \emph{et al.} [Butov ,
Nature \textbf{418}, 751 (2002)]. At higher densities, the model predicts much
richer patterns, which could be tested by future experiments.Comment: 5 Revtex4 pages, 3 figure
Coupled Fluid-Structure Interaction Analysis of Solid Rocket Motor with Flexible Inhibitors
Flexible inhibitors are generally used in solid rocket motors (SRMs) as a means to control the burning of propellant. Vortices generated by the flow of propellant around the flexible inhibitors have been identified as a driving source of instabilities that can lead to thrust oscillations in launch vehicles. Potential coupling between the SRM thrust oscillations and structural vibration modes is an important risk factor in launch vehicle design. As a means to predict and better understand these phenomena, a multidisciplinary simulation capability that couples the NASA production CFD code, Loci/CHEM, with CFDRC's structural finite element code, CoBi, has been developed. This capability is crucial to the development of NASA's new space launch system (SLS). This paper summarizes the efforts in applying the coupled software to demonstrate and investigate fluid-structure interaction (FSI) phenomena between pressure waves and flexible inhibitors inside reusable solid rocket motors (RSRMs). The features of the fluid and structural solvers are described in detail, and the coupling methodology and interfacial continuity requirements are then presented in a general Eulerian-Lagrangian framework. The simulations presented herein utilize production level CFD with hybrid RANS/LES turbulence modeling and grid resolution in excess of 80 million cells. The fluid domain in the SRM is discretized using a general mixed polyhedral unstructured mesh, while full 3D shell elements are utilized in the structural domain for the flexible inhibitors. Verifications against analytical solutions for a structural model under a steady uniform pressure condition and under dynamic modal analysis show excellent agreement in terms of displacement distribution and eigenmode frequencies. The preliminary coupled results indicate that due to acoustic coupling, the dynamics of one of the more flexible inhibitors shift from its first modal frequency to the first acoustic frequency of the solid rocket motor. This insight could have profound implications for SRM and flexible inhibitor designs for current and future launch vehicles including SLS
Trans-phonon effects in ultrafast nano-devices
We report a novel phenomenon in carbon nanotube (CNT) based devices, the
transphonon effects, which resemble the transonic effects in aerodynamics. It
is caused by dissipative resonance of nanotube phonons similar to the radial
breathing mode, and subsequent drastic surge of the dragging force on the
sliding tube, and multiple phonon barriers are encountered as the intertube
sliding velocity reaches critical values. It is found that the transphonon
effects can be tuned by applying geometric constraints or varying chirality
combinations of the nanotubes
Cosmic axion thermalization
Axions differ from the other cold dark matter candidates in that they form a
degenerate Bose gas. It is shown that their huge quantum degeneracy and large
correlation length cause cold dark matter axions to thermalize through
gravitational self-interactions when the photon temperature reaches
approximately 500 eV. When they thermalize, the axions form a Bose-Einstein
condensate. Their thermalization occurs in a regime, herein called the
`condensed regime', where the Boltzmann equation is not valid because the
energy dispersion of the particles is smaller than their interaction rate. We
derive analytical expressions for the thermalization rate of particles in the
condensed regime, and check the validity of these expressions by numerical
simulation of a toy model. We revisit axion cosmology in light of axion
Bose-Einstein condensation. It is shown that axions are indistinguishable from
ordinary cold dark matter on all scales of observational interest, except when
they thermalize or rethermalize. The rethermalization of axions that are about
to fall in a galactic potential well causes them to acquire net overall
rotation as they go to the lowest energy state consistent with the total
angular momentum they acquired by tidal torquing. This phenomenon explains the
occurrence of caustic rings of dark matter in galactic halos. We find that
photons may reach thermal contact with axions and investigate the implications
of this possibility for the measurements of cosmological parameters.Comment: 38 pages, 1 figur
- …