12,576 research outputs found
Hysteresis and Fractional Matching in Thin Nb Films with Rectangular Arrays of Nanoscaled Magnetic Dots
We have investigated the periodic pinning of magnetic flux quanta in thin Nb
films with rectangular arrays of magnetic dots. In this type of pinning
geometry, a change in the periodicity and shape of the minima in the
magnetoresistance occurs for magnetic fields exceeding a certain threshold
value. This has been explained recently in terms of a reconfiguration
transition of the vortex lattice due to an increasing vortex-vortex interaction
with increasing magnetic field. In this picture the dominating elastic energy
at high fields forces the vortex lattice to form a square symmetry rather than
being commensurate to the rectangular geometry of the pinning array. In this
paper we present a comparative study of rectangular arrays with Ni-dots,
Co-dots and holes. In the magnetic dot arrays, we found a strong fractional
matching effect up to the second order matching field. In contrast, no clear
fractional matching is seen after the reconfiguration. Additionally, we
discovered the existence of hysteresis in the magnetoresistance in the
crossover between the low and the high field regime. We found evidence that
this effect is correlated to the reconfiguration phenomenon rather than to the
magnetic state of the dots. The temperature and angular dependences of the
effect have been measured and possible models are discussed to explain this
behavior.Comment: 1 Table, 5 Figure
A Sawtooth Permanent Magnetic Lattice for Ultracold Atoms and BECs
We propose a new permanent magnetic lattice for creating periodic arrays of
Ioffe-Pritchard permanent magnetic microtraps for holding and controlling
ultracold atoms and Bose-Einstein condensates (BECs). Lattice can be designed
on thin layer of magnetic films such as . In
details, we investigate single layer and two crossed layers of sawtooth
magnetic patterns with thicknesses of 50 and 500nm respectively with a
periodicity of 1m. Trap depth and frequencies can be changed via an
applied bias field to handle tunneling rates between lattice sites. We present
analytical expressions and using numerical calculations show that this lattice
has non-zero potential minima to avoid majorana spin flips. One advantage of
this lattice over previous ones is that it is easier to manufacture.Comment: 8 pages, 6 figure
Role of shape and localized resonances in extraordinary transmission through periodic arrays of subwavelength holes: Experiment and theory
The effect of the aspect ratio of rectangular holes on the transmissivity of periodic arrays of subwavelength holes in optically thick metal films is investigated. The transmissivity is found to be highly dependent on the aspect ratio of the holes. Moreover, the wavelengths of maximum transmissivity show a monotonous shift as a function of the aspect ratio of the holes. We attribute the enhanced transmission of the periodic arrays to an interplay of surface plasmons at the surface of the metal and shape resonances (also known as localized modes) inside the holes. The importance of the shape resonances was confirmed by a comparison of transmission through periodic hole arrays and through randomly distributed holes. Dispersion curves of periodic and random hole arrays confirmed the existence of shape resonance as well. We suggest that the localized modes effectively act as waveguides and increase the coupling efficiency of surface plasmons between both sides of the film, which results in a higher transmissivity. The shift of the maxima of the transmissivity may in part be explained by the spectral position of the localized modes in the individual holes. Finally measurements on similar patterns in Ni and Ag revealed that the occurrence of shape resonances is independent of the material of the film
Radio-frequency dressed lattices for ultracold alkali atoms
Ultracold atomic gases in periodic potentials are powerful platforms for exploring quantum physics in regimes dominated by many-body effects as well as for developing applications that benefit from quantum mechanical effects. Further advances face a range of challenges including the realization of potentials with lattice constants smaller than optical wavelengths as well as creating schemes for effective addressing and manipulation of single sites. In this paper we propose a dressed-based scheme for creating periodic potential landscapes for ultracold alkali atoms with the capability of overcoming such difficulties. The dressed approach has the advantage of operating in a low-frequency regime where decoherence and heating effects due to spontaneous emission do not take place. These results highlight the possibilities of atom-chip technology in the future development of quantum simulations and quantum technologies, and provide a realistic scheme for starting such an exploration
Stability and Vortex Shedding of Bluff Body Arrays
The primary purpose of this study was to develop an understanding of the stability of laminar
flow through bluff body arrays, and investigate the nature of the unsteady vortex shedding regime
that follows. The flow was numerically investigated using a specially developed multi-domain
spectral element solver. Important criteria in the solver development were flexibility, efficiency, and
accuracy. Flexibility was critical to the functionality of the code, as arrays of varying geometry
were investigated. Efficiency with a high degree of accuracy was also of primary importance, with
the code implemented to run efficiently on today's massively parallel architectures.
Numerical two-dimensional stability analysis of the flow in several configurations of inline and
staggered array geometries was performed. The growth rate, eigenfunction, and frequency of the
disturbances were determined. The critical Reynolds number for flow transition in each case was
identified and compared to that of flow over a single body. Based on the solutions of the laminar
flow, a one-dimensional analytical analysis was performed on selected velocity profiles in the wake
region. The results of this analysis were used to guide the interpretation of the two dimensional
results and formulate a general theory of stability of inline and staggered bluff body arrays. The
nature of the flow in the unsteady regime following the onset of instability was examined for an
inline and a staggered arrangement. Particular attention was focused on the vortex shedding which
was visualized and quantified through computation of the flow swirl, a quantity which identifies
regions of rotary motion. The conditions required for the generation of leading edge vortex shedding
were identified and discussed. Finally, a third geometry related to the inline and staggered arrays
was considered. Flow solution data for this geometry is presented and its suitability as a model for
louvered arrays was discussed.Air Conditioning and Refrigeration Project 11
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