14,516 research outputs found
Vortex Molecular Crystal and Vortex Plastic Crystal States in Honeycomb and Kagome Pinning Arrays
Using numerical simulations, we investigate vortex configurations and pinning
in superconductors with honeycomb and kagome pinning arrays. We find that a
variety of novel vortex crystal states can be stabilized at integer and
fractional matching field densities. The honeycomb and kagome pinning arrays
produce considerably more pronounced commensuration peaks in the critical
depinning force than triangular pinning arrays, and also cause additional peaks
at noninteger matching fields where a portion of the vortices are located in
the large interstitial regions of the pinning lattices. For the honeycomb
pinning array, we find matching effects of equal strength at most fillings
B/B_\phi=n/2 for n>2, where n is an integer, in agreement with recent
experiments. For kagome pinning arrays, pronounced matching effects generally
occur at B/B_\phi=n/3 for n>3, while for triangular pinning arrays pronounced
matching effects are observed only at integer fillings B/B_\phi=n. At the
noninteger matching field peaks in the honeycomb and kagome pinning arrays, the
interstitial vortices are arranged in dimer, trimer, and higher order n-mer
states that have an overall orientational order. We call these n-mer states
"vortex molecular crystals" and "vortex plastic crystals" since they are
similar to the states recently observed in colloidal molecular crystal systems.
We argue that the vortex molecular crystals have properties in common with
certain spin systems such as Ising and n-state Potts models. We show that
kagome and honeycomb pinning arrays can be useful for increasing the critical
current above that of purely triangular pinning arrays.Comment: 19 pages, 22 postscript figures. Version to appear in Phys. Rev.
Honeycomb Arrays
A honeycomb array is an analogue of a Costas array in the hexagonal grid; they
were first studied by Golomb and Taylor in 1984. A recent result of Blackburn,
Etzion, Martin and Paterson has shown that (in contrast to the situation for Costas
arrays) there are only finitely many examples of honeycomb arrays, though their
bound on the maximal size of a honeycomb array is too large to permit an exhaustive
search over all possibilities.
The present paper contains a theorem that significantly limits the number of possibilities
for a honeycomb array (in particular, the theorem implies that the number
of dots in a honeycomb array must be odd). Computer searches for honeycomb
arrays are summarised, and two new examples of honeycomb arrays with 15 dots
are given
Meissner effect in honeycomb arrays of multi-walled carbon nanotubes
We report Meissner effect for type-II superconductors with a maximum Tc of 19
K, which is the highest value among those in new-carbon related
superconductors, found in the honeycomb arrays of multi-walled CNTs (MWNTs).
Drastic reduction of ferromagnetic catalyst and efficient growth of MWNTs by
deoxidization of catalyst make the finding possible. The weak magnetic
anisotropy, superconductive coherence length (- 7 nm), and disappearance of the
Meissner effect after dissolving array structure indicate that the graphite
structure of an MWNT and those intertube coupling in the honeycomb array are
dominant factors for the mechanism.Comment: 6 page
Single-atom trapping in holographic 2D arrays of microtraps with arbitrary geometries
We demonstrate single-atom trapping in two-dimensional arrays of microtraps
with arbitrary geometries. We generate the arrays using a Spatial Light
Modulator (SLM), with which we imprint an appropriate phase pattern on an
optical dipole trap beam prior to focusing. We trap single
atoms in the sites of arrays containing up to microtraps separated by
distances as small as m, with complex structures such as triangular,
honeycomb or kagome lattices. Using a closed-loop optimization of the
uniformity of the trap depths ensures that all trapping sites are equivalent.
This versatile system opens appealing applications in quantum information
processing and quantum simulation, e.g. for simulating frustrated quantum
magnetism using Rydberg atoms.Comment: 9 pages, 10 figure
Anti-ferromagnetic ordering in arrays of superconducting pi-rings
We report experiments in which one dimensional (1D) and two dimensional (2D)
arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting
transition temperature of the Nb in various magnetic fields. These pi-rings
have degenerate ground states with either clockwise or counter-clockwise
spontaneous circulating supercurrents. The final flux state of each ring in the
arrays was determined using scanning SQUID microscopy. In the 1D arrays,
fabricated as a single junction with facets alternating between alignment
parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis,
half-fluxon Josephson vortices order strongly into an arrangement with
alternating signs of their magnetic flux. We demonstrate that this ordering is
driven by phase coupling and model the cooling process with a numerical
solution of the Sine-Gordon equation. The 2D ring arrays couple to each other
through the magnetic flux generated by the spontaneous supercurrents. Using
pi-rings for the 2D flux coupling experiments eliminates one source of disorder
seen in similar experiments using conventional superconducting rings, since
pi-rings have doubly degenerate ground states in the absence of an applied
field. Although anti-ferromagnetic ordering occurs, with larger negative bond
orders than previously reported for arrays of conventional rings, long-range
order is never observed, even in geometries without geometric frustration. This
may be due to dynamical effects. Monte-Carlo simulations of the 2D array
cooling process are presented and compared with experiment.Comment: 10 pages, 15 figure
Fabrication of Artificial Graphene in a GaAs Quantum Heterostructure
The unusual electronic properties of graphene, which are a direct consequence
of its two-dimensional (2D) honeycomb lattice, have attracted a great deal of
attention in recent years. Creation of artificial lattices that recreate
graphene's honeycomb topology, known as artificial graphene, can facilitate the
investigation of graphene-like phenomena, such as the existence of massless
Dirac fermions, in a tunable system. In this work, we present the fabrication
of artificial graphene in an ultra-high quality GaAs/AlGaAs quantum well, with
lattice period as small as 50 nm, the smallest reported so far for this type of
system. Electron-beam lithography is used to define an etch mask with honeycomb
geometry on the surface of the sample, and different methodologies are compared
and discussed. An optimized anisotropic reactive ion etching process is
developed to transfer the pattern into the AlGaAs layer and create the
artificial graphene. The achievement of such high-resolution artificial
graphene should allow the observation for the first time of massless Dirac
fermions in an engineered semiconductor.Comment: 13 pages text, 8 figures, plus reference
Towards an explanation for the 30 Dor (LMC) Honeycomb nebula - the impact of recent observations and spectral analysis
The unique Honeycomb nebula, most likely a peculiar supernova remnant, lies
in 30 Doradus in the Large Magellanic Cloud. Due to its proximity to SN1987A,
it has been serendipitously and intentionally observed at many wavelengths.
Here, an optical spectral analysis of forbidden line ratios is performed in
order to compare the Honeycomb high-speed gas with supernova remnants in the
Galaxy and the LMC, with galactic Wolf-Rayet nebulae and with the optical line
emission from the interaction zone of the SS433 microquasar and W50 supernova
remnant system. An empirical spatiokinematic model of the images and spectra
for the Honeycomb reveals that its striking appearance is most likely due to a
fortuitous viewing angle. The Honeycomb nebula is more extended in soft X-ray
emission and could in fact be a small part of the edge of a giant LMC shell
revealed for the first time in this short wavelength domain. It is also
suggested that a previously unnoticed region of optical emission may in fact be
an extension of the Honeycomb around the edge of this giant shell. A secondary
supernova explosion in the edge of a giant shell is considered for the creation
of the Honeycomb nebula. A microquasar origin of the Honeycomb nebula as
opposed to a simple supernova origin is also evaluated.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Dual phononic and photonic band gaps in a periodic array of pillars deposited on a thin plate
We study theoretically the simultaneous existence of phononic and photonic band gaps in a periodic array of
silicon pillars deposited on a homogeneous thin silica plate. Several lattices, namely, square, triangular, and
honeycomb are investigated for a wide range of geometrical parameters. We discuss the most suitable cases for
dual phononic-photonic band gaps, especially in comparison to the more conventional structures constituted by
a periodic array of holes in a membrane
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