1,474 research outputs found
Full phase diagram of isolated skyrmions in a ferromagnet
Magnetic skyrmions are topological quasi particles of great interest for data
storage applications because of their small size, high stability, and ease of
manipulation via electric current. Theoretically, however, skyrmions are poorly
understood since existing theories are not applicable to small skyrmion sizes
and finite material thicknesses. Here, we present a complete theoretical
framework to determine the energy of any skyrmion in any material, assuming
only a circular symmetric 360 domain wall profile and a homogeneous
magnetization profile in the out-of-plane direction. Our model precisely agrees
with existing experimental data and micromagnetic simulations. Surprisingly, we
can prove that there is no topological protection of skyrmions. We discover and
confirm new phases, such as bi-stability, a phenomenon unknown in magnetism so
far. The outstanding computational performance and precision of our model allow
us to obtain the complete phase diagram of static skyrmions and to tackle the
inverse problem of finding materials corresponding to given skyrmion
properties, a milestone of skyrmion engineering
Quantum to classical crossover in the 2D easy-plane XXZ model
Ground-state and thermodynamical properties of the spin-1/2 two-dimensional
easy-plane XXZ model are investigated by both a Green's-function approach and
by Lanczos diagonalizations on lattices with up to 36 sites. We calculate the
spatial and temperature dependences of various spin correlation functions, as
well as the wave-vector dependence of the spin susceptibility for all
anisotropy parameters . In the easy--plane ferromagnetic region , the longitudinal correlators of spins at distance change sign
at a finite temperature . This transition, observed in
the 2D case for the first time, can be interpreted as a quantum to classical
crossover.Comment: 4 pages, 6 figures, Contribution to the Ising Centennial Colloquium,
ICM2000, Belo Horizonte, Brazil, August 200
A novel sampling theorem on the rotation group
We develop a novel sampling theorem for functions defined on the
three-dimensional rotation group SO(3) by connecting the rotation group to the
three-torus through a periodic extension. Our sampling theorem requires
samples to capture all of the information content of a signal band-limited at
, reducing the number of required samples by a factor of two compared to
other equiangular sampling theorems. We present fast algorithms to compute the
associated Fourier transform on the rotation group, the so-called Wigner
transform, which scale as , compared to the naive scaling of .
For the common case of a low directional band-limit , complexity is reduced
to . Our fast algorithms will be of direct use in speeding up the
computation of directional wavelet transforms on the sphere. We make our SO3
code implementing these algorithms publicly available.Comment: 5 pages, 2 figures, minor changes to match version accepted for
publication. Code available at http://www.sothree.or
Entanglement measurement with discrete multiple coin quantum walks
Within a special multi-coin quantum walk scheme we analyze the effect of the
entanglement of the initial coin state. For states with a special entanglement
structure it is shown that this entanglement can be meausured with the mean
value of the walk, which depends on the i-concurrence of the initial coin
state. Further on the entanglement evolution is investigated and it is shown
that the symmetry of the probability distribution is reflected by the symmetry
of the entanglement distribution.Comment: 9 pages, IOP styl
Immunohistological studies on neoplasms of female and male Onchocerca volvulus: filarial origin and absence of Wolbachia from tumor cells
Up to 5% of untreated female Onchocerca volvulus filariae develop potentially fatal pleomorphic neoplasms, whose incidence is increased following ivermectin treatment. We studied the occurrence of 8 filarial proteins and of Wolbachia endobacteria in the tumor cells. Onchocercomas from patients, untreated and treated with antibiotics and anthelminthics, were examined by immunohistology. Neoplasms were diagnosed in 112 of 3587 female and in 2 of 1570 male O. volvulus. The following proteins and other compounds of O. volvulus were expressed in the cells of the neoplasms: glutathione S-transferase 1, lysosomal aspartic protease, cAMP-dependent protein kinase, alpha-enolase, aspartate aminotransferase, ankyrin E1, tropomyosin, heat shock protein 60, transforming growth factor-beta, and prostaglandin E2. These findings prove the filarial origin of the neoplasms and confirm the pleomorphism of the tumor cells. Signs indicating malignancy of the neoplasms are described. Wolbachia were observed in the hypodermis, oocytes, and embryos of tumor-harbouring filariae using antibodies against Wolbachia surface protein, Wolbachia HtrA-type serine protease, and Wolbachia aspartate aminotransferase. In contrast, Wolbachia were not found in the cells of the neoplasms. Further, neoplasm-containing worms were not observed after more than 10 months after the start of sufficient treatment with doxycycline or doxycycline plus ivermectin
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