4,165 research outputs found
Magnetic multipole analysis of kagome and artificial ice dipolar arrays
We analyse an array of linearly extended monodomain dipoles forming square
and kagome lattices. We find that its phase diagram contains two (distinct)
finite-entropy kagome ice regimes - one disordered, one algebraic - as well as
a low-temperature ordered phase. In the limit of the islands almost touching,
we find a staircase of corresponding entropy plateaux, which is analytically
captured by a theory based on magnetic charges. For the case of a modified
square ice array, we show that the charges ('monopoles') are excitations
experiencing two distinct Coulomb interactions: a magnetic 'three-dimensional'
one as well as a logarithmic `two dimensional' one of entropic origin.Comment: 4 pages, 2 figures; v2: minor changes as in final published versio
Conditions for free magnetic monopoles in nanoscale square arrays of dipolar spin ice
We study a modified frustrated dipolar array recently proposed by M\"{o}ller
and Moessner [Phys. Rev. Lett. \textbf{96}, 237202 (2006)], which is based on
an array manufactured lithographically by Wang \emph{et al.} [Nature (London)
\textbf{439}, 303 (2006)] and consists of introducing a height offset
between islands (dipoles) pointing along the two different lattice directions.
The ground-states and excitations are studied as a function of . We have
found, in qualitative agreement with the results of M\"{o}ller and Moessner,
that the ground-state changes for , where ( is the
lattice parameter or distance between islands). In addition, the excitations
above the ground-state behave like magnetic poles but confined by a string,
whose tension decreases as increases, in such a way that for
its value is around 20 times smaller than that for . The system exhibits
an anisotropy in the sense that the string tension and magnetic charge depends
significantly on the directions in which the monopoles are separated. In turn,
the intensity of the magnetic charge abruptly changes when the monopoles are
separated along the direction of the longest axis of the islands. Such a gap is
attributed to the transition from the anti to the ferromagnetic ground-state
when .Comment: 6 pages, 7 figures. Published versio
Nambu monopoles interacting with lattice defects in two-dimensional artificial square spin ice
The interactions between an excitation (similar to a pair of Nambu monopoles)
and a lattice defect are studied in an artificial two-dimensional square spin
ice. This is done by considering a square array of islands containing only one
island different from all others. This difference is incorporated in the
magnetic moment (spin) of the "imperfect" island and several cases are studied,
including the special situation in which this distinct spin is zero (vacancy).
We have shown that the two extreme points of a malformed island behave like two
opposite magnetic charges. Then, the effective interaction between a pair of
Nambu monopoles with the deformed island is a problem involving four magnetic
charges (two pairs of opposite poles) and a string. We also sketch the
configuration of the field lines of these four charges to confirm this picture.
The influence of the string on this interaction decays rapidly with the string
distance from the defect.Comment: 7 pages, 13 figure
Proximity effect of vanadium on spin-density-wave magnetism in Cr films
The spin-density wave (SDW) state in thin chromium films is well known to be
strongly affected by proximity effects from neighboring layers. To date the
main attention has been given to effects arising from exchange interactions at
interfaces. In the present work we report on combined neutron and synchrotron
scattering studies of proximity effects in Cr/V films where the boundary
condition is due to the hybridization of Cr with paramagnetic V at the
interface. We find that the V/Cr interface has a strong and long-range effect
on the polarization, period, and the N\'{e}el temperature of the SDW in rather
thick Cr films. This unusually strong effect is unexpected and not predicted by
theory.Comment: 7 figure
Extended Source Diffraction Effects Near Gravitational Lens Fold Caustics
Calculations are presented detailing the gravitational lens diffraction due
to the steep brightness gradient of the limb of a stellar source. The lensing
case studied is the fold caustic crossing. The limb diffraction signal greatly
exceeds that due to the disk as a whole and should be detectable for white
dwarf sources in our Galaxy and it's satellites with existing telescopes.
Detection of this diffraction signal would provide an additional mathematical
constraint, reducing the degeneracy among models of the lensing geometry. The
diffraction pattern provides pico-arcsecond resolution of the limb profile.Comment: 19 pages including 17 figures, Accepted for publication in ApJ, Minor
conceptual change from previous versio
Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer
We have employed Soft and Hard X-ray Resonant Magnetic Scattering and
Polarised Neutron Diffraction to study the magnetic interface and the bulk
antiferromagnetic domain state of the archetypal epitaxial
NiFe(111)/CoO(111) exchange biased bilayer. The combination of
these scattering tools provides unprecedented detailed insights into the still
incomplete understanding of some key manifestations of the exchange bias
effect. We show that the several orders of magnitude difference between the
expected and measured value of exchange bias field is caused by an almost
anisotropic in-plane orientation of antiferromagnetic domains. Irreversible
changes of their configuration lead to a training effect. This is directly seen
as a change in the magnetic half order Bragg peaks after magnetization
reversal. A 30 nm size of antiferromagnetic domains is extracted from the width
the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and
x-ray scattering. A reduced blocking temperature as compared to the measured
antiferromagnetic ordering temperature clearly corresponds to the blocking of
antiferromagnetic domains. Moreover, an excellent correlation between the size
of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio
is found, providing a comprehensive understanding of the origin of exchange
bias in epitaxial systems.Comment: 8 pages, 5 figures, submitte
Development of a conformational histamine H(3) receptor biosensor for the synchronous screening of agonists and inverse agonists
The histamine H(3) receptor (H(3)R) represents a highly attractive drug target for the treatment of various central nervous system disorders, but the discovery of novel H(3)R targeting compounds relies on the assessment of highly amplified intracellular signaling events that do not only reflect H(3)R modulation and carry the risk of high false-positive and -negative screening rates. To address these limitations, we designed an intramolecular H(3)R biosensor based on the principle of bioluminescence resonance energy transfer (BRET) that reports the receptor's real-time conformational dynamics and provides an advanced tool to screen for both H(3)R agonists and inverse agonists in a live cell screening-compatible assay format. This conformational G-protein-coupled receptor (GPCR) sensor allowed us to characterize the pharmacological properties of known and new H(3) receptor ligands with unprecedented accuracy. Interestingly, we found that one newly developed H(3) receptor ligand possesses even stronger inverse agonistic activity than reference H(3)R inverse agonists including the current gold standard pitolisant. Taken together, we describe here the design and validation of the first screening-compatible H(3)R conformational biosensor that will aid in the discovery of novel H(3)R ligands and can be employed to gain deeper insights into the (in-)activation mechanism of this highly attractive drug target
Degeneracy analysis for a super cell of a photonic crystal and its application to the creation of band gaps
A method is introduced to analyze the degeneracy properties of the band
structure of a photonic crystal making use of the super cells. The band
structure associated with a super cell of a photonic crystal has degeneracies
at the edge of the Brillouin zone if the photonic crystal has some kind of
point group symmetry. Both E-polarization and H-polarization cases have the
same degeneracies for a 2-dimensional (2D) photonic crystal. Two theorems are
given and proved. These degeneracies can be lifted to create photonic band gaps
by changing the transform matrix between the super cell and the smallest unit
cell. The existence of the photonic band gaps for many known 2D photonic
crystals is explained through the degeneracy analysis.Comment: 19 pages, revtex4, 14 figures, p
Spin polarization of oxygen atoms in ferromagnetic Co-doped rutile TiO 2
Of central interest in the research of dilute magnetic semiconductors is the coupling mechanism leading to a ferromagnetic ground state. Using x-ray resonant magnetic scattering, we have analyzed the element specific magnetic hysteresis curves of Co, Ti, and oxygen in Co-doped Ti O2 synthesized by ion implantation. Magnetic dichroism was observed at the Co L2,3 edges, as well as at the O K edge, indicative of a spin polarization of oxygen atoms in the Ti O2 host matrix. The hysteretic shapes and the coercive field values measured at the Co L3 and O K edges are identical (1.9 kOe at 30 K). © 2006 American Institute of Physics
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