2,361 research outputs found
Phase diagrams of a classical two-dimensional Heisenberg antiferromagnet with single-ion anisotropy
A classical variant of the two-dimensional anisotropic Heisenberg model
reproducing inelastic neutron scattering experiments on La_5 Ca_9 Cu_24 O_41
[M. Matsuda et al., Phys.Rev. B 68, 060406(R) (2003)] is analysed using mostly
Monte Carlo techniques. Phase diagrams with external fields parallel and
perpendicular to the easy axis of the anisotropic interactions are determined,
including antiferromagnetic and spin-flop phases. Mobile spinless defects, or
holes, are found to form stripes which bunch, debunch and break up at a phase
transition. A parallel field can lead to a spin-flop phase.Comment: 9 pages, 9 figures; final version as accepted by Phys. Rev. B (Fig. 5
replaced, added remarks in Secs. I, III, and V
Nanostructuring lithium niobate substrates by focused ion beam milling
We report on two novel ways for patterning Lithium Niobate (LN) at
submicronic scale by means of focused ion beam (FIB) bombardment. The first
method consists of direct FIB milling on LiNbO3 and the second one is a
combination of FIB milling on a deposited metallic layer and subsequent RIE
(Reactive Ion Etching) etching. FIB images show in both cases homogeneous
structures with well reproduced periodicity. These methods open the way to the
fabrication of photonic crystals on LiNbO3 substrates
Photoevaporation of the Jovian circumplanetary disk. I. Explaining the orbit of Callisto and the lack of outer regular satellites
Context: The Galilean satellites are thought to have formed from a
circumplanetary disk (CPD) surrounding Jupiter. When it reached a critical
mass, Jupiter opened an annular gap in the solar protoplanetary disk (PPD) that
might have exposed the CPD to radiation from the young Sun or from the stellar
cluster in which the Solar System formed. Aims: We investigate the radiation
field to which the Jovian CPD was exposed during the process of satellite
formation. The resulting photoevaporation of the CPD is studied in this context
to constrain possible formation scenarios for the Galilean satellites and
explain architectural features of the Galilean system. Methods: We constructed
a model for the stellar birth cluster to determine the intracluster
far-ultraviolet (FUV) radiation field. We employed analytical annular gap
profiles informed by hydrodynamical simulations to investigate a range of
plausible geometries for the Jovian gap. We used the radiation thermochemical
code ProDiMo to evaluate the incident radiation field in the Jovian gap and the
photoevaporation of an embedded 2D axisymmetric CPD. Results: We derive the
time-dependent intracluster FUV radiation field for the solar birth cluster
over 10 Myr. We find that intracluster photoevaporation can cause significant
truncation of the Jovian CPD. We determine steady-state truncation radii for
possible CPDs, finding that the outer radius is proportional to the accretion
rate . For CPD accretion rates
yr, photoevaporative truncation explains the lack of additional
satellites outside the orbit of Callisto. For CPDs of mass , photoevaporation can disperse the disk before Callisto
is able to migrate into the Laplace resonance. This explains why Callisto is
the only massive satellite that is excluded from the resonance.Comment: 17 pages, 13 figures, accepted by A&
Spin multistability of cavity polaritons in a magnetic field
Spin transitions are studied theoretically and experimentally in a resonantly
excited system of cavity polaritons in a magnetic field. Weak pair interactions
in this boson system make possible fast and massive spin flips occurring at
critical amplitudes due to the interplay between amplitude dependent shifts of
eigenstates and the Zeeman splitting. Dominant spin of a condensate can be
toggled forth and back by tuning of the pump intensity only, which opens the
way for ultra-fast spin switchings of polariton condensates on a picosecond
timescale.Comment: 4 pages, 4 figure
When the brain takes 'BOLD' steps: Real-time fMRI neurofeedback can further enhance the ability to gradually self-regulate regional brain activation
Brain-computer interfaces (BCIs) based on real-time functional magnetic resonance imaging (rtfMRI) are currently explored in the context of developing alternative (motor-independent) communication and control means for the severely disabled. In such BCI systems, the user encodes a particular intention (e.g., an answer to a question or an intended action) by evoking specific mental activity resulting in a distinct brain state that can be decoded from fMRI activation. One goal in this context is to increase the degrees of freedom in encoding different intentions, i.e., to allow the BCI user to choose from as many options as possible. Recently, the ability to voluntarily modulate spatial and/or temporal blood oxygenation level-dependent (BOLD)-signal features has been explored implementing different mental tasks and/or different encoding time intervals, respectively. Our two-session fMRI feasibility study systematically investigated for the first time the possibility of using magnitudinal BOLD-signal features for intention encoding. Particularly, in our novel paradigm, participants (n=10) were asked to alternately self-regulate their regional brain-activation level to 30%, 60% or 90% of their maximal capacity by applying a selected activation strategy (i.e., performing a mental task, e.g., inner speech) and modulation strategies (e.g., using different speech rates) suggested by the experimenters. In a second step, we tested the hypothesis that the additional availability of feedback information on the current BOLD-signal level within a region of interest improves the gradual-self regulation performance. Therefore, participants were provided with neurofeedback in one of the two fMRI sessions. Our results show that the majority of the participants were able to gradually self-regulate regional brain activation to at least two different target levels even in the absence of neurofeedback. When provided with continuous feedback on their current BOLD-signal level, most participants further enhanced their gradual self-regulation ability. Our findings were observed across a wide variety of mental tasks and across clinical MR field strengths (i.e., at 1.5T and 3T), indicating that these findings are robust and can be generalized across mental tasks and scanner types. The suggested novel parametric activation paradigm enriches the spectrum of current rtfMRI-neurofeedback and BCI methodology and has considerable potential for fundamental and clinical neuroscience applications
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