2,140 research outputs found
Traveling waves and Compactons in Phase Oscillator Lattices
We study waves in a chain of dispersively coupled phase oscillators. Two
approaches -- a quasi-continuous approximation and an iterative numerical
solution of the lattice equation -- allow us to characterize different types of
traveling waves: compactons, kovatons, solitary waves with exponential tails as
well as a novel type of semi-compact waves that are compact from one side.
Stability of these waves is studied using numerical simulations of the initial
value problem.Comment: 22 pages, 25 figure
Graph Kernels
We present a unified framework to study graph kernels, special cases of which include the random
walk (Gärtner et al., 2003; Borgwardt et al., 2005) and marginalized (Kashima et al., 2003, 2004;
Mahé et al., 2004) graph kernels. Through reduction to a Sylvester equation we improve the time
complexity of kernel computation between unlabeled graphs with n vertices from O(n^6) to O(n^3).
We find a spectral decomposition approach even more efficient when computing entire kernel matrices.
For labeled graphs we develop conjugate gradient and fixed-point methods that take O(dn^3)
time per iteration, where d is the size of the label set. By extending the necessary linear algebra to
Reproducing Kernel Hilbert Spaces (RKHS) we obtain the same result for d-dimensional edge kernels,
and O(n^4) in the infinite-dimensional case; on sparse graphs these algorithms only take O(n^2)
time per iteration in all cases. Experiments on graphs from bioinformatics and other application
domains show that these techniques can speed up computation of the kernel by an order of magnitude
or more. We also show that certain rational kernels (Cortes et al., 2002, 2003, 2004) when
specialized to graphs reduce to our random walk graph kernel. Finally, we relate our framework to
R-convolution kernels (Haussler, 1999) and provide a kernel that is close to the optimal assignment
kernel of Fröhlich et al. (2006) yet provably positive semi-definite
Theoretical evidence for unexpected O-rich phases at corners of MgO surfaces
Realistic oxide materials are often semiconductors, in particular at elevated
temperatures, and their surfaces contain undercoordiated atoms at structural
defects such as steps and corners. Using hybrid density-functional theory and
ab initio atomistic thermodynamics, we investigate the interplay of
bond-making, bond-breaking, and charge-carrier trapping at the corner defects
at the (100) surface of a p-doped MgO in thermodynamic equilibrium with an O2
atmosphere. We show that by manipulating the coordination of surface atoms one
can drastically change and even reverse the order of stability of reduced
versus oxidized surface sites.Comment: 5 papges, 4 figure
Quantum Monte Carlo study for multiorbital systems with preserved spin and orbital rotational symmetries
We propose to combine the Trotter decomposition and a series expansion of the
partition function for Hund's exchange coupling in a quantum Monte Carlo (QMC)
algorithm for multiorbital systems that preserves spin and orbital rotational
symmetries. This enables us to treat the Hund's (spin-flip and pair-hopping)
terms, which is difficult in the conventional QMC method. To demonstrate this,
we first apply the algorithm to study ferromagnetism in the two-orbital Hubbard
model within the dynamical mean-field theory (DMFT). The result reveals that
the preservation of the SU(2) symmetry in Hund's exchange is important, where
the Curie temperature is grossly overestimated when the symmetry is degraded,
as is often done, to Ising (Z). We then calculate the spectral
functions of SrRuO by a three-band DMFT calculation with tight-binding
parameters taken from the local density approximation with proper rotational
symmetry.Comment: 9 pages, 9 figures. Typos corrected, some comments and references
adde
Magneto-optic Kerr effect in a spin-polarized zero-moment ferrimagnet
The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to
the magnetisation of a magnetically ordered metallic sample; in metallic
ferrimagnets with chemically distinct sublattices, such as rare-earth
transition-metal alloys, it depends on the difference between the sublattice
contributions. Here we show that in a highly spin polarized, fully compensated
ferrimagnet, where the sublattices are chemically similar, MOKE is observed
even when the net moment is strictly zero. We analyse the spectral ellipsometry
and MOKE of Mn 2 Ru x Ga, and show that this behaviour is due to a highly
spin-polarized conduction band dominated by one of the two manganese
sublattices which creates helicity-dependent reflectivity determined by a broad
Drude tail. Our findings open new prospects for studying spin dynamics in the
infra-red.Comment: 7 pages, 7 figure
Dynamics of Natural Killer cell receptor revealed by quantitative analysis of photoswitchable protein
Natural Killer (NK) cell activation is dynamically regulated by numerous
activating and inhibitory surface receptors that accumulate at the immune
synapse. Quantitative analysis of receptor dynamics has been limited by
methodologies which rely on indirect measurements such as fluorescence recovery
after photobleaching. Here, we report a novel approach to study how proteins
traffic to and from the immune synapse using NK cell receptors tagged with the
photoswitchable fluorescent protein tdEosFP, which can be irreversibly
photoswitched from a green to red fluorescent state by ultraviolet light. Thus,
following a localized switching event, the movement of the photoswitched
molecules can be temporally and spatially resolved by monitoring fluorescence
in two regions of interest. By comparing images with mathematical models, we
evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 +- 0.06
micron^2/s) and assessed how synapse formation affects receptor dynamics. Our
data conclude that the inhibitory NK cell receptor KIR2DL1 is continually
trafficked into the synapse and remains surprisingly stable there. Unexpectedly
however, in NK cells forming synapses with multiple target cells
simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus,
our results reveal a previously undetected inter-synaptic exchange of protein.Comment: 25 pages, 5 figure
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