496 research outputs found
Probing unitary two-time correlations in a neutral atom quantum simulator
Measuring unitarily-evolved quantum mechanical two-time correlations is
challenging in general. In a recent paper [P.~Uhrich {\em et al.}, Phys.\
Rev.~A {\bf 96}, 022127 (2017)], a considerable simplification of this task has
been pointed out to occur in spin- lattice models, bringing such
measurements into reach of state-of-the-art or near-future quantum simulators
of such models. Here we discuss the challenges of an experimental
implementation of measurement schemes of two-time correlations in quantum gas
microscopes or microtrap arrays. We propose a modified measurement protocol
that mitigates these challenges, and we rigorously estimate the accuracy of the
protocols by means of Lieb-Robinson bounds. On the basis of these bounds we
identify a parameter regime in which the proposed protocols allow for accurate
measurements of the desired two-time correlations.Comment: 15 pages, 2 figure
Master stability functions reveal diffusion-driven pattern formation in networks
We study diffusion-driven pattern-formation in networks of networks, a class
of multilayer systems, where different layers have the same topology, but
different internal dynamics. Agents are assumed to disperse within a layer by
undergoing random walks, while they can be created or destroyed by reactions
between or within a layer. We show that the stability of homogeneous steady
states can be analyzed with a master stability function approach that reveals a
deep analogy between pattern formation in networks and pattern formation in
continuous space.For illustration we consider a generalized model of ecological
meta-foodwebs. This fairly complex model describes the dispersal of many
different species across a region consisting of a network of individual
habitats while subject to realistic, nonlinear predator-prey interactions. In
this example the method reveals the intricate dependence of the dynamics on the
spatial structure. The ability of the proposed approach to deal with this
fairly complex system highlights it as a promising tool for ecology and other
applications.Comment: 20 pages, 5 figures, to appear in Phys. Rev. E (2018
The influence of dispersal on a predator-prey system with two habitats
Dispersal between different habitats influences the dynamics and stability of
populations considerably. Furthermore, these effects depend on the local
interactions of a population with other species. Here, we perform a general and
comprehensive study of the simplest possible system that includes dispersal and
local interactions, namely a 2-patch 2-species system. We evaluate the impact
of dispersal on stability and on the occurrence of bifurcations, including
pattern forming bifurcations that lead to spatial heterogeneity, in 19
different classes of models with the help of the generalized modelling
approach. We find that dispersal often destabilizes equilibria, but it can
stabilize them if it increases population losses. If dispersal is nonrandom,
i.e. if emigration or immigration rates depend on population densities, the
correlation of stability with migration rates is positive in part of the
models. We also find that many systems show all four types of bifurcations and
that antisynchronous oscillations occur mostly with nonrandom dispersal
Large two-level magnetoresistance effect in doped manganite grain boundary junctions
We performed a systematic analysis of the tunneling magnetoresistance (TMR)
effect in single grain boundary junctions formed in epitaxial
La(2/3)Ca(1/3)MnO(3) films deposited on SrTiO(3) bicrystals. For magnetic
fields H applied parallel to the grain boundary barrier, an ideal two-level
resistance switching behavior with sharp transitions is observed with a TMR
effect of up to 300% at 4.2 K and still above 100% at 77 K. Varying the angle
between H and the grain boundary results in differently shaped resistance vs H
curves. The observed behavior is explained within a model of magnetic domain
pinning at the grain boundary interface.Comment: 4 pages, 3 figures, to appear in Phys. Rev. B (Rapid Comm.
Transport anisotropy in biaxially strained La(2/3)Ca(1/3)MnO(3) thin films
Due to the complex interplay of magnetic, structural, electronic, and orbital
degrees of freedom, biaxial strain is known to play an essential role in the
doped manganites. For coherently strained La(2/3)Ca(1/3)MnO(3) thin films grown
on SrTiO(3) substrates, we measured the magnetotransport properties both
parallel and perpendicular to the substrate and found an anomaly of the
electrical transport properties. Whereas metallic behavior is found within the
plane of biaxial strain, for transport perpendicular to this plane an
insulating behavior and non-linear current-voltage characteristics (IVCs) are
observed. The most natural explanation of this anisotropy is a strain induced
transition from an orbitally disordered ferromagnetic state to an orbitally
ordered state associated with antiferromagnetic stacking of ferromagnetic
manganese oxide planes.Comment: 5 pages, 4 figure
The resolution property of algebraic surfaces
We prove that on separated algebraic surfaces every coherent sheaf is a
quotient of a locally free sheaf. This class contains many schemes that are
neither normal, reduced, quasiprojective or embeddable into toric varieties.
Our methods extend to arbitrary -dimensional schemes that are proper over a
noetherian ring.Comment: 19 page
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