9,556 research outputs found
Extracting predictive models from marked-p free-text documents at the Royal Botanic Gardens, Kew, London
In this paper we explore the combination of text-mining, un-supervised and supervised learning to extract predictive models from a corpus of digitised historical floras. These documents deal with the nomenclature, geographical distribution, ecology and comparative morphology of the species of a region. Here we exploit the fact that portions of text in the floras are marked up as different types of trait and habitat. We infer models from these different texts that can predict different habitat-types based upon the traits of plant species. We also integrate plant taxonomy data in order to assist in the validation of our models. We have shown that by clustering text describing the habitat of different floras we can identify a number of important and distinct habitats that are associated with particular families of species along with statistical significance scores. We have also shown that by using these discovered habitat-types as labels for supervised learning we can predict them based upon a subset of traits, identified using wrapper feature selection
Nonequilibrium quantum phase transition in itinerant electron systems
We study the effect of the voltage bias on the ferromagnetic phase transition
in a one-dimensional itinerant electron system. The applied voltage drives the
system into a nonequilibrium steady state with a non-zero electric current. The
bias changes the universality class of the second order ferromagnetic
transition. While the equilibrium transition belongs to the universality class
of the uniaxial ferroelectric, we find the mean-field behavior near the
nonequilibrium critical point.Comment: Final version as accepted to Phys. Rev. Let
Electrical Manipulation of Nanomagnets
We demonstrate a possibility to manipulate the magnetic coupling between two
nanomagnets with a help of ac electric field. In the scheme suggested the
magnetic coupling in question is mediated by a magnetic particle contacting
with both of the nanomagnets through the tunnel barriers. The electric field
providing a successive suppression of the barriers leads to pumping of
magnetization through the mediating particle. Time dependent dynamics of the
particle magnetization allows to to switch between ferro- and antiferromagnetic
couplings.Comment: 4 pages, 2 figure
Destruction of bulk ordering by surface randomness
We demonstrate that the arbitrarily weak quenched disorder on the surface of
a system of continuous symmetry destroys long range order in the bulk, and,
instead, quasi-long range order emerges. Correlation functions are calculated
exactly for the two- and three-dimensional XY model with surface randomness via
the functional renormalization group. Even at strong quenched disorder the
three-dimensional XY model possesses topological order. We also determine
roughness of a domain wall in the presence of surface disorder.Comment: 4 pages Revtex; Eq. (12) correcte
Long-range order in the A-like phase of superfluid 3He in aerogel
A mutual action of the random anisotropy brought in the superfluid 3He by
aerogel and of the global anisotropy caused by its deformation is considered.
Strong global anisotropy tends to suppress fluctuations of orientation of the
order parameter and stabilizes ABM order parameter. In a limit of vanishing
anisotropy these fluctuations are getting critical. It is argued that still in
a region of small fluctuations the average order parameter can acquire "robust"
component. This component maintains a long-range order even in a limit of
vanishing global anisotropy.Comment: A contribution to QFS 2007 in Kazan, revised for publication in the
Proceeding
Rectification in Luttinger liquids
We investigate the rectification of an ac bias in Luttinger liquids in the
presence of an asymmetric potential (the ratchet effect). We show that strong
repulsive electron interaction enhances the ratchet current in comparison with
Fermi liquid systems, and the I-V curve is strongly asymmetric in the
low-voltage regime even for a weak asymmetric potential. At higher voltages the
ratchet current exhibits an oscillatory voltage dependence.Comment: 5 pages, Revte
Quasi-long range order in glass states of impure liquid crystals, magnets, and superconductors
In this review we consider glass states of several disordered systems:
vortices in impure superconductors, amorphous magnets, and nematic liquid
crystals in random porous media. All these systems can be described by the
random-field or random-anisotropy O(N) model. Even arbitrarily weak disorder
destroys long range order in the O(N) model. We demonstrate that at weak
disorder and low temperatures quasi-long range order emerges. In
quasi-long-range-ordered phases the correlation length is infinite and
correlation functions obey power dependencies on the distance. In pure systems
quasi-long range order is possible only in the lower critical dimension and
only in the case of Abelian symmetry. In the presence of disorder this type of
ordering turns out to be more common. It exists in a range of dimensions and is
not prohibited by non-Abelian symmetries.Comment: 32 page
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