2,742 research outputs found
Nonequilibrium charge density wave ordering from anomalous velocity in itinerant helical magnets
The Karplus-Luttinger anomalous velocity is shown to lead to
electric-field-induced charge accumulation in nearly ferromagnetic noncollinear
magnets with itinerant electrons, like . For helical magnetic ordering,
the balance between this accumulation and the Coulomb interaction leads to a
nonequilibrium charge density wave state with the period of the helix, even
when such accumulation is forbidden by an approximate gauge-like symmetry in
the absence of electric field. We compute the strength of such charge
accumulation as an example of how unexpected many-electron physics is induced
by the inclusion of the one-electron Karplus-Luttinger term whenever the local
exchange field felt by conduction electrons does not satisfy the current-free
Maxwell equations.Comment: 5 pages and 1 figur
Raman Characterization Studies Of Synthetic And Natural Mgal 2o4 Crystals
Raman studies are reported for one natural and four synthetic MgAl 2O4 spinel crystals and symmetry assignments for the phonon modes of the spinel crystal structure are given. Deviations in the Raman selection rules are observed for the synthetic spinel crystals in the form of additional modes over and above those theoretically predicted. Also, detailed studies of one of the synthetic crystals show a probable disordering in the Mg-Al sites. Raman linewidths and violations of selection rules, as reported here, can aid in the characterization and quality control of synthetically grown crystals.5893585359
Linear PDEs and numerical methods that preserve a multi-symplectic conservation law
Multisymplectic methods have recently been proposed as a generalization of symplectic ODE methods to the case of Hamiltonian PDEs. Their excellent long time behavior for a variety of Hamiltonian wave equations has been demonstrated in a number of numerical studies. A theoretical investigation and justification of multisymplectic methods is still largely missing. In this paper, we study linear multisymplectic PDEs and their discretization by means of numerical dispersion relations. It is found that multisymplectic methods in the sense of Bridges and Reich Phys. Lett. A, 284 (2001), pp. 184-193] and Reich J. Comput. Phys., 157 (2000), pp. 473-499], such as Gauss-Legendre Runge-Kutta methods, possess a number of desirable properties such as nonexistence of spurious roots and conservation of the sign of the group velocity. A certain CFL-type restriction on might be required for methods higher than second order in time. It is also demonstrated by means of the explicit midpoint method that multistep methods may exhibit spurious roots in the numerical dispersion relation for any value of despite being multisymplectic in the sense of discrete variational mechanics [J. E. Marsden, G. P. Patrick, and S. Shkoller, Commun. Math. Phys., 199 (1999), pp. 351-395]
Critical points in edge tunneling between generic FQH states
A general description of weak and strong tunneling fixed points is developed
in the chiral-Luttinger-liquid model of quantum Hall edge states. Tunneling
fixed points are a subset of `termination' fixed points, which describe
boundary conditions on a multicomponent edge. The requirement of unitary time
evolution at the boundary gives a nontrivial consistency condition for possible
low-energy boundary conditions. The effect of interactions and random hopping
on fixed points is studied through a perturbative RG approach which generalizes
the Giamarchi-Schulz RG for disordered Luttinger liquids to broken left-right
symmetry and multiple modes. The allowed termination points of a multicomponent
edge are classified by a B-matrix with rational matrix elements. We apply our
approach to a number of examples, such as tunneling between a quantum Hall edge
and a superconductor and tunneling between two quantum Hall edges in the
presence of interactions. Interactions are shown to induce a continuous
renormalization of effective tunneling charge for the integrable case of
tunneling between two Laughlin states. The correlation functions of
electronlike operators across a junction are found from the B matrix using a
simple image-charge description, along with the induced lattice of boundary
operators. Many of the results obtained are also relevant to ordinary Luttinger
liquids.Comment: 23 pages, 6 figures. Xiao-Gang Wen: http://dao.mit.edu/~we
ACCURACY ASSESSMENT OF A UAV-BASED LANDSLIDE MONITORING SYSTEM
Landslides are hazardous events with often disastrous consequences. Monitoring landslides with observations of high spatio-temporal resolution can help mitigate such hazards. Mini unmanned aerial vehicles (UAVs) complemented by structure-from-motion (SfM) photogrammetry and modern per-pixel image matching algorithms can deliver a time-series of landslide elevation models in an automated and inexpensive way. This research investigates the potential of a mini UAV, equipped with a Panasonic Lumix DMC-LX5 compact camera, to provide surface deformations at acceptable levels of accuracy for landslide assessment. The study adopts a self-calibrating bundle adjustment-SfM pipeline using ground control points (GCPs). It evaluates misalignment biases and unresolved systematic errors that are transferred through the SfM process into the derived elevation models. To cross-validate the research outputs, results are compared to benchmark observations obtained by standard surveying techniques. The data is collected with 6 cm ground sample distance (GSD) and is shown to achieve planimetric and vertical accuracy of a few centimetres at independent check points (ICPs). The co-registration error of the generated elevation models is also examined in areas of stable terrain. Through this error assessment, the study estimates that the vertical sensitivity to real terrain change of the tested landslide is equal to 9 cm
Serum antioxidants as predictors of the adult respiratory distress syndrome in septic patients
Adult respiratory distress syndrome (ARDS) can develop as a complication of various disorders, including sepsis, but it has not been possible to identify which of the patients at risk will develop this serious disorder. We have investigated the ability of six markers, measured sequentially in blood, to predict development of ARDS in 26 patients with sepsis.
At the initial diagnosis of sepsis (6-24 h before the development of ARDS), serum manganese superoxide dismutase concentration and catalase activity were higher in the 6 patients who subsequently developed ARDS than in 20 patients who did not develop ARDS. These changes in antioxidant enzymes predicted the development of ARDS in septic patients with the same sensitivity, specificity, and efficiency as simultaneous assessments of serum lactate dehydrogenase activity and factor VIII concentration. By contrast, serum glutathione peroxidase activity and α1Pi-elastase complex concentration did not differ at the initial diagnosis of sepsis between patients who did and did not subsequently develop ARDS, and were not as effective in predicting the development of ARDS.
Measurement of manganese superoxide dismutase and catalase, in addition to the other markers, should facilitate identification of patients at highest risk of ARDS and allow prospective treatment
Universal neural field computation
Turing machines and G\"odel numbers are important pillars of the theory of
computation. Thus, any computational architecture needs to show how it could
relate to Turing machines and how stable implementations of Turing computation
are possible. In this chapter, we implement universal Turing computation in a
neural field environment. To this end, we employ the canonical symbologram
representation of a Turing machine obtained from a G\"odel encoding of its
symbolic repertoire and generalized shifts. The resulting nonlinear dynamical
automaton (NDA) is a piecewise affine-linear map acting on the unit square that
is partitioned into rectangular domains. Instead of looking at point dynamics
in phase space, we then consider functional dynamics of probability
distributions functions (p.d.f.s) over phase space. This is generally described
by a Frobenius-Perron integral transformation that can be regarded as a neural
field equation over the unit square as feature space of a dynamic field theory
(DFT). Solving the Frobenius-Perron equation yields that uniform p.d.f.s with
rectangular support are mapped onto uniform p.d.f.s with rectangular support,
again. We call the resulting representation \emph{dynamic field automaton}.Comment: 21 pages; 6 figures. arXiv admin note: text overlap with
arXiv:1204.546
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