630 research outputs found
Giant nonlinear conduction and thyristor-like negative derivative resistance in BaIrO3 single crystals
We synthesized single-crystalline samples of monoclinic BaIrO3 using a molten
flux method, and measured their magnetization, resistivity, Seebeck coefficient
and nonlinear voltage-current characteristics. The magnetization rapidly
increases below a ferromagnetic transition temperature TC of 180 K, where the
resistivity concomitantly shows a hump-type anomaly, followed by a sharp
increase below 30 K. The Seebeck coefficient suddenly increases below TC, and
shows linear temperature dependence below 50 K. A most striking feature of this
compound is that the anomalously giant nonlinear conduction is observed below
30 K, where a small current density of 20 A/cm2 dramatically suppresses the
sharp increase in resistivity to induce a metallic conduction down to 4 K.Comment: 10 pages, 4 figures Submitted to Physical Review Letter
Advanced Technology Training System on Motor-Operated Valves
This paper describes how features from the field of Intelligent Tutoring Systems are applied to the Motor-Operated Valve (MOV) Advanced Technology Training System (ATTS). The MOV ATTS is a training system developed at Galaxy Scientific Corporation for the Central Research Institute of Electric Power Industry in Japan and the Electric Power Research Institute in the United States. The MOV ATTS combines traditional computer-based training approaches with system simulation, integrated expert systems, and student and expert modeling. The primary goal of the MOV ATTS is to reduce human errors that occur during MOV overhaul and repair. The MOV ATTS addresses this goal by providing basic operational information of the MOV, simulating MOV operation, providing troubleshooting practice of MOV failures, and tailoring this training to the needs of each individual student. The MOV ATTS integrates multiple expert models (functional and procedural) to provide advice and feedback to students. The integration also provides expert model validation support to developers. Student modeling is supported by two separate student models: one model registers and updates the student's current knowledge of basic MOV information, while another model logs the student's actions and errors during troubleshooting exercises. These two models are used to provide tailored feedback to the student during the MOV course
Geometrical frustration induced (semi-)metal to insulator transition
We study the low-energy properties of the geometrically frustrated Hubbard
model on a three-dimensional pyrochlore lattice and a two-dimensional
checkerboard lattice on the basis of the renormalization group method and mean
field analysis. It is found that in the half-filling case, a (semi-)metal to
insulator transition (MIT) occurs. Also, in the insulating phase, which has a
spin gap, the spin rotational symmetry is not broken, while charge ordering
exists. The results are applied to the description of the MIT observed in the
pyrochlore system .Comment: 4 pages, 5 figure
Hemodynamic Assessment of Celiaco-mesenteric Anastomosis in Patients with Pancreaticoduodenal Artery Aneurysm Concomitant with Celiac Artery Occlusion using Flow-sensitive Four-dimensional Magnetic Resonance Imaging
ObjectivesMany pancreaticoduodenal artery (PDA) aneurysms are associated with celiac artery (CA) stenosis. The pathogenesis of PDA aneurysm may be associated with hemodynamic changes due to CA stenosis/occlusion. The aim of this study was to assess the hemodynamic changes of celiaco-mesenteric anastomosis in patients with PDA aneurysms concomitant with CA occlusion using four-dimensional flow-sensitive magnetic resonance imaging (4D-Flow).Methods4D-Flow was performed preoperatively on five patients. Seven age- and sex-matched individuals were used as controls. Hemodynamic parameters such as flow volume and maximum flow velocity in PDAs, gastroduodenal arteries, common hepatic arteries, and superior mesenteric arteries were compared between both groups. Wall shear stress (WSS) and oscillatory shear index (OSI) were mapped in both groups.ResultsIn the patient group, 4D-Flow identified retrograde flow of both gastroduodenal arteries and common hepatic arteries. Heterogeneous distribution patterns of both WSS and OSI were identified across the entire PDA in the patient group. OSI mapping showed multiple regions with extremely high OSI values (OSI > 0.3) in all patients. All PDA aneurysms, which were surgically resected, were atherosclerotic.Conclusions4D-Flow identified hemodynamic changes in celiaco-mesenteric arteries in patients with PDA aneurysms with concomitant CA occlusion. These hemodynamic changes may be associated with PDA aneurysm formation
Hole-doping effects on a frustrated spin ladder
Hole-doping effects are investigated on the {\it t-J} ladder model with the
linked-tetrahedra structure. We discuss how a metal-insulator transition occurs
upon hole doping with particular emphasis on the effects of geometrical
frustration. By computing the electron density and the spin correlation
function by the density matrix renormalization group, we show that strong
frustration triggers a first-order transition to a metallic phase, when holes
are doped into the plaquette-singlet phase. By examining spin excitations in a
metallic case in detail, we discuss whether the spin-gap phase persists upon
hole doping according to the strength of frustration. It is further shown that
the lowest excited state in a spin-gap metallic phase can be described in two
independent quasiparticles.Comment: 7 pages, 9 figure
Charge ordering in the spinels AlVO and LiVO
We develop a microscopic theory for the charge ordering (CO) transitions in
the spinels AlVO and LiVO (under pressure). The high degeneracy
of CO states is lifted by a coupling to the rhombohedral lattice deformations
which favors transition to a CO state with inequivalent V(1) and V(2) sites
forming Kagom\'e and trigonal planes respectively. We construct an extended
Hubbard type model including a deformation potential which is treated in
unrestricted Hartree Fock approximation and describes correctly the observed
first-order CO transition. We also discuss the influence of associated orbital
order. Furthermore we suggest that due to different band fillings AlVO
should remain metallic while LiVO under pressure should become a
semiconductor when charge disproportionation sets in
Stability of a metallic state in the two-orbital Hubbard model
Electron correlations in the two-orbital Hubbard model at half-filling are
investigated by combining dynamical mean field theory with the exact
diagonalization method. We systematically study how the interplay of the intra-
and inter-band Coulomb interactions, together with the Hund coupling, affects
the metal-insulator transition. It is found that if the intra- and inter-band
Coulomb interactions are nearly equal, the Fermi-liquid state is stabilized due
to orbital fluctuations up to fairly large interactions, while the system is
immediately driven to the Mott insulating phase away from this condition. The
effects of the isotropic and anisotropic Hund coupling are also addressed.Comment: 7 pages, 9 figure
Classical generalized constant coupling model for geometrically frustrated antiferromagnets
A generalized constant coupling approximation for classical geometrically
frustrated antiferromagnets is presented. Starting from a frustrated unit we
introduce the interactions with the surrounding units in terms of an internal
effective field which is fixed by a self consistency condition. Results for the
magnetic susceptibility and specific heat are compared with Monte Carlo data
for the classical Heisenberg model for the pyrochlore and kagome lattices. The
predictions for the susceptibility are found to be essentially exact, and the
corresponding predictions for the specific heat are found to be in very good
agreement with the Monte Carlo results.Comment: 4 pages, 3 figures, 2 columns. Discussion about the zero T value of
the pyrochlore specific heat correcte
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