1,388 research outputs found
Chiral mixed phase in disordered 3d Heisenberg models
Using Monte Carlo simulations, we compute the spin stiffness of a site-random
3d Heisenberg model with competing ferromagnetic and antiferromagnetic
interactions. Our results for the pure limit yield values of the the critical
temperature and the critical exponent in excellent agreement with
previous high precision studies. In the disordered case, a mixed "chiral" phase
is found which may be in the same universality class as 3d Heisenberg spin
glasses.Comment: 5 pages, 4 figures, accepted in PRB Rapid Communication
Robust adaptive tracking control for highly dynamic nanoprecision motion systems
This abstract focuses on the design and real-time implementation of advanced control strategies for motion systems with highly dynamic nanopositioning capabilities [1]. The key exemplar is the lifting and actuating unit (LAU), which integrates a pneumatic actuator for weight force compensation and a parallel electromagnetic drive to produce precision motion forces. Initial investigations cover the modeling and parametric identification of the overactuated nature of a single LAU [2]. This lifting module, integrated into a test bench, renders a 1D vertical motion system aimed to perform subnanometer positioning tasks while minimizing heat emission. To this end, we propose a control allocation strategy to assign (zero-mean) high-dynamic forces to the electromagnetic channel, producing a very low heat emission while the performance is fulfilled using an LQ-type controller plus an L1 adaptive augmentation [3]. This investigation closes with RMS positioning errors less than 0.25 nm and electrical currents less than 0.30 mA. Further investigations involve a 3D tilt-and-lift vertical motion system integrating three LAUs, each placed in each corner of a triangular payload. The key challenge of this configuration is to cope with the high cross-couplings between the degrees of freedom (DOF), i.e., vertical and rotational motion. The core of the decoupling task is the nominal LQG-type controller comprising disturbance-rejection-based observers aimed to fully compensate cross-couplings, while the L1 adaptive augmentation recovers the nominal performance in the presence of parametric uncertainties w.r.t. the input gain [4]. Given that the heat emission problem is fully solved for a single LAU (see [2] and [3]), we then focus on the performance and robustness of the 3D closed-loop system. Since full-state information of the cross-couplings is not simple to reconstruct, we adopt the output-feedback control architecture for the nominal controller and L1 adaptive augmentation [4]. The effectiveness of the proposed control strategy is verified via real-time experimentation rendering vertical RMS positioning errors of less than 0.25 nm and RMS rotational errors of less than 0.04 μrad while satisfying the heat emission constraint. The investigations conclude by exploring the outstanding performance/robustness trade-off of the L1 adaptive control theory for a nanometer planar positioning system with a travel range of ø200 mm (i.e., NPPS200) and the subsequent integration with the 3D vertical motion system, thereby transitioning to a full 6D system (i.e., NPPS200-6D) with 25 mmvertical stroke. Within this framework, the complexity of the controller design is higher because of the number of DOF, cross-couplings, external disturbances, and parametric perturbations. We completed our investigations through experimental validation with planar and vertical RMS positioning errors of less than 0.80 nm and RMS rotational errors of less than 0.05 μrad, as shown in Figure 1
2,2,2-Tris(pyrazol-1-yl)ethanol
The title compound TPE, C11H12N6O, was prepared by slow evaporation from diethyl ether. In the crystal, there is a hydrogen bond between the alcohol H atom and an N in the pyrazole ring of a neighboring molÂecule
Phase glass and zero-temperature phase transition in a randomly frustrated two-dimensional quantum rotor model
The ground state of the quantum rotor model in two dimensions with random
phase frustration is investigated. Extensive Monte Carlo simulations are
performed on the corresponding (2+1)-dimensional classical model under the
entropic sampling scheme. For weak quantum fluctuation, the system is found to
be in a phase glass phase characterized by a finite compressibility and a
finite value for the Edwards-Anderson order parameter, signifying long-ranged
phase rigidity in both spatial and imaginary time directions. Scaling
properties of the model near the transition to the gapped, Mott insulator state
with vanishing compressibility are analyzed. At the quantum critical point, the
dynamic exponent is greater than one. Correlation
length exponents in the spatial and imaginary time directions are given by
and , respectively, both assume values
greater than 0.6723 of the pure case. We speculate that the phase glass phase
is superconducting rather than metallic in the zero current limit.Comment: 14 pages, 4 figures, to appear in JSTA
Spin Gap in Two-Dimensional Heisenberg Model for CaVO
We investigate the mechanism of spin gap formation in a two-dimensional model
relevant to Mott insulators such as CaVO. From the perturbation
expansion and quantum Monte Carlo calculations, the origin of the spin gap is
ascribed to the four-site plaquette singlet in contrast to the dimer gap
established in the generalized dimerized Heisenberg model.Comment: 8 pages, 6 figures available upon request (Revtex
Modification of an Implant Material
Titanium metal is a commonly used implant material which can be colonized by bacteria. Biofilms are formed when bacteria colonizes, attaches to a surface, and immobilizes. Bacterial infections or biofilms are hard to treat once formed on the surface of a metal implant. Coating the material may minimize bacteria attachment. Self-assembled monolayers are comprised of molecules that contain both a head and tail group, and can be used to coat metal to prevent biofilm formation and bacteria growth. 12-mercaptododecylphosphonic acid self-assembled monolayers have a phosphonic acid head group and a thiol tail group. A solution of 12-mercaptododecylphosphonic acid in tetrahydrofuran was used to form self-assembled monolayers on the surface of titanium. Diffuse reflectance infrared Fourier transform spectroscopy was used to evaluate the attachment of the self-assembled monolayers. Alkyl-chain ordered self-assembled monolayers formed on the titanium surface, with the thiol tail group free at the interface for additional reactions. The monolayer attachment strength was further tested through acid, base, and tape tests
Temperature behavior of the magnon modes of the square lattice antiferromagnet
A spin-wave theory of short-range order in the square lattice Heisenberg
antiferromagnet is formulated. With growing temperature from T=0 a gapless mode
is shown to arise simultaneously with opening a gap in the conventional
spin-wave mode. The spectral intensity is redistributed from the latter mode to
the former. For low temperatures the theory reproduces results of the modified
spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting
parameters gives values of observables in good agreement with Monte Carlo
results in the temperature range 0 <= T < 0.8J where J is the exchange
constant.Comment: 12 pages, 2 figure
Cross-understanding will help complex and diverse teams achieve mutually agreeable solutions
Teams whose members have diverse backgrounds can experience differences in task knowledge, sensitivities to various aspects of the task system, as well as beliefs and preferences about how to best approach or solve a problem. How might managers deal with this? Niranjan Janardhanan, Kyle Lewis, Rhonda R. Reger, and Cynthia K. Stevens write that, rather than focusing on common ground, team leaders should emphasise cross-understanding. Understanding the bases of someone’s views will help get to the real reasons behind differences in opinion, and therefore help to achieve mutually agreeable solutions
Disordered phase of a two-dimensional Heisenberg Model with S=1
We study an anisotropic version of the model with S=1. We find a
second order transition from a N\'eel phase to a disordered phase
with a spin gap.Comment: 4 pages, 5 figure
- …