Lattice Green's function for crystals containing a planar interface

Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions. The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a $(10\bar{1}2)$ twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by \textit{ab initio} methods

Convergence rate for numerical computation of the lattice Green's function

Flexible boundary condition methods couple an isolated defect to bulk through the bulk lattice Green's function. The inversion of the force-constant matrix for the lattice Green's function requires Fourier techniques to project out the singular subspace, corresponding to uniform displacements and forces for the infinite lattice. Three different techniques--relative displacement, elastic Green's function, and discontinuity correction--have different computational complexity for a specified numerical error. We calculate the convergence rates for elastically isotropic and anisotropic cases and compare them to analytic results. Our results confirm that the discontinuity correction is the most computationally efficient method to compute the lattice Green's function.Comment: 12 pages, 4 figure

A New Mechanism for Twin Growth in Mg Alloys

Twinning is an important deformation mode in lightweight Mg alloys, but the mechanisms of nucleation and growth of twins and their interactions with solutes remain largely unresolved. Here, a new model for thermally-activated, stress-driven growth of twin boundaries is presented and the role of random and segregated solutes in controlling this growth is studied analytically and using direct molecular dynamics simulations. Twin growth occurs by the thermally-activated nucleation and expansion of twin dislocation loops on a pre-existing twin boundary. Fluctuations in the local concentration of random solutes lowers the energy barrier for this process and thus facilitates twin growth. Segregation of solutes to the twin boundary strongly increases the energy barrier and suppresses twin growth. In random solid solutions at experimental strain rates, the thermally-activated nucleation process is sufficiently fast at low applied stresses so that growth of the twin is controlled by solute strengthening of the nucleated twin loop as it expands across the twin boundary. Annealing of a deformed sample leads to solute segregation and very strong pinning so that further twinning can only be accomplished by nucleation of new twins, consistent with experimental observations. The new mechanism also (i) operates for detwinning and rationalizes complex twin shapes observed in experiments and (ii) suggests a process for dynamic strain aging commonly observed in Mg alloys at elevated temperatures

A productivity dashboard for hospitals: an empirical study

Health information systems are key assets in managing health units’ daily operations. Nevertheless, literature is scarce concerning information systems for increasing and managing hospital productivity. This study aims at filling such gap through an empirical research based on large Portuguese hospital. Specifically, a dashboard prototype is proposed addressing productivity indicators in areas such as assistance, hospitalization, surgery, among others. This dashboard is tuned using a design science research approach where health experts successively validate the prototype. Interviews are conducted to assess the benefits of using our proposal to manage productivity on a daily basis.info:eu-repo/semantics/acceptedVersio

Analysis of dissociation of < c > and < c+a > dislocations to nucleate (1 0 -1 2) twins in Mg

A mechanism for (1 0 (1) over bar 2) twin nucleation in Mg is studied in which edge and mixed lattice dislocations dissociate into a stable twin, having at least the minimum 6-layer thickness formed by three glissile twinning dislocations, plus a residual stair rod dislocation. Continuum dislocation theory is used to compute the energy of the initial and final states of the proposed dissociation process, using the twin boundary energy computed by density functional theory. For the dislocation, the proposed dissociation is energetically favorable. An alternative dissociation path into partials on two {1 0 (1) over bar 1}-type pyramidal planes is possible, as seen in an atomistic analysis, and the continuum analysis predicts this alternative path to be more favorable than the twin process. For the dislocation, the continuum model also predicts that dissociation into the twinned structure is energetically favorable for 6-layer and thicker twins. In both and cases, the equilibrium twin length is predicted to increase with increasing applied resolved shear stress and grow unstably beyond a critical stress. Atomistic simulations of these processes are then performed. For , a twinned structure is stable under zero loading but with higher energy than the alternative dissociation on two {1 0 (1) over bar 1} planes. Under a positive applied strain of 4%, resolved on the twin plane, the twinning structure grows while under a negative applied strain of -3%, it reverts back to the alternative low-energy dissociated configuration on the pyramidal planes. For the mixed dislocation, the atomistic models predict that the dissociation into twinning dislocations does not occur spontaneously at zero applied strain but there is a stable twinned region at finite applied loads. These results demonstrate that dislocation-assisted mechanisms for twinning in Mg, initiating from lattice dislocations with large Burgers vectors, are physically feasible, and therefore twin nucleation from grain boundaries is not necessarily the dominant mechanism of twinning in Mg

A New Fast and Blind Cross-Polarization Modulation Digital Compensator

We propose a novel blind feed-forward compensator of cross-polarization modulation for PDM-QPSK signals. Simulations show an improvement of 1 dB in Q-factor in a 112Gbit/s 2000km SMF dispersion-managed link

Statistical Characterization of Bit Patterning in SOAs: BER Prediction and Experimental Validation

We present a novel simulation tool for optical systems employing in-line nonlinear SOAs, able to correctly estimate the bit error rate even in the presence of significant SOA-induced intersymbol interference

DQPSK: when is a narrow filter receiver is good enough?

In this paper, we investigate experimentally and via simulation the pros and cons of a narrow filter receiver for differential quadrature phase-shift keying based on a single optical filter and eschewing the conventional asymmetrical Mach–Zehnder interferometer structure. We quantify the performance differences between the two receivers, allowing system designers and operators to determine when the less complex narrow filter receiver might be the appropriate choice.We numerically optimize the 3-dB bandwidth and center frequency of the narrow filter and show it is more robust to carrier frequency detuning than the conventional solution. We show that the narrow filter receiver is more tolerant to chromatic dispersion (CD) than the conventional one, and equally tolerant to first-order polarization-mode dispersion. We show the impact of the 3-dB bandwidth on the receiver performance when CD accumulates. Finally, we show via experiments and simulations that the 3 dB advantage of the conventional receiver vanishes when the nonlinear impairments are fiber nonlinearities; comparing the two receivers at the optimum launch power for a 25x80 km system, the difference in optical SNR margin is reduced to 1.6 dB. Experiments are conducted at 42 Gb/s using a commercially available narrow filter for reception