Simple Model Approximation Research Articles

Collective acoustic electronic excitations in LaH10 as a factor in boosting of the critical temperature of superconducting transition

The nearly room-temperature superconductivity that had been predicted theoretically for lanthanum and yttrium superhydrides at megabar pressures has been recently achieved experimentally in several superhydride compounds, including lanthanum decahydride with T _{c} of about 250 K under high pressure of about 150 GPa. Though superconductivity should be governed by the phonon mechanism in these compounds, which is evident due to the measured deuterium isotope effect in LaD10, we believe that the choice of small values of the effective Coulomb constant, used in theoretical calculations of the critical temperature, merits farther substantiation. We discuss the possibility for the collective acoustic electronic excitations (acoustic plasmons) to appear in the collective spectra of superhydrides thus facilitating the suppression of the Coulomb repulsion. In LaH10 the conditions for such mechanism arise due to the hybridization of La 4f and H 1s states near the Fermi level in the vicinity of the L-point of the Brillouin zone. A simple model approximation for the resulting conducting band allows us to show that in a certain portion of quasimomentum space an acoustic branch should appear in the spectrum of the collective electronic excitations in LaH10, arguably reducing the effective Coulomb constant.

Keep It Simple: Data-Efficient Learning for Controlling Complex Systems With Simple Models

When manipulating a novel object with complex dynamics, a state representation is not always available, for example, deformable objects. Learning both a representation and dynamics from observations requires large amounts of data. We propose Learned Visual Similarity Predictive Control (LVSPC), a novel method for data-efficient learning to control systems with complex dynamics and high-dimensional state spaces from images. LVSPC leverages a given simple model approximation from which image observations can be generated. We use these images to train a perception model that estimates the simple model state from observations of the complex system online. We then use data from the complex system to fit the parameters of the simple model and learn where this model is inaccurate, also online. Finally, we use Model Predictive Control and bias the controller away from regions where the simple model is inaccurate and thus where the controller is less reliable. We evaluate LVSPC on two tasks; manipulating a tethered mass and a rope. We find that our method performs comparably to state-of-the-art reinforcement learning methods with an order of magnitude less data . LVSPC also completes the rope manipulation task on a real robot with 80% success rate after only 10 trials, despite using a perception system trained only on images from simulation.

Informed participation in a randomised controlled trial of computed tomography screening for lung cancer

The actual lung cancer (screening) knowledge, attitudes, risk perceptions, reasons to participate in or decline participation, and informed decisions of subjects who decided to or decided not to participate in the Dutch-Belgian randomised controlled trial for lung cancer screening in high-risk subjects (the NELSON trial) were evaluated. A total of 2,500 high-risk subjects were asked to complete a questionnaire 3 weeks after they had received a brochure with information about the trial. Differences in knowledge, attitude and risk perception between participants and nonparticipants were analysed with logistic regression analyses adjusted for sex and smoking status. The questionnaire response of trial participants was 80% (n = 889) whereas the response of nonparticipants was low (7%, n = 97) and selective. Participants' responses to knowledge items on lung cancer as a disease were on average more often correct (mean+/-sd 68+/-17%) than items on lung cancer screening (49+/-29%). Participants had adequate knowledge on lung cancer screening (51%) more often than the nonparticipants (38%; p = 0.009). Of the decisions regarding participation, 49% were uninformed, mainly due to insufficient knowledge. Most of the participants (99%) and 64% of the nonparticipants had a positive attitude towards lung cancer screening. Additional efforts are required to improve the knowledge and understanding of subjects who are in the process of decision-making regarding participation in a lung cancer screening trial.

Sensitivity-based scaling for approximating structural response

This article presents a sensitivity-based linearly varying scale factor used to reconcile results from simple and refined models for analysis of the same structure. The improved accuracy of the linear scale factor compared to a constant scale factor, as well as the commonly used tangent approximation, is demonstrated. A wing-box structure is used as an example, with displacements, stresses, and frequencies correlated. The linear scale factor could permit the use of a simplified model in an optimization procedure during preliminary design to approximate the response given by a refined model over a considerable range of design changes. HE design optimization of an engineering system typi- cally requires hundreds of analyses of that system. The use of approximation s to the objective function and con- straints during portions of the design process is quite com- mon,1 because of the high computational cost of these detailed analyses. Such design approximations can be divided into two classes. First there are local, derivative-based approximations such as the linear approximation based on a Taylor-series expansion about a design point. These approximations are typically based on an accurate model to obtain the system response and its derivatives. Second, there are global ap- proximations that try to capture the behavior of the objective function or constraints over the entire design domain. Such approximations can be based on a response surface which is constructed on the basis of many analyses.2 However, global approximations are often based on a simplified theory, a coarser model, or both.3 Such global approximations are referred to here as simple-model approximations. Local approximations are typically very accurate near the design point where they are generated, but since they are based on an extrapolation procedure their accuracy can deteriorate catastrophically at a distance. Simple-model approximations are intended to cap- ture the physics of the problem at some lower, but acceptable level of accuracy over the entire design domain. Therefore, at a particular design point the simple-model approximations are generally less accurate than local approximations, but on the other hand, they typically do not experience the cata-

Theory of Transport Processes in Nearly Ferromagnetic Fermi Liquids

A theory of the low-temperature transport processes of thermal conduction and spin diffusion in Fermi liquids near the ferromagnetic state is developed on the assumption that the predominant particle scattering at low temperatures occurs from persistent spin fluctuations, or "paramagnons," of the type discussed recently by Doniach and Engelsberg and by Berk and Schrieffer. A Boltzmann equation for this type of scattering is set up and solved by the usual variational procedure to obtain expressions for the coefficients of thermal conductivity K and spin-diffusion $D$. The latter are evaluated by using a simple model approximation for the paramagnon spectral density function ${A}_{\mathrm{q}}(\ensuremath{\omega})$, based on the correct RPA result at long wavelengths. It is found that in the limit $T\ensuremath{\rightarrow}0$, $K$ varies as ${T}^{\ensuremath{-}1}$ and $D$ as ${T}^{\ensuremath{-}2}$, in accordance with the predictions of the Landau theory. As $T$ increases from absolute zero, however, $K$ and $D$ fall off increasingly less rapidly than ${T}^{\ensuremath{-}1}$ and ${T}^{\ensuremath{-}2}$, respectively, and actually increase with increasing $T$ after passing through minima. The theory can therefore account for the observed temperature dependence of the thermal conductivity and spin diffuison of liquid ${\mathrm{He}}^{3}$. In particular, the theoretical predictions are in close agreement with the low-pressure thermal-conductivity and diffusion data obtained by the Illinois group.