arXiv subjects

S. Kais

Publications and source records attributed to S. Kais.

6 recordsLinked to original sources

arXiv2021

Direct Scheme Calculation of the Kinetic Energy Functional Derivative Using Machine Learning

We report a direct scheme calculation of kinetic energy functional derivative using Machine Learning. Support Vector Regression and Kernel Ridge Regression techniques were independently employed to estimate the kinetic energy functional and its derivative. Even though the accuracy should have been a decisive factor in modeling a realistic functional, we show that at a certain level it affects the generalizability of the model. By choosing the right regularization term and by considering a reasonable interplay between it and the accuracy, we were able to deduce the functional derivative from a model that was trained to estimate the kinetic energy. Although the derivative calculations demand very high accuracy to account for small variations of the kinetic energy, the developed estimator was capable of capturing these extremely small changes of the electron density. This work pours into highly effective implementation of the orbital-free density functional theory as it employs only direct calculation scheme

physics.comp-ph↗

arXiv2015

Convergent sum of gradient expansion of the kinetic-energy density functional up to the sixth order term using Pade approximant

The gradient expansion of the kinetic energy functional, when applied for atoms or finite systems, usually grossly overestimates the energy in the fourth order and generally diverges in the sixth order. We avoid the divergence of the integral by replacing the asymptotic series including the sixth order term in the integrand by a rational function. Pade approximants show moderate improvements in accuracy in comparison with partial sums of the series. The results are discussed for atoms and Hooke law model for two electron atoms.

quant-ph↗

arXiv2013

Coupled plasmon - phonon excitations in extrinsic monolayer graphene

The existence of an acoustic plasmon in extrinsic (doped or gated) monolayer graphene was found recently in an {\it ab initio} calculation with the frozen lattice [M. Pisarra {\it et al.}, arXiv:1306.6273, 2013]. By the {\em fully dynamic} density-functional perturbation theory approach, we demonstrate a strong coupling of the acoustic plasmonic mode to lattice vibrations. Thereby, the acoustic plasmon in graphene does not exist as an isolated excitation, but it is rather bound into a combined plasmon-phonon mode. We show that the coupling provides a mechanism for the {\em bidirectional} energy exchange between the electronic and the ionic subsystems with fundamentally, as well as practically, important implications for the lattice cooling and heating by electrons in graphene.

cond-mat.mes-hall↗

arXiv2013

The interference effect of laser-assisted bremsstrahlung emission in Coulumb fields of two nuclei

In this paper, the spontaneous bremsstrahlung emission from an electron scattered by two fixed nuclei in an intense laser field is investigated in details based upon the Volkov state and the Dirac-Volkov propagator. It has been found that the fundamental harmonic spectrum from the electron radiation exhibits distinctive fringes, which is dependent not only upon the internucleus distance and orientation, but also upon the initial energy of the electron and the laser intensity. By analyzing the differential cross section, we are able to explain these effects in terms of interference among the electron scattering by the nuclei. These results could have promising applications in probing the atomic or molecular dressed potentials in intense laser fields.

physics.optics↗

arXiv2013

Optics of semiconductors and insulators: Role of local-field effects revised

We show that by nullifying the short-wave response to the long-wave excitation (local-field-effects), the adiabatic time-dependent density-functional theory (TDDFT) of optics of semiconductors and insulators can be brought into excellent agreement with experiment. This indicates that the wing elements [($\Gv,\0v)$ and $(\0v,\Gv)$, $\Gv\ne\0v$] of both the Kohn-Sham (KS) density-response function $χ^s$ and the exchange-correlation kernel $f^{xc}$ are greatly overestimated by the existing approximations to the static DFT and TDDFT, respectively, to the extent that zero is a better approximation for them than the corresponding values provided by current theories. The head element of $f^{xc}$ is thereby fixed by the static macroscopic dielectric constant $ε_M$. Our method yields accurate optical spectra including both the weakly and strongly bound excitons, while its computational cost is extremely low, since only the head element of the KS response matrix and the static dielectric constant are needed.

cond-mat.mtrl-sci↗

arXiv2009

A Venn diagram for supersymmetric, exactly solvable, shape invariant, and Infeld-Hull factorizable potentials

Supersymmetry, shape invariance, exact solubility, and the factorization method are often studied together in the literature. At the dawn of these topics confusion was present in regards to their scope of applicability and the relation among them. Considerable work have been put to study and resolve the relation among two or more of these topics. These works are scattered over the literature. While looking at the literature, one can not overlook the number of places where authors confuse these terms, and concluding implications depending on wrong assumptions of the relation between two or more of these topics. In this letter we define supersymmetry, and shape invariance, and show the relations which connects them to exact solubility and the factorization method, referring to the literature for the respective detailed work and proofs. At last we conclude our letter with a Venn diagram which illustrates those relations.

math-ph↗