Temperature dependent correlations in covalent insulators

Motivated by the peculiar behavior of FeSi and FeSb2 we study the effect of local electronic correlations on magnetic, transport and optical properties in a specific type of band insulator, namely a covalent insulator. Investigating a minimum model of covalent insulator within a single-site dynamical mean-field approximation we are able to obtain the crossover from low temperature non-magnetic insulator to high-temperature paramagnetic metal with parameters realistic for FeSi and FeSb2 systems. Our results show that the behavior of FeSi does not imply microscopic description in terms of Kondo insulator (periodic Anderson model) as can be often found in the literature, but in fact reflects generic properties of a broader class of materials.Comment: 4 pages, 4 figure

LDA+DMFT Spectral Functions and Effective Electron Mass Enhancement in Superconductor LaFePO

In this Letter we report the first LDA+DMFT results (method combining Local Density Approximation with Dynamical Mean-Field Theory) for spectral properties of superconductor LaFePO. Calculated {\bf k}-resolved spectral functions reproduce recent angle-resolved photoemission spectroscopy (ARPES) data [D. H. Lu {\it et al}., Nature {\bf 455}, 81 (2008)]. Obtained effective electron mass enhancement values $m^{*}/m\approx$ 1.9 -- 2.2 are in good agreement with infrared and optical studies [M. M. Qazilbash {\it et al}., Nature Phys. {\bf 5}, 647 (2009)], de Haas--van Alphen, electrical resistivity, and electronic specific heat measurements results, that unambiguously evidence for moderate correlations strength in LaFePO. Similar values of $m^{*}/m$ were found in the other Fe-based superconductors with substantially different superconducting transition temperatures. Thus, the dynamical correlation effects are essential in the Fe-based superconductors, but the strength of electronic correlations does not determine the value of superconducting transition temperature.Comment: 4 pages, 3 figure

Specific features of the electrical resistance of half-metallic ferromagnetic alloys Co2CrAl and Co2CrGa

It has been shown by comparing the results of studying the electrical and magnetic properties of the half-metallic ferromagnetic Heusler alloys Co2CrAl and Co2CrGa with the calculations of their electronic structure that high values of the electrical resistivity ρ are caused by a disordered distribution of atoms over the sites of the L21 cubic structure, and the anomalous behavior of ρ(T) is associated with the transformation of the electronic spectrum due to the ferromagnetic-to-paramagnetic transition. © 2013 Pleiades Publishing, Ltd

NiO: Correlated Bandstructure of a Charge-Transfer Insulator

The bandstructure of the prototypical charge-transfer insulator NiO is computed by using a combination of an {\it ab initio} bandstructure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni-d and O-p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This solves a long-standing problem in solid state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.Comment: 4 pages, 3 figur

On approximations of time-delay control systems

The paper deals with a dynamical system described by delay differential equations which is controlled under conditions of unknown disturbances. This system is approximated by a system of ordinary differential equations which is used as a leader. An aiming procedure between the delay and approximating systems is elaborated. The procedure is organized in such a way that the necessary proximity of motions of these systems is guaranteed by means of the delay system control and a certain part of the approximating system controls. Another part of the approximating system controls can be used to compensate disturbances in the initial delay system and to ensure the required quality of the control process. Thus, by using the elaborated aiming procedure, results of control theory for ordinary differential systems can be applied to delay differential systems. © 2015, IFAC Hosting by Elsevier Ltd.Russian Science Foundation, RSF15-11-1001

Optical spectroscopy and electronic structure of the GdCux compounds (x = 1, 2, 5)

Optical properties of the GdCu, GdCu2, and GdCu5 compounds with various crystal lattices have been investigated using ellipsometry in a spectral range of 0. 22-16 μm. Theoretical calculations of the electronic structure of these intermetallic compounds have been performed in the approximation of local electron density with the correction for strong electron correlations in a 4f shell of gadolinium ions. Based on the results of calculations, the interpretation of experimental spectra of interband optical conductivity has been proposed. The plasma and relaxation frequencies of conduction electrons have been determined. © 2013 Pleiades Publishing, Ltd

On guarantee optimization under conditions of integral constraints on control actions and nonterminal quality index

A linear-convex problem of dynamical guarantee optimization under conditions of unknown disturbances and with positional quality index that estimates a set of deviations of the motion of the controlled system at given instants of time from given target points is considered. Control actions are bounded by both geometrical and integral constraints. Disturbance actions are only geometrically bounded. A procedure for approximate computing of the optimal guaranteed result and of the corresponding optimal closed-loop control law is elaborated. The method is based on recurrent constructions of upper convex hulls of auxiliary program functions. Results of numerical experiments on model examples are given. © 2015, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved

Construction of Solutions to Control Problems for Fractional- Order Linear Systems Based on Approximation Models

We consider an optimal control problem for a dynamical system whose motion is described by a linear differential equation with the Caputo fractional derivative of order α ∈ (0, 1). The time interval of the control process is fixed and finite. The control actions are subject to geometric constraints. The aim of the control is to minimize a given terminal-integral quality index. In order to construct a solution, we develop the following approach. First, from the considered problem, we turn to an auxiliary optimal control problem for a first-order linear system with lumped delays, which approximates the original system. After that, the auxiliary problem is reduced to an optimal control problem for an ordinary differential system. Based on this, we propose a closed-loop scheme of optimal control of the original system that uses the approximating system as a guide. In this scheme, the control in the approximating system is formed with the help of an optimal positional control strategy from the reduced problem. The effectiveness of the developed approach is illustrated by a problem in which the quality index is the norm of the terminal state of the system. © 2020 Krasovskii Institute of Mathematics and Mechanics. All rights reserved.This work was supported by RSF (project no. 19-11-00105)

Traversing Target Points Under Lack of Information: A Game-Theoretical Approach

A dynamical object controlled under conditions of unknown disturbances or counteractions is considered. The goal is to bring the object to given target points at given times regardless of the disturbance actions. To solve this problem, the quality index is introduced that evaluates the distance between the object and target points at the indicated times, and a zero-sum differential game is considered in which the control actions minimize this quality index while the disturbance actions maximize it. The initial problem is reduced to calculating the game value and constructing a saddle point of the game. The corresponding resolving procedure is proposed that is based on the upper convex hulls method. An example is considered. Results of numerical simulations are presented. © 2018Russian Academy of SciencesThis work is supported by the Program of the Presidium of the Russian Academy of Sciences No. 1 `Fundamental Mathematics and its Applications’ under grant PRAS-18-01

Organometallic Compounds of the Lanthanides 182 [1]. Calcium and Neodymium Complexes Containing the dpp-BIAN Ligand System: Synthesis and Molecular Structure of [(dpp-BIAN)CaI(THF)2]2 and [(dpp-BIAN)NdCl(THF)2]2

Oxydation of (dpp-BIAN)Ca(THF)4 with 0.5 equiv. of I2 in THF yields [(dpp-BIAN)CaI(THF)2]2 (1). A corresponding neodymium compound [(dpp-BIAN)NdCl(THF)2]2 (2) has been obtained by reaction of (dpp-BIAN)Na2 with NdCl3 in THF. The X-ray single crystal structure analyses show 1 and 2 to be isostructural dimers containing octahedrally coordinated metal atoms bridged by the respective halides. The chelating dpp-BIAN ligand acts as a radical anion in the Ca2+ complex 1 and as a dianion in the Nd3+ complex 2, respectively.DFG, SPP 1166, Lanthanoidspezifische Funktionalitäten in Molekül und Materia