Modeling of complex oxide materials from the first principles: systematic applications to vanadates RVO3 with distorted perovskite structure

"Realistic modeling" is a new direction of electronic structure calculations, where the main emphasis is made on the construction of some effective low-energy model entirely within a first-principle framework. Ideally, it is a model in form, but with all the parameters derived rigorously, on the basis of first-principles electronic structure calculations. The method is especially suit for transition-metal oxides and other strongly correlated systems, whose electronic and magnetic properties are predetermined by the behavior of some limited number of states located near the Fermi level. After reviewing general ideas of realistic modeling, we will illustrate abilities of this approach on the wide series of vanadates RVO3 (R= La, Ce, Pr, Nd, Sm, Gd, Tb, Yb, and Y) with distorted perovskite structure. Particular attention will be paid to computational tools, which can be used for microscopic analysis of different spin and orbital states in the partially filled t2g-band. We will explicitly show how the lifting of the orbital degeneracy by the monoclinic distortion stabilizes C-type antiferromagnetic (AFM) state, which can be further transformed to the G-type AFM state by changing the crystal distortion from monoclinic to orthorhombic one. Two microscopic mechanisms of such a stabilization, associated with the one-electron crystal field and electron correlation interactions, are discussed. The flexibility of the orbital degrees of freedom is analyzed in terms of the magnetic-state dependence of interatomic magnetic interactions.Comment: 23 pages, 13 figure

Electric Field Effect in ESR Spectra of Fe³⁺ and Mn⁴⁺ Centers in Thin SrTiO<inf>3</inf> Plates

Abstract: The electric field effect in electron spin resonance spectra of Fe3+ and Mn4+ impurity ions in thin oriented single-crystal plates of strontium titanate has been studied; the crystal structure of these samples at T > 105 K is tetragonal (i.e., differs from both cubic and antiferrodistortive tetragonal structure that is inherent in SrTiO3 at T < 105 K). It is shown that the electric field effects for the investigated centers are quadratic with respect to the applied field and have identical signs and orders of magnitude, thus indicating a nonpolar character of the observed high-temperature tetragonal state. An analysis of the dependences of the axiality value on the applied field indicates that the electric field effect is not related to electrostriction and caused by modification of the wave function of the ground state of impurity centers

Critical phenomena and femtosecond ordering dynamics associated with electronic and spin-ordered phases in YVO3 and GdVO3

We present a systematic study of the electronic- and spin-excitation dynamics associated with the sequence of spin- and orbital-ordering phase transformations in the complex vanadates YVO3 and GdVO3 with ultrafast optical pump-probe reflectance spectroscopy. Relaxation dynamics occurs on a different time scale for each of the ordering transitions, which enables us to unambiguously associate the critical behavior in the dynamics with the observed ordering phenomena. Spin-ordering dynamics is observed on two time scales: τr≃2 ps (rise time) and τs≃300–3000 ps (decay time), observed below TN in both compounds. In contrast, the relaxation dynamics associated with orbital ordering occurs in YVO3 on a time scale of τo≃20–50 ps. From the temperature dependence of the dynamics, we observe that in both the G-type and C-type orbitally ordered (OO) phases of YVO3 spin order develops in a second-order mean-field fashion with the Nèel temperature of the C-OO phase found from our data TNC=83±7 K. In GdVO3 we identify the emergence of a new ordered phase within the phase-separated state below 60 K. A new response component with a lifetime of ~60 ps is observed below 60 K together with other anomalies in the ΔR/R(Δt) data. This new phase is not resolved in x-ray diffraction and is not present in YVO3.

Synthesis of Thin Niobium Films on Silicon and Study of Their Superconducting Properties in the Dimensional Crossover Region

Abstract: Niobium films with a thickness of 4–100 nm are synthesized on a silicon substrate under ultrahigh vacuum conditions. Measurements of the electrical resistance show a high superconducting transition temperature Tc in the range of 4.7–9.1 K and record-breaking small transition widths ΔTc in the range of 260–11 m. The dependences of Tc and ΔTc on the magnetic field are investigated, and the superconducting coherence lengths and mean free paths of conduction electrons for different thicknesses of the synthesized films are determined. A significant influence of the magnetic field on ΔTc is found, which reveals the transition from three- to two-dimensional superconductivity at thicknesses below 10 nm. The dependences of Tc and ΔTc on the thickness of the films and the magnitude of the magnetic field are discussed within the framework of existing theories of superconductivity in thin films of superconducting metals

Iron-implanted epitaxial palladium thin films: Structure, ferromagnetism and signatures of spinodal decomposition

We report on the formation of the dilute Pd1−xFex composites with tunable magnetic properties under ion-beam implantation of epitaxial Pd thin films. Binary Pd1−xFex alloys with a mean iron content x of 0.025, 0.035, and 0.075 were obtained by the implantation of 40 keV Fe+ ions into the palladium films on MgO (0 0 1) substrate to the doses of 0.5 × 1016, 1.0 × 1016 and 3.0 × 1016 ions/cm2, respectively. Structural and magnetic studies have shown that iron atoms occupy regular fcc-lattice Pd-sites without the formation of any secondary crystallographic phase. All the iron implanted Pd films reveal ferromagnetism at low temperatures. The observed multiple ferromagnetic resonances in the implanted Pd1−xFex films indicate a formation of a magnetically inhomogeneous state due to spinodal decomposition into regions, presumably layers, with identical crystal symmetry but different iron contents. The multiphase composite magnetic structure is robust with respect to the vacuum annealing at 770 K, though develops towards well-defined local Pd/Fe ratios

Structural and hydrogen absorption/desorption properties of Zr<inf>2</inf>(Co<inf>0.5</inf>Fe<inf>0.2</inf>Ni<inf>0.2</inf>V<inf>0.1</inf>) intermetallic alloy

The Zr2(Co0.5Fe0.2Ni0.2V0.1) intermetallic alloy was prepared by arc melting method and its physical properties and hydrogen storage performance were investigated. XRD analysis by Rietveld refinement and Mössbauer spectroscopy measurements revealed a multiphase crystal structure, in which the dominant phases are C16 Laves phase. The hydrogen-induced cracks, chemical composition, and elements distribution were analyzed by SEM and EDX mapping. The optimum conditions for the activation process, hydrogen absorption kinetics, cyclic durability performance, and disproportionation behavior were discussed. Zr2(Co0.5Fe0.2Ni0.2V0.1) alloy has a high absorption kinetic rate, with an activation energy of 10.11 kJ/mol, high hydrogen capacity (2.1 wt%) at room temperature, and good anti-disproportionation behavior (hydrogen pressure up to 8 bar and temperature below 783K). The hydrogen capacity decreased in the first three cycles (~25%) and then remained almost stable, which is likely caused by the high stability of hydride. The studied hydride has interesting features that can be considered for practical applications

Epitaxial growth and superconducting properties of thin-film PdFe/VN and VN/PdFe bilayers on MgO(001) substrates

© 2020 Mohammed et al. Single-layer vanadium nitride (VN) and bilayer Pd0.96Fe0.04/VN and VN/Pd0.92Fe0.08 thin-film heterostructures for possible spintronics applications were synthesized on (001)-oriented single-crystalline magnesium oxide (MgO) substrates utilizing a fourchamber ultrahigh vacuum deposition and analysis system. The VN layers were reactively magnetron sputtered from a metallic vanadium target in Ar/N2 plasma, while the Pd1-xFex layers were deposited by co-evaporation of metallic Pd and Fe pellets from calibrated effusion cells in a molecular beam epitaxy chamber. The VN stoichiometry and Pd1-xFex composition were controlled by X-ray photoelectron spectroscopy. In situ low-energy electron diffraction and ex situ X-ray diffraction show that the 30 nm thick single-layer VN as well as the double-layer VN(30 nm)/Pd0.92Fe0.08(12 nm) and Pd0.96Fe0.04(20 nm)/VN(30 nm) structures have grown cube-on-cube epitaxially. Electric resistance measurements demonstrate a metallic-type temperature dependence for the VN film with a small residual resistivity of 9 μΩ·cm at 10 K, indicating high purity and structural quality of the film. The transition to the superconducting state was observed at 7.7 K for the VN film, at 7.2 K for the Pd0.96Fe0.04/VN structure and at 6.1 K for the VN/Pd0.92Fe0.08 structure with the critical temperature decreasing due to the proximity effect. Contrary to expectations, all transitions were very sharp with the width ranging from 25 mK for the VN film to 50 mK for the VN/Pd0.92Fe0.08 structure. We propose epitaxial single-crystalline thin films of VN and heteroepitaxial Pd1-xFex/VN and VN/Pd1-xFex (x ≤ 0.08) structures grown on MgO(001) as the materials of a choice for the improvement of superconducting magnetic random access memory characteristics

Experimental evidences of the shape-induced structural distortion of SrTiO3 single crystals from impurity Mn4+ ions electron paramagnetic resonance

A study of high-quality SrTiO3 single crystals doped with the Mn4+ ions in the cubic phase (T > 105 K) with X-band electron paramagnetic resonance reveals direct correspondence between a shape of a sample and magnetic anisotropy of the impurity Mn4+ centers. In particular, for a sample with the shape of a square base rectangular prism, a size of (a × a × h) and faces perpendicular to the crystallographic directions, zero-field splitting parameter D is approximately proportional to (a / h − 1) quantity. Temperature dependence of D indicates that this peculiar symmetry lowering is a feature characteristic for the cubic Fm3m phase of the strontium titanate. Diminishing of the D value with the decrease of the surface roughness for a thin (001)-oriented SrTiO3:Mn platelet shows that the observed effect originates from the sample surface

ELECTRICAL PROPERTIES OF TITANIUM NITRIDE FILMS SYNTHESIZED BY REACTIVE MAGNETRON SPUTTERING

259-259A use of the four-probe resistance measurements as a tool for characterization of a quality of titanium nitride thin films deposited by the reactive dc magnetron sputtering will be discussed in the report. Few series of ~ 50 nm thick films on various substrates as fused silica, monocrystalline silicon and magnesium oxide have been deposited with several degrees of freedom (substrate temperature, magnetron chamber atmosphere and working pressure etc.) varied in a wide range. Electrical resistivity correlation with the films properties will be reported

Student online services consumption: Routine practices or mistrust to digital service?

© 2020, Anadolu University, Faculty of Communication Sciences. All rights reserved. The article aims to study the practices of students’ use of services in the conditions of digitalization. The leading methods for the study of this problem are the methods of questioning and testing, allowing to make a qualitative analysis of student consumption practices in the conditions of digitization of services. These methods allowed revealing the peculiarities of online consumption practices by students. The authors found that the majority of students who are not actively using online services, to some extent had experience using online services, but almost every tenth of them had negative result. It is established that students are distrustful of new methods to obtain services, especially when it comes to any operations related to money. It is shown that students who do not practice online services have a feeling that the provided online services are of poor quality, and informants are ready to experience some discomfort in exchange for the usual, guaranteed and high quality, in their opinion, service. In addition, students who are not actively using online services note that it is not easy for them to adapt to new technologically advanced things, but the rest of the students believe that they can adapt to innovations, but the way of providing services that is now perceived as traditional one is the most acceptable for them. It is established that the leader in the use of online services is a taxi. This service is used by all students to some extent, followed by the service of food delivery and only then online banking, online shopping and public services. The novelty and originality of the study lies in the fact that the features of consumption practices among students actively practicing online services and avoiding them are revealed. It is established that students who have little recourse to online services are wary of modern services, which is based either on the previous negative experience of use, or on the unsatisfactory experience of friends, believe that those techniques and applications that are now on the market are not optimal for them and the usual methods of obtaining services for them remain more comfortable and familiar, even though more time spent on obtaining a particular service. The practical significance lies in the fact that the data obtained in the work can be used in social psychology, age psychology, labor psychology, management, sociology, as well as for further theoretical development of this issue