Ab initio Calculations for SrTiO_3 (100) Surface Structure

Results of detailed calculations for SrTiO_3 (100) surface relaxation and the electronic structure for the two different terminations (SrO and TiO_2) are discussed. These are based on ab initio Hartree-Fock (HF) method with electron correlation corrections and Density Functional Theory (DFT) with different exchange-correlation functionals, including hybrid (B3PW, B3LYP) exchange techniques. Results are compared with previous ab initio plane wave LDA calculations. All methods agree well on both surface energies and on atomic displacements. Considerable increase of Ti[Single Bond]O chemical bond covalency nearby the surface is predicted, along with a gap reduction, especially for the TiO_2 termination

Computer Modeling of Point Defects, Impurity Self-Ordering Effects and Surfaces in Advanced Perovskite Ferroelectrics

The calculated optical properties of basic point defects - F-type centers and hole polarons - in KNbO_3 perovskite crystals are used for the interpretation of available experimental data. The results of quantum chemical calculations for perovskite KNb_xTa_(1-x)O_3 solid solutions are presented for x=0, 0.125, 0.25, 0.75, and 1. An analysis of the optimized atomic and electronic structure clearly demonstrates that several nearest Nb atoms substituting for Ta in KTaO_3 - unlike Ta impurities in KNbO_3 - reveal a self-ordering effect, which probably triggers the ferroelectricity observed in KNb_xTa_(1-x)O_3. Lastly, the (110) surface relaxations are calculated for SrTiO_3 and BaTiO_3 perovskites. The positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized using the classical shell model. Strong surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces

Tendencies in abo3 perovskite and srf2, baf2 and caf2 bulk and surface f‐center ab initio computations at high symmetry cubic structure

This research was partly funded by the Latvian Council of Science project No. LZP‐ 2020/2‐0009 (for R. Eglitis), as well as the ERAF Project No. 1.1.1.1/18/A/073. We express our gratitude for the financial support from Latvian–Ukraine cooperation Project No. Latvia–Ukraine LV‐ UA/2021/5. The Institute of Solid State Physics, University of Latvia (Latvia), as the Centre of Excellence has received funding from the European Unions Horizon 2020 Framework Pro‐ gramme H2020‐WIDESPREAD01‐2016‐2017‐Teaming Phase2 under Grant Agreement No. 739508, project CAMART2.We computed the atomic shift sizes of the closest adjacent atoms adjoining the (001) surface F‐center at ABO3 perovskites. They are significantly larger than the atomic shift sizes of the closest adjacent atoms adjoining the bulk F‐center. In the ABO3 perovskite matrixes, the electron charge is significantly stronger confined in the interior of the bulk oxygen vacancy than in the interior of the (001) surface oxygen vacancy. The formation energy of the oxygen vacancy on the (001) surface is smaller than in the bulk. This microscopic energy distinction stimulates the oxygen vacancy segregation from the perovskite bulk to their (001) surfaces. The (001) surface F‐center created defect level is nearer to the (001) surface conduction band (CB) bottom as the bulk F‐center created defect level. On the contrary, the SrF2, BaF2 and CaF2 bulk and surface F‐center charge is almost perfectly confined to the interior of the fluorine vacancy. The shift sizes of atoms adjoining the bulk and surface F‐centers in SrF2, CaF2 and BaF2 matrixes are microscopic as compared to the case of ABO3 perovskites. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.Latvian Council of Science project No. LZP‐ 2020/2‐0009; ERAF Project No. 1.1.1.1/18/A/073; Latvian–Ukraine cooperation Project No. Latvia–Ukraine LV‐ UA/2021/5. The Institute of Solid State Physics, University of Latvia (Latvia), as the Centre of Excellence has received funding from the European Unions Horizon 2020 Framework Pro‐ gramme H2020‐WIDESPREAD01‐2016‐2017‐Teaming Phase2 under Grant Agreement No. 739508, project CAMART2

On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

International audienceObservations and modelling are presented which illustrate the ability of the Finland CUTLASS HF radar to monitor the afternoon-evening equatorward auroral boundary during weak geomagnetic activity. The subsequent substorm growth phase development was also observed in the late evening sector as a natural continuation of the preceding auroral oval dynamics. Over an 8 h period the CUTLASS Finland radar observed a narrow (in range) and persistent region of auroral F- and (later) E-layer echoes which gradually moved equatorward, consistent with the auroral oval diurnal rotation. This echo region corresponds to the subvisual equatorward edge of the diffuse luminosity belt (SEEL) and the ionospheric footprint of the inner boundary of the electron plasma sheet. The capability of the Finland CUTLASS radar to monitor the E-layer SEEL-echoes is a consequence of the nearly zero E-layer rectilinear aspect angles in a region 5?10° poleward of the radar site. The F-layer echoes are probably the boundary blob echoes. The UHF EISCAT radar was in operation and observed a similar subvisual auroral arc and an F-layer electron density enhancement when it appeared in its antenna beam.Key words: Ionsophere (ionospheric irregularities) · Magnetospheric physics (auroral phenomena; magnetosphere?ionosphere interactions

Ab initio study of magnetism at the TiO2/LaAlO3 interface

In this paper we study the possible relation between the electronic and magnetic structure of the TiO2/LaAlO3 interface and the unexpected magnetism found in undoped TiO2 films grown on LaAlO$_3$. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO3 has a layered structure along the (001) direction with alternating LaO and AlO2 planes, with nominal charges of +1 and -1, respectively. As a consequence of that, an oxygen deficient TiO2 film with anatase structure will grow preferently on the AlO2 surface layer. We have therefore performed ab-initio calculations for superlattices with TiO2/AlO2 interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbour Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the LSDA and LSDA+U approximations.Comment: Accepted for publication in Journal of Materials Scienc