177 research outputs found
One-center charge transfer transitions in manganites
Get PDFIn the framework of a rather conventional cluster approach which combines the crystal field and the ligand field models we have considered different charge transfer (CT) states and O (formula presented)-Mn (formula presented) CT transitions in (formula presented) octahedra. The many-electron dipole transition matrix elements were calculated using the Racah algebra for the cubic point group. Simple “local” approximation allowed us to calculate the relative intensity for all dipole-allowed (formula presented) and (formula presented) CT transitions. We present a self-consistent description of the CT bands in insulating stoichiometric (formula presented) compound with the only (formula presented) valent state and idealized octahedral (formula presented) centers which allows us to substantially correct the current interpretation of the optical spectra. Our analysis shows the multiband structure of the CT optical response with the weak low-energy edge at 1.7 eV, associated with forbidden (formula presented) transition and a series of the weak and strong dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV, respectively, and extending up to nearly 11 eV. The most intensive features are associated with two strong composite bands near 4.6-4.7 and 8-9 eV, respectively, resulting from the superposition of the dipole-allowed (formula presented) and (formula presented) CT transitions. These predictions are in good agreement with experimental spectra. The experimental data points to a strong overscreening of the crystal field parameter (formula presented) in the CT states of (formula presented) centers. © 2002 The American Physical Society.Discussions with N.N. Loshkareva, Yu.P. Sukhorukov, E.A. Ganshina, V.S. Vikhnin, R. Hayn, and S.-L. Drechsler are acknowledged. The research described in this publication was supported in part by the Ministry of Science and Art of Saxony. The author would like to thank the Institut fur Festkorper- und Werkstofforschung, Dresden, where part of this work was made, for hospitality. The author acknowledges partial support from Grant No. REC-005 of the U.S. Civilian Research and Development Foundation for the Independent States of the Former Soviet Union (CRDF), Russian Ministry of Education, Grant No. E00-3.4-280 and No. UR.01.01.042 and the Russian Foundation for Basic Researches, Grant No. 01-02-96404
Topological Phase Separation in a Two-dimensional Quantum Lattice Bose-Hubbard System Away from Half-filling
No full textWe suppose that the doping of the two-dimensional (2D) hard-core (hc) boson system away from half-filling may result in the formation of a multicenter topological inhomogeneity (defect), such as charge order (CO) bubble domain(s) with Bose superfluid (BS) and extra bosons both localized in domain wall(s), or else in a topological CO + BS phase separation rather than a uniform mixed CO + BS supersolid phase. Starting from the classical model, we predict the properties of the respective quantum system. The long-wavelength behavior of the system is believed to be reminiscent of that of granular superconductors, charge-density wave materials, Wigner crystals, and multi-Skyrmion systems akin to the quantum Hall ferromagnetic state of a 2D electron gas. To elucidate the role played by quantum effects and that of the lattice discreteness, we have addressed the simplest nanoscopic counterpart of the bubble domain in the checkerboard CO phase of a 2D hc Bose-Hubbard (BH) square lattice. It is shown that the relative magnitude and symmetry of a multicomponent order parameter are mainly determined by the sign of the nearest-neighbor and next-nearest-neighbor transfer integrals. In general, the topologically inhomogeneous phase of the hc-BH system away from the half-filling can exhibit the signatures of the s, d, and p symmetries of the off-diagonal order.Stimulating discussions with C. Timm, S.-L. Drechsler, and T. Mishonov are acknowledged, and also support by the SMWK Grant, the INTAS Grant No. 01-0654, the CRDF Grant No. REC-005, the RME Grant Nos. E 02-3.4-392 and UR.01.01.062, and RFBR Grant No. 04-02-96077. I would also like to thank the Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, where part of this work was made, for hospitality
DFT, L(S)DA, LDA+U, LDA+DMFT, …, whether we do approach to a proper description of optical response for strongly correlated systems?
Get PDFI present a critical overview of so-called “ab initio” DFT (density fuctional theory) based calculation schemes for the description of the electronic structure, energy spectrum, and optical response for strongly correlated 3d oxides, in particular, crystal-field and charge transfer transitions as compared with an “old” cluster model that does generalize crystal-field and ligand-field theory. As a most instructive illustration of validity of numerous calculation techniques I address the prototypical 3d insulator NiO predicted to be a metal in frames of a standard LDA (local density approximation) band theory. © 2016, Pleiades Publishing, Ltd
Doped Manganites Beyond Conventional Double-Exchange Model
Get PDFThe problem of adequate electronic model for doped manganites like La1-xSrxMnO3 remains controversial. There are many thermodynamic and local microscopic quantities that cannot be explained by the conventional double-exchange model with dominantly Mn3d location of doped holes. In such a situation we argue a necessity to discuss all possible candidate states with different valent structure of manganese and oxygen atoms, as well as different valent states of octahedral MnO6 centers. In frames of rather conventional cluster approach, crystal field and the ligand field model we address different types of MnO6 centers, different types of d-d, and charge-transfer (CT) transitions. We draw special attention to the so-called CT states related to strong intra-center charge fluctuations. As we conjecture, namely these could become active valent states for doped manganites. We discuss some electric and magnetic properties of the electron MnO610-, and hole MnO68- centers with unconventional 6A1g-6T1u and 4A2g-4T2u valent manifolds, respectively. We propose two idealized theoretical models for hole system in doped manganites. The first one implies an overall oxygen localization for the doped holes occupying the non-bonding O2p orbitals. The second assumes a doping induced formation of the electron-hole Bose liquid, or a system of the electron MnO610-, and hole MnO68- centers. In a sense, this scenario resembles a well known disproportionation reaction. In both cases one might expect non-trivial magnetic behavior with strong ferromagnetic fluctuations due to anomalously strong ferromagnetic coupling of non-bonding O2p holes with Mn3d electrons. © 2002 Elsevier Science B.V. All rights reserved.The discussions with N.N. Loshkareva, Yu.P. Sukhorukov, E.A. Ganshina, A.V. Korolyov, A.A. Mukhin, P. Novak, D. Khomskii, M. Neumann, V.S. Vikhnin, S.-L. Drechsler, N.G. Bebenin, and V.R. Galakhov are acknowledged. The authors acknowledge a partial support from CRDF, Award No. REC-005, Russian Ministry of Education, grant E00-3.4-280, and Russian Foundation for Basic Researches, grant 01-02-96404
Stepwise Injection Potentiometric Determination of Ammonium-Ions in Water
Get PDFAbstract An automatized technique of stepwise injection potentiometric determination of ammonium-ions in water is developed. The technique includes consecutive stages of evolution of ammonium-ions in the form of ammonia and its liquid absorption into water phase. The determination range is of 5 to 2000 µg/l and efficiency of 7 determinations per hour
Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects
Get PDFWe reconsider the conventional Moriya approach to the Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single Cu1-O-Cu2 bond in cuprates using a perturbation scheme that provides an optimal way to account for intra-atomic electron correlations, low-symmetry crystal field, and local spin-orbital contributions with a focus on the oxygen term. The Dzyaloshinsky vector and the corresponding weak ferromagnetic moment are shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. We predict the effect of oxygen staggered spin polarization in the antiferromagnetic edge-shared CuO2 chains due to the uncompensated oxygen Dzyaloshinsky vectors. The polarization is perpendicular to both the main chain antiferromagnetic vector and the CuO2 chain normal. The intermediate 17O NMR is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. In particular, we argue that the puzzling planar 17O Knight shift anomalies observed in the paramagnetic phase of the generic Dzyaloshinsky-Moriya antiferromagnetic cuprate La2CuO4 can be assigned to the effect of the field-induced staggered magnetization. Finally, we revisit the effects of symmetric spin anisotropy, in particular, those directly induced by the Dzyaloshinsky-Moriya coupling. The perturbation scheme generalizes the well-known Moriya approach and presents a basis for reliable quantitative estimates for the symmetric partner of the Dzyaloshinsky-Moriya coupling. In contrast to the conventional standpoint, the parameters of the effective two-ion spin anisotropy are shown to incorporate the contributions of a single-ion anisotropy for two-hole configurations at both Cu and O sites. © 2007 Pleiades Publishing, Inc.04-02-96077, 06-02-17242, 06-03-90893; Citrus Research and Development Foundation, CRDF: REC-005ACKNOWLEDGMENTS I thank R. Walstedt for a stimulating and encouraging discussion, and H. Eschrig, M. Richter, and S.-L. Drechsler for their interest and the useful discussions. I thank Leibniz-Institut für Festkörper und Werk-stoffforschung Dresden, where part of this work was done, for hospitality. This work is supported in part by the CRDF (grant no. REC-005) and Russian Founda- tion for Basic Research (project nos. 04-02-96077, 06-02-17242, and 06-03-90893)
Dzyaloshinskii Interaction and Exchange-Relativistic Effects in Orthoferrites
Get PDFAbstract: We present an overview of the microscopic theory of the Dzyaloshinskii–Moriya (DM) coupling and related exchange-relativistic effects such as exchange anisotropy, electron-nuclear antisymmetric supertransferred hyperfine interactions, antisymmetric magnetogyrotropic effects, and antisymmetric magnetoelectric coupling in strongly correlated 3d compounds focusing on orthoferrites RFeO3 (R is a rare-earth ion or yttrium Y). Most attention in the paper centers around the derivation of the Dzyaloshinskii vector, its value, orientation, and sense (sign) under different types of the (super)exchange interaction and crystal field. Microscopically derived expression for the dependence of the Dzyaloshinskii vector on the superexchange geometry allows one to find all the overt and hidden canting angles in orthoferrites RFeO3 as well as corresponding contribution to magnetic anisotropy. Being based on the theoretical predictions regarding the sign of the Dzyaloshinskii vector we have predicted and study in detail a novel magnetic phenomenon, weak ferrimagnetism in mixed weak ferromagnets with competing signs of the Dzyaloshinskii vectors. The ligand NMR measurements in weak ferromagnets are shown to be an effective tool to inspect the effects of DM coupling in an external magnetic field. Along with orthoferrites RFeO3 and weak ferrimagnets RFe1 –xCrxO3, although to a lesser extent, we address such typical weak ferromagnets as α-Fe2O3, FeBO3, and FeF3. © 2021, Pleiades Publishing, Inc.Ministry of Education and Science of the Russian Federation, Minobrnauka: FEUZ-2020-0054I am grateful to I.E. Dzyaloshinskii for supporting my work and stimulating discussions. I consider it my duty to note that most of the work was done in close collaboration with A.M. Kadomtseva and her laboratory staff. I thank E.V. Sinitsyn and I.G. Bostrem for very fruitful multi-year collaboration, S.V. Maleev, A.K. Zvezdin, B.Z. Malkin, M.V. Eremin, A.A. Mukhin, B.S. Tsukerblatt, S.‑L. Drechsler, R.E. Walstedt, and V.E. Dmitrienko for stimulating and encouraging discussions.This research was funded by Act 211 Government of the Russian Federation, agreement no. 02.A03.21.0006 and by the Ministry of Education and Science, project no. FEUZ-2020-0054
SEM Investigation of ZnO and CdO–ZnO Layers Grown by Sol-Gel Technology and a Multifractal Analysis of their Surface Depending on Synthesis Conditions
Get PDFIntroduction. Super-thin films of zinc oxide regarded as transparent electrodes can be integrated in effective semiconductor heterostructures for use in modern infrared photo electronics and solar power installations. The most important parameter of zinc oxide thin layers is their surface nanorelief, which can be effectively studied using SEM spectroscopy. SEM images allow for a quantitative description of the surface depending on the synthesis conditions using the method of multifractal analysis. Such an approach reveals quantitative relationships between the fractal parameters of the surface topography of the layers in these systems and the temperature regimes used for their final annealing in conventional sol-gel technology.Aim. To reveal quantitative relationships between the fractal parameters of the surface topography of layers in the Zn–O & Zn–Cd–O systems and the temperature conditions of their final annealing. The MFA method was used for a quantitative description of the surface state depending on the synthesis conditions.Materials and methods. Super-thin films in the ZnO and ZnO–CdO systems were synthesized using a modified sol-gel technology. The temperature-concentration ranges of the parameters of the modified technological process, which allows high-quality layers of the material to be reproducibly obtained on a glass substrate, were determined. The surface morphology was investigated by SEM spectroscopy depending on the temperature of the final annealing of the layers. SEM images of the surface served as a basis for multifractal analysis (MFA) of the surface area and volume of nanoforms, which are formed on the surface of the obtained layers thus determining their surface relief.Results. Renyi’s numbers and the parameters of fractal ordering in MFA were chosen as fractal parameters for describing the nano-geometry of the layer surface. MFA was applied to the description of both the surface areas and volumes of nanoforms. Quantitative correlations between Renyi’s numbers, as well as the parameters of fractal ordering for the areas and volumes of surface nanoforms, and the temperature of the final annealing were found.Conclusion. The numerical values of Renyi’s numbers for the surface and volume characteristics of the surface of layers were used to assess the effect of the fractality of the surface on the molar surface energy of the film. Consideration of the fractal geometry of nanoforms with their characteristic sizes smaller than 5·103μm shows the possibility of both an increase in the surface energy of the resulting film and its decrease when changing the characteristic sizes of nanoforms. The latter effect is due to the formation of a highly porous surface at the nano levelIntroduction. Super-thin films of zinc oxide regarded as transparent electrodes can be integrated in effective semiconductor heterostructures for use in modern infrared photo electronics and solar power installations. The most important parameter of zinc oxide thin layers is their surface nanorelief, which can be effectively studied using SEM spectroscopy. SEM images allow for a quantitative description of the surface depending on the synthesis conditions using the method of multifractal analysis. Such an approach reveals quantitative relationships between the fractal parameters of the surface topography of the layers in these systems and the temperature regimes used for their final annealing in conventional sol-gel technology.Aim. To reveal quantitative relationships between the fractal parameters of the surface topography of layers in the Zn–O & Zn–Cd–O systems and the temperature conditions of their final annealing. The MFA method was used for a quantitative description of the surface state depending on the synthesis conditions.Materials and methods. Super-thin films in the ZnO and ZnO–CdO systems were synthesized using a modified sol-gel technology. The temperature-concentration ranges of the parameters of the modified technological process, which allows high-quality layers of the material to be reproducibly obtained on a glass substrate, were determined. The surface morphology was investigated by SEM spectroscopy depending on the temperature of the final annealing of the layers. SEM images of the surface served as a basis for multifractal analysis (MFA) of the surface area and volume of nanoforms, which are formed on the surface of the obtained layers thus determining their surface relief.Results. Renyi’s numbers and the parameters of fractal ordering in MFA were chosen as fractal parameters for describing the nano-geometry of the layer surface. MFA was applied to the description of both the surface areas and volumes of nanoforms. Quantitative correlations between Renyi’s numbers, as well as the parameters of fractal ordering for the areas and volumes of surface nanoforms, and the temperature of the final annealing were found.Conclusion. The numerical values of Renyi’s numbers for the surface and volume characteristics of the surface of layers were used to assess the effect of the fractality of the surface on the molar surface energy of the film. Consideration of the fractal geometry of nanoforms with their characteristic sizes smaller than 5·103μm shows the possibility of both an increase in the surface energy of the resulting film and its decrease when changing the characteristic sizes of nanoforms. The latter effect is due to the formation of a highly porous surface at the nano leve
Multiferroicity due to nonstoichiometry in the chain cuprate LiVCuO
Get PDFThe recently observed multiferroic behavior in the chain cuprate LiVCuO4 (≡ LiCuVO4) with edge-shared plaquettes and
helical spin ordering does not agree with the existing theories such as a spin-current scenario. We argue that the effect can be consistently
explained, if the exchange-induced electric polarization on the out-of-chain Cu2+ centers substituting for Li ions in LiVCuO4
is taken into account. These substituent centers are proved to be an effective probe of the spin incommensurability and the magnetic-field effects
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