Publications (253)853.09 Total impact
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ABSTRACT: We analyze the role of local geometry in the spin and orbital interaction in transition metal compounds with orbital degeneracy. We stress that the tendency observed for the most studied case (transition metals in O$_6$ octahedra with one common oxygen  common corner of neighboring octahedra and with $\sim 180^{\circ}$ metaloxygenmetal bonds), that ferroorbital ordering renders antiferrospin coupling, and, {\it vice versa}, antiferroorbitals give ferrospin ordering, is not valid in general case, in particular for octahedra with common edge and with $\sim 90^{\circ}$ MOM bonds. Special attention is paid to the ``third case'', neighboring octahedra with common face (three common oxygens)  the case practically not considered until now, although there are many real systems with this geometry. Interestingly enough, the spinorbital exchange in this case turns out to be to be simpler and more symmetric than in the first two cases. We also consider, which form the effective exchange takes for different geometries in case of strong spinorbit coupling.  [Show abstract] [Hide abstract]
ABSTRACT: The magnon disp ersion in Ca2RuO4 has been determined by inelastic neutron scattering on single crytals containing 1% of Ti. The dispersion is well described by a conventional Heisenberg model suggesting a local moment model with nearest neighbor interaction of J=8 meV. Nearest and nextnearest neighbor interaction as well as interlayer coupling parameters are required to properly describe the entire dispersion. Spinorbit coupling induces a very large anisotropy gap in the magnetic excitations in apparent contrast with a simple planar magnetic model. Orbital ordering breaking tetragonal symmetry, and strong spinorbit coupling can thus be identified as important factors in this system. 
Dataset: PhysRevB.91.235147
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ABSTRACT: The twodimensional AffleckKennedyLiebTasaki (AKLT) model on a honeycomb lattice has been shown to be a universal resource for quantum computation. In this valence bond solid, however, the spin interactions involve higher powers of the Heisenberg coupling (S[over →]_{i}·S[over →]_{j})^{n}, making these states seemingly unrealistic on bipartite lattices, where one expects a simple antiferromagnetic order. We show that those interactions can be generated by orbital physics in multiorbital Mott insulators. We focus on t_{2g} electrons on the honeycomb lattice and propose a physical realization of the spin3/2 AKLT state. We find a phase transition from the AKLT to the Néel state on increasing Hund's rule coupling, which is confirmed by density matrix renormalization group simulations. An experimental signature of the AKLT state consists of protected, free S=1/2 spins on lattice vacancies, which may be detected in the spin susceptibility.  [Show abstract] [Hide abstract]
ABSTRACT: The perovskite antiferromagnetic ($T_{\rm N}$ $\sim$ 220 K) insulator EuNiO$_3$ undergoes at ambient pressure a metaltoinsulator transition at $T_{\rm MI}$ = 460 K which is associated with a simultaneous orthorhombictomonoclinic distortion, leading to charge disproportionation. We have investigated the change of the structural and magnetic properties of EuNiO$_3$ with pressure (up to $\sim$ 20 GPa) across its quantum critical point (QCP) using lowtemperature synchrotron angleresolved xray diffraction and $^{151}$Eu nuclear forward scattering of synchrotron radiation, respectively. With increasing pressure we find that after a small increase of $T_{\rm N}$ ($p$ $\leq$ 2 GPa) and the induced magnetic hyperfine field $B_{\rm hf}$ at the $^{151}$Eu nucleus ($p$ $\leq$ 9.7 GPa), both $T_{\rm N}$ and $B_{\rm hf}$ are strongly reduced and finally disappear at $p_{\rm c}$ $\cong$ 10.5 GPa, indicating a magnetic QCP at $p_{\rm c}$. The analysis of the structural parameters up to 10.5 GPa reveals no change of the lattice symmetry within the experimental resolution. Since the pressureinduced insulatortometal transition occurs at $p_{\rm IM}$ $\cong$ 6 GPa, this result implies the existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We further show from the analysis of the reported high pressure electrical resistance data on EuNiO$_3$ at lowtemperatures that in the vicinity of the QCP the system behaves as nonFermiliquid, with the resistance changing as $T^{\rm n}$, with n=1.6, whereas it becomes a normal Fermiliquid, n = 2, for pressures above $\sim$15 GPa. On the basis of the obtained data a magnetic phase diagram in the ($p$, $T$) space is suggested.  [Show abstract] [Hide abstract]
ABSTRACT: We have synthesized and studied singlecrystal Ba5AlIr2O11 that features dimer chains of two inequivalent octahedra occupied by tetravalent and pentavalent ions, respectively. Ba5AlIr2O11 is a Mott insulator that undergoes a subtle structural phase transition near 210 K and a magnetic transition at 4.5 K; the latter transition is surprisingly resistant to applied magnetic fields up to 12 T, but sensitive to modest applied pressure. All results indicate that the phase transition at 210 K signals an enhanced charge order that induces electrical dipoles and strong dielectric response near 210 K. It is clear that the strong covalency and spinorbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state. The novel behavior of Ba5AlIr2O11 therefore provides unique insights into the physics of SOI along with strong covalency in competition with double exchange interactions of comparable strength.  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate that in strongly correlated systems with noniteger electron occupation there is a competition between the conventional double exchange, leading to ferromagnetism, and the tendency for electrons with strongly overlapping orbitals and large intersite hopping to form a nonmagnetic singlet molecular orbital on a dimer. This tendency is enhanced by the strong spinorbit coupling. We show that this happens in our newly synthesized singlecrystal Ba5AlIr2O11 containing dimers with Ir ions having mixed valence Ir4.5+. Singlecrystal Ba5AlIr2O11 demonstrates that the magnetic moment of a dimer is indeed considerably reduced, to 1.04 mB. Furthermore, according to our structural, transport, magnetic and specific heat measurements, it undergoes an intradimer charge ordering below TS = 210 K and an antiferromagnetic transition at TM = 4.5 K, despite its onedimensional character. Ab initio calculations correctly capture the properties of this system and confirm that molecular orbital formation in combination with spinorbit coupling counteracts, and in this case suppresses double exchange. We argue that this effect could be observed in many other, predominantly 4d and 5d systems with large electron hopping and small Hunds rule coupling.  [Show abstract] [Hide abstract]
ABSTRACT: We report on combined neutron and resonant xray scattering results, identifying the nature of the spinorbital ground state and magnetic excitations in LuVO3 as driven by the orbital parameter. In particular, we distinguish between models based on orbital Peierls dimerization, taken as a signature of quantum effects in orbitals, and JahnTeller distortions, in favor of the latter. In order to solve this longstanding puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensityanalysis of extended magnetic excitation data sets. The results of this detailed study enabled us to draw definite conclusions about classical vs quantum behavior of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of LuVO3 and similar systems.  [Show abstract] [Hide abstract]
ABSTRACT: Twodimensional AKLT model on a honeycomb lattice has been shown to be a universal resource for quantum computation. In this valence bond solid, however, the spin interactions involve higher powers of the Heisenberg coupling $(\vec{S}_i \cdot \vec{S}_j)^n$, making these states seemingly unrealistic on bipartite lattices, where one expects a simple antiferromagnetic order. We show that those interactions can be generated by orbital physics in multiorbital Mott insulators. We focus on $t_{2g}$ electrons on the honeycomb lattice and propose a physical realization of the spin$3/2$ AKLT state. We find a phase transition from the AKLT to the Neel state on increasing Hund's rule coupling, which is confirmed by density matrix renormalization group (DMRG) simulations. An experimental signature of the AKLT state consists of protected, free spins1/2 on lattice vacancies, which may be detected in the spin susceptibility. 
Article: Spinorbital interaction for facesharing octahedra: Realization of a highly symmetric SU(4) model
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ABSTRACT: Specific features of orbital and spin structure of transition metal compounds in the case of the facesharing MO$_6$ octahedra are analyzed. In this geometry, we consider the form of the spinorbital Hamiltonian for transition metal ions with double ($e_g^{\sigma}$) or triple ($t_{2g}$) orbital degeneracy. Trigonal distortions typical of the structures with facesharing octahedra lead to splitting of $t_{2g}$ orbitals into an $a_{1g}$ singlet and $e_g^{\pi}$ doublet. For both doublets ($e_g^{\sigma}$ and $e_g^{\pi}$), in the case of one electron or hole per site, we arrive at a symmetric model with the orbital and spin interaction of the Heisenberg type and the Hamiltonian of unexpectedly high symmetry: SU(4). Thus, many real materials with this geometry can serve as a testing ground for checking the prediction of this interesting theoretical model. We also compare general trends in spinorbital ("KugelKhomskii") exchange interaction for three typical situations: those of MO$_6$ octahedra with common corner, common edge, and the present case of common face.  [Show abstract] [Hide abstract]
ABSTRACT: We synthesize and study single crystals of the layered honeycomb lattice Mott insulators Na2RuO3 and Li2RuO3 with magnetic Ru4+(4d4) ions. The newly found Na2RuO3 features a nearly ideal honeycomb lattice and orders antiferromagnetically at 30 K. Singlecrystals of Li2RuO3 adopt a honeycomb lattice with either C2/m or more distorted P21/m below 300 K, depending on detailed synthesis conditions. We find that Li2RuO3 in both structures hosts a welldefined magnetic state, in contrast to the singlet ground state found in polycrystalline Li2RuO3. A phase diagram generated based on our results uncovers a new, direct correlation between the magnetic ground state and basalplane distortions in the honeycomb ruthenates.  [Show abstract] [Hide abstract]
ABSTRACT: The modelling of magnetic monopoles in solids is a hot topic nowadays. Here, I propose that in solids with the linear magnetoelectric effect there should exist, close to electric charges, magnetic textures of magnetic monopole type. Their existence can lead to rather striking consequences, such as (magneto)electric Hall effect, magnetophotovoltaic effect and so on, which can be observed experimentally. In addition, in ordinary magnetoelectric materials not only magnetic monopoles can accompany the charge, but also more complicated local magnetic objects can be created, for example, local toroics, which can also lead to unusual effects in transport and other properties of such systems.  [Show abstract] [Hide abstract]
ABSTRACT: Normally $4d$ or $5d$ transition metals are in a lowspin state. Here using firstprinciples calculations, we report on a rare case of a highspin $S$=1 magnetic state for the Ag$^{3+}$ ion in the double perovskite Cs$_{2}$KAgF$_{6}$. We also explored a possibility of a conventional lowspin $S$=0 ground state and find an associated tetragonal distortion to be 0.29 {\AA}. However, the lattice elastic energy cost and the Hund exchange loss exceed the e$_{g}$ crystalfield energy gain, thus making the lowspin tetragonal structure less favorable than the highspin cubic structure. We conclude that the compact perovskite structure of Cs$_{2}$KAgF$_{6}$ is an important factor in stabilizing the unusual highspin ground state of Ag$^{3+}$.  [Show abstract] [Hide abstract]
ABSTRACT: With the use of the band structure calculations we demonstrate that previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal and magnetic structures for NaMn$_7$O$_{12}$ are inconsistent with each other. The optimization of the crystal lattice allows us to predict a new crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFMCE type of the magnetic order stabilizes the structure with the elongated, not compressed Mn$^{3+}_B$O$_6$ octahedra, striking NaMn$_7$O$_{12}$ out of the list of the anomalous JahnTeller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.  [Show abstract] [Hide abstract]
ABSTRACT: With the use of the band structure calculations we demonstrate that previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal and magnetic structures for NaMn$_7$O$_{12}$ are inconsistent with each other. The optimization of the crystal lattice allows us to predict a new crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFMCE type of the magnetic order stabilizes the structure with the elongated, not compressed Mn$^{3+}_B$O$_6$ octahedra, striking NaMn$_7$O$_{12}$ out of the list of the anomalous JahnTeller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.  [Show abstract] [Hide abstract]
ABSTRACT: We show that in transition metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give e.g. longrange magnetic order or stay paramagnetic. Similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called orbitalselective Peierls transition. This can occur in case of strongly nonuniform hopping integrals for different orbitals and small intraatomic Hund's rule coupling JH. Yet another consequence of this picture is that for odd number of electrons per dimer there exist competition between double exchange mechanism of ferromagnetism, and the formation of singlet dimer by electron on one orbital, with remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule coupling, typical for 3d compounds, whereas the second is more plausible for 4d5d compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which orbitalselective phase seems to be realized. 
Dataset: PhysRevB.84.115138

Dataset: PhysRevLett.102.056406

Dataset: PhysRevLett.101.256403
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ABSTRACT: We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spinstate transitions in (Pr1  y Eu y )0.7Ca0.3CoO3 cobaltites. The Eu doping level y is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spinstate transitions (0.10 < y < 0.20). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope 18O, namely, with 90, 67, 43, 17, and 0% of 18O. Based on the measurements of the ac magnetic susceptibility χ( T) and electrical resistivity ρ( T), we analyze the evolution of the sample properties with a change of the Eu and 18O content. It is demonstrated that the effect of increasing the 18O content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between t 2 g and e g bands including the renormalization of this gap due to the electronphonon interaction reveals the physical mechanisms underlying this similarity.
Publication Stats
9k  Citations  
853.09  Total Impact Points  
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Institutions

19922015

University of Cologne
 II. Institute of Physics
Köln, North RhineWestphalia, Germany


19932012

University of Groningen
 Materials Science Group
Groningen, Groningen, Netherlands


2005

Universiteit Twente
 Department of Computational Materials Science (CMS)
Enschede, Overijssel, Netherlands
