arXiv subjects

D. I. Khomskii

Publications and source records attributed to D. I. Khomskii.

At least 55 records · Page 3Linked to original sources

arXiv2017

Magnetism and charge ordering in high- and low-temperature phases of Nb2O2F3

To sum up, we show in the present paper that magnetic response in Nb$_2$O$_2$F$_3$ at the high-temperatures ($T>$90 K) is related to the orbital selective regime, when part of the electrons form molecular orbitals while other electrons have local magnetic moments. The charge disproportionation, which occurs at $T\sim$90 K is seen in the GGA calculations, but its degree ($δn \sim 0.1$ electron) is far from what one would expect from naive expectations based on the formal ionic valences. The mechanism of the charge ordering is argued to be related with a sizable kinetic energy gain due to formation of two molecular orbitals in short Nb$^{3+}$-Nb$^{3+}$ dimers caused by a strong nonlinearity of the distance dependence on electron hopping. We think that this mechanism of charge ordering, stabilized not by decrease of interaction energy, but rather by the gain in kinetic energy, may be operative in many other systems, especially consisting of structural dimers.

cond-mat.mtrl-sci↗

arXiv2017

Magnon dispersion in Ca2RuO4: impact of spin-orbit coupling and oxygen moments

The magnon dispersion of Ca$_2$RuO$_4$ has been studied by polarized and unpolarized neutron scattering experiments on crystals containing 0, 1 and 10 % of Ti. The entire dispersion of transverse magnons can be well described by a conventional spin-wave model with interaction and anisotropy parameters that agree with density functional theory calculations. Spin-orbit coupling strongly influences the magnetic excitations, which is most visible in large energies of the magnetic zone-center modes arising from magnetic anisotropy. We find evidence for a low-lying additional mode that exhibits strongest scattering intensity near the antiferromagnetic zone center. This extra signal can be explained by a sizable magnetic moment of 0.11 Bohr magnetons on the apical oxygens parallel to the Ru moment, which is found in the density functional theory calculations. The energy and the signal strength of the additional branch are well described by taking into account this oxygen moment with weak ferromagnetic coupling between Ru and O moments.

cond-mat.str-el↗

arXiv2016

Covalent bonds against magnetism in transition metal compounds

Magnetism in transition metal compounds is usually considered starting from a description of isolated ions, as exact as possible, and treating their (exchange) interaction at a later stage. We show that this standard approach may break down in many cases, especially in $4d$ and $5d$ compounds. We argue that there is an important intersite effect -- an orbital-selective formation of covalent metal-metal bonds, which leads to an "exclusion" of corresponding electrons from the magnetic subsystem, and thus strongly affects magnetic properties of the system. This effect is especially prominent for noninteger electron number, when it results in suppression of the famous double exchange, the main mechanism of ferromagnetism in transition metal compounds. We study this novel mechanism analytically and numerically and show that it explains magnetic properties of not only several $4d-5d$ materials, including Nb$_2$O$_2$F$_3$ and Ba$_5$AlIr$_2$O$_{11}$, but can also be operative in $3d$ transition metal oxides, e.g. in CrO$_2$ under pressure. We also discuss the role of spin-orbit coupling on the competition between covalency and magnetism. Our results demonstrate that strong intersite coupling may invalidate the standard single-site starting point for considering magnetism, and can lead to a qualitatively new behaviour.

cond-mat.str-el↗

arXiv2016

Coupled electricity and magnetism in solids: multiferroics and beyond

The interplay of electricity and magnetism, one of the cornerstones of modern physics, takes a special form in solids in such phenomena as magnetoelectricity and the possibility of multiferroic behaviour. In this paper I give a short survey of the main notions of this field, paying special attention to microscopic aspects. Some related phenomena, such as electric activity of magnetic domain walls, etc., are also shortly discussed.

cond-mat.str-el↗

arXiv2016

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li$_2$RuO$_3$

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

cond-mat.str-el↗

arXiv2016

Suppression of magnetism in Ba5AlIr2O11: interplay of Hund's coupling, molecular orbitals and spin-orbit interaction

The electronic and magnetic properties of Ba$_5$AlIr$_2$O$_{11}$ containing Ir-Ir dimers are investigated using the GGA and GGA+SOC calculations. We found that strong suppression of the magnetic moment in this compound recently found in [J. Terzic {\it et al.}, Phys. Rev. B {\bf 91}, 235147 (2015)] is not due to charge-ordering, but is related to the joint effect of the spin-orbit interaction and strong covalency, resulting in the formation of metal-metal bonds. They conspire and act against the intra-atomic Hund's rule exchange interaction to reduce total magnetic moment of the dimer. We argue that the same mechanism could be relevant for other $4d$ and $5d$ dimerized transition metal compounds.

cond-mat.str-el↗

arXiv2015

Role of local geometry in spin and orbital structure of transition metal compounds

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 $\sim 180^{\circ}$ metal--oxygen--metal bonds), that ferro-orbital ordering renders antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give ferro-spin ordering, is not valid in general case, in particular for octahedra with common edge and with $\sim 90^{\circ}$ M--O--M 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 spin--orbital 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 spin--orbit coupling.

cond-mat.str-el↗

arXiv2015

Jahn-Teller versus quantum effects in the spin-orbital material LuVO3

We report on combined neutron and resonant x-ray scattering results, identifying the nature of the spin-orbital 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 Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity-analysis 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.

cond-mat.str-el↗

arXiv2015

Pressure-induced unusual metallic state in EuNiO$_3$

The perovskite antiferromagnetic ($T_{\rm N}$ $\sim$ 220 K) insulator EuNiO$_3$ undergoes at ambient pressure a metal-to-insulator transition at $T_{\rm MI}$ = 460 K which is associated with a simultaneous orthorhombic-to-monoclinic 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 low-temperature synchrotron angle-resolved x-ray 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 pressure-induced insulator-to-metal 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 low-temperatures that in the vicinity of the QCP the system behaves as non-Fermi-liquid, with the resistance changing as $T^{\rm n}$, with n=1.6, whereas it becomes a normal Fermi-liquid, 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.

cond-mat.str-el↗

arXiv2015

Coexisting charge and magnetic orders in the dimer-chain iridate Ba5AlIr2O11

We have synthesized and studied single-crystal 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 spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state. The 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.

cond-mat.str-el↗

arXiv2015

Spin--orbital interaction for face-sharing octahedra: Realization of a highly symmetric SU(4) model

Specific features of orbital and spin structure of transition metal compounds in the case of the face-sharing MO$_6$ octahedra are analyzed. In this geometry, we consider the form of the spin--orbital Hamiltonian for transition metal ions with double ($e_g^σ$) or triple ($t_{2g}$) orbital degeneracy. Trigonal distortions typical of the structures with face-sharing octahedra lead to splitting of $t_{2g}$ orbitals into an $a_{1g}$ singlet and $e_g^π$ doublet. For both doublets ($e_g^σ$ and $e_g^π$), 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 spin--orbital ("Kugel--Khomskii") exchange interaction for three typical situations: those of MO$_6$ octahedra with common corner, common edge, and the present case of common face, which has not been considered yet.

cond-mat.str-el↗

arXiv2015

Two-dimensional Valence Bond Solid (AKLT) states from $t_{2g}$ electrons

Two-dimensional 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 spins-1/2 on lattice vacancies, which may be detected in the spin susceptibility.

cond-mat.str-el↗

arXiv2014

Lattice-Tuned Magnetism of Ru4+(4d4) Ions in Single-Crystals of the Layered Honeycomb Ruthenates: Li2RuO3 and Na2RuO3

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. Single-crystals 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 well-defined 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 basal-plane distortions in the honeycomb ruthenates.

cond-mat.str-el↗

arXiv2014

Effect of Eu doping and partial oxygen isotope substitution on magnetic phase transitions in (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites

We study experimentally and theoretically the effect of Eu doping and partial oxygen isotope substitution on the transport and magnetic characteristics and spin-state transitions in (Pr$_{1-y}$Eu$_y$)$_{0.7}$Ca$_{0.3}$CoO$_3$ cobaltites. The Eu doping level $y$ is chosen in the range of the phase diagram near the crossover between the ferromagnetic and spin-state transitions ($0.10 < y < 0.20$). We prepared a series of samples with different degrees of enrichment by the heavy oxygen isotope $^{18}$O, namely, containing 90%, 67%, 43%, 17%, and 0% of $^{18}$O. Based on the measurements of ac magnetic susceptibility $χ(T)$ and electrical resistivity $ρ(T)$, we analyze the evolution of the sample properties with the change of Eu and $^{18}$O content. It is demonstrated that the effect of increasing $^{18}$O content on the system is similar to that of increasing the Eu content. The band structure calculations of the energy gap between $t_{2g}$ and $e_g$ bands including the renormalization of this gap due to the electron-phonon interaction reveal the physical mechanisms underlying such similarity.

cond-mat.str-el↗

arXiv2014

Rare case of magnetic Ag$^{3+}$ ion: double perovskite Cs$_{2}$KAgF$_{6}$

Normally $4d$ or $5d$ transition metals are in a low-spin state. Here using first-principles calculations, we report on a rare case of a high-spin $S$=1 magnetic state for the Ag$^{3+}$ ion in the double perovskite Cs$_{2}$KAgF$_{6}$. We also explored a possibility of a conventional low-spin $S$=0 ground state and find an associated tetragonal distortion to be 0.29 Å. However, the lattice elastic energy cost and the Hund exchange loss exceed the e$_{g}$ crystal-field energy gain, thus making the low-spin tetragonal structure less favorable than the high-spin cubic structure. We conclude that the compact perovskite structure of Cs$_{2}$KAgF$_{6}$ is an important factor in stabilizing the unusual high-spin ground state of Ag$^{3+}$.

cond-mat.str-el↗

arXiv2014

Jahn-Teller distortions and charge, orbital and magnetic orders in NaMn7O12

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 AFM-CE 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 Jahn-Teller 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.

cond-mat.str-el↗

arXiv2014

Ab initio analysis of the tight-binding parameters and magnetic interactions in Na2IrO3

By means of density functional theory (DFT) calculations (with and without inclusion of spin-orbit (SO) coupling) we present a detailed study of the electronic structure and corresponding microscopic Hamiltonian parameters of Na2IrO3. In particular, we address the following aspects: (i) We investigate the role of the various structural distortions and show that the electronic structure of Na2IrO3 is exceptionally sensitive to structural details. (ii) We discuss both limiting descriptions for Na2IrO3; quasi-molecular orbitals (small SO limit, itinerant) versus relativistic orbitals (large SO limit, localized) and show that the description of Na2IrO3 lies in an intermediate regime. (iii) We investigate whether the nearest neighbor Kitaev-Heisenberg model is sufficient to describe the electronic structure and magnetism in Na2IrO3. In particular, we verify the recent suggestion of an antiferromagnetic Kitaev interaction and show that it is not consistent with actual or even plausible electronic parameters. Finally, (iv) we discuss correlation effects in Na2IrO3. We conclude that while the Kitaev-Heisenberg Hamiltonian is the most general expression of the quadratic spin-spin interaction in the presence of spin-orbit coupling (neglecting single-site anisotropy), the itinerant character of the electrons in Na2IrO3 makes other terms beyond this model (including, but not limited to 2nd and 3rd neighbor interactions) essential.

cond-mat.str-el↗

arXiv2014

The nature of the ferromagnetic ground state in the Mn4 molecular magnet

Using ab initio band structure and model calculations we studied magnetic properties of one of the Mn$_4$ molecular magnets (Mn4(hmp)6), where two types of the Mn ions exist: Mn3+ and Mn2+. The direct calculation of the exchange constants in the GGA+U approximation shows that in contrast to a common belief the strongest exchange coupling is not between two Mn3+ ions (J_{bb}), but along two out of four exchange paths connecting Mn3+ and Mn2+ ions (J_{wb}). The microscopic analysis performed within the perturbation theory allowed to establish the mechanism for this largest ferromagnetic exchange constant. The charge ordering of the Mn ions results in the situation when the energy of the excited state in the exchange process is defined not by the large on-site Coulomb repulsion U, but by much smaller energy V, which stabilizes the charge ordered state. Together with strong Hund's rule coupling and specific orbital order this leads to a large ferromagnetic exchange interaction for two out of four Mn2+ --Mn3+ pairs.

cond-mat.str-el↗