arXiv subjects

Nikolay A. Bogdanov

Publications and source records attributed to Nikolay A. Bogdanov.

27 records · Page 2Linked to original sources

arXiv2017

Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction

Materials with strong spin-orbit interactions are presently a main target in the search for systems with novel magnetic properties. Magnetic anisotropies can be very large in such compounds, ranging from strongly frustrated Kitaev exchange in honeycomb iridates and the associated spin-liquid states to robust antisymmetric couplings in square-lattice Sr_2IrO_4 . Here we predict from ab initio quantum chemistry calculations that another highly unusual regime is realized in pyrochlore iridium oxides: the nearest-neighbor Heisenberg interaction can vanish whilst the antisymmetric Dzyaloshinskii- Moriya exchange reaches values as large as 5 meV, a result which challenges common notions and existing phenomenological models of magnetic superexchange. The resulting spin excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based pyrochlore iridates, suggesting the possibility of using these modes to store magnetic information. Indeed the magnetization dynamics indicates that these modes are unable to propagate out of the excitation region.

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arXiv2016

Covalency and vibronic couplings make a nonmagnetic j=3/2 ion magnetic

For 4$d^1$ and 5$d^1$ spin-orbit-coupled electron configurations, the notion of nonmagnetic j=3/2 quartet ground state discussed in classical textbooks is at odds with the observed variety of magnetic properties. Here we throw fresh light on the electronic structure of 4$d^1$ and 5$d^1$ ions in molybdenum- and osmium-based double-perovskite systems and reveal different kinds of on-site many-body physics in the two families of compounds: while the sizable magnetic moments and $g$ factors measured experimentally are due to both metal $d$-ligand $p$ hybridization and dynamic Jahn-Teller interactions for 4$d$ electrons, it is essentially $d$-$p$ covalency for the 5$d^1$ configuration. These results highlight the subtle interplay of spin-orbit interactions, covalency and electron-lattice couplings as the major factor in deciding the nature of the magnetic ground states of 4$d$ and 5$d$ quantum materials. Cation charge imbalance in the double-perovskite structure is further shown to allow a fine tuning of the gap between the $t_{2g}$ and $e_g$ levels, an effect of much potential in the context of orbital engineering in oxide electronics.

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arXiv2016

Orbital breathing effects in the computation of x-ray d-ion spectra in solids by ab initio wave-function-based methods

In existing theoretical approaches to core-level excitations of transition-metal ions in solids relaxation and polarization effects due to the inner core hole are often ignored or described phenomenologically. Here we set up an ab initio computational scheme that explicitly accounts for such physics in the calculation of x-ray absorption and resonant inelastic x-ray scattering spectra. Good agreement is found with experimental transition-metal $L$-edge data for the strongly correlated $d^9$ cuprate Li$_2$CuO$_2$, for which we determine the absolute scattering intensities. The newly developed methodology opens the way for the investigation of even more complex $d^n$ electronic structures of group VI B to VIII B correlated oxide compounds.

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arXiv2016

Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the $d$-level structure of layered Sr$_2$IrO$_4$ by electron spin resonance. While canonical ligand-field theory predicts $g_{\parallel}$-factors $\!<\!2$ for positive tetragonal distortions as present in Sr$_2$IrO$_4$, the experiment indicates $g_{\parallel}\!>\!2$. This implies that the iridium $d$ levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr$_2$IrO$_4$, whereas we find them in Ba$_2$IrO$_4$ to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore $d$-orbital reconstruction in the context of oxide electronics.

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arXiv2016

Spin-orbit excitation energies, anisotropic exchange, and magnetic phases of honeycomb RuCl3

Using quantum chemistry calculations we shed fresh light on the electronic structure and magnetic properties of RuCl3, a proposed realization of the honeycomb Kitaev spin model. It is found that the nearest-neighbor Kitaev exchange K is weaker than in 5d5 Ir oxides but still larger than other effective spin couplings. The electronic-structure computations also indicate a ferromagnetic K in the halide, which is supported by a detailed analysis of the field-dependent magnetization. From exact-diagonalization calculations for extended Kitaev-Heisenberg Hamiltonians we additionally find that a transition from zigzag order to a spin-liquid ground state can be induced in RuCl3 with external magnetic field.

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arXiv2013

Magnetic state of pyrochlore Cd2Os2O7 emerging from strong competition of ligand distortions and longer-range crystalline anisotropy

By many-body quantum-chemical calculations, we investigate the role of two structural effects - local ligand distortions and the anisotropic Cd-ion coordination - on the magnetic state of Cd2Os2O7, a spin S=3/2 pyrochlore. We find that these effects strongly compete, rendering the magnetic interactions and ordering crucially depend on these geometrical features. Without trigonal distortions a large easy-plane magnetic anisotropy develops. Their presence, however, reverses the sign of the zero-field splitting and causes a large easy-axis anisotropy (D = -6.8 meV), which in conjunction with the antiferromagnetic exchange interaction (J = 6.4 meV) stabilizes an all-in/all-out magnetic order. The competition uncovered here is a generic feature of pyrochlore magnets.

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arXiv2012

CaIrO3 post-perovskite, a j = 1/2 quasi-one-dimensional antiferromagnet

The 5d5 iridate CaIrO3 is isostructural with the post-perovskite phase of MgSiO3, recently shown to occur under extreme pressure in the lower Earth's mantle. It therefore serves as an analogue of post-perovskite MgSiO3 for a wide variety of measurements at ambient conditions or achievable with conventional multianvile pressure modules. By multireference configuration-interaction calculations we here provide essential information on the chemical bonding and magnetic interactions in CaIrO3. We predict a large antiferromagnetic superexchange of 120 meV along the c axis, the same size with the interactions in the cuprate superconductors, and ferromagnetic couplings smaller by an order of magnitude along a. CaIrO3 can thus be regarded as a j = 1/2 quasi-one-dimensional antiferromagnet. While this qualitatively agrees with the stripy magnetic structure proposed by resonant x-ray diffraction, the detailed microscopic picture emerging from our study, in particular, the highly uneven admixture of t2g components, provides a clear prediction for resonant inelastic x-ray scattering experiments.

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arXiv2011

Ab initio calculation of d-d excitations in quasi-one-dimensional Cu d9 correlated materials

With wavefunction-based electronic-structure calculations we determine the Cu d-d excitation energies in quasi-one-dimensional spin-chain and ladder copper oxides. A complete set of local excitations has been calculated for cuprates with corner-sharing (Sr2CuO3 and SrCuO2) and edge-sharing (LiVCuO4, CuGeO3, LiCu2O2 and Li2CuO2) CuO4 plaquettes, with corner-sharing CuF6 octahedra (KCuF3), for the ladder system CaCu2O3, and for multiferroic cupric oxide CuO. Our data compare well with available results of optical absorption measurements on KCuF3 and the excitation energies found by resonant inelastic x-ray scattering experiments for CuO. The ab initio results we report for the other materials should be helpful for the interpretation of future resonant inelastic x-ray scattering experiments on those highly anisotropic compounds.

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arXiv2011

Ab initio computation of d-d excitation energies in low-dimensional Ti and V oxychlorides

Using a quantum chemical cluster-in-solid computational scheme, we calculate the local d-d excitation energies for two strongly correlated Mott insulators, the oxychlorides TiOCl and VOCl. TiOCl harbors quasi-one-dimensional spin chains made out of S = 1/2 Ti3+ ions while the electronic structure of VOCl displays a more two-dimensional character. We find in both cases that the lowest-energy d-d excitations are within the t2g subshell, starting at 0.34 eV and indicating that orbital degeneracies are significantly lifted. In the vanadium oxychloride, spin triplet to singlet excitations are calculated to be 1 eV higher in energy. For TiOCl, the computed d-level electronic structure and the symmetries of the wavefunctions are in very good agreement with resonant inelastic x-ray scattering results and optical absorption data. For VOCl, future resonant inelastic x-ray scattering experiments will constitute a direct test of the symmetry and energy of about a dozen of different d-d excitations that we predict here.

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