Unconventional electronic ground states and novel physical properties due to strong spin-orbit couplings and correlation effects in 5d oxides

• Applicant: Dr. Liviu Hozoi
• Subject Area: Theoretical Chemistry: Electronic Structure, Dynamics, Simulation ; Theoretical Condensed Matter Physics
• Term: from 2013 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 233478970

• Final Report Year: 2017

Final Report Abstract

Large, highly anisotropic magnetic couplings in 5d and 4d transition-metal solid-state compounds and the less conventional magnetic ground-states associated with strong anisotropic intersite interactions were a main focus of this project. Much weight was put on the analysis of Kitaev anisotropic exchange in honeycomb systems. This constitutes a topic of active research in present day experimental condensed-matter physics, having as main motivation the realization of quantum spin-liquid ground states in actual materials. The Kitaev spin Hamiltonian on the honeycomb lattice is a mathematically well-understood two-dimensional model displaying various topological spin-liquid states. Its remarkable properties include protection of quantum information and the emergence of Majorana fermions. In this context, ab initio results obtained through this project provide valuable guidelines on how Kitaev interactions can be enhanced/optimized by controlling structural parameters such as bond angles and bond lengths. Our findings are described, for example, in Nature Communications 7, 10273 (2016) and Scientific Reports 5, 14718 (2015). Complementary to the investigation of symmetric exchange anisotropies such as the Kitaev coupling, we further addressed antisymmetric contributions to the magnetic Hamiltonian, also referred to as Dzyaloshinskii-Moriya (DM) exchange. We computed huge DM interaction strengths of ~20 meV in the square-lattice compound Sr2IrO4 [Nature Communications 6, 7306 (2015)] and found that the DM contribution may even outstrip the isotropic Heisenberg counterpart in pyrochlore iridate systems [Physical Review Materials 2, 074408 (2018)], a result that challenges present phenomenological models for superexchange in magnetic insulators. We additionally identified strong cation charge-imbalance effects that may compete with the effect of ligand-cage distortion, the latter of either tetragonal or trigonal type. We showed that by tuning the balance between these two effects it is in principle possible to control the sequence of the d-electron sublevels [Nature Communications 6, 7306 (2015)], the size of the t2g-eg gap [Quantum Materials 1, 16029 (2016)], the sign of the single-ion anisotropy [Physical Review Letters 110, 127206 (2013)], and the structure of the g tensor [Nature Communications 6, 7306 (2015)]. These findings are of direct relevance to the research area of stacked heterostructures and provide a simple recipe on how low-symmetry crystal fields at d-metal sites can be altered and potentially engineered through the appropriate design of successive ionic layers.

Publications

• Magnetic State of Pyrochlore Cd2Os2O7 Emerging from Strong Competition of Ligand Distortions and Longer­Range Crystalline Anisotropy, Phys. Rev. Lett. 110, 127206 (2013)
  N. A. Bogdanov, R. Maurice, I. Rousochatzakis, J. van den Brink, and L. Hozoi
  (See online at https://doi.org/10.1103/PhysRevLett.110.127206)
• Kitaev interactions between j=1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations, New J. Phys. 16, 013056 (2014)
  V. M. Katukuri, S. Nishimoto, V. Yushankhai, A. Stoyanova, H. Kandpal, S. K. Choi, R. Coldea, I. Rousochatzakis, L. Hozoi, and J. van den Brink
  (See online at https://doi.org/10.1088/1367-2630/16/1/013056)
• Mechanism of basal­plane antiferromagnetism in the spin­orbit driven iridate Ba2IrO4 , Phys. Rev. X 4, 021051 (2014)
  V. M. Katukuri, V. Yushankhai, L. Siurakshina, J. van den Brink, L. Hozoi, I. Rousochatzakis
  (See online at https://doi.org/10.1103/PhysRevX.4.021051)
• Orbital reconstruction in nonpolar tetravalent transition­metal oxide layers, Nature Commun. 6, 7306 (2015)
  N. A. Bogdanov, V. M. Katukuri, J. Romhányi, V. Yushankhai, V. Kataev, B. Büchner, J. van den Brink, and L. Hozoi
  (See online at https://doi.org/10.1038/ncomms8306)
• Strong magnetic frustration and anti­site disorder causing spin­glass behavior in honeycomb Li2RhO3, Sci. Rep. 5, 14718 (2015)
  V. M. Katukuri, S. Nishimoto, I. Rousochatzakis, H. Stoll, J. van den Brink, L. Hozoi
  (See online at https://doi.org/10.1038/srep14718)
• Covalency and vibronic couplings make a nonmagnetic j=3/2 ion magnetic, NPJ Quantum Materials 1, 16029 (2016)
  L. Xu, N. A. Bogdanov, A. Princep, P. Fulde, J. van den Brink, and L. Hozoi
  (See online at https://doi.org/10.1038/npjquantmats.2016.29)
• Kitaev exchange and field­induced quantum spin­liquid states in honeycomb RuCl3, Sci. Rep. 6, 37925 (2016)
  R. Yadav, N. A. Bogdanov, V. M. Katukuri, S. Nishimoto, J. van den Brink, and L. Hozoi
  (See online at https://doi.org/10.1038/srep37925)
• Strongly frustrated triangular spin lattice emerging from triplet dimer formation in honeycomb Li2IrO3, Nature Commun. 7, 10273 (2016)
  S. Nishimoto, V. M. Katukuri, V. Yushankhai, H. Stoll, U. K. Rössler, L. Hozoi, I. Rousochatzakis, and J. van den Brink
  (See online at https://doi.org/10.1038/ncomms10273)
• Orbital breathing effects in the computation of x­ray d­ion spectra in solids by ab initio wave­function­based methods, J. Phys.: Condens. Matter 29, 035502 (2017)
  N. A. Bogdanov, V. Bisogni, R. Kraus, C. Monney, K. Zhou, T. Schmitt, J. Geck, A. O. Mitrushchenkov, H. Stoll, J. van den Brink, and L. Hozoi
  (See online at https://doi.org/10.1088/1361-648X/29/3/035502)