NaRu2O4: A unique Quantum-Material that hosts three different phases at once - all in one at room temperature!

Emergent phenomena's from lattice, orbital, charge and spin degrees of freedom in matter surprises in various forms of ordered states through phase-transitions. Here we present a novel example of such system – NaRu2O4 with CaFe2O4-type structure and we demonstrate a novel type of Z-ordering in it.
Published in Materials
NaRu2O4: A unique Quantum-Material that hosts three different phases at once - all in one at room temperature!
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Discoveries of exotic phases at room temperature in actual materials driven by many-body interactions have motivated researchers in condensed matter physics for many decades. The exotic properties come from the interplay of lattice, orbital, charge, and spin degrees of freedom, resulting in various ordered patterns/states. The 4d transition metals have recently come to light as a fertile ground for studying emergent phenomena. The properties of such material classes result from competitions among several factors: crystal-field splitting, Coulomb repulsion, Hund’s rule, and spin-orbit coupling (SOC). Additionally, each material has its own way of making bonds between its atoms in a crystal lattice. Historically, how particular chemical bond forms in materials was a great mystery in the 19th century until new quantum mechanics explained it. Soon, new cases were reported and subsequently solved in the 1930s using novel concepts of dimerization and charge ordering (CO). These two concepts of dimerization and CO have ever since been frequently invoked to explain numerous exciting quantum effects in real materials, which are insulators almost without exceptions.

                        The valence states of metal are fundamentally important, and they play a crucial role in designing a particular property of the strongly correlated electron systems. Materials scientists discovered several mixed-valence compounds. However, mixed-valence NaRu2O4 containing Ru(III)/Ru(IV) is the first experimental example that displays a unique coexistence of charge order with a dimer formation and retains metallicity. The observed first-order phase transition occurs well above room temperature at 535 K. The structure of this material accommodates Na+ ions at different crystallographic sites inside the pseudo tunnels, which are running parallel to the crystal b-axis. We have studied this ruthenium-based oxide material and found several unique features for the first time: (i) dimerization and (ii) bond- and site-centered charge ordering (CO) all together in metallic NaRu2O4.

Crystal structure of the tunnel compound NaRu2O4 along with  the sketch of self-organized Z-order in the two-leg corner- and edge-sharing RuO6 zigzag ladders.

Figure 1: Crystal structure of the tunnel compound NaRu2O4 along with  the sketch of self-organized Z-order in the two-leg corner- and edge-sharing RuO6 zigzag ladders.

                        By combining high-resolution x-ray diffraction studies and theoretical calculations, we demonstrate that all these properties of NaRu2O4 occur through a new type of bonding, which we call Z-type ordering. The low-temperature superstructure has strong dimerization in the legs of ladders (along the b-direction), with short dimers in legs connected by forming Z-shape clusters. It can be explained by forming metal-metal bonds in a system with mixed-valence Ru3.5+ in a superstructure with the coexistence of bond- and site-centered charge ordering or charge density wave. Our results demonstrate the great flexibility of quantum materials with the intricate interplay among orbital, charge, and lattice degrees of freedom, displaying the unusual superstructures even in a metallic state. We explain through detailed LSDA+U calculations that such nontrivial behavior, due to the Ru 4d states of NaRu2O4, exhibits the delicate balance between electronic correlations and inter-site hopping. Two other factors also play a crucial role: orbital freedom, leading to orbitally-driven dimerization, and the mixed-valence of Ru, Ru3.5+, promoting site-centered charge ordering. These features seem to be the key to realizing CO and dimer coexistence in a metallic NaRu2O4. Our results will prompt renewed efforts towards the hitherto scarcely investigated Mott-Hubbard physics in metallic systems.

For more details, please read our paper.

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https://www.nature.com/articles/s43246-022-00224-8

Coexisting Z-type charge and bond order in metallic NaRu2O4

Arvind Kumar Yogi1,2,3*,#, Alexander Yaresko4*, C. I. Sathish1,2, Hasung Sim1,2, Daisuke Morikawa5, J. Nuss,4  Kenji Tsuda6, Y. Noda5,7, Daniel I. Khomskii8, and Je-Geun Park1,2,9,10#

1Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Korea

2Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea

3UGC-DAE Consortium for Scientific Research, Indore-452001, India

4Max-Planck-Institut für Festkörperforschung, 70569 Stuttgart, Germany

5Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,

Sendai 980-8577, Japan

6Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan

7J-PARC Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Ibaraki, 319-1195, Japan

8Institute of Physics II, University of Cologne, 50937 Cologne, Germany

9Center for Quantum Materials, Seoul National University, Seoul 08826, Korea

10Institute of Applied Physics, Seoul National University, Seoul 08826, Korea

* Authors with equal contributions

# Corresponding authors: jgpark10@snu.ac.kr & yogi.arvind2003@gmail.com

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