Session B54: Ruthenates

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Simultaneous control of structural and physical properties via applied electrical current poses a key, new research topic and technological significance. Studying the spin-orbit-coupled antiferromagnet Ca₂RuO₄, and its derivatives with Mn-, Fe-, Rh- and Ir-doping, we find that a small applied electrical current couples to the lattice by significantly reducing its orthorhombicity and octahedral rotations, which in turn concurrently diminishes the 125 K- antiferromagnetic transition and induces a new, orbital order below 80 K. That the current-dependence of transport and magnetic properties closely tracks that of the lattice indicates that the current-controlled lattice is the driving force of the observed novel phenomena. The coupling between the lattice and nonequilibrium driven current is interpreted in terms of t_2g orbital occupancies [1].

[1] Nonequilibrium Orbital Transitions via Applied Electrical Current in Calcium Ruthenates, Hengdi Zhao et al, Phys. Rev. B 100, 241104(R) (2019)   

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Transition metal oxides perovskites (ABO₃) along [111] direction consists of alternating AO₃ and B planes. The B sites form a triangular lattice and two such pseudo-cubic unit cells (ABO₃) form a buckled honeycomb lattice. This provides an opportunity of creating a graphene-like structure with d-electron system and is predicted to give rise to novel quantum phases. In this work, thin films of SrRuO₃ (SRO) are grown in [001] and [111] direction and their electronic and magnetic properties are investigated as a function of film thickness. Metal-insulator transition is observed at a thickness of ~3 and ~7 unit cells in SRO (001) and SRO (111), respectively. While ferromagnetism (FM) and metallicity occur concurrently in SRO (001), FM in ultrathin SRO(111) precedes metallicity which is surprising since SRO has traditionally been described as a Stoner FM. This can be related to the magnetic sublattice symmetry and more pronounced role of localized moments in SRO(111). Our study highlights the possible breakdown of itinerant magnetism picture in trigonal symmetry and the role of structure on an atomic scale will be discussed.   

Live

In 4d⁴ electron systems with t_2g orbitals, where the orbital angular momentum is not fully quenched, nonmagnetic singlet state becomes the ground state in the atomic limit due to spin-orbit coupling, while in crystals, intersite superexchange interactions could lead to the excitonic magnetic ordering due to hybridization between low-spin singlet and high-spin triplet states. Ca₂RuO₄ is a candidate material for such excitonic magnets.
In the present study, we applied the mean-field and random phase approximations to analyze the collective-mode excitations in the two-dimensional three-orbital Hubbard model for Ca₂RuO₄ derived from ab-initio calculations. We then found that when the system is in the excitonic magnetic phase, the calculated magnetic excitation spectra are in good agreement with experimental ones.
In this talk, we will present collective-mode spectra in the ordered phase of the above model and discuss their parameter dependence such as spin-orbit coupling and crystal field in detail. We will also discuss the role of excitonic magnetic ordering in inelastic neutron scattering spectra of Ca₂RuO₄. If time permits, results for resonant inelastic X-ray scattering spectra will also be presented.   

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Current-driven insulator-metal transitions are in many cases driven by Joule heating proportional to the square of the applied current. Recent experiments in Ca₂RuO₄ reveal that the non equilibrium transition can be induced by a small current and that the metal-insulator phase boundary depends on the direction of athe applied current, suggesting an important non-heating effect. We investigate the effects of an electric current in a system containing interfaces between metallic and insulating phases using a general model. We derive a heat balance equation from the Onsager transport theory and in addition to the usual Joule heating effect, we find a heating term proportional to the product of the current across the interface and the discontinuity in the Seebeck coefficient, so that heat can either be generated or removed at an interface, depending on the direction of the current relative to the change in material properties. For parameters appropriate to Ca₂RuO₄, this heating can be comparable to or larger than Joule heating. A simplified model of the relevant experimental geometry is shown to provide results consistent with the experiments.   

Live

We have systematically studied newly synthesized quasi-two-dimensional Ba₇Ru₄Cl₂O₁₅ and Ba₇Ru₄Br₂O₁₅ in single-crystal form. These two compounds with a similar crystal structure host in double-layer, face-sharing Ru₂O₉ dimers that are weakly connected with Ba-Cl/Br layers along the c-axis. The long superexchange pathway through a Ru-O-Ba-Cl-Ba-O-Ru chain along the c-axis renders a quasi-two-dimensional lattice and supports a robust antiferromagnetic state with a Néel temperature at 40K. In addition, all results indicate that the onset of the antiferromagnetic order is insensitive to the c-axis lattice parameter, leaving magnetism of these materials an intriguing topic to explore. The magnetic order is also accompanied by a sizable frustration parameter (up to 6) and a rather large Sommerfeld coefficient, suggesting significant frustration.   

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Ca₃Ru₂O₇ has been considered a material with a rich playground due to its great variety of phenomenology, displaying interesting low temperature properties such as spin-valve and giant magnetoresistance effects. It displays unconventional magnetic order and electronic transitions without changes in crystal symmetry [1]. Experimental works have suggested the emergency of novel properties under external manipulation, although still barely understood. Here, we explore the electronic properties of Ca₃Ru₂O₇ under pressure and strain using density functional theory (DFT). On the one hand, our studies suggest that several phase transitions can emerge from the interplay among magnetism, spin-orbit coupling, and Coulomb repulsion, and on the other hand, these studies help to the understanding of fundamental issues in the ground state, such as the role between the spin-orbit coupling and the lattice grades of freedom.

References
1- I. Markovic, M. D. Watson, O. J. Clark, F. Mazzola, E. A. Morales, C. A. Hooley, H. Rosner, C. M. Polley, T. Balasubramanian, S. Mukherjee, N. Kikugawa, D. A. Sokolov, A. P. Mackenzie, and P. D. C. King, PNAS 117, 15524 (2020).   

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Ca2RuO4 is a compound with a Mott insulating ground state and metallic high-temperature state, which responds to external pressure in a variety of ways. Some of these are expected (a metallisation transition to the high-T phase) while others are more surprising (expansion of the c axis). A delicate coupling between structural and electronic effects has long been acknowledged in layered Ruthenates, but computational studies have remained restricted to experimental crystal structures. Here, we present a study of the pressure-induced structural and electronic evolution of Ca2RuO4 using DFT+U calculations, which marks the first reported attempt at a fully self-consistent optimisation of Ca2RuO4. Our results generally agree well with experiment. In particular the coupling between crystal structure and Ru-4d orbital order is made abundantly clear, and the anomalous structural behaviour near the Mott transition is reproduced. Results are sensitive to the on-site repulsion U, which highlights the proximity to the Mott transition, even in the metallic phase.   

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A comprehensive theory of superconductivity (SC) in Sr2RuO4 must simultaneously explain experiments
that suggest even parity superconducting order and others that claim broken
time reversal symmetry. Completeness requires that the theory is applicable to isoelectronic Ca2RuO4.
The valence transition model, previously proposed for cuprate SC [1]
is extended to insulating Ca2RuO4 and superconducting Sr2RuO4 [2]. Within the model
Ru(4+)-to-Ru(3+) valence transition accompanies insulator-metal transition, the driving
force behind which is the unusually large ionization energy of the half-filled high-spin Ru(3+)
ion. Superconducting ruthenates are 2-component systems
in which high-spin Ru(3+) ions determine the magnetic behavior but not transport,
while the charge carriers are entirely on the layer O-ions, which have average charge
-1.5. Spin-singlet SC in Sr2RuO4 occurs in the correlated lattice-frustrated
3/4-filled band of layer O-ions, in agreement with many-body calculations that
find enhancement by the Hubbard U of superconducting pair correlations uniquely at this filling [3]. Several model-specific experimental predictions are made [2].
[1] S. Mazumdar, Phys. Rev B 98, 205153.
[2] S. Mazumdar Phys Rev Res 2, 023382.
[3. N. Gomes et al. 93, 165110.   

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Iridates and ruthenates display a wide range of electronic and magnetic states, including unconventional superconductivity, magnetism, Mott insulating behavior, and topological properties. The strong coupling between their structural (such as octahedral rotations and dimensionality) and electronic / magnetic degrees of freedom make thin films a powerful platform for manipulating the electronic properties of iridates and ruthenates. To achieve this control, we employ oxide molecular beam epitaxy (MBE) synthesis combined with in situ angle-resolved photoemission spectroscopy (ARPES) to synthesize epitaxially strained films or heterostructures of iridates and ruthenates, and directly investigate their electronic structure. We report our efforts in employing epitaxial strain to manipulate the Fermi surface topology and superconducting transition temperature of the unconventional superconductor Sr2RuO4. We also utilize epitaxial strain to convert its sister compound, Ca2RuO4, from an antiferromagnetic insulator into a ferromagnetic metal, and subsequently manipulate its magnetic anisotropy. Finally, we describe our recent efforts in employing interfacial engineering between SrIrO3 and SrRuO3, where we are able to induce charge transfer from SrIrO3 into SrRuO3, thereby shifting the Fermi level closer to the Dirac point in SrIrO3.   

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The unconventional magnetic and electronic properties of ruthenates and other transition metal oxides with 4d valence electrons often arise from a complex interplay between spin-orbit coupling, crystal field effects, and Hund's coupling. In single-layer Ca₂RuO₄ recent neutron spectroscopy experiments revealed a novel type of soft-magnetism with strong single-ion anisotropy and ‘Higgs’ amplitude fluctuations in the spin-wave spectrum. On the other hand, bilayer Ca₃Ru₂O₇ exhibits competing antiferromagnetic (AFM) Mott-insulating and AFM-metallic phases, and even minute doping by non-magnetic Ti-ions can alter the magnetic correlations significantly. Here we report on high-resolution neutron triple-axis spectroscopy measurements of critical AFM fluctuations in proximity to the Néel temperatures of Ca₂RuO_4, Ca₃Ru₂O₇, and 1% Ti-doped Ca₃Ru₂O₇. Such fluctuations are fundamentally related to the nature of the magnetic correlations and can provide new insights into the complex magnetism of single and bilayer ruthenates.   

On Demand

Three-dimensional strontium ruthenate (SrRuO3) is an itinerant ferromagnet that features Weyl points acting as sources of emergent magnetic fields, anomalous Hall conductivity, and unconventional spin dynamics. Integrating SrRuO3 in oxide heterostructures is potentially a novel route to engineer emergent electrodynamics, but its electronic band topology in the two-dimensional limit remains unknown. Here we show that ultrathin SrRuO3 exhibits spin-polarized topologically nontrivial bands at the Fermi energy. Their band anticrossings show an enhanced Berry curvature and act as competing sources of emergent magnetic fields. We control their balance by designing heterostructures with symmetric (SrTiO3/SrRuO3/SrTiO3 and SrIrO3/SrRuO3/SrIrO3) and asymmetric interfaces (SrTiO3/SrRuO3/SrIrO3). Symmetric structures exhibit an interface-tunable single-channel anomalous Hall effect, while ultrathin SrRuO3 embedded in asymmetric structures shows humplike features consistent with multiple Hall contributions. The band topology of two-dimensional SrRuO3 proposed here naturally accounts for these observations and harmonizes a large body of experimental results.