Session A70: Ruthenates and 4D Systems

Low-energy orbital texture of Ca2RuO4 and Ca3Ru2O7

Transition metal (TM) oxides with 4d valence electrons exhibit unconventional magnetic and electronic properties, which put them into the spotlight of many investigations. The origin of these intriguing properties lays in the competition of comparable energy scales set by local interactions, including the Hund’s rule and crystal field terms, together with intrinsic spin-orbit coupling (SOC) of the TM ions. By entangling the electron spin to the shape of the electronic cloud, SOC makes the electronic spin-orbital states highly sensitive to the intersite connectivity and effective dimensionality of the underlying lattice.

On this background, a wide scientific interest has been put towards the Ruddlesden-Popper type ruthenates (Sr, Ca)_n+1Ru_nO_3n+1, which show a very rich variety of exotic ground states including superconductivity and Mott physics [1-3]. This talk presents combined oxygen K-edge x-ray absorption and resonant inelastic x-ray scattering studies of the single layer Ca₂RuO₄ and the bilayered Ca₃Ru₂O₇ [4-5]. Excitations of ruthenium d orbitals were accessed indirectly through their hybridization with oxygen p orbitals. With this method, it is possible to investigate the low-lying electronic structure site and orbital specific for these closely related compounds. Whereas Ca₂RuO₄ is an antiferromagnetic band-Mott insulator at low temperature, the ground state of Ca₃Ru₂O₇ exhibits in-plane ferromagnetic order and a metallic behaviour. This direct comparison of the low-lying spin-orbital excitations demonstrates remarkable differences. Furthermore, the experimentally observed dispersion of excitations allows their interpretation as spin-orbital quasiparticle branches.

[1] Fatuzzo et al., PRB 91, 155104 (2015)

[2] Das, L. et al., PRX 8, 011048 (2018)

[3] Gretarsson et al., PRB 100, 045123 (2019)

[4] von Arx et al., PRB 102, 235194 (2020)

[5] von Arx et al., in preparation   

Wavelength dependent coherent phonon spectroscopy of Ca2RuO4

Ca₂RuO₄ is a multi-band Mott insulator featuring complex magnetic and orbital ordered states as a function of temperature. We conducted wavelength- and temperature-dependent time-resolved optical reflectivity measurements on Ca₂RuO₄ to study how optically excited coherent phonons interact with these ordered states. I will also present a microscopic model we developed to understand the dynamics of the coupled lattice and electronic order parameters.   

Renormalization in Raman spectral evolution through electron-phonon coupling in the correlated polar metal Ca3Ru2O7

Polar metal Ca3Ru2O7 undergoes two transitions: 1) at 48K where the spins reorientation followed by the reconfiguration of electronic properties near Fermi surface drive the system from metallic to pseudo-gapped phase[1]; 2) at 30K where the Fermi surface is further reconstructed [2] and the thermal transport property drastically changes due to fermiology changes[3]. Here we demonstrate two phonon modes in Ca3Ru2O7 strongly correlated with electronic states near Fermi level and are sensitive to Fermi surface reconstruction. First principal calculation explained mode-specific e-ph constants of the two modes crossed-over at 48K. Besides, experimental evidence of phonon amplitude renormalization is found at 30K. Our result suggests Raman spectroscopy as a sensitive tool to fermiology changes in strong correlated systems.

 

[1] Markovic, I. et al. Electronically driven spin-reorientation transition of the correlated polar metal Ca3Ru2O7. P Natl Acad Sci USA 117, 15524-15529, doi:10.1073/pnas.2003671117 (2020).

[2] Horio, M. et al. Electronic reconstruction forming a C2-symmetric Dirac semimetal in Ca3Ru2O7. npj Quantum Materials 6, (2021).

[3] Xing, H. et al. Existence of electron and hole pockets and partial gap opening in the correlated semimetal Ca3Ru2O7. Physical Review B 97, (2018).   

Ru2O molecules on a pyrochlore lattice

4d ruthenium compounds are a platform for exotic electronic states such as unconventional superconductivity, excitonic magnetism or molecular orbital crystal. The ground states in these systems are governed by the interplay of correlations, spin-orbit coupling, bandwidth and crystal field. We present a newly discovered pyrochlore ruthenate In₂Ru₂O₇, where yet another factor, namely the bonding character of the constituent non-magnetic “A-site” ions - the covalency of In-O bonds - plays a pivotal role in its unique ground state. While other pyrochlore ruthenates A₂Ru₂O­₇ (A: trivalent cation) with ionic A-O bonds (A = Y, Eu…) undergo long-range magnetic order, In₂Ru₂O₇ exhibits multiple structural transitions with decreasing temperature and forms a nonmagnetic ground state with semi-isolated Ru₂O units. Dominant hopping within the Ru₂O “molecules” via the Ru-O-Ru path leads to molecular orbital formation. Such molecular orbital formation is unique as it involves the O^2- anions unlike the transition metal dimers found in systems with edge- and corner-sharing octahedra. We argue that the bond disproportionation of the covalent In-O bonds plays a key role in the structural transitions and the distinct molecular orbital formation found in In₂Ru₂O₇.   

Orbital selective metallicity in the valence bond liquid phase of Li2RuO3

Physics on a honeycomb lattice is intriguing because of the potential for novel phenomena, stemming from competitive interactions between the fundamental degrees of freedom (charge, orbital, lattice, spin). One such system is Li₂RuO₃ (LRO), which forms a valence bond crystal at room temperature. At high temperature ~ 500 K, it undergoes a phase transition that involves changes to its structural and transport properties, which leads to an exotic valence bond liquid state. The orbital degrees of freedom are expected to be critical in explaining the evolution of LRO properties across the phase transition.  We report temperature dependent broadband (100 cm^-1 - 26,000 cm^-1) reflectance measurements on single crystals of LRO to elucidate structural and transport properties through the high-temperature phase transition. We observe the expected structural transition via the change of the phonon spectrum. Additionally, the optical band gap closes at high temperature, only for electrons in the d_xz/d_yz orbitals, but the d_xy electrons remain gapped. This behavior at high temperature can be associated with an orbital selective metallic state which to our knowledge has not been previously reported in LRO.

    

Observation of spin-dependent dual ferromagnetism in perovskite ruthenates

 We performed in-situ angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) experiments to investigate the relationship between electronic band structures and ferromagnetism in SrRuO₃ (SRO) thin films. Our high quality ARPES and SARPES results show clear spin-lifted band structures. The spin polarization is strongly dependent on momentum around the Fermi level, whereas it becomes less dependent at high-binding energies. This experimental observation matches our dynamical mean-field theory (DMFT) results very well. As temperature increases from low to the Curie temperature, the spin-splitting gap decreases, and band dispersions become incoherent. Based on the ARPES study and theoretical calculation results, we found that SRO possesses spin-dependent electron correlations in which majority and minority spins are localized and itinerant, respectively. Our finding explains how ferromagnetism and electronic structure are connected, which has been under debate for decades in SRO.   

Interplay of ferromagnetism and spin-orbit coupling in metamagnetic Sr4Ru3O4

The ground state of metamagnetic materials can be precisely controlled by application of magnetic field, making them exciting candidates for spintronic applications. In itinerant metamagnets, such as Sr₄Ru₃O₁₀, understanding their electronic structure is crucial for successfull manipulation and tuning of their magnetic properties.

I will show measurements using quasiparticle interference imaging in a low-temperature scanning tunnelling microscope to study the electronic structure of Sr₄Ru₃O₁₀ in magnetic field. We find a strongly anisotropic response to the in-plane magnetic field, suggesting an unusually strong effect of the orthorhombic distortion on the electronic structure. Using DFT calculations, we can model the QPI signal and find the Fermi surface dominated by bands of spin-minority character, putting constraints on theories for the origin of the metamagnetic transition. Additionally, by including strong spin-orbit coupling in the tight-binding framework, we are able to qualitatively reproduce the observed tunnnelling spectra.   

Tunable Mottness in correlated oxide monolayer with epitaxial strain

 The electron occupancy among d orbitals determines various functionalities in transition metal oxides. Especially, in correlated materials with enough Coulomb interaction, such effect will help to modulate correlated phases, such as Mott phase. Engineering orbital degeneracy lifting is one of the efficient ways to control the electron filling. For example, electrons in d orbitals of perovskite structures can be redistributed into t_2g and e_g orbitals due to crystal field splitting. Similarly, epitaxial strain can give additional anisotropic crystal field among t_2g orbitals. However, tailoring the magnitude of orbital degeneracy lifting and observing related correlated phenomena has rarely been realized.

 SrRuO₃ monolayer film can be a good system for investigating such orbital anisotropic effect. It provides various correlated phases with moderate Coulomb interaction and large crystal field splitting among t_2g levels. Here, we controlled electron filling among d orbitals and controlled Mottness via applying epitaxial strain. The electronic structures are investigated by angle resolved photoemission and optical spectroscopy. We show metal to Mott insulator transition with tetragonal crystal field effect. Our work will provide a platform for correlated physics as well as applications for electronic devices with oxide monolayer.   

Explore the possibility of new devices in nanolayered SrRuO3/SrTiO3 heterostructures: Application for spintronic and resonant tunneling

Bonding geometry engineering of metal-oxide octahedral is a simple way of designing various functional properties of transition metal oxides. Several methods, stoichiometry control, epitaxial strain, and thickness have been proposed. But these ways accompanied structural modifications. In this work, we proposed the concept of octahedral tilt engineering using atomically designed SrRuO₃/SrTiO₃​​​​​​.

 we fabricate nanolayer designing of the SrRuO₃/SrTiO₃ superlattices. In particular, the six-unit-cell-thick SrRuO₃ layers within the superlattices undergo a phase transition in crystalline symmetry from orthorhombic to tetragonal as the thickness of the SrTiO₃ layers is increase, which can be resulted in enhancing the magnetic moment and making discrete energy state of SrRuO₃ metal layers. And evaluate both magnetotransport and vertical transport of SrRuO₃/SrTiO₃ superlattices. Consequently, the magnetic anisotropy can be manipulated by structure modulation of SrRuO₃. Furthermore, vertical transport of oxide superlattices suggest understanding the correlation between oxide thickness and vertical devices performance   

Novel subbands inelectronic spectral densities of correlated systems

In spite of important recent improvements in the theoretical handling of correlated materials, it is still difficult to obtain precise and detailed theoretical electronic structure results to compare with experiments.

We calculate and resolve with high precision the electronic spectral densities at zero temperature of one of the key models for multi-orbital strongly correlated electron materials, the Kanamori-Hubbard Hamiltonian (KH), by means of the Dynamical Mean Field Theory which uses the Density Matrix Renormalization Group (DMRG) as the (effective) impurity solver. Due to the precision of this technique, we are able to observe the existence of unforeseen subbands and structure in the local density of states for finite values of the inter-orbital coupling V, which we characterize by calculating specific response functions. 

I will also show very recent results on the electronic densities of states of a one-dimensional version of the KH model calculated with DMRG, for which we also find in-gap subbands for finite values of V.

We expect that the results presented here together with the possibility of calculating more precise spectral functions for models of correlated materials will stimulate a closer study of the details of experimental results and, hence, will contribute to unveil the complex and elusive microscopic behavior of strongly correlated materials.    

Structure and transport in Perovskite molybdates

The molybdate oxides SrMoO3, and PbMoO3 are metallic perovskites that exhibit interesting properties such as high mobility, and unusual resistivity behavior. Specifically, SrMoO3 has the highest conductivity of any known oxide, and PbMoO3 has an unusual sub-linear resistivity. We use first-principles calculations based on density functional theory and dynamical mean-field theory (DFT+DMFT) to explore the structural and transport properties in these materials. We show that accurate treatment of the electronic correlations and magnetic structure is crucial for obtaining accurate structural properties. We also demonstrate the utility of DFT+DMFT for obtaining electron-electron scattering rates as well as dc conductivity in these materials.