Session G39: 5d/4d Transition Metal Systems II

Competing spin-orbital singlet states in d4 honeycomb system

4d and 5d transition-metal compounds hosting spin-orbit-entangled states have emerged as a rich platform of unconventional electronic phases. The interactions between spin-orbit-entangled pseudospins in these compounds are distinct from those of spin-only magnets, which may give rise to exotic magnetic ground states. The examples include Kitaev magnetism in d⁵ J_eff = 1/2 systems and multipolar physics in d¹ (J_eff = 3/2) and d² (J_eff = 2) compounds.

 

In this talk, I will focus on the d⁴ compounds with strong spin-orbit coupling. While spin-orbit coupling yields nonmagnetic J_eff = 0 singlet state for an isolated d⁴ ion, it has been proposed that their interactions via excited states lead to a rare example of exotic magnet called excitonic magnet. Excitonic magnets are expected to display a variety of intriguing phenomena including magnetic exciton condensation, quantum criticality and Higgs mode excitation. Particularly, d⁴ honeycomb compounds are proposed to realize frustrated exitonic magnetism where unconventional magnetic ground states such as a spin-nematic state and bosonic Kitaev liquid may emerge.

 

Despite these tantalizing perspectives, honeycomb-based excitonic magnets have remained elusive. We found that the honeycomb ruthenate Ag₃LiRu₂O₆ realizes a honeycomb lattice of spin-orbit-entangled singlet states serves as a promising starting point for frustrated excitonic magnetism. Under pressure, Ag₃LiRu₂O₆ undergoes successive phase transitions to other spin-orbital singlet phases instead of developing excitonic magnetism. While the high-pressure phase is characterized by  molecular-orbital formation as found in other honeycomb-based compounds, the intermediate phase represents a spin-orbit-entangled weak dimer state which we argue is induced by a pseudo-Jahn-Teller effect and is unique to d⁴ systems. The identification of competing spin-orbital phases on a honeycomb lattice should give a clue to the realization of unconventional magnetic ground states.   

J=0 electronic states in the low-dimensional osmium halides with the 5d4 configuration

Due to the competition among hopping t, crystal field splitting Δ, electronic correlations (U, J_H), and spin-orbit coupling l, many electronic states are possible for a d⁴ system, such as S = 2, S = 1, and J =0 states. Here, we systematically study the 1D Os-chain systems (OsCl₄, OsOCl₂) ^[1,2] and the “zero-dimensional” K₂OsX₆ (X = F, Cl, and Br) ^[3] with the 5d⁴ electronic configuration. Because of the large bandwidth or large crystal-field splitting Δ (between different t_2g orbitals), the J = 0 state is suppressed in those 1D Os-chains, resulting in an S = 1 spin-1 system ^[1,2]. However, in K₂OsX₆, the well-separated OsX₆ octahedra lead to the cubic crystal-field limit and result in dramatically decreasing hopping t among nearest neighbor Os-Os sites. In this case, the J = 0 nonmagnetic state develops in this system ^[3]. Our results provide guidance to experimentalists and theorists working on this interesting family of osmium halides.   

Magnetic properties of Cs2IrCl6 single crystals

Cs₂IrCl₆ is a member of cubic antifluorites A₂IrCl₆ (where A is an alkali metal or NH₄) featuring isolated IrCl₆ octahedra arranged on the FCC lattice. This class of material has gained renewed interest recently as a model FCC antiferromagnet with J_eff = ½ involving strong spin-orbit coupling and a combination of Heisenberg and Kitaev exchange interactions [1]. In contrast to K₂IrCl₆, very little work has been done on Cs₂IrCl₆. Here we will present the synthesis and characterization of Cs₂IrCl₆ single crystals. A paramagnetic-to-antiferromagnetic transition was revealed by specific heat and magnetic susceptibility measurements at T_n » 0.5 K. The spin structure was determined by neutron diffraction measurements. We will discuss the magnetic properties of Cs₂IrCl₆ in comparison to other cubic antifluorites in this family.    

[1] Nazir Khan, et al, Physical Review B 99, 144425 (2019).

     

Direct imaging of valence orbitals using hard x-ray photoelectron spectroscopy.

It was hypothesized already more than 40 years ago that photoelectron spectroscopy should in principle be able to image atomic orbitals. If this can be made to work for orbitals in crystalline solids, one would have literally a different view on the electronic structure of a wide range of quantum materials.

Here, we demonstrate how hard x-ray photoelectron spectroscopy can make direct images of the orbitals making up the band structure of our model system, ReO3 [1]. The images are energy specific and enable us to unveil the role of each of those orbitals for the chemical bonding and the Fermi surface topology.

The method is purely experimental, i.e. theoretical calculations are not required, and thus has a big potential for the study of the so-called strongly correlated materials, for which ab-initio theories are known to be unreliable due to complexity caused by the many-body interactions. With our imaging technique, we will still be able to obtain the local atomic many-body wavefunction information.

The orbital image information is complementary to that from angle-resolved photoemission and thus completes the determination of the electronic structure of materials.

[1] D. Takegami et al. Phys. Rev. Research 4, 033108 (2022); https://doi.org/10.1103/PhysRevResearch.4.033108   

Competing energy scales in a rhodium oxide spinel

Materials possessing frustrated crystal structures in conjunction with interacting spin, charge, and orbital degrees of freedom offer an avenue toward non-trivial quantum phenomenology. The spinel structure type is one that possesses frustration through its intertwined diamond and pyrochlore sublattices, and the spinel LiRh2O4 has been demonstrated to possess spin dimerization, charge ordering, and orbital ordering at tangible energy scales [1]. Furthermore, recent measurements of the local structure in this material indicate that the charge ordering arises from strong correlations and is similar to that of magnetite, satisfying the ‘Anderson condition’ for pyrochlore lattices [2]. In this talk, we will demonstrate the effect of electron and hole doping in the structural and physical properties of LiRh2O4, with an emphasis on the effect of dopants on the formation of dimers via changes in the local structure as indicated by pair distribution function measurements. Our results demonstrate an opportunity to study competing energy scales on a frustrated, mixed-valent pyrochlore lattice.

[1] Okamoto, et al., Phys. Rev. Lett. 101, 086404 (2008).

[2] Shiomi, et al., Phys. Rev. B 105, L041103 (2022).

    

Probing the Ground States of the Kitaev-Heisenberg Model in RuCl3 through Guarded Electrical Measurements

Alpha Ruthenium Chloride (α-RuCl3) is part of a special class of Mott insulators, Kitaev materials, that exhibit strong spin orbit coupling and bond directional exchange interactions. In particular, the Kitaev-Heisenberg model predicts this material to host interesting ground states such as a Quantum Spin Liquid (QSL) with fractionalized excitations making it ideal for applications in topological quantum computing, as well as magnetic ordered states. Through guarded measurements of RuCl3 thin crystal electronic devices, we find that electronic transport is dominated by thermally activated transport at high temperatures and Efros-Shklovskii Variable Range Hopping transport at low temperatures down to the known zig zag antiferromagnetic ordering transition for RuCl3. Measurements demonstrate that this antiferromagnetic ordering is robust up to fields of 11T out of the plane. Future experiments will explore the effects of a magnetic field in the plane of the sample and its effect on the electronic transport mechanism.   

Metallic Bi-Rh-O thin films with p-type conduction

Transition metal oxides with partially filled 4d and 5d orbitals have recently been viewed as promising target materials for the discovery of novel properties, due to their strong spin-orbit coupling that can generate exotic electronic states. Here, we report the fabrication of three kinds of Bi-Rh-O compounds. They are synthesized by the deposition of amorphous films at room temperature by pulsed laser deposition and subsequent annealing in a tube furnace under O₂ atmosphere. On Y-ZrO₂ (111) substrates, two novel compounds with supercell structures [Bi_nO_n]-[RhO₂] (n = 3, 2) are synthesized at annealing temperatures (T_anneal) of 700 °C and 900 °C, respectively. When a buffer layer of pyrochlore oxides is deposited on Y-ZrO₂, single crystalline pyrochlore Bi₂Rh₂O₇ is stabilized at T_anneal of 1,000 °C. These results demonstrate the critical role of the interface-engineering in stabilizing certain crystal structures, which will pave the way for further exploration of Rh oxides and materials design of transition metal oxides. Hall resistivity at 2 K is merely linear to an applied magnetic field with a positive slope for all the compounds, and the estimated carrier densities are p = 3×10²¹ cm^-3 ([Bi₂O₂]-[RhO₂]), 6×10²¹ cm^-3 ([Bi₃O₃]-[RhO₂]), and 1×10²³ cm^-3 (Bi₂Rh₂O₇). The magnetoresistance at 2 K shows quadratic ([Bi₂O₂]-[RhO₂]) and linear ([Bi₃O₃]-[RhO₂], Bi₂Rh₂O₇) field dependence, suggesting strong spin-orbit coupling.   

Temperature and pressure driven structural phase transition in GaPt5P.

Recent discoveries of novel magnetism in the TMPt₅P and TMPd₅P (TM = 3d transition metal) based families [1-3] has brought renewed interest to the wider class of M(Pt/Pd)₅X (M = TM, IIIA elements, and rare earths; X = P, As) materials. IIIA element-based M(Pt/Pd)₅X, where M = Al, Ga, and In, are non-magnetic analog of magnetic TM(Pt/Pd)₅X and other than structure, are relatively un-characterized. Here, we present detail studies of GaPt₅P single crystals using ambient and high pressure resistivity measurements as well as ambient pressure magnetization and X-ray diffraction measurements. At ambient pressure, a step-like feature at about T ~ 80 K in both resistivity and magnetization measurements indicates a possible structural change. The hysteretic nature suggests the transition to be the first-order type. Furthermore, X-ray diffraction measurements confirm the first-order structural transition to a lower symmetry structure with a volume collapses. Hydrostatic pressure increases the transition temperature bringing it to room temperature by ~ 2.3 GPa. [1] X. Gui et. al., Appl. Electron. Matter. 3, 3501 (2021). [2] X. Gui et. al., Chem. Mater. 32, 3922 (2020). [3] T. J. Slade and P. C. Canfield Z. Anorg. Allg. Chem. 648, e202200145, 2022.   

Observation of orbital fluctuation inducing unconventional magnetic order in a correlated insulator

Intertwined electronic and orbital degrees of freedom in strongly correlated materials with strong spin-orbit-coupling has shown to give rise to many exotic quantum phenomenon, especially to emergent quantum phases and transitions ^[1,2], such as quantum spin liquids, multipolar orderings, cooperative Jahn-Teller (JT) effects, and correlated topological semi-metals. 4d and 5d transition metal compounds have provided a rich playground for realizing these correlated quantum phenonmenon. In these materials, it is often difficult to determine the driving mechanism for the emergent phases. Here we implement a multi-modal technique ^[3] that is able to independently probe different symmetry-related order parameters to determine the primary driving order parameter in such a system. Examples of the application of the technique to real materials will be discussed. 

[1] Witczak-Krempa, William, et al. "Correlated quantum phenomena in the strong spin-orbit regime." Annu. Rev. Condens. Matter Phys. 5.1 (2014): 57-82.

[2] Chen, Gang, Rodrigo Pereira, and Leon Balents. "Exotic phases induced by strong spin-orbit coupling in ordered double perovskites." Physical Review B 82.17 (2010): 174440.

[3] Carr, Stephen, et al. "Multi-modal spectroscopy of phase transitions." arXiv preprint arXiv:2208.10987 (2022).

 

    

Strategy to Develop the High-Performance Thermomagnetic Gd-Si-Ge Alloys for Low-grade Waste Heat Energy Harvesting

Searching for new technology for energy harvesting from low-grade waste heat via the use of magnetic phase change materials is one of the focused research areas within the domain of renewable energy resources. The key challenges to be addressed simultaneously are (i) a large change in magnetization with temperature, (ii) magnetic phase transition at/near room temperature, and (iii) high thermal conductivity. In this work, we established SeS₂ as an extraordinary sintering aid that effectively densifies the high-performing Gd₅Si_2.4Ge_1.6 thermomagnetic material via low-temperature spark plasma sintering at 200 °C. The weight ratio of SeS₂ per unit gram is optimized to improve the density by 32%, thermal conductivity by 90%, and maintained the large magnetization in the Gd₅Si_2.4Ge_1.6 compound. The magnetic phase transition of 33 °C is successfully achieved for (Gd₅Si_2.4Ge_1.6)_0.9(SeS₂)_0.1 material.

In order to further improvement in the magnetic and thermal properties of Gd₅Si_2.4Ge_1.6 compound, a constructive screening of the best composition with doping at Gd site is desirable. In this direction the thermomagnetic properties are estimated by partially filled 3d, 4d, and 4f elements from transition metal and rare-earth elements by using the first principle modeling based tools implemented in KKR-CPA and VASP codes. Computational results show that by co-doping of V and Co element at Gd site the magnetic properties improves by 56 % as compared to un-doped Gd₅Si_2.4Ge_1.6 compound.

    

Crystal Structure of NaOsO3 under High Pressure Determined by Single Crystal X-ray Diffraction

The 5d transition metal oxide NaOsO₃ attracts many attentions due to its metal-insulator transition, the origin of which is still under debate [1-3]. At ambient condition, NaOsO₃ crystallizes into an orthorhombic perovskite structure (Pbnm; a = 5.32817 Å, b = 5.38420 Å, c = 7.58038 Å). A high-pressure structural study was performed on polycrystalline NaOsO₃, which reported a highly unusual phase transition from nonpolar phase (Pbnm) to polar phase (Pna₂₁) at ~18 GPa [4]. Motivated by this report, we conducted the single-crystal synchrotron XRD measurement at high pressures up to 40 GPa and analyze the pressure effect on the crystal structure of NaOsO₃. Pressure enlarges the orthorhombic distortion, i.e. splitting the lattice parameters a, b, and c/, shown in figure 1. The single-crystal diffraction data of NaOsO₃ can be refined reasonably well with the Pbnm (No. 62) structural model up to ~40 GPa. In this talk, we will also compare the pressure effect on the orthorhombic perovskite NaOsO₃ with other 3d and 4d orthorhombic perovskite compounds.

[1] Y. G. Shi, Y. F. Guo, S. Yu, M. Arai, A. A. Belik, A. Sato, K. Yamaura, E. Takayama-Muromachi, H. F. Tian, H. X. Yang, J. Q. Li, T. Varga, J. F. Mitchell, and S. Okamoto, Continuous metal-insulator transition of the antiferromagnetic perovskite NaOsO₃, Phys. Rev. B. 80, 161104 (2009).

[2] S. Calder, V. O. Garlea, D. F. McMorrow, M. D. Lumsden, M. B. Stone, J. C. Lang, J. W. Kim, J. A. Schlueter, Y. G. Shi, K. Yamaura, Y. S. Sun, Y. Tsujimoto, and A. D. Christianson, Magnetically Driven Metal-Insulator Transition in NaOsO₃, Phys. Rev. Lett. 108, 257209 (2012).

[3] J. G. Vale, S. Calder, C. Donnerer, D. Pincini, Y. G. Shi, Y. Tsujimoto, K. Yamaura, M. M. Sala, J. van den Brink, A. D. Christianson, and D. F. McMorrow, Evolution of the Magnetic Excitations in NaOsO₃ through its Metal-Insulator Transition, Phys. Rev. Lett. 120, 227203 (2018).

[4] R. Sereika, P. Liu, B. Kim, S. Kim, J. Zhang, B. Chen, K. Yamaura, C. Park, C. Franchini, Y. Ding, and H.-k. Mao, Aberrant electronic and structural alterations in pressure tuned perovskite NaOsO₃, npj Quantum Mater. 5, 66 (2020).