Investigating the electronic and magnetic ground state of 4d³ double perovskite ruthenates

Asha Ann Abraham¹ and Soham Manni¹

¹Department of Physics, Indian Institute of Technology Palakkad, India

Ruthenates, a prominent class of 4d TMOs lie at the crossover regime of strong spin orbit coupling (SOC) and electron-electron correlation (U). This delicate balance facilitates an exotic array of emergent quantum phenomena ranging from unconventional superconductivity to quantum spin liquid behaviour in ruthenates¹. Double perovskite (DP) ruthenates (A₂BRuO₆) provides a highly tunable structural framework (owing to their site-tunability) to systematically investigate these interactions and corresponding ground states. Among them, 4d³ systems lie at the intermediate region of LS and jj coupling where the effects of SOC are unpredictable². The interplay of these competing interactions leads to non-trivial complexities as evidenced by superconductivity³ and unconventional multipolar ordering⁴ in DP ruthenates. Therefore, investigating these systems is vital for resolving the fundamental mechanisms that drive emergent quantum phenomena in the ordered DP-ruthenates.

One of the least explored series of 4d³ DP ruthenates are the A₂LaRuO₆ (A = Ca, Sr, Ba) system⁵ and Ca₂ScRuO₆⁶. Through a thorough investigation using bulk probes such as magnetization, heat capacity, resistivity, NPD measurements along with density functional theory (DFT) calculations and a highly sensitive RIXS measurements, our results show that both, A₂LaRuO₆ (A = Ca, Sr, Ba) system and Ca₂ScRuO₆ have an antiferromagnetic (AFM) Mott insulating ground state with a non - negligible effect of SOC and an unusual T² dependence of the magnetic heat capacity. Our findings establish a high frustration (f ~ 13-15) for the A₂LaRuO₆ system and an unusual magnetic ground state in Ca₂ScRuO₆, where a long range ordered AFM coexists with small magnetic clusters which manifests an unusual hyperbolic high temperature inverse magnetic susceptibility. Our results provide a clear and direct picture of a Mott insulating ground state with non-negligible effect of SOC underscoring the role of competing energy scales in 4d³ DP ruthenates. Detailed understanding of the different energy scales in Mott insulators, the parent compounds of superconducting oxides (Sr₂RuO₄⁷, Cu-doped Sr₂YRuO₆³) is crucial for resolving the mechanisms of superconductivity in 4d TMOs.

References
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