Orbital-driven spin-singlet dimerization in La₂Ru₄O₁₀

La₄Ru₂O₁₀ exhibits an interplay between spin, orbital, and lattice degrees of freedom. This quasi-two-dimensional compound consists of LaRuO₄ layers, formed by the corner-sharing RuO₆ octahedra, separated by buckled LaO layers. At T_S = 160 K, La₄Ru₂O₁₀ undergoes a strong first-order structural transition, accompanied by an orbital ordering of the Ru 4d electrons, resulting in a non-magnetic ground state due to the dimerization of Ru⁴⁺ S=1 spins.

In this talk, we present results from inelastic neutron scattering (INS) and X-ray diffraction studies on single-crystal La₄Ru₂O₁₀. In the INS data, we observed well-separated bands of oscillatory excitations centered at around 45 meV and 60 meV, which are indicative of the presence of strongly coupled Ru-Ru dimers, with weak inter-dimer couplings. We apply the pair distribution function (PDF) analyses on the X-ray diffraction data at various temperatures, to explore the correlations between the spin-singlet dimerization and the lattice distortion through the monoclinic to triclinic structural phase transition.