Spin-Orbit Coupling Controlled J=3/2 Electronic Ground State in 5d^{3} Oxides

Ferro-octupolar Order and Low-Energy Excitations in d^{2} Double Perovskites of Osmium

Conflicting interpretations of experimental data preclude the understanding of the quantum magnetic state of spin-orbit coupled d^{2} double perovskites. Whether the ground state is a Janh-Teller-distorted order of quadrupoles or the hitherto elusive octupolar order remains debated. We resolve this uncertainty through direct calculations of all-rank intersite exchange interactions and inelastic neutron scattering cross section for the d^{2} double perovskite series Ba_{2}MOsO_{6} (M=Ca, Mg, Zn). Using advanced many-body first-principles methods, we show that the ground state is formed by ferro-ordered octupoles coupled by superexchange interactions within the ground-state E_{g} doublet. Computed ordering temperature of the single second-order phase transition is consistent with experimentally observed material-dependent trends. Minuscule distortions of the parent cubic structure are shown to qualitatively modify the structure of gaped magnetic excitations.

Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet

Dimensionality and symmetry play deterministic roles in the laws of Nature. They are important tools to characterize and understand quantum phase transitions, especially in the limit of strong correlations between spin, orbit, charge, and structural degrees of freedom. Here, using newly-developed, high-pressure resonant X-ray magnetic and charge diffraction techniques, we have discovered a quantum critical point in Cd₂Os₂O₇ as the all-in-all-out antiferromagnetic order is continuously suppressed to zero temperature and, concomitantly, the cubic lattice structure continuously changes from space group Fd-3m to F-43m. Surrounded by three phases of different time reversal and spatial inversion symmetries, the quantum critical region anchors two phase lines of opposite curvature, with striking departures from a mean-field form at high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle excitations interact in the quantum critical region, we argue that they present the necessary components for strongly-coupled quantum criticality in this three-dimensional compound.

The critical behavior of a continuous phase transition is determined by both symmetry and dimensionality. Wang et al. present evidence that the cubic all-in-all-out spin order in Cd₂Os₂O₇ leads to a realization of a non-mean-field strongly-coupled quantum phase transition in three dimensions.