Crystal and magnetic structure ofCa2RuO4:Magnetoelastic coupling and the metal-insulator transition

Abstract

The crystal and magnetic structure of Ca 2 RuO 4 ͑CRO͒ has been studied by powder neutron diffraction. CRO was synthesized in two different modifications ͑stoichiometric and containing excess oxygen͒ whose crystal structures are, in spite of strong differences in the lattice constants, closely related. Both structures are derived from the ideal K 2 NiF 4 structure type by a rotation of the RuO 6 octahedra around the long axis, combined with a tilt around an axis lying in the RuO 2 plane. The orientation of the tilt axis seems to distinguish the two room-temperature symmetries; the excess oygen compound is characterized by a smaller tilt angle and shorter Ru-O in-plane bond distances. Stoichiometric CRO undergoes large structural changes on cooling, though no symmetry change was detected. In contrast, the excess oxygen containing compound undergoes a first-order structural phase transition accompanied by a change from metallic to insulating behavior in the electric resistivity. Both compounds exhibit antiferromagnetic order below 110-150 K; for the stoichiometric sample, the onset of magnetic order is associated with several structural anomalies.

Evolution of magnetism in single-crystalCa2Ru1−xIrxYuan1, Terzic2, Wang3 et al. 2015Phys. Rev. B493751View full textAdd to dashboardCite

“…Ca 2 RuO 4 adopts a very peculiar distortion of the K 2 NiF 4 -prototype with a Pbca (61) space group consisting of layers of RuO 6 octahedra separated by Ca atoms [4,14].…”

“…Extensive investigations of Ca 2 RuO 4 [4,5] have established that a strong cooperative Jahn-Teller distortion removes the degeneracy of the three Ru t 2g orbitals (d xy , d yz , d zx ) via a transition to orbital order that, in turn, drives the metal-insulator transition at T MI = 357 K [6][7][8][9][10][11][12][13][14]. Classic Mott insulators undergo simultaneous transitions to antiferromagnetic (AFM) order and an insulating state at T MI .…”

Magnetic excitations in the metallic single-layer ruthenates Ca2−xSrxRuOSteffens1, Friedt2, Sidis3 et al. 2011Phys. Rev. BSelf Cite

“…Even though the details of the real band structure might slightly change this estimation, it imposes a very low boundary of the shift of electrons among the orbitals, and is thus in clear contradiction to the proposed scenario of an orbital selective Mott transition, 10 which requires integer filling of the bands. 35 and has, apart from a minimum in the center, a broad and flat plateau in between. These features are observed in scans at a number of different energy transfers and temperatures.…”

Higgs mode and its decay in a two-dimensional antiferromagnet

“…1). Because the local symmetry around the Ru(IV) ion is very low 8,9 (having only inversion symmetry), it is widely believed that the orbital moment is completely quenched by the crystalline electric field [10][11][12][13] , which is dominated by the compressive distortion of the RuO 6 octahedra along the c-axis (Fig. 1).…”

“…This term also acts towards suppressing the magnetic order by favouring theS z = 0 singlet ground state-known in the literature as 'spin nematic' 22 -and is responsible for the significant reduction of the moment size from the nominal 2µ B for pure S = 1 moments to approximately 1.3µ B (ref. 8). Other terms allow fine tuning of the spin-wave dispersions; the pseudodipolar term accounts for the weak dispersion along the magnetic zone boundary ( π 2 , π 2 )-(π,0), and is responsible for gapping the transverse mode.…”