Orbital ordering transition in Ca$_2$RuO$_4$ observed with resonant x-ray diffraction

The chemical and magnetic structures of the series of compounds Ca$_{2-x}$La$_x$RuO$_4$ [$x = 0$, $0.05(1)$, $0.07(1)$, $0.12(1)$] have been investigated using neutron diffraction and resonant elastic x-ray scattering. Upon La doping, the low temperature S-Pbca space group of the parent compound is retained in all insulating samples [$x\leq0.07(1)$], but with significant changes to the atomic positions within the unit cell. These changes can be characterised in terms of the l...

X-ray diffraction with photon energies near the Ru L$_2$-absorption edge was used to detect resonant reflections characteristic of a G-type superstructure in RuSr$_2$GdCu$_2$O$_8$ single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the RuO$_2...

The electric-current stabilized semi-metallic state in the quasi-two-dimensional Mott insulator Ca$_2$RuO$_4$ exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and X-ray diffraction, we show that this non-equilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure and epitaxial strain, which in turn leads to a distinct electronic band stru...

We studied the crystal and magnetic structure of Ca2RuO4 by different diffraction techniques under high pressure. The observed first order phase transition at moderate pressure (0.5 GPa) between the insulating phase and the metallic high pressure phase is characterized by a broad region of phase coexistence. The following structural changes are observed as function of pressure: a) a discontinuous change of both the tilt and rotation angle of the RuO6-Octahedra at this transit...

The alloy Ca$_{2-x}$Sr$_x$RuO$_4$ exhibits a complex phase diagram with peculiar magnetic metallic phases. In this paper some aspects of this alloy are discussed based on a mean field theory for an effective Kugel-Khomskii model of localized orbital and spin degrees of freedom. This model results from an orbital selective Mott transition which in the three-band system localized two orbitals while leaving the third one itinerant. Special attention is given to the region around...

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ~0. (iii) I...

The crystal structure of Ca_{2-x}Sr_xRuO_4 with 0.2 < x < 1.0 has been studied by diffraction techniques and by high resolution capacitance dilatometry as a function of temperature and magnetic field. Upon cooling in zero magnetic field below about 25 K the structure shrinks along the c-direction and elongates in the a, b planes (0.2 < x < 1.0), whereas the opposite occurs upon cooling at high-field (x = 0.2 and 0.5). These findings indicate an orbital rearrangement driven by...

The ground-state orbital occupancy of the Ru$^{4+}$ ion in Ca$_{2-x}$La$_x$RuO$_4$ [x=0, 0.05(1), 0.07(1) and 0.12(1)] was investigated by performing X-ray absorption spectroscopy (XAS) in the vicinity of the O K edge as a function of angle between the incident beam and the surface of the crystals. A minimal model of the hybridization between the O 2p states probed at the K edge and the Ru 4d orbitals was used to analyze the XAS data, allowing the ratio of hole occupancies $n...

We utilize near-infrared femtosecond pulses to investigate coherent phonon oscillations of Ca2RuO4. The coherent Ag phonon mode of the lowest frequency changes abruptly not only its amplitude but also the oscillation-phase as the spin order develops. In addition, the phonon mode shows a redshift entering the magnetically ordered state, which indicates a spin-phonon coupling in the system. Density functional theory calculations reveal that the Ag oscillations result in octahed...

Under zero magnetic field, a quadrupolar order parameter at q_Q=(1/2,1/2,1/2) in a typical antiferro-quadrupole (AFQ) ordering compound CeB6 has been observed for the first time by means of a resonant X-ray scattering (RXS) te\ chnique. The RXS is observed at the 2p->5d dipole transition energy of the Ce L3-edge. Using this RXS technique to observe the pure order parameter of the AFQ state, the magnetic phase diagram of Phase II is first determined.