Orbital ordering inCa2RuO4investigated byR99

Abstract

The ␥-␥ perturbed angular correlation technique ͑PAC͒ using 99 Ru was applied to investigate the orbital ordering of Ru 4d electrons in Ca 2 RuO 4 . It was confirmed that the ferro-orbital ordering characterized by the double occupation of d xy orbitals at every Ru ion is realized in the stoichiometric Ca 2 RuO 4 . A strong temperature dependence of the quadrupole interaction below the metal-insulator transition around 360 K reflects variation in the occupation of crystal field orbitals enhanced by very pronounced change in the lattice parameters. Temperature dependence of the hyperfine magnetic field H hf at the Ru site shows very steep increase just below the ordering temperature, which could suggest a two-dimensional character of the magnetic ordering in Ca 2 RuO 4 .

Anomalous Thermoelectric Response in an Orbital-Ordered Oxide Near and Far from Equilibrium

“…The Mott insulator Ca 2 RuO 4 [10,11] offers a unique playground to study the non-equilibrium effect on correlated electron systems. In equilibrium, this compound exhibits a firstorder metal-insulator transition at T MI ≃ 360 K [12] and an antiferromagnetic transition at T N = 110 K. The optical conductivity [13], resonant x-ray [14] and perturbed angular correlation [15] experiments have revealed that a ferro-type d xy orbital ordering is realized below T MI , as is expected from flattening of the RuO 6 octahedra at low temperatures [16][17][18][19][20], while different orbital patterns are also suggested [21][22][23]. Recently, Nakamura et al reported that this compound exhibits an electric-field-induced insulator-to-metal transition at E th ∼ 40 V/cm at room temperature [24], which is very low threshold field compared with that for Zener and avalanche breakdown [25].…”

Competition of three-dimensional magnetic phases in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

“…22 Perturbed Angular Correlation measurement also suggested two-dimensional character of the magnetic ordering in the parent compound. 48 Spin ordering in similar structures, such as high-T c cuprate La 2 CuO 4 49 and iron-pnictide BaFe 2 As 2 50 , have been demonstrated to arise from two-dimensional spin fluctuations. In the Fe-dope CRO compound, the two phases further undermines each other resulting in two separate transitions.…”