Charge correlations in cobaltates La_2–xSr_x CoO₄

Abstract

We report on an elastic neutron scattering study of the charge correlations in La2–xSrx CoO4 with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La2–xSrx CoO4. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Electronic and Magnetic Nano Phase Separation in Cobaltates La2−x Sr x CoO4

“…However, using 100 keV hard X-rays at the synchrotron, we were able to show that in La 5/3 Sr 1/3 CoO 4 the charge correlations essentially occur with CBCO character only, see Fig. 1b [13,14]. Also, in La 1.6 Sr 0.4 CoO 4 , the charge ordering peaks were not observed at the incommensurate positions that correspond to the magnetic peak positions, but, instead at half-integer peak positions typical for CBCO [8,14].…”

“…1b [13,14]. Also, in La 1.6 Sr 0.4 CoO 4 , the charge ordering peaks were not observed at the incommensurate positions that correspond to the magnetic peak positions, but, instead at half-integer peak positions typical for CBCO [8,14]. The CBCO character of the charge correlations in La 5/3 Sr 1/3 CoO 4 and La 1.6 Sr 0.4 CoO 4 is also reflected in the topmost 1 Co-O bond-stretching phonon dispersions that resemble the one in the half-doped CBCO ordered cobaltate La 1.5 Sr 0.5 CoO 4 where an anomalous phonon softening can be observed at half-integer propagation vectors, see magenta arrow in Fig.…”

“…Within the undoped islands the nearest-neighbour exchange interactions J are quite large. However, in the surrounding CBCO regions, only weak exchange iteractions J across the holes exist [14] whereas the remaining hole-rich regions can be excited only at lower energies such that the so-called hour-glass spectrum is formed from the coupling of these two very different type of excitations.…”

Single-crystal growth and physical properties of 50% electron-doped rhodate Sr1.5La0.5RhO4

“…Besides of a comparative study of Sr 2 RhO 4 single crystals isotypic with Sr 2 RuO 4 mainly polycrystalline Sr 2−x La x RhO 4 samples were studied over a wider doping range x = 0 ∼ 1.0 [9][10][11] . Regarding the recent observations of charge order and nano phase separation in the 3d cobaltate system La 2−x Sr x CoO 4 [12][13][14][15] also the availability of single crystals of the 4d Rhodium analogue with hole-doping levels away from the end member Sr 2 RhO 4 and, especially, around 50% electron doping (Sr 1.5 La 0.5 RhO 4 ), would be of interest for future studies of the charge correlations.…”

Single Crystal Growth of Pure Co3+ Oxidation State Material LaSrCoO4

“…Hard X-ray diffraction measurements for x = 1/3 [3] and neutron measurements for x = 0.4 [2], however, were not able to detect any charge stripes. Instead, checkerboard charge correlations were detected, and the hourglass spectrum could be explained in by a novel nano phase separation scenario based on La 2 CoO 4 -like undoped clusters and La 1.5 Sr 0.5 CoO 4 -like checkerboard charge ordered clusters [2][3][4][5][6]. Nevertheless, the discussion of the possible presence or absence of additional fractions of charge stripes in these cobaltates still continues [7].…”