Abstract
In low dimensional cuprates several interesting phenomena, including high Tc superconductivity, are deeply connected to electron correlations on Cu and the presence of the Zhang-Rice (ZR) singlet state. Here, we report on direct spectroscopic observation of the ZR state responsible for the low-energy physical properties in two isostructural A-site ordered cuprate perovskites, CaCu3Co4O12 and CaCu3Cr4O12 as revealed by resonant soft x-ray absorption spectroscopy on the Cu L3,2- and O K-edges. These measurements reveal the signature of Cu in the high-energy 3+ (3d8), the typical 2+ (3d9), as well as features of the ZR singlet state (i.e., 3d9L, L denotes an oxygen hole). First principles GGA + U calculations affirm that the B-site cation controls the degree of Cu-O hybridization and, thus, the Cu valency. These findings introduce another avenue for the study and manipulation of cuprates, bypassing the complexities inherent to conventional chemical do** (i.e. disorder) that hinder the relevant physics.
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Introduction
The many-body ZR singlet state1,2,3,4,5,6,7,8,9, a doped hole on the oxygen site coupled antiferromagnetically with a hole on the copper site, can be manipulated with conventional solid-state chemistry methods by partial removal of oxygen or by non-isovalent cation exchange (e.g. La3+ → Sr2+). Such chemical routes of manipulation make the the doped cuprates prone to chemical disorder causing strong structural distortions or even changes in crystal symmetry, which in turn may severely alter the properties associated with Cu d-electron derived electronic and magnetic structures. En route to the goal of realizing the ZR state without chemical disorder or lattice distortion, while the majority of the Cu oxide compounds have the Cu ions in the B-site of the perovskite ABO3 structure, the A-site ordered perovskites with chemical formula (ACu3)B4O12 are intriguing candidates for investigation due to the unique Cu A-site arrangement (see Fig. 1(a)). In this structure, a surprisingly rich set of interesting physics phenomena
Results
Resonant x-ray absorption spectroscopy (XAS) in the soft X-ray regime is an ideal tool, extensively used in the past, to investigate the Cu and O electronic structure and the effects of electron-electron correlations in high Tc cuprates2,6,7,8,25,26,27. Results of our soft XAS measurements on the Cu L3,2- and O K-edge are shown in Fig. 2 for the two A-site ordered perovskites CaCu3Co4O12 (CCCoO) and CaCu3Cr4O12 (CCCrO) along with that of the optimally doped superconducting YBa2Cu3O7−δ (YBCO) and metallic LaCuO3 (LCO) perovskite with a unique formal Cu3+ (3d8) oxidation state. From the absorption line shape in Fig. 2, it is readily apparent that in CCCoO and CCCrO a mixed valency of Cu is present; we will discuss the CCCrO compound first. The sharp peak ~ 930 eV corresponds to the transition from the d9 ground state, featuring a single hole in the Cu eg band, to the cd10 (here c denotes a core hole) excited state. The shoulder seen at 931.5 eV, a signature of the ZR singlet state, is a transition from the ground state d9L to the cd10L excited state2,5,7,26,27. The key difference between these two transitions being that while the Cu still maintains a nearly 2+ (or d9) valency, a ligand hole is distributed over the neighboring oxygens as illustrated in Fig. 1(e). The interaction between this ligand hole and the core hole created by the photon absorption raises the energy required to promote the core electron to the unoccupied state in the eg band resulting in the observed high-energy state marked by red arrows. Furthermore, to corroborate this, we show the absorption data on LCO with the formal Cu charge state of 3+. Indeed, in-line with our previous statement, the main feature of the LCO XAS around the L3-edge at ~ 931.5 eV is the transition from the d9L state, thus making the ZR state the dominant contribution to the ground state in good agreement with previous reports28,29. The aforementioned doped ligand holes are the charge carriers essential to the physics of high Tc cuprates; for instance, in hole doped cuprates the superconducting transition temperature is strongly dependent on the amount of do** present2,30,31. To clarify the connection between these compounds and the high-temperature superconducting cuprates, XAS data was taken for an optimally doped YBCO sample. As seen in Fig. 2, the Cu L-edge spectrum of CCCrO shows remarkable resemblance to YBCO, namely the dominant d9 initial states followed by the higher energy ZR state shoulder. This result highlights the capacity of the A-site ordered cuprate perovskites to mimic the effects of chemical do** without provoking unwanted lattice and stoichiometry deviations inherent to conventional chemical do**. In addition, the multiplet split peak at ~ 940 eV corresponds to a transition from the metastable 3d8 state to the cd9 excited state, indicating the presence of the ionically Cu3+ state entirely absent in YBCO.
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Acknowledgements
JC is supported by DOD-ARO Grant No. 0402-17291. JSZ and JBG is supported by NSF Grant. No. DMR-1122603. TS-D would also like to thank, CSIR and DST, India for funding. Work at the Advanced Photon Source, Argonne is supported by the U.S. Department of Energy, Office of Science under Grant No. DEAC02-06CH11357. Thanks to Dr. Bogdan Dabrowski for the SrCoO2.9 (~4+) sample. JGC acknowledges the support from the Japan Society for the Promotion of Science (Grant No. 12F02023) and the Chinese Academy of Sciences.
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D.M., J.W.F. and J.C. acquired the experimental data. S.M. and T.S.-D. did the theoretical calculations. J.G.C., J.S.Z. and J.B.G. grew the samples. S.M. and B.A.G. analyzed data and provided notes on the initial versions of the manuscript. D.M., T.S.-D. and J.C. wrote the final version of the manuscript.
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Meyers, D., Mukherjee, S., Cheng, JG. et al. Zhang-Rice physics and anomalous copper states in A-site ordered perovskites. Sci Rep 3, 1834 (2013). https://doi.org/10.1038/srep01834
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DOI: https://doi.org/10.1038/srep01834
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