Charge±spin±orbitalcouplinginCuO X.G.Zheng a, * ,C.N.Xu b ,E.Tanaka c ,Y.Tomokiyo c ,M.Suzuki a ,E.S.Otabe d a Department of Physics, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan b Kyushu National Industrial Research Institute, Saga 841-0052, Japan c High Voltage Electron Microscopy Laboratory, Kyushu University, Fukuoka 812-8581, Japan d Department of Computer Science and Electronics, Kyushu Institute of Technology, Iizuka 820-8502, Japan Received16October2000;accepted23November2000 Abstract The electron pairing mechanism for superconductivity in cuprates is generally attributed to magnetic interactions becausesuperconductivityoccursonthevergeoffrustratedlow-dimensionalantiferromagneticorderinCu±Oplanesor chains.CuOisthesimplestcopperoxidethatmaybethefundamentalonenotjustbecausetillnowonlycompounds containingtheCu±O±Cubondhaveproducedthehigh-T c superconductivitybutalsobecauserecentlywefoundthat charge stripes exist in this simplest compound. Here we report strong charge±spin±orbital coupling in CuO, which showsthatCuOisareferencesystemforstudyingtheelectroncorrelationincuprates. Ó 2001PublishedbyElsevier ScienceB.V. PACS: 71.30.h;71.70.Ej;74.10.v;74.25.-q;75.40.-s;75.50.Ee Keywords: Cupric oxide;Electroncorrelation;Charge±spin±orbitalcoupling 1. Introduction In transition-metal oxides with anisotropic- shaped d-orbital electrons, the orbital degree of freedom occasionally plays an important role in metal±insulatortransitionsandpropertiessuchas high-temperature superconductivity high-T c )and colossalmagnetoresistanceCMR)throughstrong coupling with charge, spin, and lattice dynamics [1].Cupricoxide,CuO,hasamonoclinicstructure withfournearestOatomsandtwodistantapical O atoms forming a very distorted octahedron around the Cu atom illustrated in Fig. 1) [2]. Cu 2 3d 9  inCuOresemblesMn 3 3d 4  inman- ganese oxides in having e g orbital degeneracy in anoctahedral®eld,andhenceundergoingalarge JTdistortion.LikeCu 2 inanumberofbinuclear cupriccomplexessuchasincupricacetatemono- hydrate [3,4], this distortion would lift the e g or- bitaldegeneracywithhigherenergyford x 2 y 2 than for d z 2 orbital due to the distant apical O atom coordination, which causes a similar situation as the Mn 3 3d 4  in that only the d x 2 y 2 orbital is relevant.Theplanar-coordinatedCuisalsosimilar to that in high-T c copper oxides such as YBa 2 - Cu 3 O 7 . Previous research has clari®ed two mag- netic transitions in CuO; long-range AF ordering at T N1  230Kfollowedbyanincommensurateto commensuratetransitionwithspinreorientationat PhysicaC357±3602001)181±185 www.elsevier.com/locate/physc * Corresponding author. Tel.: +81-952-28-8588; fax: +81- 952-28-8547. E-mail address: zheng@cc.saga-u.ac.jpX.G.Zheng). 0921-4534/01/$-seefrontmatter Ó 2001PublishedbyElsevierScienceB.V. PII:S0921-453401)00228-3