Electronic structure of one-dimensional cuprates K. Maiti and D. D. Sarma* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India T. Mizokawa and A. Fujimori Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Received 24 February 1997; revised manuscript received 8 August 1997 We have investigated the electronic structures of one-dimensional antiferromagnetic insulators Ca 2 CuO 3 and Sr 2 CuO 3 combining electron spectroscopic measurements and various calculations. While calculations based on a local-spin-density approach for the real magnetic structures fail to yield an insulating state, from our experiments we estimate the intrinsic band gaps in these materials to be about 1.7 eV (Ca 2 CuO 3 ) and 1.5 eV (Sr 2 CuO 3 ). Analysis of the core-level and the valence-band spectra in terms of model many-body Hamilto- nians show that the charge-transfer energy  for these one-dimensional systems is significantly smaller than other cuprates, such as the high-T c oxides two-dimensional and CuO three-dimensional. Such a small  suggests the presence of the bare upper Hubbard band within the oxygen p bandwidth and thus provides an example of a correlated covalent insulator. S0163-18299802103-1 I. INTRODUCTION The one-dimensional cuprates and nickelates have at- tracted much attention in recent times due to the presence of interesting electronic and magnetic properties. 1–11 There have been extensive experimental investigations in such sys- tems involving the transport and magnetic properties, 5–9 structural properties, 10,11 and the spin dynamics. 1–4 However, there have been few investigations using various techniques of high-energy spectroscopies. It is well known that such electron spectroscopic investigations, in conjunction with model many-body calculations, yield estimates of various electronic interaction strengths, such as the charge-transfer energy, Coulomb interaction strengths, and the hopping in- teraction strengths, thereby providing a quantitative descrip- tion of the ground state in such strongly correlated electron systems. We have carried out detailed investigations of two closely related one-dimensional cuprates, Ca 2 CuO 3 and Sr 2 CuO 3 , using various forms of electron spectroscopic tech- niques. From our studies we have estimated different inter- action strengths and we discuss the similarities and differ- ences in the electronic structures of these two compounds vis a vis cuprates with two- or three-dimensional crystal struc- tures. Our results indicate interesting ground states in these compounds, particularly for Sr 2 CuO 3 with a characteristi- cally reduced charge-transfer energy. The structure of Sr 2 CuO 3 and Ca 2 CuO 3 , shown in Fig. 1a, is K 2 NiF 4 -like space group Immm with the vacancy at the oxygen sites along the a direction. 12 The lattice constants a , b , and c are 3.48 Å, 3.91 Å, and 12.68 Å in Sr 2 CuO 3 and 3.263 Å, 3.783 Å, and 12.262 Å in Ca 2 CuO 3 , respectively. The CuO 4 polyhedra are linked by sharing corners with the Cu-O-Cu angle to be 180° only along the b direction. The CuO 4 units are perfect square in Sr 2 CuO 3 , but slightly dis- torted in Ca 2 CuO 3 with a smaller Cu-O distance along the chain ( b ) direction. 10 The parallel Cu-O chains shifted along the a direction in this structure do not couple significantly due to the absence of oxygen atoms in the edge center posi- tions along the a axis. The chains shifted along the c axis also do not interact with each other due to the large separa- tions involved. Thus, the electronic structure of these com- pounds is essentially governed by the one-dimensional Cu-O chains. Magnetic properties are also influenced by these structural aspects. The intrachain exchange interaction is very strong whereas the interchain interaction is very weak. The three-dimensional antiferromagnetic long-range order sets in only at a very low temperature ( T N  5K, 7 though FIG. 1. a The crystal structure of Sr 2 CuO 3 and Ca 2 CuO 3 show- ing the one-dimensional character of the system. b The one- dimensional oxygen sublattice considered in the present problem obtained from the chain shown with the crystal structure. The rel- evant oxygen orbitals as well as the Cu d x 2 -y 2 orbital at the impu- rity site are also shown. The dotted line encloses the unit cell of the host oxygen 2p lattice considered in the impurity calculation. PHYSICAL REVIEW B 15 JANUARY 1998-I VOLUME 57, NUMBER 3 57 0163-1829/98/573/15727/$15.00 1572 © 1998 The American Physical Society