Magnetic excitations in the spin gap system KCuCl 3 and TlCuCl 3 T. Kato a, * , A. Oosawa a , K. Takatsu a , H. Tanaka a , W. Shiramura b , K. Nakajima c , K. Kakurai c a Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan b Department of Physics, Sophia University, Tokyo 102, Japan c Neutron Scattering Laboratory, ISSP, The University of Tokyo, Ibaraki 319-11, Japan Abstract Magnetic excitation spectra of the spin S  1/2 system of the KCuCl 3 -family crystals are studied by neutron inelastic scattering experiments. The obtained dispersion curves of KCuCl 3 for Q changed in the a*– c* plane showed that this compound is likely to be a quasi-two-dimensionally coupled antiferromagnetic-dimer spin system.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; C. Neutron scattering; D. Magnetic properties 1. Introduction Quantum spin systems on some characteristic low-dimen- sional lattice have been investigated by many researchers with theoretical and experimental techniques. As the spin S  1/2 system on zigzag-type double-chain lattice, KCuCl 3 and its isomorphic copper compounds were of recent inter- est and their magnetic properties were precisely studied [1– 5]. The crystal structure of KCuCl 3 is the distorted mono- clinic structure (space group P2 1 /c) [6] from the orthorhom- bic NH 4 CdCl 3 -type (Pnma) which involves the zigzag-type double chains of edge-sharing octahedra CdCl 6 . For the copper compound KCuCl 3 and its isomorph TlCuCl 3 , the octahedra CuCl 6 are distorted by the Jahn–Teller effect, which changes the double chains to the stacked planar dimers Cu 2 Cl 6 . These dimers in the KCuCl 3 crystal structure is illustrated in Fig. 1(a) and (b). It is natural that the intra- dimer exchange interactions must be stronger than the inter- dimer interactions in each double chain. When the interdimer interactions along the double-chain are antiferro- magnetic, the KCuCl 3 -family should exhibit the character of the spin ladder. The previous measurements of the magnetic susceptibilities and the magnetization processes of KCuCl 3 and TlCuCl 3 revealed that these crystals have the singlet ground state with the spin gap of 2.74 and 0.64 meV, respectively [1–3]. However, the properties observed in these magnetic measurements are common to the spin gap systems, such as the dimers, the spin ladders and also the chain system with alternating exchange interactions. In order to select the suitable interaction model and also to determine the interaction strengths, the dispersion curves of the magnetic excitation spectra are needed. Thus, we performed the neutron inelastic scattering of KCuCl 3 , the results of which was partly published in Ref. [7]. We had shown in Ref. [7] the dispersion curves of KCuCl 3 for the scattering vectors Q on the reciprocal lines of (h, 0, 0), (h, 0, 1), (0.5, 0, l) and (1, 0, l). Our conclusion there was as follows: (i) the lowest excitation energy was found at the wavevectors Q  (0, 0, -1) and (1, 0, ^1) and evaluated as 2.70 meV, which corresponds to the spin gap. These wave- vectors contrast to that giving the gap for the case of spin ladder system, (1/2, 0, 0); (ii) the magnon band width of the dispersion curve of KCuCl 3 along the a*-axis is the same order of the gap energy, so the interdimer interactions are important; (iii) the width of the dispersion curves along the c*-axis showed also the same order of the energy band- width as the gap. Then KCuCl 3 is described as the coupled dimer system with the comparable inter- and intradimer interactions. In the present brief report, we show some addi- tional data taken at the same time and discuss the magnetic dimensionality of KCuCl 3 . A preliminary result of the inelastic neutron scattering of isomorphic compound TlCuCl 3 will be briefly mentioned towards the end. Journal of Physics and Chemistry of Solids 60 (1999) 1125–1128 0022-3697/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(99)00072-4 * Corresponding author.