PHYSICAL REVIEW 8 VOLUME 50, NUMBER 2 Soft longitudinal modes in spin-singlet CuGe03 1 JULY 1994-II J. E. Lorenzo, K. Hirota, G. Shirane, and J. M. Tranquada Department of Physics, Brookhaven National Laboratory, Upton, ¹to York Iltt73 M. Hase and K. Uchinokura Department of Applied Physics, The Uniuersity of Tokyo, Bunkyo ku-, Tokyo II3, Japan H. Kojima, I. Tanaka, and Y. Shibuya Institute of Inorganic Synthesis, Yamanashi Uniuersity, Kofu-shi, Yamanashi 400, Japan (Received 28 January 1994) We present results from a neutron-scattering study of lattice anomalies associated with the transition to the gapped spin-singlet state observed in the spin- ~ linear-chain compound Cuoe03. Instead of the lattice dimerization along the chain direction (c axis) expected for a spin-Peierls system, we observe a re- markable softening of the longitudinal acoustic phonons in a direction (along the b axis) perpendicular to the chains. A spontaneous strain appears in the same direction below the magnetic transition tempera- ture of 14 K. Hase, Terasaki, and Uchinokura' recently reported the discovery of an unusual magnetic transition in CuGe03, an inorganic compound containing linear spin- —, Cu-0 chains. Below the transition temperature T, of 14 K, the magnetic susceptibility drops isotropically toward zero, indicating the development of an energy gap for spin ex- citations. Inelastic neutron-scattering experiments by Nishi, Fujita, and Akimitsu have confirmed the ex- istence of a gap, which reaches a value of 2. 1 meV well below T, . The singlet nature of the ground state has been directly deinonstrated by Nishi et al. , 3 who have shown that the minimum-energy excitation splits into three when a magnetic field is applied, as expected for single- triplet excitations. Measurements of the magnetic-field dependence of T„when scaled appropriately, agree quite well with both theoretical predictions and experi- mental results for organic spin-Peierls systems. It appears that CuGe03 might be an example of an inorganic spin-Peierls system. However, in a proper spin-Peierls transition, the pairing of spins in singlets occurs in the presence of a concomitant structural dimer- ization of the chains. Nishi, Fujita, and Akimitsu have found no evidence for such a chain distortion, which would double the unit cell along the c axis (parallel to the chains). They have also discovered a significant inter- chain exchange interaction, with a magnitude of approxi- mately 10% of the intrachain coupling. The coupling be- tween chains may be responsible for the very rapid growth of the gap below T, . We present here the results of a neutron-scattering study on a high-quality CuGe03 single crystal. Contrary to expectations, we observe lattice anomalies along the b direction, perpendicular to the chain direction. In partic- ular, the longitudinal-acoustic (LA) mode in this direc- tion is found, quite surprisingly, to be softer than a transverse-acoustic (TA) mode propagating in the same direction. Furthermore, a spontaneous strain along the b axis appears at the magnetic transition. Whether the dis- tortion is coupled to a modulation of the exchange along the chains or between them is not yet clear. Measurements were performed on the triple-axis spec- trometer H8 located in the High-Flux Beam Reactor at Brookhaven National Laboratory. The single crystal used for the experiment was grown by the fioating zone method and has dimensions 4X 3 X 2 mm . Although it is rather small for inelastic neutron studies, it is an excep- tionally high-quality crystal, with a mosaic of less than 3 min. The lattice parameters at room temperature are a =4. 81 A, b=8. 47 A, and c =2. 941 A, 's with c the chain axis. The sample was mounted inside an aluminum can with the crystallographic (Okl) zone in the scattering plane of the spectrometer. The sample can, filled with He to favor thermal exchange, was attached to the cold finger of a two-stage He displex. The lowest attainable temperature with this setup is 3. 3 K. The incident energy was selected by the (002) refiection of a pyrolitic graph- ite (PG) monochromator. We first characterized the phase transition by measur- ing the temperature dependence of the intensity at 1 meV, repeating the measurements of Nishi, Fujita, and Akimitsu. Measurements were done at constant final neutron energy (E& = 14.7 meV) with a PG filter after the sample and horizontal collimations of 40'-40'-80'-80', as seen from reactor to sample. The results are shown in Fig. 1. The energy gap, its temperature dependence, and the phase transition temperature are identical to those re- ported by Nishi, Fujita, and Akimitsu. Extensive elastic studies below T, were carried out along directions (0, 0.5, l) and (O, k, l+0. 5) to investigate the existence of new su- perlattice reflections. We did not find any new peaks, in complete agreement with previous results. Measure- ments of TA and LA phonons propagating along c' indi- cated a normal behavior. In particular, we did not find any anomalies in the energy widths of the longitudinal 0163-1829/94/50(2)/1278(4)/$06. 00 50 1278 1994 The American Physical Society