INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 18 (2006) 4065–4070 doi:10.1088/0953-8984/18/16/012 Phonon analysis of the S = 1 quantum spin systems Ni 5 Te 4 O 12 X 2 (X = Cl and Br) G Caimi 1 , L Degiorgi 1 , H Berger 2 and L Forr´ o 2 1 Laboratorium f¨ ur Festk¨ orperphysik, ETH Z¨ urich, CH-8093 Z¨ urich, Switzerland 2 Institut de Physique de la Mati` ere Complexe (IPMC), EPF Lausanne, CH-1015 Lausanne, Switzerland Received 22 December 2005 Published 7 April 2006 Online at stacks.iop.org/JPhysCM/18/4065 Abstract We report our investigations of the electrodynamic response of the S = 1 quantum spin systems Ni 5 Te 4 O 12 X 2 (X = Cl and Br), which develop a magnetic ordered state below 30 K. We measure the optical reflectivity over a broad spectral range extending from the far infrared up to the ultraviolet. Besides identifying the electronic interband transitions, we primarily focus our attention on the lattice dynamics, emphasizing the phonon modes spectrum and its temperature dependence. Our findings do not reveal any direct link between possible structural anomalies and the transition into the magnetically ordered state at low temperatures. (Some figures in this article are in colour only in the electronic version) Low-dimensional quantum spin systems have attracted considerable attention over the past few years from both the theoretical and experimental points of view [1]. In these systems the long-range magnetically ordered phases (ferro- or antiferromagnetic) are suppressed by the effect of quantum fluctuations. There are various origins for quantum fluctuations: the reduced dimensionality of the investigated system (chains or layers), the small spin ( S = 1/2 or 1) and/or the effect of magnetic frustration (competition between exchange processes). The interest in such phenomena has mainly been motivated by the investigation of the two- dimensional superconducting cuprates (HTC) as well as of related transition metal oxides. Very often the two-dimensional exchange topologies provide the basis for an effective interplay between competing interactions with remaining spin anisotropies. This leads to rich phase diagrams of short- and long-range ordered phases dependent on certain coupling parameters [1]. Several materials have been designed and developed so far as prototype examples of low- dimensional quantum spin systems. There has been much experimental activity directed at S = 1/2 materials involving Cu 2+ ions with a 3d 9 configuration, or Ti 3+ and V 4+ systems in the d 1 configuration. To give a few examples we start by mentioning the spin-Peierls CuGeO 3 system and the charge ordered NaV 2 O 5 compound, representing the class of materials with broken translational symmetry [1]. Moreover, the two-leg spin ladder system SrCu 2 O 3 0953-8984/06/164065+06$30.00 © 2006 IOP Publishing Ltd Printed in the UK 4065