Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,C.pdfVIP

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Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,C.pdf

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Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,C

a r X i v : c o n d - m a t / 0 3 0 1 1 1 9 v 1 9 J a n 2 0 0 3 Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu2Te2O5X2 (X=Br,Cl) Roser Valent??1, T. Saha-Dasgupta2, Claudius Gros1 and H. Rosner3,? 1Fakulta?t 7, Theoretische Physik, University of the Saarland, D-66041 Saarbru?cken, Germany. 2S.N. Bose National Centre for Basic Sciences, JD Block, Sector 3, Salt Lake City, Kolkata 700098, India. 3 Department of Physics, University of California, Davis CA 95616 (February 2, 2008) Motivated by recent discussion on possible quantum critical behavior in the coupled Cu- tetrahedra system Cu2Te2O5Br2, we present a comparative ab initio study of the electronic prop- erties of Cu2Te2O5Br2 and the isostructural Cu2Te2O5Cl2. A detailed investigation of the copper- copper interaction pathes reveals that the halogen-ions play an important role in the inter-tetrahedral couplings via X4-rings (X=Br, Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange pathes mediated by the X4-rings. PACS numbers: 75.30.Gw, 75.10.Jm, 78.30.-j The recently discovered [1] spin-tetrahedral com- pounds Cu2Te2O5X2 (X=Cl, Br) open the possibility to study the interplay between localized many-body tetra- hedral cluster excitations and inter-tetrahedral magnetic couplings leading to a quantum-phase transition with various possible ordered states [2]. Transitions to ordered states have been observed experimentally [3] in these compounds with T (Br) N = 11.4 K and T (Cl) N = 18.2 K. These phase-transitions exhibit unusual magnetic-field dependences which have been linked to the closeness to a quantum-critical point [3,4]. Unconventional Raman- scattering has been found in the magnetic channel [2,3] and the occurrence of low-lying singlet excitations has been proposed [1] and observed by Raman together with a longitudinal magnon [4]. The nature of the ordered states in Cu2Te2O5X2 has not yet been definite