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TT: Tiefe Temperaturen

TT 15: Metall-Isolator-Überg
änge, Phasenüberg
änge in Quantensystemen I

TT 15.9: Vortrag

Mittwoch, 13. März 2002, 16:45–17:00, H20

From Singlet to Néel State - a Substitution Study on the Spin Tetrahedra System Cu₂Te₂O₅(Br_1−xCl_x)₂ — •P. Lemmens^1,2, K.Y. Choi², G. Güntherodt², H. Kageyama³, Z. Hiroi³, W. Schäfer⁴, C. Geibel⁵, W. Brenig⁶, C. Gros⁷, R. Valenti⁷, P. Millet⁸, K.W. Törnroos⁹, M. Johnsson¹⁰, and F. Mila¹¹ — ¹IMNF, TU Braunschweig — ²2. Phys. Institut, RWTH Aachen — ³ISSP Tokyo, Kashiwa, Chiba, Japan — ⁴MIN/ZFR-FZ Jülich — ⁵MPI-CPfS, Dresden — ⁶Theor. Physik, TU Braunschweig — ⁷Theor. Physik, Univ. Saarbrücken — ⁸CEMS-CNRS, Toulouse, France — ⁹Dept. Chemistry, Univ. of Bergen, Norway — ¹⁰Dept. Inorg. Chemistry, Stockholm Univ., Sweden — ¹¹Inst. de Phys. Theor., Univ. Lausanne, Switzerland

The recently found quantum spin system Cu₂Te₂O₅(Br_1−xCl_x)₂ with weakly coupled s=1/2 spin tetrahedra allows in a unique way to interpolate between two very different ground states as the unit cell volume and the inter-tetrahedra coupling is dependent on the stoichiometry x [1]. With x=0 and x=1 a singlet or a Néel ground state is realized, respectively, as evidenced by spectroscopic and thermodynamic properties. Samples with 0<x<1 show a continuous tuning of the phonon frequencies without an enlargement of their linewidth. The magnetic excitation spectrum of the system, however, changes drastically as for x ≈ 1/3 a large number of low energy modes are induced. Further thermodynamic studies support this observation of a regime with different low temperature properties. We acknowledge support by DFG SPP1073.
P. Lemmens, et al, PRL87, 227201 (2001).