Berlin 2005 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 9: Solids at Low Temperature - Quantum Liquids, Bose-Einstein Condensates, Ultracold Atoms, ...
TT 9.6: Vortrag
Freitag, 4. März 2005, 18:30–18:45, TU H3027
Magnetoelasic Coupling in the Spin Dimer System TlCuCl3 — •N. Johannsen1, T. Lorenz1, A. Vasilev2, A. Oosawa3, and H. Tanaka4 — 1II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln — 2Departement of Low-Temperature Physics, Moscow State University, Moscow 119899, Russia — 3Advanced Science Research Center, Japan Atomic Energy Research Institue, Tokai, Ibaraki 319-1195, Japan — 4Departement of Physics Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-8551, Japan
We present high-resolution measurements of the thermal expansion and the magnetostriction on a single crystal of TlCuCl3 which shows a field-induced magnetic order. In this material Cu2+ ions form S=1/2 spin dimers with an antiferromagnetic coupling. The spin gap between the singlet ground state and the first excited triplet state amounts only to about 0.65 meV. Therefore, a moderate magnetic field H > 6 T induces antiferromagnetic order with a staggered magnetization perpendicular to the applied field. The phase transition causes pronounced anomalies of opposite signs in the field and temperature dependencies of different lattice directions signaling a large magnetoelastic coupling. These anomalies allow for a precise determination of the phase boundaries, which turns out to be extremely sensitive to pressure, e.g. the transition field may change up to ±200 %/GPa depending on the direction of uniaxial pressure. This drastic effect can be unambiguously traced back to changes of the intradimer coupling under pressure, whereas the interdimer couplings remain essentially unchanged. On the other hand, a replacement of the Tl by smaller K ions causes a strong change in the interdimer couplings.