Regensburg 2007 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 2: Quantum-Critical Phenomena
TT 2.4: Vortrag
Montag, 26. März 2007, 10:15–10:30, H19
Anomalous thermodynamics of spin-gap systems close to quantum phase transitions — •T. Lorenz1, S. Stark1, O. Heyer1, H. Tanaka2, and K. Krämer3 — 1II. Phys. Inst., University of Cologne, Germany — 2Dep. of Phys., Tokyo Inst. of Tech., Japan — 3Dep. of Chem. and Biochem., Univ. of Bern, Switzerland
Very rich and fascinating physical properties are predicted for various theoretical models of coupled spin-1/2 dimers. TlCuCl3 and (C5H12N)2CuBr4 can be viewed as model systems for three- and one-dimensionally coupled spin-1/2 dimers, respectively [1]. Both have nonmagnetic singlet ground states, but moderate fields of about 6 Tesla are sufficient to close the spin-gap to the first excited triplet states. Due to the three-dimensional couplings larger fields induce a Néel order in TlCuCl3, while in the spin-ladder material (C5H12N)2CuBr4 a crossover takes place to a state, which can be mapped on a Luttinger-liquid phase. In the zero-temperature limit these transitions represent examples of quantum phase transitions, whose control parameter may be easily tuned by variation of the magnetic field. We present a high-resolution study of the thermal expansion and magnetostriction on both compounds and compare our experimental results to the theoretically predicted behavior in the vicinity of a pressure-dependent quantum critical point [2].
This work was supported by the DFG through SFB 608.
[1] T. Lorenz et al., cond-mat/0609348 (2006), B.C. Watson et al., PRL 86, 5168 (2001). [2] L.J. Zhu et al., PRL 91, 066404 (2003), M. Garst and A. Rosch , PRB 72, 205129 (2005).