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TT: Fachverband Tiefe Temperaturen
TT 22: Correlated Electrons: Quantum-Critical Phenomena 2
TT 22.1: Invited Talk
Wednesday, March 25, 2009, 09:30–10:00, HSZ 03
Thermal expansion and magnetostriction close to quantum criticality — •Markus Garst — Institut für Theoretische Physik, Universität zu Köln, Zülpicher Str. 77, 50937 Köln
Quantum phase transitions occur in materials at zero temperature upon tuning an external parameter, e.g., magnetic field to an instability of the ground state. A coupling of the quantum critical fluctuations to the lattice degrees of freedom can be exploited to probe quantum criticality at finite temperatures. We discuss the resulting anomalous signatures in thermal expansion, magnetostriction and the Grüneisen parameter, which provide a valuable tool not only to detect but also to classify a quantum phase transition. A smoking gun for the existence of such a transition is, e.g., the divergence of the Grüneisen parameter with an exponent characteristic for its universality class. We also explain that a negative thermal expansion naturally accompanies such transitions, and that its sign changes indicate the entropy distribution in the phase diagram. As examples, we discuss (a) quantum critical metamagnetism, a concept introduced for the bilayer ruthenate Sr3Ru2O7, and (b) the critical properties of the spin-ladder compound (C5H12N)2CuBr4, that exhibits a diverging thermal expansion.