Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 18: Correlated Electrons: Low-dimensional Systems - Materials 2
TT 18.4: Vortrag
Dienstag, 26. Februar 2008, 16:00–16:15, H 2053
Extending Grüneisen’s relation on systems with two magnetic energy scales — •Andreas Brühl1, Bernd Wolf1, Christoph Gross1, Wolf Aßmus1, Roser Valenti2, Stefan Glocke3, Andreas Klümper3, and Michael Lang1 — 1Physikalisches Institut, Universität Frankfurt, D-60438 Frankfurt(M) — 2Institut für Theoretische Physik, Universität Frankfurt, D-60438 Frankfurt(M) — 3Theoretische Physik, Universität Wuppertal, D-42097 Wuppertal
The high-pressure phase of (VO)2P2O7 (abbr. HP-VOPO) consists of alternating S=1/2 spin chains with exchange constants J1 and J2 of similar size (≈ 120 K). This gives rise to a second, well-separated energy scale in the system, the spin gap Δ= 32 K. Thermal expansion measurements on HP-VOPO reveal distinct anomalies related to these two energy scales. However, the relative size of the low-temperature anomaly, corresponding to the spin gap, is very much enlarged compared to its size in the specific heat. Therefore, the commonly used Grüneisen relation, assuming a proportionality between the magnetic contributions αmag and Cmag to thermal expansion and specific heat, does not apply. Instead, an extended form of this relation is presented, which contains an additional term in αmag proportional to the derivative of the magnetic entropy with respect to the spin gap. Using accurate T-DMRG calculations of the required thermodynamic quantities, this leads to a good description of the thermal expansion of HP-VOPO in the whole temperature range. This method could be generalized to cover other spin systems with two or more exchange constants, e. g. to determine their values via thermal expansion measurements.