Based on high-resolution measurements of the thermal expansion and the magnetostriction we analyze the magnetoelastic coupling of TlCuCl3. We find that the phase boundary between the low-field spin liquid phase and the magnetic-field-induced Néel phase is extremely sensitive to pressure and would change by about +/-185%/GPa depending on the direction of uniaxial pressure. This drastic effect can unambiguously be traced back to the pressure dependencies of the intradimer coupling, whereas changes of the interdimer couplings under pressure play a minor role. The observed strong decrease of the interdimer coupling, when Tl is substituted by K, is therefore not a consequence of chemical pressure.

No abstract

Motivated by the observation of a giant Nernst effect in URu 2 Si 2 , the thermoelectric response of the related system UPt 2 Si 2 was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect, and electrical resistivity. Unlike URu 2 Si 2 , UPt 2 Si 2 is neither superconducting nor exhibits a "hidden-order" state. Nevertheless a pronounced Nernst effect anomaly is found to coincide with the onset of the antiferromagnetic order in UPt 2 Si 2 . Although the absolute values are substantially lower, its appearance and characteristics can favorably be compared to the giant Nernst effect in URu 2 Si 2 , indicating the common feature of a partial Fermi surface gap.The mysterious nature of the "hidden-order" ͑HO͒ phase in URu 2 Si 2 , discovered more than 20 years ago, is still unresolved. 1-3 Initially HO was associated with a weak antiferromagnetic order setting in at T HO Ӎ 17.5 K, 4,5 however recent experiments have shown this transition to be essentially nonmagnetic. 6 Transport properties such as the Seebeck effect or the Nernst and Hall effects are very sensitive in detecting slight changes in the charge carrier spectrum at the Fermi energy. The appearance of a giant Nernst effect 7 emerging with the onset of the HO impelled the investigation of these transport properties in the related system UPt 2 Si 2

We present a thermal expansion and magnetostriction study of TlCuCl3, which shows a magnetic-field induced transition from a spin gap phase to a Néel ordered phase. Using Ehrenfest relations we derive huge and strongly anisotropic uniaxial pressure dependencies of the respective phase boundary, e.g. the transition field changes by about ±185 %/GPa depending on the direction of uniaxial pressure.