Thermische Ausdehnung und Magnetostriktion von CeCu6-xAux bei sehr tiefen Temperaturen

Abstract

A well-studied magnetic quantum critical point (QCP) exists at the onset of antiferromagnetic order in the heavy-fermion system CeCu 6-x Au x with a critical gold concentration of x c ≈ 0.1. Due to the instability at the QCP the entropy S shows at finite temperatures a maximum as a function of x, volume V, or pressure p. The maximum leads to a sign change of the thermal expansion coefficient, α = -(1/V)(∂S/∂p), and of the Griineisen parameter Γ, the ratio of a and specific heat. This feature and the divergence of Γ at T → 0 are important thermodynamic probes to detect and classify QCPs. This work describes the set-up of a high-resolution dilatometer in a 3 He/ 4 He dilution refrigerator and reports low-temperature thermal expansion and magnetostriction measurements on the critical concentration x = 0.1 and x = 0.15 with a Neel temperature of T N ≈ 89 mK. The thermal expansion was measured in a temperature range of 30mK < T < 300 K in magnetic fields parallel to the c- and the a-axis of up to 3 T. The thermal expansion shows clearly the strong NFL behavior and a diverging Γ. A comparison with theoretical predictions for a spin density wave model though reveals a small mismatch and a slightly shifted maximum in S(x). The possibility of an additional energy scale E* is studied in the pressure and strain dependencies of a and Γ respectively. It is possible to map out a phase diagram for x = 0.15 from magnetostriction measurements. Comparing the change of different control parameters, such as pressure p and magnetic field B, the measurements reveal good scaling behavior for small changes of p and B. For larger changes a clear discrepancy in the critical behavior of p and B is measured.