U. Wyder

High field study of the magnetic phase transition of URu2Si2

We investigate the phase transition of URu2Si2 at T0 = 17.5 K by thermal expansion and heat capacity in magnetic fields, H, up to 25 T. At T0 an energy gap of Δ ⋍ 115 K opens, which decreases to 65 K in 25T. The H dependence of T0 scales with Δ, whereas a different H dependence and energy scale was observed for the ordered dipole moment. The anomalous H dependence of the thermal expansion coefficients may discriminate between the various scenarios for the primary order parameter of URu2Si2.

Magnetism and superconductivity of the U(Pt, Pd)3 system

The superconducting phase diagram of UPt3 has been studied by a variety of techniques, all leading to three different phases in the B T plane that meet the normal phase in a tetracritical point. Superconductivity has been reported to coexist with small-moment antiferromagnetism. The effects of substitutions on either the U site or the Pt site on the superconducting transition have been addressed in several studies. Pd substitutions for Pt play a particular role since the zero-field splitting between two superconducting phases increases from about 60 mK for pure UPt3 to about 120 mK for the 0.2 at % Pd alloy. On further increasing the Pd content, superconductivity is suppressed above 0.5 at % Pd, whereas between 2 and 10 at % Pd long-range antiferromagnetic order is observed with values of the uranium moment up to 0.6p~. The relevant questions in these Pd substitution studies are: by which mechanism is the splitting between the two zero-field superconducting phases enhanced and, secondly, in which way does the small-moment antiferromagnetism evolve into the large-moment antiferromagnetic order at higher Pd concentrations. In this contribution, the salient features are reviewed with respect to the occurrence of magnetic order and superconductivity in the pseudo-binary system or U(Pt, Pd)3 alloys, with an emphasis on dilatation experiments.

Spin fluctuations and heavy fermion behavior

A heuristic approach to the relation of spin fluctuations (SF) and the heavy fermion (HF) state will be presented. The appearance of SF in the eigenstates of the periodic Anderson model is discussed in terms of relevant length scales, something already known for the Kondo problem but this time presented in a slightly more general framework. It will be shown that a specific form of SF can produce a HF-like state. The magnetic ordering, its small magnetic moment (using a Stoner-like criterion), charge transfer, and the magnetic field effect will be discussed.

Length change experiments on UPt3 in high magnetic fields

We present thermal-expansion and forced-magnetostriction experiments on UPt3 in the temperature range 1.5–17 K and in magnetic fields parallel to the crystallographic a-axis up to 25 T, well above the metamagnetic field, B∗, of 20 T. For this field direction the length changes along the a-, b- and c-axis have been determined. We observe that the Gruneisen parameter, reflecting the relative strain dependence of the effective mass, collapses abruptly above B∗. In addition, we observe a feature for B > B∗ that is reminiscent of a re-entrance of the heavy-fermion state.

Thermal expansion and magnetostriction of CePt2Si2

We have measured the coefficients of linear thermal expansion of a single-crystalline sample of the tetragonal Kondo-lattice compound CePt2Si2 over a wide temperature range (0.3 < T < 220 K). The coefficient of volume expansion exhibits two broad anomalies centered at 70 and 180 K. Magnetostriction data taken at 0.4 K (B ‖ [110]), confirm the presence of a metamagnetic-like transition at 2.6 T.

High field dilatation experiments on U(Pt0.95Pd0.05)3; Magnetic ordering versus heavy fermion behaviour

Abstract A still open question is whether long-range magnetic ordering as observed in some heavy fermion (HF) systems is intimately related to the HF state or is an extra phenomenon interacting with it. This problem might be addressed by studying the compound U(Pt0.95Pd0.05)3. Doping UPt3 with 5% Pd induces long range antiferromagnetic order (LRAFO) which coexists with the heavy-fermion (HF) behavior. We present thermal expansion and magnetostriction data on U(Pt0.95Pd0.05)3 along all three crystallographic axes in magnetic fields ( B¶a- axis ) up to 17 T and the temperature range 1.5–12 K covering the critical fields and temperatures of both the LRAFO (TN = 5.8 K, BAF = 13 T) and the HF state ( T ∗ ≈ 8.2 K , B ∗ ≈ 12 T ). We observe features related to the HF-behaviour and to the LRAFO separately, but also features which we interpret as originating from the interaction between LRAFO and HF-behaviour.