P. Hellmann

New observations concerning magnetism and superconductivity in heavy-fermion metals

Abstract Ce-based heavy-fermion superconductors (HFSC) exhibit magnetic phase diagrams differing from those of both the ideal Kondo lattice and substitutional alloys with intact Ce sublattice. While for the Ce-based HFSC (with well localized 4f shell) superconductivity and antiferromagnetic order seem to compete with each other, both phenomena typically coexist for U-based HFSC (with less localized 5f shell).

Ground state properties of UNi2Al3 and UPd2Al3

Abstract We present a status report on our investigation of the two new heavy fermion superconductors UNi 2 Al 3 and UPd 2 Al 3 . We discuss in particular the magnetic phase diagram, crystal field effects and the properties of the superconducting state.

The effect of composition on the occurrence of a second phase transition in the vicinity of Tc in CeCu2Si2

We report on measurements of the thermal expansion and specific heat on a series of polycrystalline CeCu 2 Si 2 samples with small variations in the stoichiometry. The occurrence of superconductivity and/or the surrounding phase labeled »A«, were found to be most sensitively controlled by the Ce:Cu:Si composition

Classification of strongly correlated f-electron systems

Ce-based heavy-fermion (HF) metals behave as Kondo-lattice systems and can be classified with the aid of a single coupling parameter, ¦J ¦/W,where J < 0is the local exchange integral and W the conduction-band width. Depending on its actual composition, the exemplary material CeCu2Si2chooses one out of two ground states: HF superconductivity and a new magnetic HF phase “A”. In a narrow composition range, these two phases are nearly degenerate and expell each other upon varying either the temperature or the external magnetic field. The one-parameter-scaling approach appears inapplicable to the U-based HF metals. For the exemplary material UPd2Al3, antiferromagnetic ordering between seemingly local 5f moments coexists, on a microscopic scale, with HF superconductivity. Whether this coexistence can be explained by assuming 5 f states localized on a tetravalent U-ion with non-magnetic crystal-field ground state remains to be shown. We discuss arguments which invokeitinerant 5 f states in the U-based HF metals to be distinguished from thelocalized 4f states in the Ce-based counterparts.

Scaling of the “Cp α T 1nT” dependence in Ce systems

We analyze the T 1n T dependnece of the specific heat (Cp) in several Ce binaries and ternaries. We find that these systems can be directly compared by using a scaling law of the type: Cp/t = −D log t + ET0, where t = T/T0, with the scaling temperature T0 ∼ TK, D = 7.2 J(mol K) and 0 < E < 0.14 J/mol K2). This general function describes those systems within the 0.005<t<0.5 range and acounts for nearly 12 of the expected entropy of the Ce-doublet.

“Non-Fermi-liquid” phenomena in heavy-fermion CeCu2Si2 and CeNi2Ge2

Abstract We report low-temperature results of specific-heat and resistivity measurements on the heavy-fermion (HF) compounds CeCu 2 Si 2 and CeNi 2 Ge 2 . “Non-Fermi-liquid” effects are observed which suggest the nearness of an antiferromagnetic quantum critical point (QCP) in either system. The observed deviations from the properties of a Landau Fermi liquid (FL) agree with theoretical predictions and point to an anomalous energy dependence of both the quasiparticle mass and the quasiparticle-quasiparticle scattering cross section. However, the complexity of the B - T phase diagram of CeCu 2 Si 2 as well as the specific-heat results for CeNi 2 Ge 2 measured at high pressure (1.7 GPa) indicate that the physics of HF metals is richer than anticipated in the theoretical models.

Pressure studies of quantum critical effects in CeCu2Si2 and CeNi2Ge2

We have investigated the effect of pressure and magnetic field on the specific heat of CeCu2Si2 and CeNi2Ge2, two undoped compounds close to an antiferromagetic (afm) quantum critical point. In a CeCu2Si2 sample showing an afm phase at p=0, application of a pressure of p=0.7 GPa results in a replacement of this phase by superconductivity and to non-Fermi-liquid (NFL) behaviour in C/T. In CeNi2Ge2 the NFL behaviour present at p=0 is suppressed by either a magnetic field B=8 T or a pressure p=1.3 GPa. For both compounds, the NFL effects are consistent (as T→0) with C/T=γ0-αT1/2 as expected when TN→0.

Multiple magnetic phase transitions in Ce(Cu1-xNix)2Ge2

We present a reinvestigation of the complex magnetic phase diagram of the heavy-fermion system Ce(Cu1-xNix)2Ge2 by employing measurements of the specific heat, C(T), and thermal expansion, α(T), on high-quality single crystals with x = 0, 0.02, 0.05, 0.5 and 0.8 in magnetic fields B ⩽ 8T. In addition to what was observed in polycrystalline samples multiple phase-transition anomalies could be resolved in these single crystals for x < 0.5 and 1.5 K <T <TN. Sharp anomalies in C(T) and thermal hysteresis in α(T) give evidence that these transitions are of first order (x = 0.02, 0.05). Our findings demonstrate that the structure of the antiferromagnetically ordered state depends on subtle changes of the hybridization of the Ce-4f state to its local environment, while the interplay of Kondo- and RKKY-interaction, which determines the existence of a long-range ordered state is governed by volume and/or density-of-states effects.

Phase diagram of the magnetic heavy-fermion superconductor UPd2Al3

Abstract The following issues concerning UPd2Al3 are addressed: (i) its anisotropic magnetic phase diagram which can be explained by domain-reorientation effects, (ii) the coexistence between magnetically ordered local 5f moments and Cooper pairs formed by weakly delocalized 5f states and (iii) distinct anomalies in the sample length measured both vs T and B, suggesting a strongly undercooled first-order transition at B ∗ (T) ≲ B c2 (T) for T ∗ ⋍ 0.8 T c .