We have carried out 2D angular correlation of positron annihilation radiation measurements on oriented single crystals of the heavy-fermion superconductor CeCu2Si2. Measurements have been made at 50 and 4 K which are above and below the Kondo temperature, T-K similar to 15 K, for this compound. The measurements show a small temperature dependence. Experimental results have been compared with linear muffin-tin orbital band-structure calculations for CeCu2Si2 and the isostructural non-f-electron compound, LaCu2Si2, The measured anisotropies agree well with the calculations. In addition, at both temperatures, the measured [001] projected k-space occupation densities reflect clearly the Fermi-surface breaks predicted by the calculations for CeCu2Si2. This implies that local-density approximation calculations provide a good description of the electronic structure in CeCu2Si2. We have also measured the positron lifetime in both superconducting (S-type) and magnetic (A-type) samples of CeCu2Si2. The lifetime and intensity components in the S- and A-type polycrystals are different, suggesting that the nature of the defects that trap positrons is different in the two phases.

Anomalous properties of the magnetization, the crystal dilatation, and other physical quantities observed in the mixed state of compounds such as UPd2Al3 and CeRu2 suggest the occurrence of a new state near H-c2. In such compounds with large spin susceptibility and high H-c2, a new inhomogeneous superconducting state is theoretically predicted to occur in high magnetic fields. In this inhomogeneous state which is shown to be a generalized Fulde-Ferrell-Larkin-Ovchinnikov state, the order parameter is spatially modulated, and planar nodes of the order parameter are periodically aligned perpendicular to the vortices. Various theoretical predictions are compared with experimental results on UPd2Al3 and CeRu2 related to this new superconducting high-field state.

Results of 2D-ACAR experiments on the heavy fermion superconductor CeCu2Si2 above and below the Kondo temperature, T-K are presented. Experimental data are compared with LMTO band structure calculations on both CeCu2Si2 and the isostructural f-electron free compound, LaCu2Si2.

The results of the magnetization, magnetostriction and AC-susceptibility experiments are presented for both the antiferromagnetic heavy-Fermion superconductor UPd2Al3 and the valence fluctuation compound CeRu2. For (H > 10 kOe). These results are discussed on the basis of a generalized Fulde-Ferrell-Larkin-Ovchinnikov superconducting state.

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).

An experimental study based upon magnetic and dilatometric measurements is presented for the clean high-re superconductors UPd2Al3, CeRu2 and V3Si. All three compounds show an enhanced spin susceptibility. Their superconducting state is strongly Pauli limited, and an anomalous peak effect is observed at T<(0.8-0.9)T-c, slightly below H-c2(T). This phenomenon appears to be qualitatively consistent with a first-order transition between weak and collective pinning, caused by the formation of a staggered order parameter in a generalized Fulde-Ferrell-Larkin-Ovchinnikov phase (M. Tachiki et al., Z. Phys. B, in press).

We report an investigation of the magnetic and dilatometric properties of single crystals of the superconductors UPd2Al3 and CeRu2, both compounds exhibiting enhanced spin susceptibilities. Our results suggest for both systems a first-order transition between weak and strong pinning at T < 0.9Tc, somewhat below Hc2(T). We argue that these observations are compatible with a staggered order parameter due to the formation of a "generalized Fulde-Ferrell-Larkin-Ovchinnikov state".

We report on measurements of the thermal expansion and specific heat on a series of polycrystalline CeCu2Si2 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.

By means of ultrahigh-resolution dilatometry we have studied the linear expansion coefficients at the superconducting transition for directions along and perpendicular to the superconducting layers. A pronounced discontinuity was found along the interlayer direction which corresponds to an extraordinary large uniaxial-pressure dependence of dT c /dp a ∗ = −4.8K/kbar . This indicats a most sensitive coupling between superconductivity and interlayer spacing in this layered superconductor.

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) less than or similar to B-c2(T) for T < T* similar or equal to 0.8 T-c.

We have investigated the superconducting and the antiferromagnetic transition of polycrystalline UNi2Al3 by means of high-resolution dilatometry. At Tc = 0.95 K and TN = 4.6 K the thermal-expansion coefficient α shows second-order phase-transition anomalies of negative sign. Investigations of the antiferromagnetic transition under magnetic fields in dicate a complex magnetic phase diagram reminiscent of that of the Pd homolog.

We report dilatation and vibrating-reed experiments on monocrystalline UPd2Al3. Anomalies found at high fields near Tc may reflect the transition to the Fulde-Ferrell-Larkin-Ovchinnikov superconducting state. We discuss possible effects due to the pinning of flux lines.

The magnetic phase diagram of the antiferromagnetically ordered heavy-fermion superconductor UPd 2 Al 3 ( T N = 14K , T c = 2K ) is highly anisotropic. Three different magnetic phases are only observed for magnetic fields applied in the easy basal plane of the hexagonal PrNi 2 Al 3 structure ( B ⊥ c ). For B || c , only one transition at the Néel temperature T N is found. A tetravalent ( 5f 2 ) uranium-configuration is inferred from a crystal-electric field analysis of the temperature dependence of the anisotropic paramagnetic susceptibility.

We report on thermal expansion and magnetostriction of the new hexagonal heavy fermion magnetic superconductor UPd 2 Al 3 . Measurements were taken on monocrystalline samples between 0.1K and 20K in magnetic fields up to 4T. Both thermal expansion and magnetostriction are strongly anisotropic and are dominated by magnetic effects. In an applied magnetic field, the thermal expansion anomaly observed near T c seems to be of first order.

We present a status report on our investigation of the two new heavy fermion superconductors UNi2Al3 and UPd2Al3. We discuss in particular the magnetic phase diagram, crystal field effects and the properties of the superconducting state.

We report thermal-expansion, magnetostriction and ultrasound measurements on monocrystalline samples of the hexagonal heavy fermion compound UPd2Al3. At high magnetic fields anomalies are found within the superconducting phase. The absolute values of the elastic constants were determined using ultrasound techniques.

We discuss the superconducting and other phase transitions in CeCu2Si2 and UT2Al3 (T: Ni,Pd). For CeCu2Si2, a novel lattice instability is found to limit the highest Tc attainable by appropriate heat treatment. In the UT2Al3 homologs, antiferromagnetism develops at TN = 4.6K (14K) and possibly coexists with heavy-fermion superconductivity below Tc = 1K (2K) for T = Ni(Pd).

Heavy-fermion superconductivity and other cooperative effects have been explored by thermal expansion, specific heat and ultrasound measurements on CeCu2Si2 single crystals. Crystals annealed under Cu atmosphere show sharp superconducting transitions at Tcmax = 0.63 K. At the same temperature the "as grown", i.e., non-bulk-superconducting, crystals reveal a pronounced phase-transition anomaly, presumably of structural origin. This new transition is associated with an expansion of the volume upon cooling and gives rise to magnetic correlations. Our results indicate a complex interplay between lattice instability, magnetic phenomena and superconductivity in CeCu2Si2.

We report measurements on polycrystalline and, for the first time, single-crystalline samples of UPd2Al3, a new heavy-fermion superconductor (HFS) with a record-T(c) of 2K and an antiferromagnetic transition at T(N) = 14K.