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We discuss the superconducting and other phase transitions in CeCu 2 Si 2 and UT 2 Al 3 (T: Ni,Pd). For CeCu 2 Si 2 , a novel lattice instability is found to limit the highest T c attainable by appropriate heat treatment. In the UT 2 Al 3 homologs, antiferromagnetism develops at T N = 4.6K (14K) and possibly coexists with heavy-fermion superconductivity below T c = 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.

Magnetic phase diagrams have been explored in heavy-fermion (HF) compounds by controlled changes of the stoichiometry (UCu4+xAl8−x), dopant concentration (Ce(Cu1−xNix)2Ge2) and magnetic field (CeCu2Si2). The results demonstr competition (i) between Kondo and RKKY interactions in the former two compounds and (ii) between HF superconductivity and some cooperative state, presumably HF band magnetism, in the latter.

We review (i) an itinerant antiferromagnetic phase transition below 4 K in Ni-rich Ce(Cu 1− x Ni x ) 2 Ge 2 systems, (ii) the coincidence at T = 0.63 K of both a structural lattice instability in “as-grown” (non-superconducting) CeCu 2 Si 2 single crystals and bulk superconductivity in annealed ones as well as (iii) antiferromagnetic and superconducting transitions at T N = 4.6 K and T c = 1 K, respectively, in the heavy-fermion compound UNi 2 Al 3 .

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

Measurements of the resistivity, susceptibility and specific heat are reported which clearly show that the hexagonal compound UNi2Al3 is a heavy-fermion magnet belowT N=4.6 K and a heavy-fermion superconductor belowT c=1 K.

UPd2Al3 is a new heavy-fermion superconductor with a recordTc of 2 K. In addition, it shows a transition to long-range antiferromagnetic order atTN=14 K. Its Sommerfeld coefficient is reduced from γp=210mJ/K2 mole in the paramagnetic to γ0=150mJ/K2 mole in the antiferromagnetic phase.

CeCu2Si2 single crystals (Tc : 0.63−0.68 K) have been studied utilizing calorimetric, dilatometric and ultrasound experiments. The strain dependencies of Tc along the a- and c- axis have opposite sign and indicate that superconductivity is favoured upon compensation of the local 4f moments. According to the c11(T,B) behaviour, Tc appears limited by an additional, presumably antiferromagnetic, instability of the Fermi liquid.

We report measurements of (i) the DC magnetization of La1.85Sr0.15CuO4 and YBa2Cu3O7, (ii) thermal properties of La2-xSrxCuO4 (x = 0.15) and (iii) transport properties of YBa2Cu3O7. We arrive at the following results: (i) The shielding magnetization of the sintered pellets studied is dominated by inter-grain Josephson contacts, but we find no evidence for additional intra-grain Josephson contacts. (ii) High-Tc superconductivity is a bulk effect. (iii) For YBa2Cu3O7 the effective mass m* of those carriers involved in the superconducting transition is of the order of 4m0.

We report measurements of the upper critical magnetic field,Bc2(T), on single crystal and polycrystalline samples of the heavy-fermion superconductors CeCu2Si2, UPt3 and UBe13. Comparison is made with experimental results for a “conventional” superconductor (CeRu3Si2) and with results of the weak-coupling theory applied to both ans-wave superconductor in the “dirty-limit” and ap-wave superconductor. New anomalies are discovered to add to the unusualBc2(T) behavior.

We report results on the normal-state (n-state) and the superconducting properties for CeCu2.2Si2 and the quasi-binary alloys Ce1-xMxCu2.2Si2 with M=La (x⩽11 at%) and M=Y(x⩽3 at%). Upon increasing the dopant concentration, the specific heat jump height, ΔC, is depressed much more strongly than Tc. This proves a pair-breaking effect of the La- and Y-induced “Kondo holes” — characterized by temperatures Th, La and Th, Y, that show an anticorrelation to the characteristic temperatures (T∗La, T∗Y) of the underlying Ce1-xMxCu2.2Si2 systems. Compared with La, Y impurities are more efficient in pair breaking and in destroying coherence in the normal state, presumably owing to a less anisotropic scattering potential.

Specific heat and thermal conductivity in the superconducting state of the Kondo-lattice system CeCu2Si2 and the paramagnon system UPt3 point to substantial mass anisotropies at the Fermi surface (FS): while in CeCu2Si2 bare band electrons at certain portions of the FS seem to remain normal, in UPt3 heavier fermions seem to stay normal and Cooper pairs are formed by somewhat less heavy ones. The upper critical field of UPt3 shows no influence of Pauli limiting and can reasonably well be explained with S = 0 as well as S = 1 pairing. CeCu2Si2 is strongly Pauli limited, independent of orientation. This substantiates recent Josephson experiments which show that CeCu2Si2 is a singlet superconductor.

While searching for new Ce- and U-based “heavy fermion” systems, we have investigated the following intermetallic compounds: CeCu 2 Ge 2 , CeCu 2 Sn 2 , CeAg 2 Ge 2 , CeZn 2 Al 2 (tetragonal ThCr 2 Si 2 -structure and modifications thereof); CeRu 3 Si 2 , URu 3 Si 2 (hexagonal LaRu 3 Si 2 -structure); Ce 2 Cu 7 Al 10 , U 2 Cu 7 Al 10 (trigonal Th 2 Mn 17 -structure); CeCu 4 Al 8 , CeAg 4 Al 8 , UCu 4 Al 8 (tetragonal CeMn 4 Al 8 -structure); LaCu 13− x Al x with x = 3, 5.5, and 6.5, as well as CeCu 6.5 Al 6.5 (cubic NaZn 13 -structure). Resistive and inductive measurements were undertaken between T = 30 mK and room temperature, and two new superconductors were found: LaCu 6.5 Al 6.5 ( T c = 0.65 K) and CeRu 3 Si 2 ( T c = 1.2 K). The specific heat and thermopower of two of these systems have also been measured. CeCu 4 Al 8 shows interesting features that are presumably related to fine structure in the “heavy-fermion” density of states near the Fermi level. CeCu 6.5 A1 6.5 shows a magnetic transition at T = 0.5 K which resembles a “spin-glass freezing” rather than a formation of long-range magnetic order.

We report measurements of Tc and ΔC for Ce1−xMxCu2.22Si2 (M = La, Y, Gd) and U1−xMxBe13 (M: Th, La, Y). “Diamagnetic” pair breaking by nonmagnetic dopants is observed which implies an anisotropic nature of both the superconducting order parameter and the impurity-scattering mechanism. Gd impurities in CeCu2.2Si2 give rise to additional “paramagnetic” pair breaking.

The present status concerning lower and upper critical magnetic fields, Bc1(T) and Bc2(T), and specific heats, CS(T), of heavy-fermion superconductors is reviewed. For CeCu2Si2, the data near Tc are found to be in full accord with GL theory. Yet, a surprising enhancement of the elastic mean free path over that in the normal state is inferred. Structure in both Bc1(T) and Bc2(T) of UBe13 appears related to a bump in the normal-state specific heat (B = 0) around T = 0.5 Tc. This may be a precursor to the lower of the two calorimetric transitions in U0.97Th0.03Be13 which are confirmed by new Bc1(T)‖ results.

In Ce 1− x Gd x Cu 2.2 Si 2 spin-glass ordering coexists superconductivity below the “critical” ( T c → 0) concentration x cr = 5.1 at %. For samples with slightly lower Gd content, a small volume fraction of ⋍ 10% remains superconducting presumably due to demagnetizing effects. Thus, the full T c ( x ) dependence is accessible. It closely follows the AG-result and proves the existence of a “paramagnetic” pair-breaking mechanism.

Measurements are reported on the electrical resistivity, susceptibility, dc-Meissner effect and specific heat of the tetragonal U-compound URu2Si2. At T N ≃17 K, an antiferromagnetic phase transition is observed. This magnetically ordered state appears to coexist with superconductivity, which is found below T c ≃1.5 K. An analysis of the upper critical field data reveals rather unusual properties of both the normal-and the superconducting state. The effective mass of the quasiparticles at low temperatures is substantially enhanced (m*≃50 m 0), though the measured low-temperature specific heat coefficient γ does not exceed 65 mJ/K2mol.

Non-magnetic La and Y impurities in CeCu 2.2 Si 2 give rise to a “diamagnetic” pair-breaking effect and, in addition, Gd causes “paramagnetic” pair breaking. In dilute (0 1-x La x Cu 2.2 Si 2 alloys an “alloying-induced depression of incoherent scattering” is observed and related to the formation of antiferromagnetic order at several K.