U. Rauchschwalbe

Superconductivity and Magnetic Order in a Strongly Interacting Fermi-System: URu2Si2

Measurements are reported on the electrical resistivity, susceptibility, dc-Meissner effect and specific heat of the tetragonal U-compound URu2Si2. AtTN≅17 K, an antiferromagnetic phase transition is observed. This magnetically ordered state appears to coexist with superconductivity, which is found belowTc≅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 m0),though the measured low-temperature specific heat coefficient γ does not exceed 65 mJ/K2mol.

Heavy fermions in Kondo lattice compounds (invited)

Even in the absence of both magnetic order and superconductivity, Ce‐based Kondo lattice systems show distinct anomalies in the low‐temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl3 and normal (n) state CeCu2Si2 on the one hand as well as CeRu2Si2 and CeCu6 on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field Bc2(T) for the heavy‐fermion superconductor CeCu2Si2 (Tc≂0.6 K) shows a flat maximum at ∼0.2 Tc, which is ascribed to coherence‐derived structure in the n‐state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu2Si2...

Investigation of new lanthanum-, cerium- and uranium-based ternary intermetallics

Abstract 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.

Superconductivity in CeCu2Si2 (invited)

The specific heat, dc magnetization, and resistivity have been measured on a number of samples of CeCu2Si2, which was previously found to assume a novel superconducting state below 0.5 K [F. Steglich et al., Phys. Rev. Lett. 43, 1892 (1979)]. By comparing the transition temperatures of samples which have undergone different heat treatments, we demonstrate that small fractions of spurious phases hinder the onset of superconductivity but are gradually removed with annealing. With a very pure sample obtained in this way, we have fully confirmed all the characteristics of the superconducting phase transition reported before on less pure samples. However, even after this heat treatment, the samples show clear signs of striking inhomogeneities, manifested by a static Meissner effect of bulk samples representing typically only a few percent of the volume. An increase of this Meissner signal up to 60 vol% is observed after powdering the samples. In addition to measurements in the superconducting state, the intrin...

Gapless superconductivity and variation of Tc in the heavy-fermion system CeCu2Si2☆

Abstract Polycrystalline samples of CeCu2Si2 exhibit bulk superconductivity carried by “heavy fermions”. While usually Tc ≊ (0.55 ± 0.15) K, a few samples showed considerably lower transition temperatures (Tc ⩾ 0.11 K), which could, however, be raised to usual values by proper heat treatment. We found that lower Tcs are accompanied by both slightly reduced values of the spin-fluctuation temperature of the Ce3+ ions and the phenomenon of “gapless superconductivity”. In agreement with recent results by Aliev et al. [5], a CeCu2Si2 single crystal did not become superconducting above 20 mK, but showed a 20% deficiency in Cu occupation.

Specific heat and transport properties of UPt3

Specific heat, resistivity, thermal conductivity and thermoelectric power measurements are reported for the normal and superconducting regimes of the heavy-fermion superconductor UPt3. A comparison is made with uranium-based spin-fluctuation and cerium-based Kondo-lattice compounds.

Lower critical field of U0.97Th0.03Be13: evidence for two coexisting superconducting order parameters

Evidence is presented for the coexistence of two superconducting order parameters in U0.97Th0.03Be13 below Tcb = 0.26 K. Measurements of the lower critical magnetic field performed for the first time and published data on the specific heat prove that the lower transition in U0.97Th0.03Be13 is a superconducting transition below which a new order parameter adds to the one existing below Tca = 0.48 K. Our conclusion contrasts the picture where one order parameter is replaced by another one.

Upper critical magnetic fields of the heavy fermion superconductors CeCu2Si2, UPt3, and UBe13: Comparison between experiment and theory

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.

Magnetic properties of the high-Tc superconductor La1.85Sr0.15CuO4

For the high-Tc superconductor La1.85Sr0.15CuO4, we present results for the low-field magnetization and the lower and upper critical fieldsBc1(T) andBc2(T). ForBc1(0) we find a value of about 20 mT. Extrapolating our resistive data for the transition midpoint taken in fields up to 12 T, we deduce a value of about 50 T forBc2(0). TheBc2 values obtained by inductive measurements are significantly lower. We explain this by a largeBc2 anisotropy due to the layered structure of these compounds. Losses in the Meissner state observed in magnetization and susceptibility measurements disappear after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample.

High-magnetic-field and high-pressure effects in monocrystalline URu2Si2

Abstract High-field magnetisation and high-pressure resistivity measurements have been performed on monocrystalline samples of the superconducting compound URu2Si2 just above Tc. At 1.4 K, well below the magnetic ordering temperature of about 17.5 K, two sharp transitions at 36 and 40 T have been observed in the magnetisation versus field curves along the tetragonal c-axis, whereas the a-axis magnetisation curve remains linear up to 40 T. These transitions point to a complex magnetic behaviour. Above the second transition the magnetic moment per uranium atom exceeds a value of 1.3 μB. The resistivity versus temperature curve is distinguished by two characteristic temperatures of 75 and 17.4 K, corresponding with the onset of coherent scattering and the magnetic transition, respectively. Both characteristic temperatures increase with pressure.