U. Ahlheim

Heavy-fermion superconductivity atT c =2K in the antiferromagnet UPd2Al3

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.

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.

A new heavy-fermion superconductor: UNi2Al3

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

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.

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.

UPd2Al3-a new heavy-fermion superconductor with Tc = 2K

Abstract 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.

Phase transitions in the heavy Fermi-liquid state: CeCu2Si2, UNi2Al3 and UPd2Al3

Abstract 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).

CeCu2Si2 and UPt3: Two different cases of heavy fermion superconductivity☆

Abstract 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.

Sm3Se4: a heavy-fermion system without charge carriers

Abstract Specific heat measurements down to 80 mK in magnetic fields up to 8 T have been done on Sm 3 Se 4 to check the heavy-fermion character in non-metallic materials. Heavy-fermion character with a large γ-value, 0.79 J/K 2 mol Sm 3+ , is observed. Low-field magnetization measurements show magnetic ordering, presumably of frustrated kind, due to competing exchange interactions. Both the exchange mechanism and the possible Kondo state are discussed.

Magnetic and Superconducting Phase Diagramof Heavy Fermion Superconductors Ce1-xThxCu2.2Si2–Cu NMR and NQR Study

The magnetic and superconducting phase diagram of Th-doped Ce 1- x Th x Cu 2.2 Si 2 has been established by Cu NMR and NQR technique and is compared with that of CeCu 2.02 Si 2 . Static magnetic ordering with T N =2.4 K and 4.5 K for x =0.08 and 0.12, respectively, is evidenced by a pronounced broadening of the 63 Cu-NQR and -NMR spectra and a pronounced anomaly in the specific heat. In contrast, for the lightly Th-doped compounds with x ≤0.064, there has been observed neither a signature of a broadening of the NQR and NMR spectra nor an anomaly of the specific heat, but a dramatic reduction of the NMR and NQR intensities below a temperature ranging from 0.9 K to 1.3 K upon increasing Th-content. It is emphasized that the unusual “ magnetic transition ” found in undoped and lightly Th-doped CeCu 2.2 Si 2 should be distinguished from the static magnetic ordering in the heavily Th-doped systems.