A. de Visser

Superconductivity on the Border of Weak Itinerant Ferromagnetism in UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

UPt3, heavy fermions and superconductivity

Abstract This review considers studies of the normal and superconducting state of the heavy-fermion superconductor UPt 3 ( γ = 422 mJ/K 2 molU, T c = 0.5 K). We discuss a large variety of experiments, most of them performed on good quality single crystals. Concerning the normal state we concentrate on the magnetic, thermal, elastic and transport properties. These properties can be fairly described in a spin-fluctuation model. Experiments concerning the superconducting phase reveal several unusual features. The possibility of an unconventional pairing mechanism is discussed. On alloying UPt 3 with Pd, additional anomalies are observed, suggesting that UPt 3 is close to an antiferromagnetic instability.

Resistivity of single-crystalline UPt3 and its pressure dependence; Interpretation by a spin-fluctuation model

Abstract Resistivity measurements on a series of uranium-platinum compounds were carried out in the range 1.3 up to 300 K and pressures up to 5 kbar. The resistivity of single-crystalline UPt3 is strongly anisotropic and points to pronounced spin-fluctuation effects that are partly suppressed at a pressure of 4 kbar. In paramagnetic UPt2 and UPt5 spin-fluctuation effects are less pronounced, whereas in ferromagnetic UPt the spin-disorder scatter is dominant above 27 K.

Low-temperature properties of the heavy-fermion compound CeRu2Si2 at the metamagnetic transition

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu2Si2 are reported. The field variation of the linear term of the specific heat γ derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT → 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB →B*. Similar experiments performed on a single-crystalline Ce0.95La0.05Ru2Si2 sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu2Si2 approaches a magnetic instability atB* forT → 0.

Unusual upper critical field of the ferromagnetic superconductor UCoGe

We report upper critical field B(c2)(T) measurements on a single-crystalline sample of the ferromagnetic superconductor UCoGe. B(c2)(0) obtained for fields applied along the orthorhombic axes exceeds the Pauli limit for B parallela,b and shows a strong anisotropy B(c2)(a) approximately B(c2)(b)>>B(c2)(c). This provides evidence for an equal-spin pairing state and a superconducting gap function of axial symmetry with point nodes along the c axis, which is also the direction of the uniaxial ferromagnetic moment m(0)=0.07micro(B). An unusual curvature or kink is observed in the temperature variation of B(c2) which possibly indicates UCoGe is a two-band ferromagnetic superconductor.

Thermal expansion and elastic constants of single-crystalline UPt3

Thermal expansion measurements have been performed on single-crystalline UPt3 between 1.4 and 100K. Spin-fluctuation terms dominate the volume expansion at low temperatures. The linear terms in the thermal expansion and the specific heat are compared and a giant value is obtained for the appropriate Gruneisen parameter. The elastic constants of UPt3 at room temperature have been determined from sound velocity measurements. A value of 0.48 Mbar-1 results for the nearly isotropic compressibility.

Electron−phonon coupling in uranium compounds

Abstract We present results of elastic measurements for UN, UPd 3 , UBe 13 and UPt 3 . Exchange striction effects are seen for UN and crystal field anomalies for UPd 3 . Polycrystalline UBe 13 exhibits an anomalously low Poisson ratio. UPt 3 shows low temperature anomalies in various elastic constants and strong anharmonic effects for the c 44 mode.

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.

Specific Heat, Susceptibility and High-Field Magnetisation Experiments on Heavy Fermion UPt3 Alloyed with Pd

Abstract Specific heat, susceptibility and high-field magnetisation experiments have been performed on a number of pseudobinary U(Pt1−xPdx)3 compounds with x⩽0.30. For low Pd concentrations (x⩽0.10) the spin-fluctuation contribution to the specific heat is enhanced with respect to pure UPt3. For x⩾0.15 the spin-fluctuation phenomena are lost. On alloying, the anomalies present for UPt3 in the susceptibility at 17 K and in the high-field magnetisation at 21 T (at 4.2 K), shift towards lower temperatures and fields, respectively, and have not been observed in a compound with x=0.15. Superconductivity has not been found down to 40 mK in a U(Pt0.995Pd0.005)3 sample.

Enhancement of spin fluctuations by alloying heavy-fermion UPt3 with Pd

Abstract In the course of our investigation of the low-temperature properties of heavy-fermion UPt 3 , a number of pseudobinary U(Pt 1- x Pd x ) 3 compounds has been prepared and their specific heat measured in the temperature range 1.2–30 K. For low Pd concentrations the linear term in the specific heat increases, pointing to an enhancement of the spin-fluctuation effects at low temperatures. Superconductivity has not been observed down to 40 mK in the U(Pt 0.99 Pd 0.01 ) 3 sample.