N.H. van Dijk

Thermodynamic stability of the superconducting phase diagram of UPt3

The superconducting phase diagram ofUPt3, as determined by accurate dilatometry as function of temperature and magnetic field (B‖c andB⊥c), exhibits three superconducting phases which meet in a tetracritical point. We show that the measured phase diagrams are thermodynamically stable and that all four phase lines are of second order. Via Ehrenfest relations the pressure dependence of the various phases has been determined. The so-calledC phase is the most stable phase under uniaxial pressure along the c-axis (pc>2.5 kbar). The resulting phase diagram in thepc-T plane yields stringent constraints on the fourth order coupling constants in the Ginzburg-Landau scenario of a vector superconducting order parameter coupled to a symmetry breaking field.

Dilatometry study of the superconducting phases of UPt3

Abstract Magnetostriction and thermal expansion measurements on single-crystalline UPt3(ΔL∥c) have been performed for magnetic fields in the basal plane. From these experiments a detailed superconducting phase diagram (field versus temperature) has been constructed. Via the Ehrenfest relations an accurate prediction of the pressure dependence of the phase diagram is obtained.

The influence of Gd and Ni impurities on the superconducting transition temperature of UPt3

Abstract The effect of Gd and Ni impurities on the upper superconducting transition temperature of UPt3 (Tc+ = 0.55 K) has been studied by means of resistivity measurements. Surprisingly, we find that paramagnetic Gd impurities suppress Tc+ at the same rate as non-magnetic Y or Th impurities, yielding further support for non-s-wave pairing. Ni impurities also suppress Tc+, but do not dissolve homogeneously in the UPt3 matrix.

Uniaxial pressure dependence of the superconducting phase diagram of UPt3

Thermal expansion and magnetostriction techniques have been applied in order to determine the superconducting phase diagram of UPt3 (B | c and B perp c). The uniaxial pressure dependence of the various phases, as determined with the Ehrenfest relations is strongly anisotrpic. For pressure along the c-axis we obtain dΔTc/dpc = −22.3 mK/kbar, and for pressure along the a-axis, dΔTc/dpa = 4.9 mK/kbar (ΔTc = Tc+ − Tc−). The phase diagrams are discussed in view of the relevant Ginzburg Landau models.

On the magnetic and superconducting phases of heavy-fermion UPt3

Abstract The electronic instability in the height correlated electron system UPt3 gives rise to various intriguing phenomena at liquid-helium temperatures, among which the occurrence of unconventional superconductivity (T+c = 0.49 K, T-c = 0.44 K) has attracted most attention. In a magnetic field three superconducting phases are observed that meet at a tetracritical point. We here present a discussion of a number of puzzling aspects of a superconducting phase diagram (B⊥c) of unprecedented accuracy, determined by dilatometry. The possible interplay of superconductivity and reduced moment antiferromagnetism (TN = 5K) is investigated. Furthermore, we report on recent studies of the suppression of Tc, by alloying with Pd, Y, Th, Au and Ir.

Absence of antiferromagnetic order in UBe13

The linear magnetostriction (λ‖ and λ⊥) of a single-crystalline sample of the heavy-fermion compound UBe13 has been measured for fields B < 8 T (B‖[100]) in the temperature interval 0.3 < T < 12 K. We find neither evidence for the antiferromagnetic order (TN = 8.8 K) nor for the magnetostrictive oscillations, that were reported recently. Instead λ varies proportional to B2 as expected for a normal paramagnetic metal.

Magnetism and superconductivity of the U(Pt, Pd)3 system

The superconducting phase diagram of UPt3 has been studied by a variety of techniques, all leading to three different phases in the B T plane that meet the normal phase in a tetracritical point. Superconductivity has been reported to coexist with small-moment antiferromagnetism. The effects of substitutions on either the U site or the Pt site on the superconducting transition have been addressed in several studies. Pd substitutions for Pt play a particular role since the zero-field splitting between two superconducting phases increases from about 60 mK for pure UPt3 to about 120 mK for the 0.2 at % Pd alloy. On further increasing the Pd content, superconductivity is suppressed above 0.5 at % Pd, whereas between 2 and 10 at % Pd long-range antiferromagnetic order is observed with values of the uranium moment up to 0.6p~. The relevant questions in these Pd substitution studies are: by which mechanism is the splitting between the two zero-field superconducting phases enhanced and, secondly, in which way does the small-moment antiferromagnetism evolve into the large-moment antiferromagnetic order at higher Pd concentrations. In this contribution, the salient features are reviewed with respect to the occurrence of magnetic order and superconductivity in the pseudo-binary system or U(Pt, Pd)3 alloys, with an emphasis on dilatation experiments.

Dilatometry study of the heavy-fermion superconductor URu2Si2

Dilatation measurements have been performed on a single-crystalline sample of the heavy-fermion superconductor URu2Si 2 (T c = 1.2 K). Thermal expansion measurements indicate a thermal electronic Griineisen parameter F x = 27. A comparison with the magnetic electronic Gr/ineisen parameter derived from the magnetostriction, F a = 26, points to a single energy scale. Magnetostriction measurements in the superconducting state reveal hysteresis with a peculiar change of sign close to the upper critical field, Be2.

Low-temperature phase diagram of the heavy-fermion superconductor UPt3

The transition to the superconducting state of UPt3 has been studied in specific-heat experiments on polycrystalline and single-crystalline samples in dependence of heat treatment, purity of starting materials and deviations from stoichiometry, as well as on substituted compounds. The concepts behind the splitting of the transition in two anomalies that are 60 mK apart are critically reviewed. The superconducting phase diagram in the B-T plane has been studied with the use of a sensitive dilatometric technique. The three superconducting phases meet at a tetracritical point and all four phase lines are of second order. From the length anomalies at the phase boundaries the pressure derivatives of the phase lines have been determined and the most stable phase under uniaxial pressure along the c-axis has been identified.

Meta-stable states in the low-field magnetostriction of UPt3

Detailed magnetostriction measurements on single-crystalline UPt 3 reveal an anomaly at a field of 0.1 T in the normal and the superconducting state, both for BIIc and BZc. The magnetostriction in a hysteresis loop reveals several transitions between history dependent meta-stable states below 1.5 K. In the superconducting state, the anomaly in the magnetostriction coincides with a kink in one of the phase lines for B±c, indicating a coupling between the meta-stable states and the superconductivity.