Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with magnetism

The effect of Pd doping on the superconducting phase diagram of the unconventional superconductor UPt3 has been measured by (magneto)resistance, specific heat, thermal expansion and magnetostriction. Experiments on single- and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We find that DTc(x) correlates with an increase of the weak magnetic moment m(x) upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios with magnetism as the symmetry breaking field, i.e. the 2D E representation and the 1D odd parity model. Only for small splittings DTc is proportional to m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T plane persists till at least x= 0.002 for B perpendicular to c, while it is rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at the expense of the C phase.Comment: 25 pages text (PS), 8 pages with 14 figures (PS), submitted to Phys.Rev.

Suppression of Superconductivity in Single Crystals of UPt_3 by Pd Substitution

The suppression of superconductivity by substitution effects has been measured in high quality single crystals of U(Pt_{1-x} Pd_x)_3 with 0 <= x <= 0.002. While the superconducting transition temperature T_c varies linearly with residual resistivity r_0, consistent with pair-breaking by impurity potential scattering, the rate of suppression of T_c with r_0 is much larger for Pd substitution than for other impurity substitutions or by increased defect density. This effect is correlated with an increase in the inelastic scattering coefficient, and may be related to Pd-induced changes in the magnetic fluctuation spectrum.Comment: 12 page in manuscript, plus 4 figure

Onset of antiferromagnetism in UPt_{3} via Th-substitution studied by muon spin spectroscopy

Muon spin spectroscopy has been used to study in detail the onset of large-moment antiferromagnetism (LMAF) in UPt_{3} as induced by Th substitution. Zero-field experiments have been carried out on a series of polycrystalline U_{1-x}Th_{x}Pt_{3} (0 <= x <= 0.05) samples in the temperature range 0.04 - 10 K. At low Th content (x <= 0.002) magnetic ordering on the time scale of the uSR experiment (10^{-8} s) is not detected. For x = 0.005 a weak magnetic signal appears below T = 2 K, while for 0.006 <= x <= 0.05, spontaneous oscillations in the uSR spectra signal the presence of the LMAF phase. The data are well described by a two-component depolarization function, combining the contribution of a polycrystalline antiferromagnet and a Kubo-Lorentzian response. However, the transition into the antiferromagnetic phase is quite broad. For x = 0.01 and 0.02, a weak magnetic signal appears below about 7 K, which is well above the mean-field transition temperatures. The broadening may be a result of the effects of disorder on the time fluctuations associated with anomalous small-moment antiferromagnetism.Comment: 29 pages, 9 figure

Influence of chlorination of poly(vinyl chloride) on miscibility with poly(methyl methacrylate)

The improvement of the miscibility of poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC) with moderate chlorination of PVC was demonstrated by determination of cloud-point curves and glass transitions of the mixtures. Heats of mixing of low-molar-mass analogues for PMMA and for the various structural units present in chlorinated PVC (CPVC) were determined in a microcalorimeter along with Fourier transform infra-red spectroscopy of these mixtures. The results show that the improvement of miscibility is caused by a specific interaction between the carbonyl group of PMMA and predominantly the CHCl group of CPVC. The specific interactions present in these systems are best described as a Lewis acid-base type of interaction, considering the carbonyl oxygen of the ester as an electron donor and the hydrogen of the CHCl group as an electron acceptor: CO … HCCl. The decrease in miscibility observed when more strongly chlorinated PVC is used can be ascribed to the relatively high concentration of CCl2 groups, which have a less favourable interaction with the carbonyl groups

Association of stereoregular poly(methylmethacrylates): 5. Stereo-complex formation in dilute solutions

Further study was carried out on the stereocomplex formation in dilute solution using viscometry, osmometry, light scattering and gel permeation chromatography. It appears that the process can be divided in two steps. The primary step involves the formation of very compact particles by association of several isotactic and syndiotactic PMMA chains in a base-molar ratio of . The radius of gyration of these particles remains about the same as the additive value of the original components, whereas their molecular weight increases. The secondary step involves the aggregation of the particles which is reflected by an increase of the radius of gyration. It is proposed that this aggregation is caused by interparticle stereo-association of noncomplexed chain parts