We report the magnetic field dependence of the dc susceptibility and resistivity in superconducting single crystals of the hexagonal heavy-fermion superconductor . From the susceptibility we derive a crystalline-electric-field scheme, which closely resembles that of . Furthermore, we examine the magnetic phase transition into the incommensurably ordered state at and construct the magnetic phase diagram for the three crystallographic directions. While for fields B parallel to the a- or c-axis is monotonically lowered, this is not the case for . Instead, here we find a field-induced magnetic transition into a, presumably, commensurate ordered magnetic state. We discuss our results on in comparison to those for the related system .

Neutron-diffraction experiments have been carried out on single-crystalline samples of the heavy-fermion pseudobinaries U(Pt1-xPdx)(3) (x less than or equal to 0.05) in order to investigate the evolution of magnetic order. At low Pd contents (x = 0.002 and 0.005), small-moment magnetic order is observed below similar to 6 K,just like in pure UPt3. For x = 0.02 and 0.05, the ordered moment is about one order of magnitude larger compared to UPt3. The compound with x = 0.01 represents a most intriguing intermediate case as it exhibits a cross-over behaviour from small-moment to large-moment antiferromagnetic order.

We have performed the thermal expansion α(T) and elastic constant c(T) measurements of the heavy electron system URu2Si2, with special attention to the tetragonal symmetry of Γ3(x2 − y2) and Γ4(xy). The experimental results of c66, 12(c11 − c12) and α(T) along [± 1 1 0] do not confirm a sizable uniform distortion for these two types of symmetry through a puzzling phase transition at To = 17.5 K. On the other hand, the 12(c11 − c12) data, which are missing in the previous detailed study, are found to show a weak but significant tendency of softening below about 70 K, suggesting an elastic response of the quadrupolar moment of Jx2 − Jy2.

We investigate the magnetic phase transition in UNi4B by specific heat, c(T), thermal expansion and resistivity under pressure. These experiments reveal that the hexagonal symmetry is not broken at TN = 20 K, and predicts a hydrostatic pressure dependence of TN of − 29 mK/kbar, close to the result from resistivity. The anomalous upturn of c/T to 470 mJ/mol K2 at 0.4 K is reduced to 160 mJ/mol K2 in 16 T ∥ b-axis. The specific heat critical exponent α = −0.15(2) suggests 3D Heisenberg universality in zero field.

We investigate the phase transition of URu2Si2 at T0 = 17.5 K by thermal expansion and heat capacity in magnetic fields, H, up to 25 T. At T0 an energy gap of Δ ⋍ 115 K opens, which decreases to 65 K in 25T. The H dependence of T0 scales with Δ, whereas a different H dependence and energy scale was observed for the ordered dipole moment. The anomalous H dependence of the thermal expansion coefficients may discriminate between the various scenarios for the primary order parameter of URu2Si2.

We have studied non-Fermi liquid properties of the tetragonal diluted uranium alloys Th1−xUxRu2Si2 (x ≤ 0.07), by means of thermal expansion measurements in the temperature range 0.5–100 K. As the temperature is lowered below about 10 K, the volume effect αv(T)/T (=(αc + 2αa)/T) is found to exhibit the tendency to diverge logarithmically, similar to other quantities of the system: C/T ∼ − ln T, ϱ ∼ −ln T (or T12). Interestingly, the sign of the anomaly in αv(T) is negative, opposite to the usual Kondo shrink behavior. As a possible interpretation, we discuss the two-channel Kondo model, proposing c-f hybridization effects dominated by the virtual f2-f3 charge fluctuations.

The low-temperature thermal expansion of the heavy-fermion system with the formula (x = 0 and x = 0.05) close to the magnetic instability is analysed in terms of the renormalization group and self-consistent renormalized spin-fluctuation models. The Grüneisen parameter calculated using the renormalized Fermi temperature is compared with the effective one which is determined from thermal expansion measurements and previously obtained specific heat data.

We report on a μSR study of the evolution of antiferromagnetism in the heavy-fermion pseudobinary series U(Pt, Pd)3. For pure UPt3 adn U(Pt0.998Pd0.002)3 no small-moment antiferromagnetism has been observed in the present experiment: the zero-field μSR data yield a temperature-independent Kubo-Toyabe line width σKT = 0.06 ± 0.01 μs−1 (T ⩽ 8 K). As recent neutron-diffraction experiments on UPt3 and U(Pt0.998Pd0.002)3 do reveal magnetic order, we suggest that the muons stop at high symmetry sites where the dipole fields cancel. However, for the large-moment compound U(Pt0.95Pd0.05)3), the μSR data unambiguously show magnetic signals: below TN two frequencies appear (ν1 ⋍ 8.0 MHz and ν2 ⋍ 2.0 MHz, for T → 0), indicating two magnetically inequivalent muon stopping sites.

We have performed thermal expansion and elastic constant measurements of the heavy electron system URu2Si2, focusing attention on the Gamma(3)(x(2) - y(2)) and Gamma(4)(xy) symmetry of the tetragonal group D-4h. It is reconfirmed that there is no sizable uniform spontaneous distortion for these two types of symmetry through the puzzling phase transition at T-o = 17.5 K. On the other hand, a weak but significant tendency of softening are found below about 70 K in the transverse (C-11 - C-12)/2 mode. The results strongly suggest the presence of a lattice instability in the Gamma 3 symmetry. From the results, proposed crystalline-electric-field models are also discussed.

We investigated the influence of the over-oxidation process on the resistivity and structure of graphite intercalation compounds with sulphuric acid. Staging phenomena and amorphization were observed. The magnetoresistance and Hall effect at low temperature of low density carbon foils, prepared from exfoliated graphite with different structural defects were studied. The heat treatment temperature and time of oxidation appear to be the parameters which govern the degree of 3D or 2D ordering in the carbon foils. The weak localization phenomenon for 2D electronic systems is invoked to explain the negative magnetoresistance as well as the low temperature dependence of the resistivity of samples.

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 0.6μB. The relevant questions in these Pd substitution studies are: by which mechanism is the splitting between the two zero-filed 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

The temperature and magnetic-field dependences of the conductivity of heterostructures alternatively doped with silicon - delta-doping of GaAs and simultaneous uniform doping of the AlxGa1-xAs layer - are investigated. The concentration and mobility of the two-dimensional electrons in quantum-well subbands are determined from the experimental data. The band diagrams of the structures with different distances of the delta layer from the heteroboundary are calculated. It is shown that such structures can have a much higher conductivity than ordinary structures. (C) 1996 American Institute of Physics.

The magnetoresistance of n- and p-type alloys Pb1-xSnxTe(In) with different compositions and of Pb1-xMnxTe(In) alloys in pulsed magnetic fields up to 40 T has been investigated. Localization of nonequilibrium charge carriers in a magnetic field has been observed in n- and p-type Pb1-xSnxTe(In). The amplitudes of a large-scale potential well in Pb1-xSnxTe(In) are estimated. A model which relates the localization mechanism to the tunneling of charge carriers from the allowed band to a single-electron metastable impurity state is proposed. A localization effect has not been observed in Pb1-xMnxTe(In). (C) 1996 American Institute of Physics.

In the course of an investigation of pseudobinary compounds based on heavy-fermion UPt3, we have prepared U(Pt0.98Pd0.02)3 in single-crystalline form. The location in the B-T plane of the antiferromagnetic phase boundary and of the metamagnetic-like transition field has been measured by means of high-field magnetoresistance experiments (B < 25 T, B ; b). The new results are discussed with reference to the magnetic phase diagrams reported for other compounds in the U(Pt, Pd)3 series.

Magnetization, magnetic susceptibility, electrical resistivity, and specific heat measurements of UNiAl point to antiferromagnetic ordering below 19.3 K, which is confirmed by neutron-diffraction experiments. The enhanced γ value of 167 mJ/mol K2 is reflecting pronounced presence of magnetic fluctuations, which influence also the other electronic properties down to low temperatures. The thermal expansion of UNiAl was measured in the temperature range 1.5–210 K on a single crystal along the a and c axis. Similar to other bulk properties, also the thermal expansion of UNiAl is highly anisotropic. The a axis is monotonously expanding with increasing temperature in the whole temperature range. Along the c axis, the lattice first collapses with increasing temperature up to 35 K. Around this temperature, the thermal expansion coefficient αc changes sign and continuous expansion with further increasing temperature is then observed. The sharp anomaly of α near 19.3 K present in both directions is consistent with the magnetic phase transition. The results are discussed in terms of the presence of anisotropic magnetic fluctuations.

The effect of Gd and Ni impurities on the upper superconducting transition temperature of UPt(3) (T(c)(+) = 0.55 K) has been studied by means of resistivity measurements. Surprisingly, we find that paramagnetic Gd impurities suppress T(c)(+) at the same rate as non-magnetic Y or Th impurities, yielding further support for non-s-wave pairing. Ni impurities also suppress T(c)(+), but do not dissolve homogeneously in the UPt(3) matrix.

The magnetostriction on polycrystalline superconducting NbTi perpendicular to the field is measured to study lattice deformation effects caused by fluxoids exerting forces on the lattice through pinning centers. A ''dip'' for increasing and a ''peak'' for decreasing fields in its length at 80% of the upper critical field (B-c2 = 12 T) was observed. Its temperature dependence is well explained by universal B-c2(T) scaling laws for pinning forces and elastic constants. Identical features are observed in the magnetization. In critical current no anomaly is found. An explanation in terms of an anomaly in elastic constants due to a field induced degeneracy of the ground state is discussed.

Measurements of galvanomagnetic effects in the temperature range 4.2-300 K and photoinduced ''transient thermoelectric effect'' (TLE) along the C-2 axis at 300 K have been made for two types of solid solutions of semiconductors Sb2-xInxTe3 (0 less than or equal to x less than or equal to 0.4) and Sb2Te3-ySey (0 less than or equal to y less than or equal to 1.8). By incorporating In atoms into the Sb2Te3 lattices, Hall coefficients, Hall mobilities, and the frequencies of Shubnikov-de Haas (SdH) oscillations are varied systematically. For Sb2Te3-ySey, the Hall mobility is decreased with y up to y = 0.7 and then increased appreciably in the range 0.7 < y < 1.8, and a frequency component of SdH oscillations is observed for y greater than or equal to 0.25. The observed TTE voltages decay exponentially with time, showing a multirelaxation process with characteristic relaxation times tau(i) (i = 1,2,...) for thermal diffusions of photoinduced conduction carriers, whose analyses give valuable information about carrier mobilities and effective masses. In the host material Sb2Te3, four relaxation times tau(i) (i = 1-4) are found, which are attributable to holes in the anisotropic upper and lower valence bands with effective-mass anisotropies of about 3. In addition, we have found two kinds of extra relaxation times tau(i) (i = 5 and 6) for y > 0.6 in Sb2Te3-ySey, confirming the existence of a valence band, whose anisotropy in the effective mass along the C-2 direction is evaluated to be of the order of 2-2.5. Based on these experimental data we have proposed the most probable band model for these solid solutions.

In order to investigate the suppression of the heavy-fermion state of CeCu6 by a magnetic field (B ∥ c) we have measured the coefficients of thermal expansion of a single-crystalline sample (αa, αb, αc) in the temperature range 0.3–10 K in fields up to 8 T. The electronic Grüneisen parameter (Γhf = Vmav/κγ), which amounts to 80 in zero field (T → 0), is reduced by a factor 10 in a field of 8 T. This enormous drop of Γ is attributed to a rapid suppression of the magnetic inter-site correlations.