G.J. Nieuwenhuys

Magnetic and electrical properties of several equiatomic ternary U-compounds

Magnetisation, specific heat, electrical resistivity, magnetoresistivity and Hall effect were measured for several equiatomic ternary (1-1-1) intermetallic compounds of formula RTX with R = U, Th, Hf and Ti, T a transition metal (Co, Ni, Ru, Rh, Pd, Ir, Pt and Au), and X = Al, Ga, Sn and Sb. These compounds crystallize in three different crystal structures: the cubic MgAgAs-type, and the hexagonal Fe2P- and CaIn2-types. All U-compounds, which we focus upon in this paper, exhibit magnetic moments of about 3μB/U at high temperature and encompass U-U distances from 3.51 to 4.68 A. For the compounds which the largest U-U distances, Kondo-lattice behaviour was observed. The specific heat coefficient γ shows typical metallic values with some enhancement. Remarkably these compounds have an electrical resistivity up to three orders of magnitude larger than that expected and usually found for U-based intermetallic compounds. The Hf- and Th-based compounds serve as nonmagnetic reference materials, however, they also exhibit anomalously larger resistivities.

Interrelation of Structure, Dimensionality and Superconductivity in the High-Temperature Superconductor YBa2Cu3O7-δ

We have measured x-ray diffraction (XRD), high-resolution electron microscopy (HREM), resistivity and susceptibility for a series of differently prepared YBa2Cu3O7-δ samples. The high-temperature superconductivity was studied as a function of variable quench temperature. Quenching above 820 °C leaves the sample in the tetragonal phase and destroys the superconductivity. We suggest that the supercondutivity is located in the CuO chains which are extremely sensitive to oxygen loss.

Antiferromagnetism and crystal‐field effects in CeCuX3 (X=Al,Ga) compounds

We have studied two new tetragonal compounds, CeCuAl3 and CeCuGa3, which both display very large specific‐heat, c(T), values at low temperature. CeCuAl3 orders antiferromagnetically below TN=3.0 K, while CeCuGa3 remains paramagnetic down to 0.4 K. The large values of c(T) in these and other cerium intermetallics are interpreted as resulting from an unusually small crystal‐field splitting and the possible presence of atomic disorder.

Antiferromagnetism and crystal-field effects in polycrystal CePd2Al3

Abstract We present an experimental study of the structural and magnetic properties of various polycrystalline samples of the new heavy-fermion antiferromagnet CePd2Al3, TN = 2.7 K. The relevant energy scales of the Kondo effect and crystal-field splitting are determined by comparing them with nonmagnetic LaPd2Al3. The critical dependence of the magnetic properties of rare-earth heavy-fermions on sample preparation is discussed using Doniachs Kondo lattice model.

Resistivity anomalies in heavy-fermion CeCu2Sb2 and CeNi2Sb2

Abstract Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat of the tetragonal compounds CeCu2Sb2 and CeNi2Sb2 have been studied. The resistivity of both compounds displays a sharp maximum followed by a shallow minimum at higher temperature, indicative of heavy-fermion behavior. The possibility of anomalous low-energy crystalline electric field states is discussed by comparison with other cerium-based 1-2-2 compounds.

μ+SR studies of the magnetic and superconducting states of URu2Si2

We have performed μ+SR studies on single-crystal URu2Si2 in the temperature range 0.05–300 K. Zero-field (ZF) measurements show an increase in relaxation rate below TN = 17.5 K. Transverse-field (TF) measurements in H = 2900 G along the a and c directions display a strongly anisotropic Knight shift which does not scale with the bulk susceptibility for T < 50 K. Low-T measurements reveal a penetration depth of ∼7000–10 000 A with a temperature-dependent linewidth in which two steps are observed near 0.3 and 0.9 K.

Characterization of the Superconducting Transition in Single-Crystal URu2Si2

To investigate the nature of the superconducting transition in URu2Si2 we have made careful studies of the specific heat on oriented samples cut from a high-quality single crystal. Our high-resolution a.c.-specific-heat experiments establish the existence of only one sharp superconducting transition. Measurements in applied fields of up to 5 Tesla show that fields along both the a and c axes induce no splitting of the transition. From these Cp(H) results the upper critical field can be obtained, elucidating for the first time the bulk behaviour of Hc2(T). The large anisotropy (Hc2,a Hc2,c) and the small region of positive curvature near Tc for H||a agree with the results obtained from resistivity and a.c.-susceptibility measurements.

Magnetic, Thermal and Transport Properties of URh2Ge2

Using a high-quality single crystal of URh2Ge2 we have measured its susceptibility, magnetization, resistivity and specific heat. The materials has a highly anisotropic magnetization and resistivity, a negative-temperature coefficient of resistivity down to 35 mK, a large electronic specific-heat coefficient, and does not order magnetically. There is also no superconductivity above 35 mK. Our experiments suggest that URh2Ge2 is a unique Kondo-lattice material that neither orders magnetically nor forms a coherent ground state. A crystal-electric-field (CEF) scheme with a singlet ground state is proposed to qualitatively explain some of the results. However, for a full comparison Kondo-lattice effects and hybridization of the CEF-levels due to the strongly correlated electrons must be incorporated into a more sophisticated model.

Disorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems

Muon spin rotation and relaxation (μSR) experiments have yielded evidence that structural disorder is an important factor in many f-electron-based non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFL behaviour are suggested by the observed broad and strongly temperature-dependent μSR (and NMR) linewidths in several NFL compounds and alloys. Local disorder-driven theories (Kondo disorder, Griffiths–McCoy singularity) are, however, not capable of describing the time-field scaling seen in muon spin relaxation experiments, which suggest cooperative and critical spin fluctuations rather than a distribution of local fluctuation rates. A strong empirical correlation is established between electronic disorder and slow spin fluctuations in NFL materials.

Kondo effect and quadrupolar interactions in single-crystal CePd2Al3

Abstract We have studied the magnetic anisotropy on a single crystal of the new heavy-fermion compound CePd 2 Al 3 . The magnetic moments lie in the hexagonal ( a , b )-plane, but do not order antiferromagnetically, in contrast to annealed polycrystalline samples. The Kondo effect can account for the large specific heat at low temperatures. The small crystal-field splitting causes a field-induced electric-quadrupole moment, which orders in fields larger than 4.3 T.