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Progressive substitution of Cu for Al in UCu 4+ x Al 8 −x yields as increasing compression of the unit cell. This results in a reduction of the Neel temperature from ifT N = 42 K for x = 0 to ifT N = 15 K for x = 1.0. No magnetic ordering is found for x ⩾ 1.5. Rather the T -dependence of the resistivity shows a maximum in the range 6 to 15 K and indicates the formation of a Kondo-lattice state. The latter is characterized by heavy fermions (γ( T a0)≅0.8 J/K 2 mol for x = 1.75).

Elastic and inelastic neutron scattering studies of the Kondo lattice CeCu2Ge2 were performed. AtTN=4.1 K an incommensurate magnetic order develops with an ordering wave vectorq0=(0.28, 0.28, 0.54) and an ordered moment µs=0.74 µB. The crystalline electric field splits the 4f1-J-multiplet of the Ce ion into a ground state doublet and a quartet at 191 K. The wave function of the ground state yields an ordered moment of 1.54µB. Thus, due to the onset of the formation of a Kondo singlet the magnetic moment is considerably reduced. The magnetic relaxation rate Λ was investigated via quasielastic neutron scattering. The temperature dependence of Λ(T) is characteristic of heavy-fermion systems with a high temperature square root dependence and a limiting low temperature value, yielding a Kondo temperatureTK≈10K. The quasielastic component of the scattered neutron intensities persists down to the lowest temperatures, well belowTN. This quasielastic line is regarded as a characteristic feature of heavy-fermion systems and corresponds to the enhanced value of the linear term of the specific heat.

Magnetic, calorimetric and transport measurements characterize CeCu2Ge2 as a Kondo–lattice system which orders antiferromagnetically below TN = (4.15 ± 0.05) K. The low-temperature specific heat shows a magnon-derived T3 term and, in addition, an electronic contribution γ(T)T. γ(T) extrapolates to 0.1 J/K2mol as T → 0 and assumes a large maximum of 0.22 J/K2mol at T = 0.45 K.

Magnetic fluctuations in CeCu2Si2 were directly observed, both above the superconducting transition temperatureTC as well as below. Spatial correlations of the magnetisation density belowTC are weak, resulting in an essentially wavevector independent magnetic response. The intensity observed within an energy window of ±5 THz centred on ω=0 THz corresponds to 1µβ per Ce atom. Higher energy excitations peaking between 7–10 THz were observed and are probably due to higher crystal field levels.

Ce${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ single crystals prepared with an excess of Cu exhibit low residual resistivity ${\ensuremath{\rho}}_{0}$, low susceptibility $\ensuremath{\chi}$, small unit-cell volume $V$, and bulk "heavy-fermion" superconductivity below 0.65 K, while crystals grown from stoichiometric melt show higher ${\ensuremath{\rho}}_{0}$ and $\ensuremath{\chi}$, larger $V$, but no superconductivity. Ce${\mathrm{Cu}}_{2}$${\mathrm{Si}}_{2}$ behaves as an $s$-state superconductor with strongly temperature-dependent pair breaking and almost isotropic Fermi surface.

We present the results of two types of inelastic neutron scattering experiments on the heavy-fermion system ${\mathrm{CeCu}}_{2\ifmmode\pm\else\textpm\fi{}0.2}$${\mathrm{Si}}_{2}$ to investigate the quasielastic scattering (low-energy response) and the crystal-field (CF) excitations (high-energy response). A characteristic energy width of about 0.7 meV is found for the magnetic low-energy response at low temperatures, and the CF scheme given earlier for ${\mathrm{CeCu}}_{2}$${\mathrm{Si}}_{2}$ is roughly confirmed. The inelastic spectrum of ${\mathrm{CeCu}}_{2.2}$${\mathrm{Si}}_{2}$ gives some indications for a coexistence of \ensuremath{\gamma}-cerium and \ensuremath{\alpha}-cerium. The scattering intensities are compared with static susceptibility measurements.

An inelastic neutron scattering study is performed on the new magnetically ordered heavy-fermion systems CeCu2Ge2 and CeAg2Ge2. The temperature dependence of the quasielastic line can be interpreted in terms of a Kondo type of behavior. In addition crystalline electric field transition energies are reported.

The magnetic relaxation rates Γ as determined via inelastic neutron scattering experiments are reported for the ternary compounds CeM 2 X 2 (M  Ni, Cu, Ag, Au, Ru; X  Si, Ge). Strongly enhanced values of Γ and deviations from a Korringa-type of behaviour were found. A close correlation between the unit cell volume and the 4f-local spin / conduction electron hybridization is demonstrated.

We report results of the specific heat, dc susceptibility, thermal expansion, thermopower and resistivity for a number of Ce(Cu1−xNix)2Ge2 alloy. Upon increasing x, a non-linear increase of the heavy-fermion band width T∗ and a stron non-monotonic Neel temperature TN(x) are found ( T N = 4.1 K for x = 0 and T N K for x ≧ 0.75 ).

The spin relaxation rates Γ in magnetically ordered Kondo lattices have been determined using neutron scattering techniques. Enhanced values of Γ and distinct deviations from a Korringa-type of relaxation behaviour were found. Gaussian contributions to the quasielastic scattered intensities were interpreted as being due to magnetic fluctuations.

Non-magnetic La and Y impurities in CeCu 2.2 Si 2 give rise to a “diamagnetic” pair-breaking effect and, in addition, Gd causes “paramagnetic” pair breaking. In dilute (0 1-x La x Cu 2.2 Si 2 alloys an “alloying-induced depression of incoherent scattering” is observed and related to the formation of antiferromagnetic order at several K.

For the high-Tc superconductors La1.85Sr0.15CuO4 and YBa2Cu3O7 we present results for the low-field magnetization and the lower and upper critical fields Bc1(T) and Bc2(T). Losses in the Meissner state observed in magnetization and susceptibility measurements disappeared after powdering the sample. This is taken as strong evidence for intergrain Josephson coupling in the bulk sample. The Bc2(T) obtained by inductive measurements indicates a large anisotropy due to the layered structure of these compounds.