G. Sparn

Heavy fermions in Kondo lattice compounds (invited)

Even in the absence of both magnetic order and superconductivity, Ce‐based Kondo lattice systems show distinct anomalies in the low‐temperature specific heat and thermopower. These features can be understood, in the frame of a microscopic treatment of the asymmetric Anderson model, by taking into account the periodicity of the Ce sites. A substantial variation in the observed phenomena between CeAl3 and normal (n) state CeCu2Si2 on the one hand as well as CeRu2Si2 and CeCu6 on the other highlight the importance of realistic band structure calculations for the quasiparticles (‘‘heavy fermions’’) in these different compounds. The temperature dependence of the upper critical magnetic field Bc2(T) for the heavy‐fermion superconductor CeCu2Si2 (Tc≂0.6 K) shows a flat maximum at ∼0.2 Tc, which is ascribed to coherence‐derived structure in the n‐state quasiparticle density of states. A dc Josephson effect with a critical pair current of ordinary size is observed for the first time on a weak link between CeCu2Si2...

Specific heat and transport properties of UPt3

Specific heat, resistivity, thermal conductivity and thermoelectric power measurements are reported for the normal and superconducting regimes of the heavy-fermion superconductor UPt3. A comparison is made with uranium-based spin-fluctuation and cerium-based Kondo-lattice compounds.

Low-temperature transport properties of Kondo lattices

Abstract The low-temperature behavior of the resistivity, thermal conductivity and thermoelectric power (TEP) is compared for three Kondo lattices, i.e. antiferromagnetic CeAl 2 , enhanced Pauli paramagnetic CeAl 3 and superconducting CeCu 2 Si 2 . For the latter two compounds, coherence leads to a hitherto not observed change of sign in the TEP below T = 0.5 K .

Alloying-induced transition from local-moment to itinerant heavy fermion magnetism in Ce(Cu1−xNix)2Ge2

Abstract A monotonous increase of the Kondo temperature in Ce(Cu1−xNix)2Ge2 from 7 (x = 0) to 30 K (x = 1) is accompanied by drastic changes of ground state properties: for x⩽0.2, a modulated magnetic structure (q01 = (0.28, 0.28, 0.54)) involving Kondo-reduced local Ce moments ( μ s = 0.74μ B Ce for x = 0) forms below TN1(x). TN1 = 4. 1 K for CeCu2Ge2 is strongly depressed upon increasing x. At x ≲ 0.2, a different modulation develops below TN2(x) which becomes maximum (≃4 K) for x = 0.5. Since this is characterized by a very small value of q02 (=(0, 0, 0.13) at x = 0.5) and a gradually decreasing ordered moment (reaching μs ≲ 0.2μB/Ce for x ⩾0.65), we ascribe it to “heavy fermion band magnetism”. For 0.02⩽x⩽0.3, the two modulations seem to be superposed at sufficiently low temperatures. A non-magnetic heavy fermion ground state exists for x > 0.75.

Ground-state properties of new U-Pt-Si and Ce-Pt-Si compounds

We have prepared and investigated several new ternary Ce-Pt-Si and U-Pt-Si compounds. The results are discussed with respect of their ground-state properties. Most interesting features are found in U 2 Pt 15 Si 7 where we observe the formation of a Kondo lattice at low temperature.

Seebeck coefficient of heavy fermion compounds

The thermoelectric power (TEP) of the magnetically ordered Kondo lattices CeCu2Ge2, CeCu4Al8 and CePb3 was measured up to 350 K and compared with that of heavy-fermion UBe13. New low-T TEP-anomalies, presumably coherence-derived, are found to show up in CePb3 and UBe13, but to be absent in U0.97Th0.03Be13.

The effect of composition on the occurrence of a second phase transition in the vicinity of Tc in CeCu2Si2

We report on measurements of the thermal expansion and specific heat on a series of polycrystalline CeCu 2 Si 2 samples with small variations in the stoichiometry. The occurrence of superconductivity and/or the surrounding phase labeled »A«, were found to be most sensitively controlled by the Ce:Cu:Si composition

Alloying experiments on heavy fermion compounds

Abstract This paper is intended to demonstrate the usefulness of controlled alloying for the understanding of heavy-fermion physics: (1) Th-substitution for Ce in CeCu2Si2 emphasizes the dominating role of the dopant-induced strain fields in generating incoherent scattering and pair breaking, (2) replacement of Cu by Ni in Ce(Cu1-xNix)2Ge2 leads to phenomena which are interpreted as derived from a transition between local-moment and itinerant heavy-fermion magnetism, and (3) increasing Cu concentration in UCu4+xAl8-x is accompanied by an antiferromagnetic to nonmagnetic transition near xcr = 1.5 similar to what has been found before for several Ce-based systems. A heavy Fermi-liquid phase with incipient coherence of the quasiparticles is established for x⪆xcr.

YbNiAl: a new Yb-based heavy-fermion antiferromagnet

Abstract We have investigated the compound YbNiAl, which crystallizes in the same structure as the well-known mixed-valent system YbCuAl, and have found that it is the first example of a clear cut Yb-based heavy-fermion antiferromagnet. The resistivity increases logarithmically by 25% between 100 and 10 K, C/T extrapolates below 0.4 K to a γ value of 350 mJ/K2 mol and the entropy at the antiferromagnetic ordering temperature TN = 3 K reaches only 0.45R In 2. We compare the properties of this compound with those of YbPtAl, where Yb is trivalent.

Heavy fermion effects in Ce(Cu1−xNix)2Ge2

Abstract 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 ).