S. Horn

Transport coefficients of intermediate valent Cenix intermetallic compounds

Abstract The resistivity and thermoelectric power of intermediate valent CeNi x intermetallic compounds and their La homologs were determined in the temperature ranges 1.5 to 350 K and 1000 K, respectively. The results are discussed in the frame of a theory for the quasiparticle density of states of intermediate valent systems.

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.

Spin dynamics of metallic Ce systems

Abstract Neutron-scattering experiments on CeCu2Si2, CeB6, CeAl2, and (Ce,M)Al2 with M = La, Y, Sc reveal a common temperature dependence of the reduced quasielastic magnetic linewidth for these substances, i.e. Γ/Γ(T→0)= const +( T 1K ) 1 2 . Our results strongly suggest that all these systems are of “Kondo” rather than “intermediate-valence” type.

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.

Intermediate valence and Kondo effect in Ce1−xScxAl2

Results of lattice parameter, thermal expansion, thermopower and magnetic susceptibility measurements are reported for Ce1 − xScxAl2, a system exhibiting a solubility gap over a wide composition range. Two isostructural phases are identified within the gap. For the Sc-rich systems, both phases contain intermediate valent (IV) Ce ions, whereas in the Ce-rich systems (x < 0.5), a low-volume IV phase coexists with a high-volume phase containing Ce3+ ions. The temperature-induced overall valence change of the IV ions amounts to 15–20%. A factor-of-three increase of the spinfluctuation temperature is found upon increasing the Sc concentration from x = 0.5 to 0.95. For the Ce-rich systems, we have discovered a hitherto not known “Kondo anomaly” in the temperature dependence of the thermal expansivity.

Manifestation of valence change in Ce1-xScxAl2 by neutron scattering

Abstract Neutron-scattering experiments demonstrate that the Ce-ions in Ce1-xScxAl2 are trivalent up to x = 0.4, undergo a change into an intermediate-valence state between x=0.4 and 0.5, and show, upon further increasing x, a continuous increase of valence, eventually reaching a completely demagnetized state of Ce for dilute (Ce, Sc )Al2 alloys.

Thermopower of mixed-valent CeNi5: 4f-electron instability vs band structure effects

Abstract A thermopower anomaly which has the outward appearance of those prevariously observed in mixed-valent systems is observed in mixed-valent CeNi5 and Ce 1−x La x Ni 5 . However, by means of both band structure calculations and Cu-alloying experiments it is demonstrated that the observed anomaly is dominated by electron scattering from Ni-derived 3d states rather than by effects due to the 4f electron instability. These results dramatize a caveat which should be exercised when interpreting various observed properties of those mixed-valent systems which possess a high density of d states at the Fermi energy.

Transition from heavy-fermion behaviour to magnetic ordering in CePtSi1−xGex

Abstract A transition from a heavy-fermion to an antiferromagnetically ordered ground state has been observed in CePtSi 1−x Ge x . The highest value of γ = 1.7 J/(K 2 mol) is observed for x = 0.2. The crossover to the magnitically ordered state occurs at the highest γ value.

Electronic transport properties of UCu4+xAl8−x at low temperatures

Abstract Magneto-resistivity measurements show that in the compound UCu4+x, which undergoes a transition from a magnetically ordered to a heavy fermion groundstate with increasing x, single ion Kondo processes dominate the low temperature properties. A comparison to Bethe Ansaty calculations indicates an angular momentum of the groundstate larger than two.