S. Mock

Non-Fermi-liquid behavior in strongly correlated electron systems

Abstract Non-Fermi-liquid (NFL) behavior in three-dimensional metals with strong electron correlations can have different origins, viz. collective or single-ion effects. The former is manifest at a magnetic instability where the system changes from a nonmagnetic to a magnetic ground state as a function of some parameter like concentration or pressure. We review the properties of the exemplary NFL heavy-fermion system CeCu6−xAux at this magnetic instability, i.e. a logarithmic divergence of the specific-heat coefficient γ(T) = C/T, a cusp in the magnetic DC susceptibility χ(T) for T → 0, and a T-linear contribution Δϱ to the electrical resistivity, as opposed to the Fermi-liquid behavior γ ∼ χ ≈ const, Δϱ ∼ T2 found in many heavy-fermion systems, including the parent compound CeCu6. Single-ion effects leading to NFL behavior are overscreening of magnetic or quadrupolar degrees of freedom of a magnetic impurity or a distribution of Kondo temperatures in a disordered alloy. Similarities and differences of the NFL phenomenology between these scenarios will be reviewed.

Pressure dependence of the Néel temperature in antiferromagnetic CeCu6-xAux for 0.3≤x≤1.3

Abstract The volume dependence of the Neel temperature TN of CeCu6−xAux is investigated in detail with magnetic-susceptibility and specific-heat measurements under hydrostatic pressure p up to 9 kbar. In the concentration range 0.3 ⩽ x ⩽ 1.3, TN (p) decreases roughly linearly with increasing p, with dTN/dp changing monotonically from −58 to −5 mK/kbar with increasing x. In contrast, TN(x) passes over a sharp maximum for x = 1 while the atomic volume of the CeCu6−xAux alloys as determined from X-ray diffraction increases monotonically. A subtle structural change at x = 1 (the concentration dependence of the orthorhombic b-axis changes from contraction to expansion) must be at the origin of the TN(x) maximum.

Low-temperature properties of CeTAl3 (T = Au, Cu, Pt) and CeAuGa3

Measurements of the magnetization, specific heat, and electrical resistivity on CeTAl3(T=Au, Cu, Pt) and CeAuGa3polycrystals are reported. For all alloys, magnetic order is found. In CeTAl3the antiferromagnetic ordering temperature TNincreases from 1.5 K (T=Au) to 2.9 K (T=Cu) while spin-glass order is found at 0.8 K for T=Pt. This tendency resembles the nonmonotonic dependence of the ordering temperature on the hybridization between Ce moments and ligand atoms in terms of the Doniach picture. CeAuGa3(TN=2.4 K) behaves similarly to CeAuAl3, thus underscoring the dominant role of transition metal ligands in ternary Ce compounds.

Magnetization and specific heat of CeCu6−xMx (MNi, Pd, Pt)

Abstract Polycrystalline alloys of CeCu 6 with Ni, Pd, and Pt have been studied with magnetization ( M ) and specific-heat ( C ) measurements in the concentration range where they still retain the orthorhombic structure. Sharp maxima in M ( T ) and C ( T ) for Pd alloys with x > 0.07 indicate antiferromagnetic order. At the critical concentration where magnetic order sets in, C varies as C / T ∼ − ln( T / T 0 ) and M shows a cusp for T → 0, indicative of non-Fermi-liquid behavior. Similar features are found for Pt alloys. Ni alloys, on the other hand, evolve towards intermediate valence behavior with increasing x . These differences are discussed in terms of changes of volume and band structure upon alloying.

Magnetic and thermodynamic properties CeCu6−xAux for 0 ≤ x ≤ 2

The Neel temperature TN(x) of the heavy-fermion alloys CeCu6−xAux exhibits a sharp maximum, TN = 2.2 K for x = 1, coinciding with a change of the lattice constant b(x) from a slight contraction for x 1. The samples remain single-phase orthorhombic (at room temperature) up to x = 1.4 after annealing at 700°C and up to x = 2 directly after preparation. Measurements of the specific heat indicate that C/T at low T shows a nonmonotonic behavior with a maximum at x ≈ 0.25 and a minimum at x = 1. These extrema can be qualitatively understood by considering the contributions from Kondo quasiparticle and magnetic excitations.

Specific heat and magnetic susceptibility of UCu4Pd at very low temperatures

Abstract UCu 5− x Pd x alloys are heavy-fermion materials with indications of non-Fermi liquid (NFL) behavior for x = 1 and 1.5. We report on the specific heat C of UCu 4 Pd measured between 0.08 and 3.5 K which shows NFL behavior C / T ∼ −1n( T / T 0 ) down to 0.2 K but indicates spin-glass freezing around 0.2 K. This is confirmed by measurements of the DC magnetization and the AC susceptibility.

Low-temperature magnetic and thermal properties of CePd2In in magnetic fields

A detailed study of the low-temperature magnetization M and specific heat C of CePd2In is reported. For B ≈ 0, M and C show maxima at TN≈ 1.25 K indicating an antiferromagnetic phase transition as observed previously. In a magnetic field B the transition seen in M is shifted to lower T while TN as determined from the specific-heat maximum is essentially unaltered up to fields of 6 T. This different behavior is attributed to magnetic anisotropy of the polycrystalline samples. The hyperfine contribution to C is discussed in terms of nuclear quadrupole splitting as well as Zeeman splitting by applied and transferred hyperfine fields. A rough estimate of the Kondo temperature from C sufficiently above TN yields TK ≈ 1.5 K.