T. Pietrus

Heavy-fermion systems at the magnetic-nonmagnetic quantum phase transition

Abstract Heavy-fermion systems are characterized by their proximity to magnetic order which can often be induced or suppressed by changing a parameter such as concentration or pressure. Hence, these systems provide ideal testing grounds for a magnetic phase transition occurring at T = 0 as a function of these parameters. We will review the salient features of the exemplary system CeCu 6− x Au x where antiferromagnetism can be induced by alloying with Au above a critical concentration x c = 0.1. At the quantum-critical point x = x c , unusual features are observed, e.g. a specific-heat coefficient γ = − ln(T/T 0 ) that diverges logarithmically for T → 0, and a resistivity Δρ varying linearly with T , in marked contrast to the canonical Fermi-liquid behaviour.

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.

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.

Magnetic properties of the one-dimensional Heisenberg antiferromagnet tetraphenylverdazyl

Abstract (1,3,5,6)-tetraphenylverdazyl (Ph-TPV) is an organic Heisenberg antiferromagnet with quasi-one-dimensional properties. We measured the specific heat and electron spin resonance and determined an intrachain exchange constant of J 1 d / k B = −17 K and the onset of magnetic long-range order at T N = 0.92 K .

Hall effect in the doped Kondo insulator (Ce1−xLax)3Bi4Pt3

Abstract Single crystals of (Ce1−xLax)3Bi4Pt3 have been grown by using a flux grow technique. The Hall effect measured between T=1.5 and 80 K in B=±5 T indicates that the carriers are electron-like with a concentration n increasing from n=8.5×10 20 cm −3 for x=0 to 5.6×10 21 cm −3 for x=1. The T and x dependences of the electrical resistivity, Hall effect and mobility of the carriers suggest that the critical La concentration for the metal–insulator transition is less than x=0.006, where an impurity band at the Fermi energy is already formed by La doping.

Electrical resistivity of the non-Fermi-liquid alloy CeCu5.9Au0.1

Abstract The temperature and directional dependence of the electrical resistivity ϱ of single-crystalline CeCu 5.9 Au 0.1 is investigated. We find non-Fermi-liquid behavior, ϱ = ϱ 0 + A ′ T , between 15 mK and ∼0.5 K for all current directions. The residual resistivity ϱ 0 , as well as the coefficient A ′ are largest for the current parallel to the a direction which is the direction of the magnetic ordering wave vector for the magnetically ordered compounds CeCu 6− x Au x with x > 0.1. This anisotropy is the same as in CeCu 6 . The data are compared with predictions for spin-fluctuation theories.