O. Stockert

TWO-DIMENSIONAL FLUCTUATIONS AT THE QUANTUM-CRITICAL POINT OF CECU6-XAUX

The heavy-fermion system CeCu_{6-x}Au_x exhibits a quantum critical point at x_c = 0.1 separating nonmagnetic and magnetically ordered ground states. The pronounced non-Fermi-liquid behavior at x_c calls for a search for the relevant quantum critical fluctuations. Systematic measurements of the inelastic neutron scattering cross section S(q,omega) for x = 0.1 reveal rod-like features in the reciprocal ac plane translating to two-dimensional (2d) fluctuations in real space. We find 3d magnetic ordering peaks for x = 0.2 and 0.3 located on these rods which hence can be viewed as 2d precursors of the 3d order.

Mechanism for the Non-Fermi-Liquid Behavior in CeCu{sub 6{minus}{ital x}}Au{sub {ital x}}

We propose an explanation for the recently observed non-Fermi-liquid behavior of metallic alloys CeCu_{6-x}Au_x: near x=0.1, the specific heat c is proportional to T ln (T_0/T) and the resistivity increases linearly with temperature T over a wide range of T. These features follow from a model in which three-dimensional conduction electrons are coupled to two-dimensional critical ferromagnetic fluctuations near the quantum critical point, x_{c}=0.1. This picture is motivated by the neutron scattering data in the ordered phase (x=0.2) and is consistent with the observed phase diagram.

Investigation of non-Fermi-liquid behavior in CeCu6−xAux

Abstract Non-Fermi-liquid (NFL) behavior has been observed in CeCu 6−x Au x at the onset of antiferromagnetic order, either for x = x c = 0.1 at pressure p = 0, or for x = 0.3 by pressure-tuning the Neel temperature T N to zero. We report on a detailed study in the vicinity of x c exploring the NFL behavior, i.e. the specific heat C divided by temperature T varying as C / T ≈ − ln( T / T 0 ) and the magnetization M measured in B = 0.1 T exhibiting a cusp for T → 0. A magnetic field which likewise suppresses magnetic order does not induce NFL behavior when T N → 0 as shown for a sample with x = 0.2. This supports the idea that NFL behavior arises from an abundance of low-lying spin excitations. The Kondo temperatures are determined from the high-field specific heat.

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.

Incommensurate antiferromagnetism and magnetic correlations in CeCu6-xAux☆

Magnetic ordering and correlations in single crystals of the heavy-fermion alloys CeCu5.8Au0.2 and CeCu5.9Au0.1 were investigated by elastic and inelastic neutron scattering. CeCu5.8Au0.2 orders with an incommensurate magnetic structure and a wave vector q = (± 0.79 0 0). Only dynamic antiferromagnetic correlations along a∗ are observed in CeCu5.9Au0.1 which evolve into a static sine modulation in CeCu5.8Au0.2.

Magnetic fluctuations in CeCu6−xAux at the nonmagnetic–magnetic quantum phase transition

Abstract When the heavy-fermion system CeCu 6 is doped with Au, long-range antiferromagnetic order develops above a threshold concentration x =0.1, with T N reaching 2.3xa0K for x =1. The magnetic ordering vector of CeCu 6− x Au x as determined from elastic neutron scattering is almost independent of x for x =0.15, 0.2 and 0.3, Q ≈(0.63 0 0.26), and jumps to the a ∗ axis for x =0.5. At the quantum critical point x =0.1, the specific heat C exhibits a divergence of C / T ∼−ln( T / T 0 ) towards low temperatures T , and the electrical resistivity varies linearly with T . Inelastic neutron scattering experiments were carried out at the ILL Grenoble and at Riso in order to investigate whether the critical fluctuations for x =0.1 can account for this anomalous behavior. We review some salient features of the both data sets. The observed reduced dimensionality of the critical fluctuations indeed leads to the above anomalous behavior. A detailed study of the energy and temperature dependence of the dynamic structure factor reveals unusual dynamics. The orthorhombic–monoclinic distortion in CeCu 6 is suppressed rapidly with increasing x and is shown to be unrelated to the quantum phase transition.

Low-temperature thermal conductivity and thermopower of CeCu6−xAux near the magnetic quantum phase transition

Abstract The thermal conductivity κ and thermopower S in the heavy-fermion alloy CeCu6−xAux at the critical concentration x=0.1 separating magnetically ordered and non-magnetic ground states, has been measured in the range 0.1–2.5xa0K at zero magnetic field and B=6xa0T. κ(T) is found to be linear at high T in close agreement with pure CeCu6. In the limit of low T, κ is strongly suppressed. In contrast to x=0, for x=0.1κ increases only very weakly in a magnetic field of 6xa0T. The (positive) thermopower is enhanced with respect to x=0, dropping off only well below 0.5xa0K. The magnetic field reduces strongly the value of S and leads to a collapse of S(T) for x=0 and 0.1 onto identical curves.

Specific heat of NdCu2 at very low temperatures

Abstract NdCu 2 undergoes successive magnetic transitions at 6.5 and 4.1 K, respectively. We extend the previous specific-heat measurements to below 1 K and to magnetic fields up to 6 T. The specific heat C below 3 K decreases exponentially, indicating a gap of 10.3 K· k B in the magnetic excitation spectrum and hence a rather strong magnetic anisotropy. Below 0.8 K C increases towards low T because of the hyperfine contribution of Nd. In a magnetic field of 6 T applied parallel to the b direction of orthorhombic NdCu 2 , the gap increases to 12.8 K· k B . Determination of the magnetic transitions by measurements of C in a magnetic field and by the magnetocaloric effect complements the ( B , T ) phase diagram of NdCu 2 .

μSR studies of Kondo disorder in UCu5−xPdx and CeCu5.9Au0.1

Abstract Muon spin rotation (μSR) spectra in the non-Fermi-liquid (NFL) heavy-fermion alloys UCu5−xPdx (x = 1.0 and 1.5) and CeCu5.9Au0.1 yield estimates of the rms width (δχ)rms of the inhomogeneous susceptibility distribution in these systems. In UCu5−xPdx, the temperature dependence of (δχ)rms indicates a wide distribution of Kondo temperatures TK, resulting in a significant number of uncompensated U spins with low TK which cause NFL behavior. In contrast, μSR lines in CeCu5.9Au0.1 are too narrow for such “Kondo disorder” to account for NFL properties unless, as seems unlikely, the correlation length for the susceptibility disorder is long in this alloy.