There are two main theoretical descriptions of antiferromagnets. The first arises from atomic physics, which predicts that atoms with unpaired electrons develop magnetic moments. In a solid, the coupling between moments on nearby ions then yields antiferromagnetic order at low temperatures. The second description, based on the physics of electron fluids or 'Fermi liquids' states that Coulomb interactions can drive the fluid to adopt a more stable configuration by developing a spin density wave. It is at present unknown which view is appropriate at a 'quantum critical point' where the antiferromagnetic transition temperature vanishes. Here we report neutron scattering and bulk magnetometry measurements of the metal CeCu(6-x)Au(x), which allow us to discriminate between the two models. We find evidence for an atomically local contribution to the magnetic correlations which develops at the critical gold concentration (x(c) = 0.1), corresponding to a magnetic ordering temperature of zero. This contribution implies that a Fermi-liquid-destroying spin-localizing transition, unanticipated from the spin density wave description, coincides with the antiferromagnetic quantum critical point.

PACS numbers: * Electronic address: stockert@cpfs.mpg.de 1The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu 2 Si 2 . A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the major driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point.While conventional superconductivity (SC) is generally incompatible with magnetism, magnetic excitations seem to play an important role in the Cooper pair formation of unconventional superconductors such as the high-T c cuprates or the low-T c organic and heavyfermion (HF) superconductors. Since the discovery of SC in CeCu 2 Si 2 1 , antiferromagnetic (AF) spin excitations have been proposed as a viable mechanism for SC 2-4 . The discovery of SC at the boundary of AF order in CePd 2 Si 2 5 has pushed this notion into the framework of AF quantum criticality 6 . Unfortunately, such quantum critical points (QCPs) proximate to HF superconductors typically arise under pressure, which makes it difficult to probe their magnetic excitation spectrum.Here, we report a detailed study of the magnetic excitations in CeCu 2 Si 2 , which exhibits SC below T c ≈ 0.6 K. This prototypical HF compound is ideally suited for our purpose, since SC here is in proximity to an AF QCP already at ambient pressure (cf. Fig. 1(a)).As displayed in Fig. 1(b) CeCu 2 Si 2 crystallises in a structure with body-centred tetragonal symmetry and is one of the best studied HF superconductors and well characterised by low-temperature transport and thermodynamic measurements 7 . Moreover, those measurements in the field-induced normal state have already provided evidence that the QCP in this compound is of the three-dimensional (3D) spin-density-wave (SDW) type 8 . The spatial anisotropy of the spin fluctuations in superconducting CeCu 2 Si 2 was measured at T = 0.06 K and at an energy transfer ω = 0.2 meV and is shown in Fig. 1(c). These magnetic correlations display only a small anisotropy (a factor of 1.5) in the correlation lengths 2 between the [110] and the [001] direction. Therefore, these quite isotropic spin fluctuations are in line with thermodynamic and transport measurements exhibiting C/T = γ 0 − a √ T or ρ − ρ 0 = AT α , α = 1 − 1.5 8,9 , and strongly support a three-dimensional quantum critical SDW scenario 10 . We are able to identify the magnetic excitations in the normal state of paramagnetic, ...

The heavy-fermion system CeCu6−xAux exhibits a quantum critical point at xc ≈ 0.1 separating nonmagnetic and magnetically ordered ground states. The pronounced non-Fermi-liquid behavior at xc calls for a search for the relevant quantum critical fluctuations. Systematic measurements of the inelastic neutron scattering cross section S(q, ω) 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. 75.30.Mb, 71.27.+a, 75.20.Hr Continuous quantum phase transitions which occur in a strict sense only at temperature T = 0 are driven by quantum fluctuations instead of thermal fluctuations as for ordinary classical phase transitions [1,2]. This leads to unusual and rich behavior even at finite temperatures in the neighborhood of the critical point. Because of the uncertainty principle the energy scale of fluctuations introduces a time scale which leads to an intricate coupling of static and dynamic critical behavior. For instance, the critical behavior of the specific heat will depend on the dynamical critical exponent z relating the typical lifetime ξ τ and correlation length ξ of critical fluctuations, ξ τ ∝ ξ z . Such a quantum phase transition can be achieved by changing a coupling parameter which plays a role analogous to temperature in ordinary phase transitions. In recent years, many physical realizations of quantum phase transitions have been found. The case of a magnetic-nonmagnetic transition in heavy-fermion metals is particularly interesting because of the involvement of itinerant electrons. Excitations of a system of interacting itinerant electrons in a metal, i.e. quasiparticles, are usually described within the Fermi-liquid theory, with the specific heat C ∝ T , a Pauli susceptibility independent of T , and an electrical resistivity contribution ∆ρ ∝ T 2 due to quasiparticle-quasiparticle scattering. Interactions renormalize the quasiparticle masses with respect to the free-electron mass m 0 . Even in heavy-fermion systems with quasiparticle masses as high as several 100 m 0 , Fermi-liquid behavior is the rule rather than the exception [3]. In heavy-fermion systems, the coupling parameter tuning the magnetic-nonmagnetic transition is the (antiferromagnetic) exchange interaction J between 4f or 5f magnetic moments and conduction electrons [3]. If it is strong, a local singlet state is formed via the Kondo effect around each 4f or 5f site, leading to a nonmagnetic ground state. On the other hand, a weak (but non-zero) exchange interaction leads to a Rudermann-Kittel-Kasuya-Yosida coupling between moments and hence to magnetic order. In the exemplary system CeCu 6−x Au x doping of CeCu 6 with the larger Au atom leads -via lattice expansion -to a weakening of the Kondo effect and hence to long-range antiferromagnetic order for x > x c ≈ 0.1, with a linear increase of the 1 [4].At x c where T N vanishes, i...

Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.