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In the prototypical heavy-fermion system CeCu6-xAux, a magnetic quantum critical point can be tuned by Au concentration x, hydrostatic pressure p, or magnetic field B. A striking equivalence of the tuning behavior with x or p had been found with respect to thermodynamic and transport properties. By means of elastic neutron scattering on single crystalline CeCu5.5 Au05, we demonstrate this x -- p equivalence on a microscopic level by showing that the magnetic ordering wave vector qm can be tuned accordingly. At ambient pressure,CeCu5.5Au05 orders at qm ~ (0.5900). Upon applying p = 4.1 kbar, qm ≈ (0.6100.21) is found corresponding to CeCu5.6Au0.4 at ambient pressure. The transition seems to occur in a first-order fashion and to be governed by slight changes in the nesting properties of the Fermi surface. [ABSTRACT FROM AUTHOR]

The 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 CeCu2Si2. 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 main driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point. [ABSTRACT FROM AUTHOR]

The 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 CeCu2Si2. 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 main driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point. [ABSTRACT FROM AUTHOR]

Phonons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron{phonon coupling. YN2B2C was chosen as a test example because it is a conventional superconductor with a fairly high Tc (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron{phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth. [ABSTRACT FROM AUTHOR]

Abstract: We report on inelastic neutron scattering experiments on the heavy-fermion superconductor CeCu2Si2 to study the magnetic excitation spectrum in single crystals. Elastic short-range magnetic correlations have been observed in the superconducting state. While the low-energy magnetic response seems to be quasielastic in the normal state above , a truly inelastic response is detected in the superconducting state well below . This inelastic response is discussed in connection to the properties of the superconducting state. [Copyright &y& Elsevier]

(The EPR signal from localized ytterbium ions was observed in an undoped YbRh)2(Si)2 compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb(3+)(4f (13)) ion with the activation energyD = 115 K. [ABSTRACT FROM AUTHOR]

Single-crystal like nuclear magnetic resonance spin-echo spectra are obtained for powder samples of the antiferromagnet CeCu5Au in the paramagnetic phase. Line shifts and quadrupolar splittings are analyzed for 63Cu and 65Cu. The influence of the extreme magnetic anisotropy of the CeCu6-xAux compounds on nuclear spin relaxation is discussed. [ABSTRACT FROM AUTHOR]

We report on extensive elastic neutron scattering to determine the wave vector of the magnetic order in CeCu6-xAux single crystals for x > 0.1. For all values of x investigated ( 0.2, 0.3, 0.5, 1.0) we find long-range incommensurate antiferromagnetic order with an ordering vector ( 0.625 0.275) for x =0.2, nearly unchanged for x =0.3, and ( 0.59) for x =0.5, staying roughly the same for x =1.0. In addition, short-range correlations are observed for x =0.2, reminiscent of those found previously for x =0.1. The ordered magnetic moment is found to increase rapidly for small x, and more slowly for the larger x values. The increase of the specific-heat anomaly at the ordering temperature with x is in qualitative accord with this behavior. Finally, data of the electrical resistivity for current flow along the three crystallographic directions are presented, showing a clear signature of the magnetic order. A theoretical interpretation of the interplay of magnetic order and transport in terms of (i) the partial suppression of the Kondo effect by the staggered magnetization and (ii) the anisotropic band structure induced by the staggered field is shown to account well for the data, provided the ordering vector is close to 2 k F, where k F is a typical Fermi momentum. [ABSTRACT FROM AUTHOR]

Abstract: Depending on the actual stoichiometry, shows either antiferromagnetic order, superconductivity or both phenomena. We investigated a single crystal, where are roughly degenerate. A custom-made in-situ AC-susceptibility setup was utilized for an elastic neutron scattering experiment to directly correlate antiferromagnetic intensity and AC-susceptibility. We found antiferromagnetic satellite peaks together with a diamagnetic signal below . The results can be understood by a temperature and magnetic field dependent spatial separation of antiferromagnetic and superconducting volume fractions. [Copyright &y& Elsevier]

Abstract: Here we report on neutron diffraction experiments in magnetic fields on a large A-type single crystal of . The aim was to investigate the field dependence of the antiferromagnetic order below . The intensity of the observed antiferromagnetic satellite peak decreases continuously to zero with increasing field, indicating a second order phase transition. [Copyright &y& Elsevier]

We report inelastic neutron-scattering experiments on RNi2B2C single crystals (R=Tb and Ho) to investigate the dispersion of low-energy phonons. Both compounds order antiferromagnetically at low temperatures. For superconducting HoNi2B2C we observe a strong softening of two phonon branches similar to other superconducting RNi2B2C compounds. In contrast, for the non-superconducting TbNi2B2C we could not detect significant changes in the phonon spectra as a function of temperature. This correlation between superconductivity and the observed phonon softening indicates a strong electron–phonon interaction and strongly supports the BCS-like description of these compounds. [Copyright &y& Elsevier]