A. Furrer

Quantum magnets under pressure: controlling elementary excitations in TlCuCl3.

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.

FOCUS: a hybrid TOF-spectrometer at SINQ☆

Abstract The concept of the SINQ-time-of-flight (TOF) spectrometer FOCUS foresees a hybrid-TOF solution that combines a doubly focusing crystal monochromator with a Fermi-chopper. The spectrometer can be operated in time focusing (TF) and monochromatic focusing (MF) mode such that it can be optimised for quasielastic and inelastic scattering applications, respectively. By the use of two interchangeable monochromators (PG and MICA) access is provided to incident energies in the range 0.25 meV

Long range f.c.c. antiferromagnetic ordering of type III in YbAs

Abstract Neutron diffraction investigations performed at temperatures T ⩾ 7 mK on a single crystal of the rare-earth compound YbAs prove the existence of long-range f.c.c. antiferromagnetic ordering of type III, corresponding to k = [1, 0, ± 1 2 ] . The ordered magnetic saturation moments of magnitude μYb = 0.86(3) μB are oriented perpendicular to [0, 0, 1]. The phase transition at TN = 0.7 K is second-order, but the temperature dependence of the staggered magnetization exhibits a most unusual critical behaviour. A mean-field calculation indicates that the magnetic moments are stabilized by quadrupolar interactions.

Antiferromagnetism in the ytterbium monopnictides YbX (X = N, P, As, Sb)

Neutron diffraction investigations prove the existence of long-range antiferromagnetic ordering of type III in stoichiometric YbN and YbAs and of type II in nonstoichiometric YbP. For stoichiometric YbSb no long-range antiferromagnetic ordering was found down to 7 mK. The crystal-field level scheme was established by neutron spectroscopy to be Γ6-Γ8-Γ7 for all compounds.

Barocaloric effect: The use of pressure for magnetic cooling in Ce3Pd20Ge6

The barocaloric effect found in many rare-earth compounds opens the way to novel magnetic refrigeration techniques which are rather based on the application of external pressure than on the application of large external magnetic fields as needed for cooling by the magnetocaloric effect. Here we report on the first observation of this effect in a heavy-fermion system, namely the Kondo lattice compound Ce3Pd20Ge6. Upon isothermal application of uniaxial pressure the compound lowers the magnetic entropy, subsequent adiabatic release of the pressure leads to a cooling by as much as −0.75 K per 0.3 GPa pressure release at 4.4 K. Based on entropy considerations the observed cooling effect can be explained by an increase of the Kondo temperature and by a structurally induced splitting of the Γ8 ground-state quartet upon uniaxial pressure.

F.c.c. type-III antiferromagnetism in YbN

AbstractNeutron diffraction experiments performed for YbN in the temperature range from 7 mK to room temperature prove the existence of long-range f.c.c. antiferromagnetic ordering of type III belowTN=(0.790±0.005) K. The low ordered magnetic moment of Yb at saturation, μ=(0.39±0.05)μ> B, is presumably caused by Kondo hybridization. By means of inelastic neutron scattering the crystalfield level scheme was established to beΓ6 −Γ8(33 meV)−Γ7(81 meV). TheΓ6 −Γ8 transition was found to be split into two lines which may be due to a bound state between the crystal-field excitation and phonons.

CRYSTAL-FIELD EXCITATIONS IN ND2CUO4

Abstract Inelastic neutron scattering has been employed to study the crystal- field interaction in Nd2CuO. The energetic ordering of the crystal- field levels and the crystal-field parameters are determined from the observed transitions. J-admixture effects turn out to be important and are included in the calculations. At 1.5 K the ground-state doublet is found to be split by about 0.35 meV due to the long-range magnetic ordering of the Nd3+ ions.

Crystal-field and magnetic properties of the distorted perovskite NdGaO3

Inelastic neutron scattering and magnetic susceptibility measurements have been performed on the distorted perovskite NdGaO3. The magnetic susceptibility data show a Curie-Weiss behaviour with an effective magnetic moment close to 3.6 mu B per mole of Nd ions. No long-range magnetic ordering was detected in the temperature range 2-300 K. The inelastic neutron spectra observed at T=12 K exhibit four peaks of magnetic origin between 11 and 7D meV which can be unambiguously assigned to the complete crystalline-electric-field splitting pattern in the ground-state J multiplet 4I9/2 of the Nd3+ ions. We analysed the spectra in terms of geometrical considerations based on the actual C2 site symmetry of Nd3+. The best agreement between the experimental spectra and the calculated level structure was obtained for a model that takes into account the three nearest-neighbouring coordination polyhedra associated with the O2-, Ga3+ and Nd3+ ions as well as J-mixing between all multiplets of the 4I term. We conclude that single-particle crystal-field theory adequately explains the majority of magnetic and crystal-field properties of NdGaO3.

The crystalline electric field as a direct probe for the electron doping process in Nd2-χCeχCuO4

Abstract The neutron spectroscopic data available for the cuprates Nd 2-χ Ce χ CuO 4 are analyzed towards establishing the crystalline electric field (CEF) interaction around the Nd 3+ sites, which are situated close to the CuO 2 planes and thus constitute a local probe of the charge distribution in these planes. Through a comparison of the CEF parameters in the insulating (χ=0) and superconducting (χ=0.15) state we find direct evidence for the addition of electrons to the CuO 2 planes by doping Nd 2 CuO 4 with Ce 4+ . While this compound is widely believed to be favourable for electron doping onto copper, our results indicate that the oxygen sites are also involved in the electron doping mechanism.

Oxygen-vacancy induced changes of the crystal-field interaction in ErBa2Cu3Ox(6.1⩽x⩽7.0) determined by inelastic neutron scattering

Abstract Inelastic neutron scattering has been employed to study the crystalline electric field (CEF) interaction in the high-T c superconductor ErBa 2 Cu 3 O x (6.1 ⩽ x ⩽ 7.0). The second-order CEF parameters are found to exhibit abrupt changes vs. x , which correlates with the observed two-plateau structure of T c . Our observations are interpreted in terms of charge redistributions in the superconducting CuO 2 planes, consistent with the recently proposed concept of charge transfer from the chains to the planes.