W. Henggeler

Quantum magnetic interactions in S = 1/2 KCuCl3

KCuCl3 is an S = 1/2 magnetic insulator with a singlet ground state and a finite spin excitation gap. Above the gap, dispersive triplet excitation modes propagate in the whole reciprocal space. From single-crystal inelastic neutron investigations the three-dimensional coupling scheme is rationalized in the framework of a dimer Heisenberg model, and related to the structural features of KCuCl3. The experimental and theoretical characterization presented completes earlier works on the compound under investigation, providing also higher-order expressions for the singlet-triplet dispersion relation. The latter may also be of relevance for the parent quantum systems TlCuCl3 and NH4CuCl3, albeit at different coupling ratios with respect to KCuCl3.

New elaboration technique, structure and physical properties of infinite-layer Sr1−xLnxCuO2(Ln = Nd, Pr)

Abstract The infinite-layer compounds of the (Sr, Ln)CuO 2 family have been prepared by a new synthesis method from the low-pressure phase of SrCuO 2 and NdCuO 2 as starting materials. The crystal structure, microstructure and superconductivity of these samples have been studied with use of neutron powder diffraction, optical micrographs, energy-dispersive X-ray analysis. magnetic-susceptibility and electrical-resistivity measurements. The ratio of the lattice constants a/c as an empirical parameter for the existence of superconductivity in the electron-doped infinite-layer compounds is discussed.

Neutron spectroscopy in RBa2Cu3Ox (R=rare earth; 6≤x≤7) and R2-xCexCuO4-δ (0≤x≤0.2) compounds: charge transfer, cluster formation, percolative superconductivity, charge fluctuations

Abstract Neutron spectroscopic experiments performed for the high-T c superconducting hole-doped RBa 2 Cu 3 O x (R=rare earth; 6≤x≤7) and electron-doped R 2-x Ce x CuO 4-δ (0≤x≤0.2) compounds are discussed. In these systems the R ions are situated close to the superconducting CuO 2 planes, thus the crystal-field interaction at the R site constitutes an ideal local probe of the charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the observed crystal-field spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation, i.e., the superconductivity is an inhomogeneous property in the persovskite-type compounds. From a line-width analysis of the observed crystal-field transitions we derive the evolution of the fractal sizes of the clusters versus doping. At high doping the neutron spectroscopic data reveal anomalies which are interpreted in terms of copper-oxyde charge fluctuations.

Magnetic excitations of Nd in Nd2−xCexCuO4 (x = 0 and 0.13)

Abstract We have studied the wave vector dependence of the magnetic excitation spectrum of Nd in Nd 2− x Ce x CuO 4 ( x = 0,0.13) by inelastic neutron scattering experiments on single crystals. The results are analyzed with the help of model calculations which are performed in the context of the mean field random-phase approximation (RPA). This enables us to obtain direct information on the coupling constants between the rare earth ions. In Nd 2 CuO 4 we observe a dependence of the coupling constants on the initial and final state that is involved in a crystal field excitation. Furthermore, our results indicate that the interaction of the Nd spins is opposite to the ordering enforced by the NdCu exchange. We argue that this will lead to instability of the Nd-ordering in the Ce-doped systems which might be important for the onset of high γ-values.

Neutron spectroscopy in ErBa2Cu3Ox (6 ⩽ × ⩽ 7) and Pr2 − xCexCuO4 − δ (0 ⩽ × ⩽ 0.2): Charge transfer, cluster formation and percolative superconductivity

Abstract Neutron spectroscopic data obtained for ErBa 2 Cu 3 O x (6 ⩽ × ⩽ 7) and Pr 2 − x Ce x CuO 4 − δ (0 ⩽ × ⩽ 0.2) are discussed. Since the rare-earth ions in these compounds are situated in close proximity to the superconducting CuO 2 -planes, crystal-field spectroscopy constitutes an ideal local probe of the charge distribution and therefore monitors directly changes of the carrier concentration induced by doping. Clear evidence for cluster formation and percolative superconductivity is provided by the observed separation of crystal-field transitions — observed in hole- and electron-doped superconductors — into different local components whose spectral weights depend on the doping level.

Magnetic excitations and spin gap in KCuCl3

Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to be a necessity in characterizing unconventional quantum spin systems — in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S= 1 2 Cu 2+ compound KCuCl3. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet–triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl3 as a quantum spin liquid near to the dimer limit.

Neutron crystal-field spectroscopy of RNi2 11B2C (R=Ho, Er, Tm)

Inelastic neutron scattering experiments of the crystal-electric-field (CEF) splitting of the R3+-ions (R=Ho, Er, Tm) in the superconductor RNi211B2C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the magnetic properties of these systems. Both the easy axis and the size of the zero-field moments are correctly reproduced by our calculations. Our CEF parameter set also reproduces the behaviour of specific heat and M/H measured for single crystals.

Phase space transformation on ultra cold neutrons

We present the results of a Monte Carlo simulation and analytical calculations for a phase space transformer, which accelerates ultra-cold neutrons (UCNs) to cold or thermal neutron energies. We show that due to the sharp energy distribution of UCNs this kind of neutron device is able to produce monochromatized neutrons with intensities competing todays most performing time-of-flight instruments, an energy resolution of the order of δE/E∼0.3%, and a horizontal divergency of ∼2° at the planned spallation source for UCNs (PSI, Switzerland).

The phase space transformer instrument

We performed a feasibility study for a phase space transformer (PST) instrument to be implemented at a cold neutron guide at SINQ (PSI, Switzerland). The study is based on both analytical calculations and Monte-Carlo simulations with particular accent on the energy resolution and intensity. We show that the PST instrument is able to deliver thermal neutrons by up-scattering, with fluxes comparable to the todays best thermal time-of-flight spectrometers, but with considerably better energy resolution.

Influence of Ce-doping on the magnetic excitations of Nd in Nd2−xCexCuO4

Abstract We present a study of the spin-wave excitations of Nd in Nd2−xCexCuO4 (x = 0, 0.09, 0.15, 0.18) by inelastic neutron scattering. Upon Ce-doping one observes a softening of the spin-wave branches. This is mainly explained by the decrease of the CuNd exchange field at the Nd site, while the NdNd exchange interactions remain almost unchanged. Consequently a high density of states of spin-wave excitations at low energies is created. The resulting calculated specific-heat shows a large γ = C/T-value at low temperatures, which indicates that the novel heavy-fermion behavior in these compounds is mainly due to magnetic correlations of the Nd ions.