C. Vettier

Investigation of the spin dynamics in YBa2Cu3O6+x by inelastic neutron scattering

Abstract Inelastic neutron scattering experiments have been carried out on YBa2Cu3O6+x single crystals to investigate the spin dynamics in the pure AF-state (x = 0.15, TN = 410 K), in the doped AF-state (x = 0.37, TN = 180 K), in the metallic weakly doped superconducting state (x = 0.45, Tc = 37 K; x = 0.51, Tc = 47 K) and in the Tc = 60 K superconducting phase (x = 0.69, Tc = 59 K). The main results are summarized: the two dimensional character of the spin-wave spectrum in the AF-state, strong renormalization of low-energy excitations and overdamping of in-plane spin fluctuations in the doped AF-state, dynamical AF correlations in the superconducting state, evidence for a pseudo-gap induced by superconductivity and existence of superconducting correlations above Tc.

Neutron and magnetization studies of CeSb and CeSb1−xTex solid solutions

Abstract The unusual properties of CeSb are reviewed and new results presented. In particular the magnetic phase diagram has been investigated under high magnetic fields where a new phase FP′ (+++0) has been discovered and under high pressure which increases strongly TN (TN = 30.5 K at P = 21 kbar), changes the ordering to type I and suppresses the non-magnetic planes. At low T the magnetic excitation spectra give evidence of strongly anisotropic in-plane interactions and weak inter-plane ones. At high T, excitations in non-magnetic planes have been observed similar to the paramagnetic state. The studies of the CeSb1−xTex solid solutions strongly support the p-f mixing model: the CEF splitting increases, the ordering temperature drops together with anisotropic properties with only a small amount of Te. For larger Te-concentrations an important Kondo-effect is present.

Inelastic neutron scattering study of cerium heavy fermion compounds

We will review inelastic neutron scattering experiments performed on single crystals of the heavy fermion compounds CeRu2Si2 and CeCu6. At high temperatures, the magnetic scattering can be described by a single quasi-elastic Lorentzian peak. At low temperatures antiferro and incommensurate magnetic correlations develop below 70 and 10 K in CeRu2Si2 and CeCu6, respectively; the associated wave vectors are k1 = (0.3, 0, 0) and k2 = (0.3, 0.3, 0)for CeRu2Si2 and k1 = (0, 0, 1) and k1 = (0, 0, 1) andk2 = (0.85, 0, 0) for CeCu6. These magnetic correlations are destroyed by a magnetic field applied along the easy axis (Hc ≈ 25 kOe for CeCu6, Hc ≈ 83 kOe for CeRu2Si2). These high field experiments allow us to establish that in both compounds, at low T and H = 0, the magnetic scattering is the superposition of two contributions: i) a q-independent (single site) quasi-elastic contribution of Lorentzian type, slowly decreasing at high field, ii) a strongly peaked inelastic contribution associated with magnetic correlations, centered at a finite energy ħω0 with characteristic energy width Γ ≈ ħω0 ≈ 1.2 meV and 0.2 meV for CeRu2Si2 and CeCu6, respectively.

Magnetic ordering in CexLa1−xRu2Si2 solid solutions

Abstract CeRu 2 Si 2 is a moderate heavy fermion (HF) system which exhibits neither magnetic nor superconducting ordering. The substitution of Ce by La suppresses progressively the HF character. Ce x La 1− x Ru 2 Si 2 with x = 0.08, 0.10, 0.13 and 0.20, have been investigated by elastic neutron scattering. They all order in structure of wave vector ik = (0.309, 0, 0). The magnetic moment and the transition temperature ( m 0 ⋍ 1.2μ B and T n ⋍ 5.8 K for x = 0.20) decrease continuously with x .

Energy-Dispersive Phase Plate for Magnetic Circular Dichroism Experiments in the X-ray Range

A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2 keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

Inelastic neutron scattering study of the spin dynamics in the YBa2Cu3O6+χ system

Abstract Inelastic neutron scattering experiments have been carried out on YBa 2 Cu 3 O 6+ξ single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes: the pure and doped AF-states (ξ = 0.15, 0.37), the weakly doped (χ = 0.45, 0.51), the heavily doped (χ = 0.69, 0.92) and the overdoped (χ = 1) metallic states. In the insulating state, the hole doping strongly affects the AF-order and yields a strong renormalization of the spin wave velocity. In the metallic state, dynamical AF-correlations remain, but the in-plane correlation length becomes very short (ξ/a ≌ 0.8) as the doping increases. However, the AF-coupling between the two CuO 2 -layers is not affected, except in the overdoped regime. At low temperatures the spin excitation spectrum exhibits an energy gap in any superconducting samples. Its value, E G / kT c ≌ 3.5, becomes much weaker close to the I-M transition. Moreover, a pseudo-gap persists above T c in the heavily doped regime.

Neutron scattering study of YBa2Cu3O6+x single crystals

Abstract Inelastic neutron scattering studies of YBa2Cu3O6+x single crystals are reported. In the pure AF phase (x = 0.15, TN = 410 K) the 2D-character of the spin wave spectrum and the exchange parameters have been established. A small amount of p holes in CuO2 planes (x = 0.37, TN = 180 K,m nh = 1.8%) strongly modifies the spin dynamics at low q: strong damping of in-plane spin excitations and renormalization of the spin wave velocity. In the superconducting state (x = 0.45, Tc = 34.8 K) we have found dynamical magnetic correlations and an anomalous decrease of the intensity at low energy in the vicinity of Tc.

Neutron scattering study of magnetic fluctuations in Ce1−xLaxRu2Si2(x = 0, x = 0.2)

Abstract We report neutron scattering experiments on single crystals of Ce1−xLaxRu2Si2 with x = 0 and x = 0.2. In CeRu 2Si2 the magnetic scattering is the superposition of two contributions: (i) a single site (q-independent) quasi-elastic contribution, and (ii) a strongly peaked inelastic contribution ascribed to magnetic correlations. The correlation lengths and the fluctuation rates of both contributions have been studied as a function of temperature. In Ce0.8La0.2Ru2Si2, the quasi-elastic neutron scattering measurements as a function of temperature show the coexistence of strong magnetic fluctuations at low temperature together with the incommensurate long range order.

Spin dynamics in the high-Tc system YBa2Cu3O6+x

Abstract We report on inelastic neutron scattering experiments carried out on single crystals of the YBa 2 Cu 3 O 6+ x system. The spin dynamics has been systematically investigated in five typical regimes: the pure and doped AF states ( x =0.15, 0.37), the weakly doped ( x =0.45, 0.51, 0.53) and the heavily doped ( x =0.69, 0.92) metallic states which all exhibit a superconducting ground state. The hole doping strongly affects the AF order, dynamical AF correlations remain in the metallic state which, surprisingly, involves only in-plane spin components. An energy gap has been found in the spin excitation spectrum of any superconducting samples, however, it becomes weaker close to the I-M transition. A quite unusual T dependence of the spin excitation spectra was found.

Spin fluctuations in the metallic and superconducting states in the high-Tc system YBa2Cu3O6+x

Abstract The spin dynamics in single-crystals of YBa 2 Cu 3 O 6+ x has been successfully investigated by inelastic neutron scattering experiments, as a function of temperature in the metallic state over the whole doping range from the weakly doped ( x =0.45, 0.51) to the heavily doped ( x =0.69, 0.92) and the overdoped ( x =1) regimes. Dynamical AF correlations remain in the metallic state but the in-plane correlation length becomes very short ( ξ a ≅0.8) as the doping increases. However, the AF coupling between the two CuO 2 layers is not affected, except in the overdoped regime. At low temperatures, the spin excitation spectrum exhibits an energy gap in any superconducting sample. Its value, E G kT c ≅3.5 , becomes much weaker close to the I-M transition. Moreover, a pseudo-gap persists above T c in the heavily doped regime. The results are compared with the magnetic excitation spectra found for the typical heavy fermion materials CeCu 6 and CeRu 2 Si 2 .