J. Rossat-Mignod

Evidence for high temperature antiferromagnetic ordering in YBa2Cu3O6

Abstract Neutron diffraction experiments, performed on a powder sample, have shown that the reduced compound YBa 2 Cu 3 O 6 orders with an antiferromagnetic structure at a temperature as high as 500 K. Magnetic moments of about 0.48±0.05 μ B are aligned along the tetragonal c -axis and exhibit an antiferromagnetic coupling both in the basal plane ( k 0 =[ 1 2 1 2 0] ) and in between the two CuO 2 layers of the unit cell. A discussion of the results is presented.

Neutron scattering study of the antiferromagnetic ordering in YBa2Cu3O6+x powder and single crystal samples

Abstract Neutron diffraction experiments have been performed on powder and single crystal (2×5×2×1.5 mm 3 ) samples of YBa 2 Cu 3 O 6+x compounds. The reduced compound YBa 2 Cu 3 O 6 is found to order antiferromagnetically belowT N = 420 ± 10K. The magnetic moment is about 0.6 ± 0.05 μ B on the Cu 2+ sites and aligned perpendicular to the tetragonal c-axis. The magnetic structure is characterized by an in-plane wave vector|1/2 1/2 0| and an antiferromagnetic coupling between the two CuO 2 planes of the unit cell. The addition of oxygen induces only a decrease of T N and the moment value (290 ± 10 K, 0.45 ± 0.05 μ B and 245 ± 10 K, 0.28± 0.05 μ B , for x = 0.25 ± 0.03 and 0.38 ± 0.03, respectively). The long range order disappears abruptly aroundx = 0.40.

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.

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 .

Magnetic excitations in SmB6 single crystals

Abstract Two, essentially dispersionless, magnetic excitations, at energies ℏ ω 1 ≃ 36 and ℏ ω 2 ≃ 14 meV, have been investig ated by inelastic neutron scattering measurements on a single crystal of mixed-valent 154 Sm 11 B 6 . The latter one decreases markedly upon heating, or with increasing | Q | . It is ascribed to the spin-flip of 5d electrons which form a bound state with the 4f electrons of a given site by tunnelling over the octahedron defined by its six nearest Sm neighbours.

Polarised neutron study of high-Tc superconductors

Abstract The magnetization density induced by an applied magnetic field in the high-Tc superconductor YBa2Cu3O6+x has been measured by polarized neutron diffraction. Measurements have been carried out on single crystals intercalated with different oxygen compositions: x = 0.52 and x = 1.00, at different fields up to 10 tesla and different temperatures. The magnetic density is located mainly on the copper sites, and the behaviour of these magnetic sites depends strongly on temperature. On the copper of the CuO2 planes, the superconductivity is associated with a quenching of the spin part of the density: this quenching begins below Tc for YBa2Cu3O7 (Tc = 90 K) but far above Tc for YBa2Cu3O6.52 (Tc = 48 K). This is in accordance with the variations of the Knight shift measured by NMR. On the copper of the chains, the magnetization measured by neutrons shows a combination of Curie and Pauli paramagnetism, which practically does not depend on the onset of superconductivity and reflects the order of the chain fragments.

NONLINEAR EXCITATIONS IN ld AND 2d MAGNETIC SYSTEMS

We present experimental results which show clearly the existence and the role played by nonlinear excitations in the quasi-1d compounds TMMC and CsCoCl 3 (solitons) and quasi-2d compounds BaM 2 (XO 4 ) 2 (M = Co, Ni; X = P, As). The role of the collisions between solitons and magnons and the applicability of the Kosterlitz-Thouless theory are discussed in the light of new inelastic neutron scattering experiments.

Local singlet bound state and magnetic excitations in mixed-valence SmB6

Abstract The recent observation of an unconventional magnetic excitation at h z.shtsls; ω = 13–14 meV in SmB 6 may be one of the key results for understanding the nature of the mixed-valence state in this system. A detailed neutron scattering study of this feature has been carried out as a function of momentum transfer and temperature, and the results obtained support a picture in which the electron released from the 4f shell forms a local bound state on an extended orbital around the original site.

Polarized neutron determination of the ground state of Ce3+ in CeSb

Abstract CeSb exhibits very unusual magnetic properties which can be explained by an important exchange anisotropy, as in some uranium compounds. To better understand the magnetic behaviour of this compound microscopic studies have been undertaken by means of the determination of the magnetization density. Using polarized neutron diffraction, accurate measurements of the magnetic form factor have been carried out. In the paramagnetic region the results show that the crystalline field ground state is the Γ8 quartet in contradiction with the usual assumption. In the ferromagnetic region the 4f moment is found to be only 1.91 μB; the reduction of the 4f moment observed in both regions, and the anomalous value of the crystalline field parameter, are discussed in terms of a mixing of 4f electrons with p-electrons of Sb.

Spin dynamics in the normal and superconducting states of YBa2Cu3O6+x

Abstract The spin dynamics of CuO 2 -layers in the high-T c superconducting system YBa 2 Cu 3 O 6+x has been extensively investigated by inelastic neutron scattering on single crystals. In this paper we will review the most important results recently obtained on the evolution of magnetic excitation spectra in the weakly, heavily and overdoped regimes, from both sides of T c .