Spin Fluctuations in Magnetically Coupled Bi-layer Cuprates

We propose a possible mechanism of pseudo spin gap anomaly(PSGA) in magnetically coupled bi-layer cuprates without any fermion pairing instability. In our proposal PSGA does not necessarily require the spin-charge separation or the breakdown of the Fermi liquid description of a normal state of the cuprate superconductor.The low energy magnetic excitations are mainly governed by the {\it itinerant nature of the intra-layer system} and {\it the inter-layer antiferromagnetic coupling}. No matter how weak the bare inter-layer coupling is, it can be dramatically enhanced due to the intra-layerspin fluctuations. As the temperature decreases near the antiferromagnetic phase boundary the strongly enhanced inter-layer correlation induces the inter-layer particle-hole exchange scattering processes that tend to enhance the inter-layer spin singlet formation and kill the triplet formation. We propose that the coupling of spin fluctuations on the adjacend layers via the strong repulsive interaction between parallel spins travelling on each layer give rise to the dynamical screening effects. As a result the low energy part of the spin excitation spectrum is strongly suppressed as the temperature decreases near the antiferromagnetic phase boundary. We ascribe PSGA to this dynamical screening effects.Comment: 30 page, latex, figures are available upon reques

Inelastic neutron scattering peak in Zn substituted YBa2Cu3O7

The effects of nonmagnetic impurities on the neutron scattering intensity are studied for a model of the copper oxide layers in the normal state. The contribution to the Q=(pi,pi) neutron scattering intensity from processes involving the scattering of the spin fluctuations from an impurity with large momentum transfers are calculated within the random phase approximation. It is shown that this type of scatterings could lead to a peak in the neutron scattering intensity in the normal state.Comment: 4 pages, 4 figure

Spin-fluctuation mechanism of superconductivity in cuprates

The theory of superconductivity within the t-J model, as relevant for cuprates, is developed. It is based on the equations of motion for projected fermionic operators and the mode-coupling approximation for the self-energy matrix. The dynamical spin susceptibility at various doping is considered as an input, extracted from experiments. The analysis shows that the superconductivity onset is dominated by the spin-fluctuation contribution. We show that T_c is limited by the spin-fluctuation scale $\Gamma$ and shows a pronounced dependence on the next-nearest-neighbor hopping t'. The latter can offer an explanation for the variation of T_c among different families of cuprates.Comment: Color figure

Spin Dynamics for the t-J Model

The spin dynamics at the finite temperature for the t-J model in the underdoped and optimal doped regimes is studied within the fermion-spin theory. It is shown that the dynamical spin structure factor spectrum at the antiferromagnetic wave vector $Q=(\pi,\pi)$ are separated as low- and high-frequency parts, respectively, but the high-frequency part is suppressed in the dynamical susceptibility spectrum $\chi^{''}(Q,\omega)$, while the low-frequency part is the temperature dependent, which are in qualitative agreement with the experiments and numerical simulations.Comment: 3 pages, three figures are adde

The SO(5) theory of high T_c superconductivity

This paper gives a simple introduction to the SO(5) theory of high T_c superconductivity. Current status and relation to experiments are summarized.Comment: Invited Talk presented at the International Conference on Materials and Mechanisms of Superconductivity, to be publsihed in Physica

Regards croisés sur les placements sous contrainte concernant les personnes souffrant de troubles psychiques: le placement à des fins d’assistance, les mesures thérapeutiques institutionnelles et l’organisation du travail en réseau

Cette recherche exploratoire porte sur les placements sous contrainte dans le Valais romand, des personnes souffrant de troubles psychiques et traite plus spécifiquement des placements à fin d’assistance, faisant partie du nouveau droit de la protection de l’adulte du Code civil, ainsi que des mesures thérapeutiques institutionnelles pour le traitement des troubles mentaux tel que défini par l’art. 59 du Code pénal. Partant du constat que ces mesures mobilisent de nombreux acteurs provenant des domaines sociaux, médicaux, juridiques et administratifs, cette recherche s’articule autour des enjeux poursuivis par ces mesures et des interactions au sein des réseaux décisionnels, entre les acteurs concernés. Le cadre théorique présente les notions d’autodétermination et d’aide contrainte avant d’aborder plus en détails les mesures de placement sous contrainte civiles et pénales, ainsi que les processus et modalités de mise en oeuvre des placements sous contrainte dans le Valais romand. La partie pratique de ce travail de recherche repose sur une approche qualitative, basée sur sept entretiens semi-dirigés, auprès de représentants des différents domaines impliqués dans les processus et les réseaux décisionnels concernant les mesures de placement sous contrainte. Les résultats démontrent que les acteurs partagent globalement les mêmes objectifs de soin et de sécurité envers les mesures de placement sous contrainte. Et bien que les mesures de placement civiles ou pénales en institution sociale constituent, à l’heure actuelle, encore une problématique nécessitant une réflexion et une mobilisation des réseaux d’action publique, les acteurs font part de leur volonté et relèvent l’importance du travail en réseau en lien avec la complexité de ces situations. Cette recherche permet d’entrevoir de nouveaux modèles d’intervention basés sur le développement de réseaux interinstitutionnels autour de ces situations de placement sous contrainte. Elle ouvre, également, des perspectives de recherche tant au niveau des mesures de placement sous contrainte que des pratiques professionnelles des travailleurs sociaux

Quantum fluctuations in the effective pseudospin-1/2 model for magnetic pyrochlore oxides

The effective quantum pseudospin-1/2 model for interacting rare-earth magnetic moments, which are locally described with atomic doublets, is studied theoretically for magnetic pyrochlore oxides. It is derived microscopically for localized Pr^{3+} 4f moments in Pr_2TM_2O_7 (TM = Zr, Sn, Hf, and Ir) by starting from the atomic non-Kramers magnetic doublets and performing the strong-coupling perturbation expansion of the virtual electron transfer between the Pr 4f and O 2p electrons. The most generic form of the nearest-neighbor anisotropic superexchange pseudospin-1/2 Hamiltonian is also constructed from the symmetry properties, which is applicable to Kramers ions Nd^{3+}, Sm^{3+}, and Yb^{3+} potentially showing large quantum effects. The effective model is then studied by means of a classical mean-field theory and the exact diagonalization on a single tetrahedron and on a 16-site cluster. These calculations reveal appreciable quantum fluctuations leading to quantum phase transitions to a quadrupolar state as a melting of spin ice for the Pr^{3+} case. The model also shows a formation of cooperative quadrupole moment and pseudospin chirality on tetrahedrons. A sign of a singlet quantum spin ice is also found in a finite region in the space of coupling constants. The relevance to the experiments is discussed.Comment: 18 pages including 14 figures; Comparison with the magnetization curve on Pr2Ir2O7 included; to appear in Phys. Rev.

Phenomenological theory of spin excitations in La- and Y-based cuprates

Motivated by recent inelastic neutron scattering (INS) experiments on La-based cuprates and based on the fermiology theories, we study the spin susceptibility for La-based (e.g., La$_{2-x}$Sr$_x$CuO$_4$) and Y-based (e.g., YBa$_2$Cu$_3$O$_y$) cuprates, respectively. The spin excitation in YBa$_2$Cu$_3$O$_y$ is dominated by a sharp resonance peak at the frequency 40 meV in the superconducting state. Below and above the resonance frequency, the incommensurate (IC) peaks develop and the intensity of the peaks decreases dramatically. In the normal state, the resonant excitation does not occur and the IC peaks are merged into commensurate ones. The spin excitation of La$_{2-x}$Sr$_x$CuO$_4$ is significantly different from that of Y-based ones, namely, the resonance peak does not exist due to the decreasing of the superconducting gap and the presence of the possible spin-stripe order. The spectra are only enhanced at the expected resonance frequency (about 18 meV) while it is still incommensurate. On the other hand, another frequency scale at the frequency 55 meV is also revealed, namely the spectra are commensurate and local maximum at this frequency. We elaborate all the results based on the Fermi surface topology and the d-wave superconductivity, and suggest that the spin-stripe order be also important in determining the spin excitation of La-based cuprates. A coherent picture for the spin excitations is presented for Y-based and La-based cuprates.Comment: 8 pages, 8 figure

Magnetic-Field-Induced 4f-Octupole in CeB6 Probed by Resonant X-ray Diffraction

CeB6, a typical Gamma_8-quartet system, exhibits a mysterious antiferroquadrupolar ordered phase in magnetic fields, which is considered as originating from the T_{xyz}-type magnetic octupole moment induced by the field. By resonant x-ray diffraction in magnetic fields, we have verified that the T_{xyz}-type octupole is indeed induced in the 4f-orbital of Ce with a propagation vector (1/2, 1/2, 1/2), thereby supporting the theory. We observed an asymmetric field dependence of the intensity for an electric quadrupole (E2) resonance when the field was reversed, and extracted a field dependence of the octupole by utilizing the interference with an electric dipole (E1) resonance. The result is in good agreement with that of the NMR-line splitting, which reflects the transferred hyperfine field at the Boron nucleus from the anisotropic spin distribution of Ce with an O_{xy}-type quadrupole. The field-reversal method used in the present study opens up the possibility of being widely applied to other multipole ordering systems such as NpO2, Ce_{x}La_{1-x}B_{6}, SmRu_{4}P_{12}, and so on.Comment: 5 pages, 4 figures, submitte

Double dispersion of the magnetic resonant mode in cuprates

The magnetic excitation spectra in the vicinity of the resonant peak, as observed by inelastic neutron scattering in cuprates, are studied within the memory-function approach. It is shown that at intermediate doping the superconducting gap induces a double dispersion of the peak, with an anisotropy rotated between the downward and upward branch. Similar behavior, but with a spin-wave dispersion at higher energies, is obtained for the low-doping case assuming a large pairing pseudogap.Comment: 4 LaTeX pages, 4 figure