845 research outputs found
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 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 are separated as low- and
high-frequency parts, respectively, but the high-frequency part is suppressed
in the dynamical susceptibility spectrum , 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., LaSrCuO) and Y-based (e.g.,
YBaCuO) cuprates, respectively. The spin excitation in
YBaCuO 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
LaSrCuO 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
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