Evidence for two distinct energy scales in the Raman spectra of YBa2(CuNi)O6.95

We report low energy electronic Raman scattering from Ni-substituted YBa2Cu3O6.95 single crystals with Tc ranging from 92.5 K to 78 K. The fully symmetrical A1g channel and the B1g channel which is sensitive to the dx2-y2 gap maximum have been explored. The energy of the B1g pair-breaking peak remains constant under Ni doping while the energy of the A1g peak scales with Tc (EA1g/Tc=5). Our data show that the A1g peak tracks the magnetic resonance peak observed in inelastic neutron scattering yielding a key explanation to the long-standing problem of the origin the A1g peak.Comment: 10 pages, 4 figures and 1 tabl

Microscopic origin of local moments in a zinc-doped high-TcT_{c} superconductor

The formation of a local moment around a zinc impurity in the high-$T_{c}$ cuprate superconductors is studied within the framework of the bosonic resonating-valence-bond (RVB) description of the $t-J$ model. A topological origin of the local moment has been shown based on the phase string effect in the bosonic RVB theory. It is found that such an $S=1/2$ moment distributes near the zinc in a form of staggered magnetic moments at the copper sites. The corresponding magnetic properties, including NMR spin relaxation rate, uniform spin susceptibility, and dynamic spin susceptibility, etc., calculated based on the theory, are consistent with the experimental measurements. Our work suggests that the zinc substitution in the cuprates provide an important experimental evidence for the RVB nature of local physics in the original (zinc free) state.Comment: The topological reason of local moment formation is given. One figure is adde

Magnetic Resonant excitations in High-{Tc\rm T_c} superconductors

The observation of an unusual spin resonant excitation in the superconducting state of various High-Tc ~copper oxides by inelastic neutron scattering measurements is reviewed. This magnetic mode % (that does not exist in conventional superconductors) is discussed in light of a few theoretical models and likely corresponds to a spin-1 collective mode.Comment: 4 figures, Proceedings conference MSM'03 (september 2003) in Monastir (Tunisia) to be published in Phys. Stat. Solid

Spin reorientation in Na-doped BaFe2_2As2_2 studied by neutron diffraction

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.Comment: 5 pages, 4 figure

Field-induced paramagnons at the metamagnetic transition in Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic ransition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4 has, therefore, to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field

Characterization of the Intra-Unit-Cell magnetic order in Bi2Sr2CaCu2O8+d

As in YBa2Cu3O6+x and HgBa2CuO8+d, the pseudo-gap state in Bi2Sr2CaCu2O8+d is characterized by the existence of an intra-unit-cell magnetic order revealed by polarized neutron scattering technique. We report here a supplementary set of polarized neutron scattering measurements for which the direction of the magnetic moment is determined and the magnetic intensity is calibrated in absolute units. The new data allow a close comparison between bilayer systems YBa2Cu3O6+x and Bi2Sr2CaCu2O8+d and rise important questions concerning the range of the magnetic correlations and the role of disorder around optimal doping.Comment: 12 pages, 8 figures, submitted to physical review