We present muon spin relaxation (µSR) measurements on a large YBa2Cu3O6.6 single crystal in which two kinds of unusual magnetic order have been detected in the pseudogap region by neutron scattering. A comparison is made to measurements on smaller, higher quality YBa2Cu3Oy single crystals. One type of magnetic order is observed in all samples, but does not evolve significantly with hole doping. A second type of unusual magnetic order is observed only in the YBa2Cu3O6.6 single crystal. This magnetism has an ordered magnetic moment that is quantitatively consistent with the neutron experiments, but is confined to just a small volume of the sample (∼ 3 %). Our findings do not support theories that ascribe the pseudogap to a state characterized by loop-current order, but instead indicate that dilute impurity phases are the source of the unusual magnetic orders in YBa2Cu3Oy.PACS numbers: 74.72. Bk, 74.25.Ha, 76.75.+i It is widely believed that the mysterious pseudogap region of high-transition temperature (high-T c ) copper oxide superconductors is caused by a "hidden order". Varma [1,2] has proposed that the pseudogap is caused by a circulating-current (CC) state that breaks timereversal symmetry and is characterized by a unique longrange pattern of loop currents in the CuO 2 planes that breaks rotational symmetry, but preserves the translational symmetry of the lattice (TSL). Alternatively, Chakravarty et al. [3] has attributed the pseudogap to a competing d-density wave (DDW) order. The DDW phase also breaks time reversal and rotational symmetries, but has an orbital current pattern that breaks the TSL. In both models the loop-current order is predicted to weaken with increased doping, and to vanish at a quantum critical point somewhat above optimal doping. Seemingly direct evidence for DDW order comes from neutron scattering experiments on underdoped YBa 2 Cu 3 O y (YBCO) with y = 6.6 [4, 5] and y = 6.45 [6], which reveal a weak antiferromagnetic (AF) ordered magnetic moment predominantly directed perpendicular to the CuO 2 planes. However, no such static magnetic order was observed by Stock et al. [7] in a neutron study of YBa 2 Cu 3 O 6.5 . A second unusual magnetic order recently detected in the pseudogap region of YBCO and HgBa 2 CuO 4+δ (Hg1201) by polarized neutron scattering [8-10] does not break the TSL, and hence is instead qualitatively consistent with the CC phase. However, the ordered moment in this case is not perpendicular to the CuO 2 planes as expected for the CC phase, but rather has a large in-plane component. Spin-orbit coupling [11] or orbital currents involving the apical oxygens [12] have been offered as possible reasons for why the magnetic moments are severely canted. The first scenario has also been invoked [13] to explain the onset of an accompanying weak ferromagnetism near T * , which has been detected in YBCO by high-resolution polar Kerr effect (PKE) experiments [14].What is most surprising is the lack of evidence from previous zero-field (ZF) µSR experiments for the existence of ...

An air-bridged, 120-nm-thick semiconductor slab with a two-dimensional (2D) square array of through holes on a 480 nm pitch (Λ) was fabricated using selective wet etching techniques. The second order photonic resonances of the structure were studied by comparing broadband optical scattering data with numerical solutions of Maxwell’s equations. Features observed in these spectra over a 1200 cm−1 range, near 9500 cm−1, indicate that the 2D texture splits the energy degeneracy of slab modes with propagation constants {±2π/Λ,0} and {0,±2π/Λ} by as much as 14%.

The real and imaginary photonic band structure of modes attached to two-dimensionally textured semiconductor membranes is determined experimentally and theoretically. These porous waveguides exhibit large ͑1000 cm Ϫ1 at 9500 cm Ϫ1 ) second-order optical gaps, highly dispersive lifetimes, and bands with well-defined polarization along directions of high symmetry.

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We report that in YBa2Cu3Oy and La2-xSrxCuO4 there is a spatially inhomogeneous response to the magnetic field for temperatures T extending well above the bulk-superconducting transition temperature Tc. An inhomogeneous magnetic response is observed above Tc even in ortho-II YBa2Cu3O6.50, which has highly ordered doping. The degree of the field inhomogeneity above Tc tracks the hole-doping dependences of both Tc and the density of the superconducting carriers below Tc, and therefore is apparently coupled to superconductivity.