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 ...

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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.

We report on muon spin rotation (µSR) measurements of the internal magnetic field distribution n(B) in the vortex solid phase of YBa2Cu3Oy (YBCO) single crystals, from which we have simultaneously determined the hole doping dependences of the in-plane Ginzburg-Landau (GL) length scales in the underdoped regime. We find that Tc has a sublinear dependence on 1/λ 2 ab , where λ ab is the in-plane magnetic penetration depth in the extrapolated limits T → 0 and H → 0. The power coefficient of the sublinear dependence is close to that determined in severely underdoped YBCO thin films, indicating that the same relationship between Tc and the superfluid density is maintained throughout the underdoped regime. The GL coherence length ξ ab (vortex core size) is found to increase with decreasing hole doping concentration, and exhibit a field dependence that is explained by proximity-induced superconductivity on the CuO chains. Both λ ab and ξ ab are enhanced near 1/8 hole doping, supporting the belief by some that stripe correlations are a universal property of high-Tc cuprates.

It is shown that attempts to accurately deduce the magnetic penetration depth λ of overdoped BaFe1.82Co0.18As2 single crystals by transverse-field muon spin rotation (TF-µSR) are thwarted by field-induced magnetic order and strong vortex-lattice disorder. We explain how substantial deviations from the magnetic field distribution of a nearly perfect vortex lattice by one or both of these factors is also significant for other iron-based superconductors, and this introduces considerable uncertainty in the values of λ obtained by 74.25.Ha, 76.75.+i TF-µSR is routinely used to determine the magnetic penetration depth λ of type-II superconductors in the vortex state for the purpose of obtaining indirect information on the energy gap structure [1]. The magnetic field distribution n(B) in the sample is determined by detecting the decay positrons from implanted positive muons that locally probe the internal fields, and λ is subsequently determined by modeling the contribution of the vortex lattice (VL) to n(B). However, even in conventional superconductors the VL contribution is not known a priori, and one must rely on phenomenological models to deduce what is really an "effective" penetration depthλ. One reason for this is that only cumbersome microscopic theories account for the effects of low-energy excitations on n(B) [2]. Extrapolating low-temperature measurements ofλ to zero field to eliminate the effects of intervortex transfer of quasiparticles, as well as nonlocal and nonlinear effects, has been demonstrated to be an accurate way of determining the "true" magnetic penetration depth λ [3,4]. Yet an underlying assumption is always that the VL is highly ordered and that other contributions to n(B) are relatively minor. The purpose of this Letter is to point out that this is not the case in many of the recently discovered iron-based superconductors, making a reliable determination of λ by TF-µSR extremely difficult.Here we report on representative TF-µSR measurements of BaFe 1.82 Co 0.18 As 2 (T c = 21 K) single crystals grown from a FeAs flux, as described elsewhere [5]. High-statistics TF-µSR spectra of 20 million muon decay events were collected in magnetic fields H = 0.02 T to 0.5 T applied transverse to the initial muon spin polarization P (t = 0), and parallel to the c-axis of the crystals. The TF-µSR signal is the time evolution of the muon spin polarization, and is related to n(B) as followswhere γ µ is the muon gyromagnetic ratio. Generally, the TF-µSR signal is fit in the time domain, with the inverse Fourier transform or "TF-µSR line shape" given byproviding a visual approximation of the internal field distribution. The field distribution of a perfectly ordered VL is characterized by sharp cutoffs at the minimum and maximum values of B(r), and a sharp peak at the saddlepoint value of B(r) [1]. These features are not observed in polycrystalline samples, where the orientation of the crystal lattice varies with respect to H, but are observed in single crystals when a highly-ordered VL exists and other co...