The B-site ordered double perovskite Ba2CaOsO6 was studied by dc magnetic susceptibility, powder neutron diffraction and muon spin relaxation methods. The lattice parameter is a = 8.3619(6) Å at 280 K and cubic symmetry [Formula: see text] is retained to 3.5 K with a = 8.3462(7) Å. Curie-Weiss susceptibility behaviour is observed for T > 100 K and the derived constants are C = 0.3361(3) emu K mol(-1) and ΘCW = -156.2(3) K, in excellent agreement with literature values. This Curie constant is much smaller than the spin-only value of 1.00 emu K mol(-1) for a 5d(2) Os(6+) configuration, indicating a major influence of spin-orbit coupling. Previous studies had detected both susceptibility and heat capacity anomalies near 50 K but no definitive conclusion was drawn concerning the nature of the ground state. While no ordered Os moment could be detected by powder neutron diffraction, muon spin relaxation (µSR) data show clear long-lived oscillations indicative of a continuous transition to long-range magnetic order below TC = 50 K. An estimate of the ordered moment on Os(6+) is ∼ 0.2 μB, based upon a comparison with µSR data for Ba2YRuO6 with a known ordered moment of 2.2 μB. These results are compared with those for isostructural Ba2YReO6 which contains Re(5+), also 5d(2), and has a nearly identical unit cell constant, a = 8.36278(2) Å-a structural doppelgänger. In contrast, Ba2YReO6 shows ΘCW = - 616 K, and a complex spin-disordered and, ultimately, spin-frozen ground state below 50 K, indicating a much higher level of geometric frustration than in Ba2CaOsO6. The results on these 5d(2) systems are compared to recent theory, which predicts a variety of ferromagnetic and antiferromagnetic ground states. In the case of Ba2CaOsO6, our data indicate that a complex four-sublattice magnetic structure is likely. This is in contrast to the spin-disordered ground state in Ba2YReO6, despite a lack of evidence for structural disorder, for which theory currently provides no clear explanation.

We have performed transverse field muon spin rotation measurements of single crystals of Ba͑Fe 0.926 Co 0.074 ͒ 2 As 2 with the applied magnetic field along the ĉ direction. Fourier transforms of the measured spectra reveal an anisotropic line-shape characteristic of an Abrikosov vortex lattice. We have fit the SR spectra to a microscopic model in terms of the penetration depth and the Ginzburg-Landau parameter . We find that as a function of temperature, the penetration depth varies more rapidly than in standard weak-coupled BCS theory. For this reason we first fit the temperature dependence to a power law where the power varies from 1.6 to 2.2 as the field changes from 0.02 to 0.1 T. Due to the surprisingly strong field dependence of the power and the superfluid density we proceeded to fit the temperature dependence to a two-gap model, where the size of the two gaps is field independent. From this model, we obtained gaps of 2⌬ 1 = 3.77k B T C and 2⌬ 2 = 1.57k B T C , corresponding to roughly 6 and 3 meV, respectively.

Superfluid density and field-induced magnetism inWe report muon spin rotation ͑SR͒ measurements of single-crystal Ba͑Fe 1−x Co x ͒ 2 As 2 and Sr͑Fe 1−x Co x ͒ 2 As 2 . From measurements of the magnetic field penetration depth we find that for optimally and overdoped samples, 1 / ͑T → 0͒ 2 varies monotonically with the superconducting transition temperature T C . Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

We present the results of muon spin relaxation/rotation, transmission electron microscopy, and neutron diffraction measurements performed on several specimens of BaTi 2 (As 1−x Sb x ) 2 O, which is known to have either charge density or spin density wave ordering at T DW for all x, and superconductivity below T c ≈ 1 K for x = 1. Zero-field muon spin relaxation measurements show no significant increase in relaxation rate at the density wave ordering temperature for any composition, indicating that the density wave is of the charge rather than spin type. The absence of any superstructure peaks in selected area electron and high-resolution neutron diffraction measurements below T DW suggests that the charge density wave does not involve modulation of atomic arrangement. Transverse field muon spin rotation measurements reveal a robust superconducting state below T c ≈ 1 K for x = 1. PACS number(s): 76.75.+i, 74.90.+n, 68.37.Og Layered oxypnictide systems, such as ATi 2 P n 2 O (A = Na 2 , Ba, (SrF) 2 , (SmO) 2 ; P n = As, Sb, Bi), have been found to possess interesting electronic and magnetic properties, including spin/charge density wave (S/CDW) ordering and superconductivity. 1-14 Although the superconducting phase transition in these systems occurs at low temperatures ( 5 K), 7-11 and in some cases not at all, they nevertheless share certain structural and electronic similarities with the cuprate and iron-pnictide compounds exhibiting hightemperature superconductivity. 15,16 These similarities include the presence of planar sheets of Ti 2 O square nets, in analogy to the CuO 2 sheets found in the cuprates, an electron configuration of 3d 1 that is electron-hole symmetric with the 3d 9 configuration of the cuprates, and the close proximity of the S/CDW instability to the superconducting state, as also observed in many cuprates and iron-pnictides. Given these similarities, these oxypnictide systems have the potential to yield important insights into high-temperature superconductivity.Resistivity and magnetic susceptibility measurements recently performed on isovalent systems for BaTi 2 P n 2 O, with P n = As 3− (T c = 0 K), Sb 3− (T c = 1.2 K), Bi 3− (T c = 4.6 K), have revealed an intriguing phase diagram with S/CDW ordering present for BaTi 2 (As 1−x Sb x ) 2 O, which is quickly suppressed and replaced by a superconducting state in BaTi 2 (Sb 1−y Bi y ) 2 O (Fig. 1). 10 Remarkably, the superconducting phase exhibits a two-dome structure as observed in iron pnictide, 17 suggesting the presence of multiple bands at the Fermi surface, in accordance with band calculations revealing contributions of d x 2 −y 2 , d z 2 , and d xy orbitals. 18-20 On the other hand, the hole-doped systems via aliovalent cation and anion substitution, (Ba 1−x Na x )Ti 2 Sb 2 O 8 and BaTi 2 (Sb 1−x Sn x ) 2 O, 11 respectively, display a different phase diagram with a robust S/CDW state even while supporting superconductivity with T c as high as 5.5 K in (Ba 0.67 Na 0.33 )Ti 2 Sb 2 O. In addition, any two-dome structure in the supercondu...