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Heteroepitaxially grown bilayers of ferromagnetic La0.7Ca0.3MnO3 (LCMO) on top of superconducting YBa2Cu3O7 (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness dY and constant LCMO thickness dL = 50 nm, show a significant reduction of the superconducting transition temperature Tc only for dY < 10 nm,with only a slightly stronger Tc suppression in the bilayers, as compared to the single layers. X-ray magnetic circular dichroism (XMCD) measurements confirm recently published data of an induced magnetic moment on the interfacial Cu by the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu and Mn moments. However, we observe a significantely larger Cu moment than previously reported, indicating stronger coupling between Cu and Mn at the interface. This in turn could result in an interface with lower transparency, and hence smaller spin diffusion length, that would explain our electric transport data, i.e. smaller Tc suppression. Moreover, linear dichroism measurements did not show any evidence for orbital reconstruction at the interface, indicating that a large change in orbital occupancies through hybridization is not necessary to induce a measurable ferromagnetic moment on the Cu atoms.

Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of X-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the moststudied perovskites, La 0.7 Ca 0.3 MnO 3 (LCMO) and YBa 2 Cu 3 O 7 (YBCO), at the L 2,3 edge of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length λ and the ratio λ/d have been independently determined between 50 and 300 K. The extracted sampling depth d LCMO ≈ 3 nm for LCMO at high temperatures in its polaronic insulator state (150 -300 K) is not much larger than values reported for pure transition metals (d Co or Ni ≈ 2 -2.5 nm) at room temperature, but is smaller than d YBCO ≈ 3.9 nm for metallic YBCO that is in turn smaller than the value reported for Fe 3 O 4 (d Fe3O4 ≈ 4.5 nm). The measured d LCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.

Two superconductors separated by a thin tunneling barrier exhibit the Josephson effect and can be characterized by a single macroscopic wavefunction that allows charge transport at zero voltage 1 , typically with no phase shift between the superconductors in the lowest energy state. Recently, Josephson junctions with the ground state phase shifts of π proposed by theory three decades ago 2 , have been demonstrated 3-5 . Enclosed in superconducting loops, π-junctions cause spontaneous circulation of persistent currents 2 , making such loops analogous to spin-1/2 systems 6 . Here we use a scanning SQUID (Superconducting QUantum Interference Device) microscope 7 to image spontaneous currents in superconducting networks of temperature-controlled π-junctions based on weakly ferromagnetic barriers 3 . By combining cells with even and odd numbers of πjunctions, we construct non-uniformly frustrated arrays that have previously not been attained. We find an onset of spontaneous supercurrents at the 0-π transition temperature of the junctions T π ~ 3 K. Control over both geometry and interaction strength in these arrays makes them attractive as model systems for studies of the exotic phases of the 2D XY-model 8-9 and for applications in scalable architectures of adiabatic quantum computers 10 .

We report the growth and characterization of ultra-smooth, highly ordered, strained La2/3Ca1/3MnO3±d thin films (approximately 600 Å in thickness). The thin films were deposited by ozone-assisted, block-by-block, molecular beam epitaxy on SrTiO3 (001) substrates without any post-deposition annealing. Scanning tunneling microscopy images showed oriented, unit-cell-high terraces characteristic of a step-flow growth mechanism. The root-mean-square roughness of the surface of an imaged film was determined to be 2 Å. This same film showed negative magnetoresistance values, [R(H=0)−R(Happlied)]/R(H=0), at 150 K of 93% and 5% in applied magnetic fields of 5.12 and 4×10−2 T, respectively.