We have synthesized and characterized laser-deposited film samples of perovskite BaCrO_{3}, a missing member of the perovskite-chromate family. The BaCrO_{3} films have a substantially larger lattice constant than other chromates, are insulating, and
exhibit weak ferromagnetism likely associated with canted antiferromagnetism. Comparison with the other sister compounds CaCrO_{3} and SrCrO_{3} suggest an anomalous Mott transition where magnetism is independent of whether the compound is metallic or insulating
We report on the local electronic structure of oxygen incorporated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, intially grown films are measured to be non-superconducting, but become sup
erconducting following oxygen incorporation. In FeSe the opposite happens, initially grown films are measured to be superconducting, but experience a quenching of superconductivity following oxygen incorporation. Total Fluorescence Yield (TFY) X-ray absorption experiments show that oxygen incorporation changes the initial Fe valence state in both the initially grown FeTe and FeSe films to mainly Fe3+ in the oxygen incorporated films. In contrast we observe that while Te moves to a mixed Te0/Te4+ valence state, the Se always remains Se0. This work highlights how different responses of the electronic structure by the respective chalcogenides to oxidation could be related to the mechanisms which are inducing superconductivity in FeTe and quenching superconductivity in FeSe.
We have studied the effect of tensile strain on the superconductivity in FeSe films. 50 nm, 100 nm, and 200 nm FeSe films were grown on MgO, SrTiO$_3$, and LaAlO$_3$ substrates by using a pulsed laser deposition technique. X-ray diffraction analysis
showed that the tetragonal phase is dominant in all of our FeSe films. The 50 nm FeSe films on MgO and SrTiO$_3$ are under tensile strain, while the 50 nm FeSe film on LaAlO$_3$ and the other thick FeSe films are unstrained. Superconducting transitions have been observed in unstrained FeSe films with T$_{onset}$ $approx$ 8 K, which is close to the bulk value. However, no sign of superconductivity has been observed in FeSe films under tensile strain down to 5 K. There is evidence to show that tensile strain suppresses superconductivity in FeSe films.
We have used high-resolution Extended X-ray Absorption Fine-Structure and diffraction techniques to measure the local structure of strained La$_{0.5}$Sr$_{0.5}$CoO$_3$ films under compression and tension. The lattice mismatch strain in these compound
s affects both the bond lengths and the bond angles, though the larger effect on the bandwidth is due to the bond length changes. The popular double exchange model for ferromagnetism in these compounds provides a correct qualitative description of the changes in Curie temperature $T_C$, but quantitatively underestimates the changes. A microscopic model for ferromagnetism that provides a much stronger dependence on the structural distortions is needed.
We have successfully grown epitaxial La$_{1.67}$Sr$_{0.33}$NiO$_4$ films with a small crystalline mosaic using pulsed laser deposition. With synchrotron radiation, the x-ray diffraction peaks associated with charge stripes have been successfully obse
rved for relatively thick films. Anomalies due to the charge-ordering transition have been examined using four-point probe resistivity measurements. X-ray scattering provides direct evidence for suppression of the stripe phase in thin samples; the phase disappears for film thicknesses $leqslant$ 2600 ~AA{}. The suppression appears to be a result of shrinking the stripe phase domains. This may reflect the stripe phase progressing from nematic to isotropic.
We have studied the magnetic characteristics of a series of super-oxygenated La2-xSrxCuO4+y samples. As shown in previous work, these samples spontaneously phase separate into an oxygen rich superconducting phase with a TC near 40 K and an oxygen poo
r magnetic phase that also orders near 40 K. All samples studied are highly magnetically reversible even to low temperatures. Although the internal magnetic regions of these samples might be expected to act as pinning sites, our present study shows that they do not favor flux pinning. Flux pinning requires a matching condition between the defect and the superconducting coherence length. Thus, our results imply that the magnetic regions are too large to act as pinning centers. This also implies that the much greater flux pinning in typical La2-xSrxCuO4 materials is the result of nanoscale inhomogeneities that grow to become the large magnetic regions in the super-oxygenated materials. The superconducting regions of the phase separated materials are in that sense cleaner and more homogenous than in the typical cuprate superconductor.
