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Sumner Harris1 Renato Camata1

1, The University of Alabama at Birmingham, Birmingham, Alabama, United States

The intense research activity underway in iron-based superconductors, has included significant attention to FeSe due in part to its relative structural simplicity and promising characteristics for elucidating the physics of superconductivity in these materials. These characteristics include the highest critical temperature observed so far in iron-based superconductors (Tc ~ 65 K for an FeSe monolayer grown on SrTiO3), a nematic phase that is not suppressed when frustrated magnetic interactions suppress magnetic ordering, and Cooper pairing with orbital selectivity. FeSe-based alloys and chemical doping of FeSe can lead to additional attractive features such as control of Tc, altered nematic phase structure, possibly distinct pairing of charge carriers, and high Hc and Jc values, which are valuable in high magnetic field applications. In particular, doping of FeSe with Cu has been shown to be an effective way of switching superconductivity on and off in bulk samples of Fe1-xCuxSe. In this work, Fe1-xCuxSe thin films were grown on MgO (100) substrates via pulsed laser deposition. A KrF excimer laser (1.4–3.4 J/cm2) was used to ablate a pressed, sintered target produced by mixing powders of FeSe and Cu2Se precursor compounds at various concentrations. Targets were sintered in sealed quartz ampules at 700°C for 12 hours. Targets were ablated at pressures below 3.0x10-6 Torr with substrate temperature kept at 500°C. The thickness of all films was targeted at 400 nm as determined by deposition rate and scanning electron microscopy thickness calibrations. Films are analyzed by atomic force microscopy for surface morphology, and X-ray diffraction to characterize the crystalline structure of the films. We show that the surface morphology and crystallographic phase and orientation of the Fe1-xCuxSe films vary greatly with deposition conditions. Virtually all samples grown with x = 0 exhibit both, the tetragonal β-FeSe phase and the hexagonal δ-FeSe phase. It is also found that increasing the laser fluence in the 1.4–3.4 J/cm2 range, with the substrate temperature kept at 500°C, increased the fraction of β-FeSe in the films. Features of Stranski–Krastanov island growth are apparent in these samples for all deposition conditions explored with x = 0, suggesting that c-axis oriented β-FeSe islands grow on a δ-FeSe facet with a close epitaxial relationship to the tetragonal phase. This effect may allow the controllable growth of arrays of superconducting dots. Fe1-xCuxSe films with x = 0.010 and x = 0.015 are shown to be exclusively tetragonal, with the c-axis of the tetragonal structure oriented normal to the MgO substrate. These synthesis conditions correspond to the enrichment of selenium in the ablation target, since the Cu precursor used in the process (Cu2Se powder) is also a source of Se, which is consistent with the known sensitivity of the Fe:Se ratio on the stability of the crystal structure of this FeSe-based system.

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