Structural Stability and Superconductivity in the Iron Chalcogenides

We have developed processes to grow Fe$_{1+x}$Se single crystals and epitaxial films. X-ray diffraction measurements show that the plate side thin films of the crystal is tetragonal $\beta $-FeSe. The as grown crystals show a superconducting transition T$_{c}$ at 8 K. In addition, superconducting Fe$_{1+x}$(Se$_{1-y}$Te$_{y})$ thin films have also been fabricated by pulsed laser deposition on MgO. All Fe$_{1+x}$(Se$_{1-y}$Te$_{y})$ films show preferred orientation and smooth surface morphology. However, a strong orientation and thickness dependence of Tc was found in Fe$_{1+x}$Se thin films deposits at low substrate temperature. Detailed x-ray structural studies on both the single crystal and epitaxial thin films show that the existence of a low temperature structural distortion is essential for the occurrence of superconductivity.