Transition from insulator to superconductor and significant enhancement of superconductivity in FeSe films via tuning Fe-vacancy disorders
In contrast to its bulk crystals, the FeSe film or layer exhibits better superconductivity performance, which attract much interest in its fundamental research as well as potential application. In present work, transition from insulator to superconductor and significant enhancement of superconductivity were achieved in the high quality (00l) oriented FeSe films via controlling the thickness as well as the Fe/Se ratio. The highest Tc up to 15.2 K (almost 2 times higher than those of bulk crystals) and Hc2 up to 35.5 T are obtained in our atmosphere-stable FeSe thin film with practical thickness (240 nm), implying their great potential application in the electronic devices at high magnetic fields. More importantly, it was found that the Fe-vacancy disorder in FeSe films is the intrinsic factor determining the evolution of the superconductivity, rather than thickness effect. In our non-superconducting FeSe film with Fe/Se ratio of 1.00:1.09, insulating \b{eta}-Fe1-xSe phase with iron-vacancy disorders is the main phase and more likely to be the parent phase of FeSe superconducting system. Tuning the Fe-vacancy disorders via changing the Fe/Se ratio can dramatically vary the concentration of charge carrier and introduce proper electron doping, which finally leads to the transition from insulator to superconductor and further enhancement in the superconductivity. Intriguingly, our results also indicate that when the Fe/Se ratio of film is beyond a critical value, superconducting FeSe films will become instable, and phase separation occurs with new non-superconducting phase precipitating in the superconducting matrix, causing the degradation in the superconductivity. The results in present work help us to well understand the intrinsic mechanism of superconductivity among Fe-Se superconducting system and provide a new strategy to further pursue higher Tc in these materials.