Superconductivity in oxygen doped iron telluride by molecular beam epitaxy

iron base superconductor;iron chalcogenide;oxygen doping;molecular beam epitaxy

Zheng, Mao

关键词: iron base superconductor;    iron chalcogenide;    oxygen doping;    molecular beam epitaxy;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46712/Mao_Zheng.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Iron base superconductor have gained much attention in the research community. They offer great potentials to improveour understanding of the subject of superconductivity by having another family of high temperature superconductorsto compare and contrast to the cuprates. Practically, the iron based superconductors seems to be even bettercandidates for applications in power generation and power transmission. Iron telluride is regarded as the parent compoundof the ”11” family, the family of iron chalcogenide that has the simplest structure. Iron telluride itself is not asuperconductor, by can become one when doped with oxygen.In this investigation, we developed the growth recipe of thin film iron telluride by Molecular Beam Epitaxy (MBE).We found the growth to be self-regulated, similar to that of GaAs. The initial layers of growth seem to experience aspontaneous crystallization, as the film quickly go from the initial polycrystalline phase to highly crystalline in justa few unit cells. We studied oxygen doping to the iron telluride thin films and the resultant superconductivity. Wecharacterized the sample with AFM, XRD, transport, and STEM-EELS, and we found that interfacial strain is notan essential ingredient of superconductivity in this particular case. We investigated the doping conditions for twocandidate oxygen doping modes: substitution and interstitial. We found that substitution occurs when the film grownin oxygen, while interstitial oxygen is primarily incorporated during annealing after growth. The substitutional oxygenare concentrated in small local regions where substitution is around 100%, but does not contribute to superconductivity.We estimated substitutional oxygen to be about 5%, and is the proximate cause of superconductivity.Hall experiment on our sample showed a shift of dominant carrier type from holes to electrons around 35 K, butthe transition was set in motion as early as the structural phase transition around 70 K. We believe the shift is a resultof enhanced mobility of electrons at low temperatures.Using the capability of MBE to make pristine and abrupt interfaces, we grow two film structures: FeTe:Ox/AlOx/Auand FeTe:Ox/Al/AlOx/Au. We explored processing recipes to fabricate these films into tunel junctions devices.FeTe:Ox/AlOx/Au type of devices turned out to be suffering from nanoshorts and exhibit point contact spectroscopyjunction behaviors. We observed evolution of enhanced conduction peaks around 20mV, consistent with publishedliterature. FeTe:Ox/Al/AlOx/Au junctions behave differently, showing a evolving energy gap around 3mV. The factthat the energy gap evolved together with the superconducting transition, and the close match of gap size to these of the other iron chalcogenide superconductors, gives evidence of proximity coupling between the iron telluride layerand the aluminum layer.