Superconductivity in oxygen doped iron telluride by molecular beam epitaxy

Zheng, Mao

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https://hdl.handle.net/2142/46712 Copy

Description

Title

Superconductivity in oxygen doped iron telluride by molecular beam epitaxy

Author(s)

Zheng, Mao

Issue Date

2014-01-16T17:59:59Z

Director of Research (if dissertation) or Advisor (if thesis)

Eckstein, James N.

Doctoral Committee Chair(s)

Greene, Laura H.

Committee Member(s)

• Fradkin, Eduardo H.
• Thaler, Jonathan J.
• Eckstein, James N.

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

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

Abstract

Iron base superconductor have gained much attention in the research community. They offer great potentials to improve our understanding of the subject of superconductivity by having another family of high temperature superconductors to compare and contrast to the cuprates. Practically, the iron based superconductors seems to be even better candidates for applications in power generation and power transmission. Iron telluride is regarded as the parent compound of the ”11” family, the family of iron chalcogenide that has the simplest structure. Iron telluride itself is not a superconductor, 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 a spontaneous crystallization, as the film quickly go from the initial polycrystalline phase to highly crystalline in just a few unit cells. We studied oxygen doping to the iron telluride thin films and the resultant superconductivity. We characterized the sample with AFM, XRD, transport, and STEM-EELS, and we found that interfacial strain is not an essential ingredient of superconductivity in this particular case. We investigated the doping conditions for two candidate oxygen doping modes: substitution and interstitial. We found that substitution occurs when the film grown in oxygen, while interstitial oxygen is primarily incorporated during annealing after growth. The substitutional oxygen are 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, but the transition was set in motion as early as the structural phase transition around 70 K. We believe the shift is a result of 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/Au and 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 spectroscopy junction behaviors. We observed evolution of enhanced conduction peaks around 20mV, consistent with published literature. FeTe:Ox/Al/AlOx/Au junctions behave differently, showing a evolving energy gap around 3mV. The fact that 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 layer and the aluminum layer.

Graduation Semester

2013-12

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http://hdl.handle.net/2142/46712

Copyright and License Information

Copyright 2013 Mao Zheng

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