Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Pouyan Ghaemi Mohammadi

Committee Members

V. Parameswaran Nair

Javad Shabani

Sarang Gopalakrishnan

Alexios Polychronakos

Subject Categories

Condensed Matter Physics | Quantum Physics

Keywords

odd-frequency pairing, iron-based superconductivity, doped topological insulators, superconducting Dirac materials, geometric resonance, spin-orbit coupling

Abstract

Multiband superconducting materials, such as iron pnictides and doped topological insulators, have shown to be particularly promising platforms for realizing unconventional electronic behavior of both fundamental and practical importance. Similarly, new innovations in the engineering of gapped topological phases, like semiconductor based Kitaev chains and topological insulator based heterostructures, have opened new directions for solid-state design. Central to much of the excitement generated by such multifaceted electronic systems is a rich interplay between various inherent structural ordering tendencies and topologically non-trivial properties.

In some classes of pnictides, spin density wave order coexists with superconductivity over a range of doping and temperature, while surfaces and domain walls host novel Andreev-bound and topological surface states which can exhibit anomalous correlations such as odd-frequency spin-triplet Cooper pairing, due to the combined breaking of spin-rotation and translation symmetry by the spin-density-wave and edge termination, respectively, showcasing pnictides as a natural platform for realizing odd frequency superconductivity without heterostructures.

Recent experimental studies on Niobium-doped Bismuth Selenide have shown that the magnetic properties of dopant atoms can strongly affect the resulting superconducting phase. This effect is mainly due to the development of Yu-Shiba-Rusinov bound-states at the magnetic atoms and the coupling of such states through the superconducting environment. We show that the doping level can critically affect the type of magnetic coupling between dopant atoms in superconducting doped topological insulators, leading to enhancement, within the superconducting phase, of multiple competing magnetic orders with critical phase boundaries appearing in the vicinity of the critical doping level as determined by the degree of band inversion in the parent topological insulator.

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