Searching For Intrinsic And Device-based Topological Superconductivity

Recently,searching for topological superconductors and the associated Majorana bound states?MBSs?at their boundaries have become a very hot topic in condensed matter physics.A MBS is the equal superposition of an electron state and a hole state,obeying Non-Abelian statistics.MBSs must appear in pairs to ensure the global fermionic statistic of the system.Two MBSs in a pair are non-locally entangled,creating degenerate ground states,based on which topological qubits can be defined.Topological quantum computing is a promising scheme of quantum computing which immunes to local perturbations from the environment.Topological superconductivity could occur as an intrinsic property of materials such as Sr₂RuO₄-a material that has been intensively studied and is believed to have a chiral p_x±ip_y-wave order parameter,with broken time reversal symmetry.Topological superconductivity could also exist in artificial devices.For example,p_x+ip_y-like superconductivity could be induced via proximity effect by placing a topological insulator?TI?adjacent to an s-wave superconductor.In this dissertation work,we explored topological superconductivity in both uniform Bi/Ni material and TI-based devices.This dissertation contains four chapters:In chapter I,I will briefly review the concept of topology and how it was introduced into condensed matter physics.I will review the research progresses on intrinsic and device-based topological superconductivity,including those related to MBSs.In chapter II,I will present the results of our studies on the superconductivity of Bi/Ni bilayer via superconducting quantum interference devices?SQUIDs?.The Bi/Ni bilayer demonstrates the coexistence of ferromagnetism and superconductivity at low temperatures,although neither Bi nor Ni alone is superconducting at the temperature investigated.The superconductivity is likely triggered by ferromagnetic fluctuations at the interface of Bi and Ni.In such case,the time reversal symmetry should be broken,so that superconductivity should be topologically non-trivial and chiral-there will be a circulating current and out-of-plane magnetization at the edge of the superconductor due to orbital moments of Cooper pairs.To detect possibly existing edge magnetizations of Bi/Ni bilayer,we designed and fabricated SQUIDs by in situ involving Bi/Ni bilayer in the interference path of the devices.An anomalous“advanced”instead of retarded hysteresis was found in the interference pattern of the SQUID,revealing the formation of chiral superconducting domains and domains-wall motion in the Bi/Ni bilayer.This is the first experiment that quantitatively studies the domains-wall motion of a topological superconductor.In chapter III,we report our progresses on searching for topological superconductivity and MBSs in Pb-Bi₂Te₃-Pb Josephson devices.When an s-wave superconductor is brought adjacent to a topological insulator?Bi₂Te₃?,p_x+ip_y-like superconductivity with time reversal symmetry could be induced,as proposed by theorists.Through contact resistance measurement,we found that the minigap between electron-like and hole-like Andreev bound states could be fully closed with varying magnetic field.The result reveals the high transparent nature of these in-gap states on the surface of Bi₂Te₃ in the junction area,presumably being topologically protected away from backscattering.Further experimental investigation is needed to clarify whether the charge transport in the junction is via Andreev bound states or via MBSs.In chapter IV,I will give a summary of my research and a prospect of the field.