Date of Degree

2-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor(s)

Lia Krusin-Elbaum

Committee Members

Maria Tamargo

Vinod Menon

Jim Hone

Vadim Oganesyan

Elisa Riedo

Subject Categories

Condensed Matter Physics

Abstract

Topological quantum matter is a unique and potentially transformative protectorate against disorder-induced backscattering. The ultimate disorder limits to the topological state, however, are still not known - understanding these limits is critical to potential applications in the fields of spintronics and information processing. In topological insulators spin-orbit interaction and time-reversal-symmetry invariance guarantees - at least up to a certain disorder strength - that charge transport through 2D gapless Dirac surface states is robust against backscattering by non-magnetic disorder. Strong disorder may destroy topological protection and gap out Dirac surface states, although recent theories predict that under severe electronic disorder a quantized topological conductance might yet reemerge. Very strong electronic disorder, however, is not trivial to install and quantify, and topological matter under such conditions thus far has not been experimentally tested.

This thesis addresses the behavior of three-dimensional (3D) topological insulator (TI) films in a wide range of structural and electronic disorder. We establish strong positional disorder in thin (20-50 nm) Sb2Te3 films, free of extrinsic magnetic dopants. Sb2Te3 is a known 2nd generation topological insulator in low-disorder crystalline state. It is also a known phase-change material that undergoes insulator-to-metal transition with the concurrent orders of magnitude resistive drop, where a huge range of disorder could be controllably explored. In this work we show that even in the absence of magnetic dopants, disorder may induce spin correlations detrimental to the topological state.

Chapter 1 contains a brief introduction to the topological matter and describes the role played by disorder. This is followed by theory considerations and a survey of prior experimental work. Next we describe the motivation for our experiments and explain the choice of the material.

Chapter 2 describes deposition techniques used for material growth, including the parameters significance and effects on the material properties. Chapter 3 describes structural and electrical characterization techniques employed in the work.

In Chapter 4-5 we discuss the experimental results. Sb2Te3 films at extreme disorder, where spin correlations dominate the transport of charge, are discussed in Chapter 4. We employ transport measurements as our main tool to explore disorder-induced changes in the Sb2Te3. In addition we directly detect disorder-induced spin response in thin Sb2Te3 films free of extrinsic magnetic dopants; it onsets at a surprisingly high temperature (200 K) and vanishes when disorder is reduced. Localized spins control the hopping (tunneling) transport through spin memory induced by the non-equilibrium charge currents. The observed spin-memory phenomenon emerges as negative magnetoresistance distinct from orbital quantum interference effects. The hopping mechanism and spin correlations dominate transport over an extensive disorder range. Spin correlations are eventually suppressed by the restoration of positional order in the (bulk) crystalline state, implying a disorder threshold to the topological state.

As disorder is reduced the material undergoes structural and electronic transitions, which are discussed in Chapter 5. We obtain a number of characteristic attributes that change sharply at the structural and electronic transitions: localization length, dimensionality, and the nature of conductance. Structural transition is clearly seen in the changes in lattice vibrations tracked by Raman spectroscopy, which we use here as a metric of disorder. The significance of the disorder-induced localization transition is discussed. Next we investigate the effects of structural and electronic disorder on the bulk and surfaces in the crystalline state of Sb2Te3.

The nontrivial topology of this strongly spin-orbit coupled material comes from the band inversion in the bulk. One of the key transport signatures of topological surfaces is weak antilocalization (WAL) correction to conductivity; it is associated with the topological Berry phase and should display a two-dimensional (2D) character. In our work, we establish the disorder level at which 2D WAL appears. The conduction at this threshold is one conduction quantum; it corresponds to the topological quantum channel.

Finally, we summarize our key findings and discuss open questions and next steps toward the understanding of disorder-induced correlations in the spin and charge channels that can alter the emergent behaviors of the topological states.

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