Dissertations, Theses, and Capstone Projects

Date of Degree

6-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Lia Krusin-Elbaum

Committee Members

Maria C. Tamargo

Vadim Oganesyan

Kyungwha Park

James C. Hone

Subject Categories

Condensed Matter Physics | Physics

Keywords

Quantum anomalous Hall effect, Intrinsic magnetic topological insulators, Topological insulators, Hydrogenation, Chemical potential tuning, Topological ferromagnetic superlattice

Abstract

In the past several years, a new field of symmetry-protected topological materials has emerged in condensed matter physics, based on the wide range of consequences that result from the realization that certain properties of physical systems can be expressed as topological invariants, which are insensitive to local perturbations. This new class of materials hosts unique surface/edge states, such as the first known topological system – quantum Hall insulator with dissipationless chiral edge states, and massless spin-helical Dirac surface states in 3D topological insulators that are unlike any other known 1D or 2D electronic systems. In this thesis, to understand the role and significance of topology in real materials we focus on time-reversal symmetry (TRS) protected 3D topological insulators (TIs) and on the chiral edge states that emerge in the quantum anomalous Hall (QAH) state after TRS is broken.

While the interior/bulk of topological insulators is no different from ordinary semiconductors with narrow bandgap, the spins of surface electrons are locked at a 90◦ to their momentum and the conduction channels for up-spin and down-spin particles are separated, akin to a two-way highway with a barrier in between to prevent the collisions. This phenomenon is a consequence of the TRS protection at the surface. If we break this symmetry and remove one of the conduction channels, for example with an out-of-plane magnetization, a QAH state with the dissipationless chiral edge state can emerge, while both the interior of the surface and the bulk are now free of itinerant charge carriers.

These dissipationless chiral conduction channels are technologically important as they can transform modern electronics into quantum electronics for supreme energy efficiency. Even though QAH was shown possible theoretically on a 2D honeycomb lattice under staggered magnetic flux by Haldane in 1988, it has been only recently realized in a topological magnet after the discovery of topological insulators. The first quantum anomalous Hall demonstration was under most restrictive materials engineering constraints, severely limiting the exploration of fundamental physics and technological applications.

The access to the chiral channels in magnetically doped TIs (such as Cr, V heavily doped (Bi, Sb)2Te3 thin films) is challenging – it requires tuning the Fermi level precisely into the small Dirac mass gap (~10 meV) uniformly across a mesoscopic sample. The doping disorders limit the observed QAH temperature to ∼300 mK range and only to samples with thicknesses of 5 - 10 nm. A recently discovered new class of van der Waals (vdW) materials in the MnBi2Te4 and MnSb2Te4 class opened vast new opportunities in materials design. These intrinsic topological magnets do not require doping with magnetic ions, for their crystal structure comprises seven atomic layers (septuple layers, SL) blocks with a single Mn layer in the middle, each aligned ferromagnetically (FM) out-of-plane. Such FM exchange interaction has been predicted to open a very large Dirac mass gap (∼70 meV), making it in principle easier to achieve QAH state. However, the interaction between SLs was determined to be antiferromagnetic (AFM), and QAH has been possible only in the small odd number of SLs where the net surface magnetic order was still FM-like.

This new class of topological magnets can potentially host chiral edge states at higher temperatures without thickness limits, but currently it has two outstanding challenges, namely tuning the Fermi level into the Dirac exchange gap in the presence of naturally occurring charged defects, and the AFM coupling in the bulk. In this thesis, we aim to resolve these two challenges by (1) modifying the magnetism of the bulk to ferromagnetic by inserting a topological Bi2Te3 non-magnetic spacer in between each SL layer, i.e., MnBi2Te4/Bi2Te3 superlattice, and (2) developing a new chemical potential tuning method | hydrogenation using an aqueous solution of hydrogen chloride to tune the Fermi level of the bulk without impacting the surface electron mobility.

This dissertation consists of four chapters. In Chapter 1 we give an introduction to topological effects, with a focus on the theoretical and experimental progress in our understanding of QAH. We then summarize the current status of knowledge regarding the recently discovered intrinsic topological magnets and lay out the challenges in this new quantum materials class. In Chapter 2 we describe the experimental methods used in our study. In Chapter 3 we present our discovery of a previously unknown Berry-curvature-driven anomalous Hall regime (‘Q-window’) at above-Kelvin temperatures in the magnetic topological bulk crystals where through growth Mn ions self-organize into a period-ordered MnBi2Te4/Bi2Te3 superlattice. In Chapter 4 we discuss a new chemical tuning method involving hydrogen ions. We demonstrate that hydrogenation resolves an outstanding challenge in chalcogenide classes of three-dimensional (3D) topological insulators and magnets | the control of intrinsic bulk conduction that denies access to quantum surface transport. We demonstrate those carrier densities are easily tuned by over 1020 cm-3, allowing moving the Fermi level into the bulk bandgap to enter surface/edge current channels. We show that the hydrogen-tuned topological materials are stable at room temperature and tunable disregarding bulk size, opening a breadth of platforms for harnessing emergent topological states.

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