Engineering a robust quantum spin Hall state in graphene via adatom deposition

By C. Weeks, Jun Hu, J. Alicea, M. Franz, R. Wu. 2011

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PHYSICAL REVIEW X 1, 021001 (2011) Engineering a Robust Quantum Spin Hall State in Graphene via Adatom Deposition Conan Weeks, 1 Jun Hu, 2 Jason Alicea, 2 Marcel Franz, 1 and Ruqian Wu 2 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada Department of Physics and Astronomy, University of California, Irvine, California 92697 (Received 17 May 2011; published 3 October 2011; corrected 30 March 2012) The 2007 discovery of quantized conductance in HgTe quantum wells delivered the field of topological insulators (TIs) its first experimental confirmation. While many three-dimensional TIs have since been identified, HgTe remains the only known two-dimensional system in this class. Difficulty fabricating HgTe quantum wells has, moreover, hampered their widespread use. With the goal of breaking this logjam, we provide a blueprint for stabilizing a robust TI state in a more readily available two-dimensional material—graphene. Using symmetry arguments, density functional theory, and tight-binding simulations, we predict that graphene endowed with certain heavy adatoms realizes a TI with substantial band gap. For indium and thallium, our most promising adatom candidates, a modest 6% coverage produces an estimated gap near 80 K and 240 K, respectively, which should be detectable in transport or spectroscopic measurements. Engineering such a robust topological phase in graphene could pave the way for a new generation of devices for spintronics, ultra-low-dissipation electronics, and quantum information processing. DOI: 10.1103/PhysRevX.1.021001 Subject Areas: Condensed Matter Physics, Graphene, Topological Insulators I. INTRODUCTION When certain physical properties of a system depend on global topology—and not on local details, such as disor- der—then the system is said to realize a topological phase. By virtue of their universality, rooted in this independence of microscopic details, topological phases are both para- digmatically interesting and thought to possess seeds of important future technologies. Until rather recently, experimental studies of topological phases were confined to the quantum Hall effect, seen in high-quality two- dimensional electron systems subjected to strong magnetic fields. The advent of topological insulators (TIs), a class of two- and three-dimensional nonmagnetic crystalline solids enjoying strong spin-orbit coupling [1–3], opened a fasci- nating new chapter in the field. Two-dimensional TIs are commonly referred to as quan- tum spin-Hall (QSH) states [1–3] due to their similarity to integer quantum Hall liquids. A defining signature of a QSH state is the existence of gapless edge states which are protected from elastic backscattering and localization by time-reversal symmetry ðT Þ. This feature, combined with the spin-filtered nature of the edge modes, renders such systems technologically very promising. Furthermore, when in contact with a superconductor, the edges are predicted to host Majorana fermions [4], which play an important role in topological quantum computation schemes [5]. The QSH state was first predicted to arise in graphene [6], although later it was realized that, due to Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. carbon’s small atomic number, spin-orbit coupling is too weak to produce an observable effect under realistic con- ditions. (First-principles calculations [7–11] on pristine graphene all predict sub-Kelvin gaps for this phase.) The QSH state was subsequently predicted in several other materials, including HgTe and InAs/GaSb quantum wells [12,13] and bilayer bismuth [14]. While landmark experi- ments indeed observed this phase in HgTe [15], experi- mental activity on the QSH effect has remained limited by notorious practical difficulties associated with HgTe- based structures. Given the comparative ease with which graphene can be fabricated, it would be highly desirable to artificially en- hance graphene’s spin-orbit coupling strength to elevate the gap protecting the QSH state to observable levels. A prac- tical means of doing so would pave the way to a new generation of QSH experiments accessible to a broad spec- trum of researchers worldwide. Previous work has explored the possibility that strong interactions [16–18] or radiation [19] might drive graphene into a robust topological phase. Our goal in this manuscript is to explore a practical new route to this end. Specifically, we ask whether graphene can ‘‘inherit’’ strong spin-orbit coupling from a dilute concen- tration of heavy adatoms (whose innate spin-orbit coupling strength can be on the electron-volt scale) deposited ran- domly into the honeycomb lattice. The basic principle underlying our proposal can be understood by considering processes in which an electron from graphene tunnels onto an adatom—whereupon it ‘‘feels’’ enormous spin-orbit coupling—and then returns to the graphene sheet. Such processes in effect locally enhance graphene’s spin-orbit strength manyfold. A viable proposal must, however, address numerous competing ef- fects. For instance, adatoms often form local magnetic Published by the American Physical Society

Subfields: Atomic and Molecular Physics, and Optics, Materials Chemistry

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