Session B2: Theory and Experiment on Topological Insulators

Experimental Realization of a Three-Dimensional Topological Insulator

Three-dimensional(3D) topological insulators (TIs) are a new state of quantum matter with a bulk gap generated by the spin orbit interaction and odd number of relativistic Dirac fermions on the surface. The robust surface states of TIs can be the host for many striking quantum phenomena, such as an image magnetic monopole induced by an electric charge and Majorana fermions induced by the proximity effect from a superconductor. Recently, a class of stoichiometric materials, Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$, and Sb$_{2}$Te$_{3}$, were theoretically predicted to be the simplest 3D TIs whose surface states consist of a single Dirac cone. By investigating the surface state of Bi$_{2}$Te$_{3}$ with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. These results establish that Bi$_{2}$Te$_{3}$ is a simple model system for the 3D TI with a single Dirac cone on the surface. The large bulk gap of Bi$_{2}$Te$_{3}$ also points to promising potential for high-temperature spintronics applications.   

Topological Insulator Nanoribbons and Nanocrystals

Following the discovery of two-dimensional topological insulator edge states in HgTe quantum wells at cryogenic temperatures, three dimensional (3D) topological insulators were recently discovered in Bismuth Selenide (Bi$_{2}$Se$_{3})$ and related compounds. Theoretical prediction and angle resolved photon emission spectroscopy studies show quantum spin Hall surface states in these 3D topological insulator materials. However, all the studies thus far have been on bulk size materials and it is challenging to observe directly the surface topological state conduction since its effects are masked by the residue bulk carrier. Here I present our study on topological insulator nanostructures, which can manifest the surface conduction states due their large surface-to-volume ratios. We show unambiguous transport evidence of topological surface states through periodic quantum interference effects in layered single-crystalline Bi$_{2}$Se$_{3}$ nanoribbons. Pronounced Aharonov-Bohm oscillations in the magnetoresistance clearly demonstrate the coverage of two-dimensional electrons on the entire surface. I will discuss our recent exciting study on topological insulator nanocrystals. Our results suggest that topological insulator nanoribbons and nanocrystals afford novel promising materials for future spintronic devices.   

Molecular beam epitaxy growth and novel properties of topological insulator films of Bi2Te3 and Bi2Se3

In this talk, we summarize our recent activity in state-of-art molecular beam epitaxy (MBE) growth and characterization of nontrivial surface states of topological insulator films of Bi2Te3 and Bi2Se3 on Si(111) and 6H-SiC(0001) substrates. We studied the growth dynamics and epitaxial relationship under different flux ratios and substrate temperatures, and identified the optimized conditions for stoichiometric and layer-by-layer MBE deposition of both Bi2Te3 and Bi2Se3 films by real time reflection high energy electron diffraction (RHEED). We demonstrate the atomically flat morphology and intrinsic topological property of the resulted films by angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS). By direct imaging standing waves associated with magnetic and nonmagnetic impurities and steps on Bi2Te3 and Bi2Se3 (111) surfaces, we show that the topological states have a surface nature and are protected by the time reversal symmetry. We also studied the growth of conventional superconductor and magnetic thin films on Bi2Te3 and Bi2Se3. Implication on probing Majorana states and topological magneto-electric effect will be discussed. Work done in collaboration with Xi Chen, Jinfeng Jia, Xucun Ma, Ke He, Lili Wang, Yayu Wang, Xi Dai, Zhong Fang, Xincheng Xie, Shunqing Shen, Qian Niu, Ying Liu, Xiao-Liang Qi, and Shou-Cheng Zhang.   

Visualizing Topological Surface States using Scanning Tunneling Microscopy and Spectroscopy

Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated by strong spin-orbit coupling. These novel materials are distinguished from ordinary insulators by the presence of gapless metallic boundary states, akin to the chiral edge modes in quantum Hall systems, but with unconventional spin textures. Angle resolved photoemission experiments and theoretical efforts have provided strong evidence for bulk topological insulators and their spin-chiral surface states in several Bi-based compounds. We have performed scanning tunneling microscopy and spectroscopic studies of topological surface states on a range of different compounds. I will describe how these experiments illustrate the importance of the spin-texture of these novel states on their scattering and quantum confinement. Experiments demonstrate that these states are protected from backscattering between opposite spin states due to their chiral spin textures. [1]. More recently, our studies were extended to determine the interplay between the influence of spin symmetry on scattering and the possibility of energy level quantization due to geometric confinement for topological surface states. [2] Work was done in collaboration with P. Roushan, J. Seo, H. Beidenkopf, Y.-S. Hor, C. Parker, D. Hsieh, D. Qian, and A. Richardella, M. Z. Hasan, R. Cava. Supported by ARO, ONR, and MRSEC through PCCM. \\[4pt] [1] P. Roushan et al. Nature 460, 1106 (2009). \\[0pt] [2] J. Seo et al. submitted (2009).   

Spin liquid phases in exactly solvable models: proof of principle

``Spin liquids'' are elusive quantum states of matter, characterized by topological order and a spectrum of fractionalized excitations rather than by broken symmetries. As the study of spin-liquids is still in a formative stage, it is useful to construct exactly solvable models with various flavors of spin liquid ground state so as to establish their stability as phases of matter and to derive essential features of their physical properties. Here we study three such models: Firstly, we show that two distinct chiral spin liquid phases are realized in a spin-1/2 Kitaev model on a decorated honeycomb lattice (also know as the star lattice). Depending on coupling parameters, its vortex excitations (visons) obey Abelian or non-Abelian statistics. The quantum phase transition between the two phases, although purely topological, is never-the-less continuous. Secondly, we introduce a spin-3/2 Kitaev-like model on the square lattice. This model has a ``half-integer'' spin per unit cell, and an algebraic spin liquid ground state. Remarkably, by fine-tuning some parameters in this spin-3/2 model, fermionic excitations with an emergent Fermi surface can be realized. Thirdly, we establish genuine spin-charge separation in an extended Hubbard model of electrons on the Kagome lattice. We show how this extended Hubbard model potentially can be realized in ultra-cold gases with dipolar interactions.