Session Q27: Invited Session: DCMP Prize Session: Buckley, Isakson, MGM

Oliver E. Buckley Condensed Matter Prize Lecture: Topological Insulators

A topological insulator is a material that is an insulator on its interior, but has special conducting states on its surface. These surface states are unlike any other known two dimensional conductor. They are characterized by a unique Dirac type dispersion relation and are protected by a topological property of the material's underlying electronic band structure. In this talk we will outline our path to the theoretical discovery of this phase and describe the physical properties of the two dimensional topological insulator - also known as a quantum spin Hall insulator - as well as its three dimensional generalization. We will then go on to discuss more recent developments, including the topological classification of point and line defects in topological insulators and superconductors. The latter may provide a venue for observing Majorana fermion states and for realizing proposals for topological quantum computation.   

HgTe as a Topological Insulator

HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating. Our transport data provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator. To turn the material into a 3-dimensional topological insulator, we utilize growth induced strain in relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a direct observation of the quantum transport properties of the 2-dimensional topological surface states.   

Oliver E. Buckley Condensed Matter Prize Lecture: Topological insulators and superconductors

In this talk I shall briefly review the basic concepts of topological insulators and superconductors, and recall the history of the discovery of the first topological insulator in nature, the HgTe material. I will then describe some striking physical properties of topological insulators and their possible applications. \\[4pt] X. L. Qi, S. C. Zhang, Phys. Today 63, 33 (2010). \newline X. L. Qi, S. C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).   

Frank Isakson Prize for Optical Effects in Solids Lecture: Infrared nano-spectroscopy and nano-imaging of Dirac plasmons in graphene

We have applied antenna-based infrared (IR) nano-spectroscopy and nano-imaging to investigate Dirac plasmons in monolayer graphene. This experimental technique enables IR imaging with nano-scale spatial resolution, and also allows one to investigate electromagnetic phenomena at wave-vectors on the order of the Fermi wave-vector in gated graphene. Nano-spectroscopy and nano-imaging experiments have uncovered rich optical effects associated with the Dirac plasmons of graphene [\textit{Fei et al. Nano Letters 2011}]. We were able to directly image Dirac plasmons propagating over sub-micron distances and reflecting from the edges of graphene flakes, all with a spatial resolution far exceeding the plasmon wavelength. Furthermore, we employed new IR nano-optics capabilities to demonstrate the gate-tunable plasmonic properties of graphene and to investigate the coupling between Dirac plasmons and the phonon modes of polar substrates.   

Maria Goeppert Mayer Award Lecture: Spectroscopy of Hybrid Superconductor-Carbon Nanostructure Systems

The electronic properties of carbon nanotubes and graphene have excited much interest, for both fundamental science and technological applications. In this talk, I will discuss how coupling superconductors to these carbon nanostructures can enable new spectroscopic tools. In particular, I will discuss our experiments demonstrating that superconducting probes on carbon nanotube quantum dots can enhance weak spectroscopic features. I will also show how superconducting tunnel probes enable direct measurements of electron-electron interactions in carbon nanotubes. Finally, I will present data showing that connecting graphene to superconductors allows for the spectroscopy of individual, tunable superconducting (Andreev) bound states.