Session A20: Invited Session: Advanced Quantum Materials for Future Information Technology

Graphene-based Electronics and Optoelectronics

Graphene has rather unique electrical and properties and there is currently strong interest in taking advantage of these properties for technological applications. In my talk I will review some of the key properties of free graphene, how these properties are affected by environmental interactions and under technologically relevant conditions, and how they can be utilized in electronics and optoelectronics. In electronics, I will focus on high frequency ($\ge $ 300 GHz) graphene transistors and simple IC circuits, as well as related device physics problems, such as the role of electrical contacts, scattering effects, graphene topology, device size scaling, energy dissipation, etc. I will then review the key optical properties of graphene and their use in optoelectronics. Specifically, I will focus on the far-infrared and THz range of the spectrum, on ways of controlling graphene's absorption in this spectral range and provide examples of corresponding applications. I will also discuss photocurrent generation in graphene and its use in ultrafast graphene photodetectors.   

Visualizing Helical Metals on Topological Insulators

During the last few years, it has become apparent that there can be a distinct type of insulator, which can occur because of the topology of electronic wavefunctions in materials comprised of heavier elements. Strong interaction between the spin and the orbital angular momentum of electrons in these compounds alters the sequence in energy of their electronic states. The key consequence of this topological characteristic (and the way to distinguish a topological insulator from an ordinary one) is the presence of metallic electrons with helical spin texture at their surfaces. I will describe experiments that directly visualize these novel quantum states of matter and demonstrate their unusual properties through spectroscopic mapping with the scanning tunneling microscope (STM). These experiments show that the spin texture of these states protects them against backscattering and localization. These states appear to penetrate through barriers that stop other electronic states. I will also describe more ongoing efforts focused on unraveling the physics of topological surface states and their potential for device-like applications. \\[4pt] References: \\[0pt] P. Roushan et al, Nature \textbf{460 }1106 (2009). \\[0pt] J. Seo et al, Nature \textbf{466 }343 (2010). \\[0pt] H. Beidenkopf et al, to appear Nature Physics (2011).   

Quantum information processing with defect spins in diamond and silicon carbide

Many proposals for quantum information technologies require quantum systems that can be easily manipulated by an outside observer, while remaining largely unaffected by destructive interactions with the surrounding environment. One system that matches this description is a defect in the crystal lattice of diamond known as the nitrogen-vacancy (NV) center. Electrons trapped at this defect form an atomic scale spin state that can be used as an individually addressable, solid state quantum bit (qubit) even at room temperature. The exceptional quantum properties of the diamond NV have motivated recent efforts to search for similar defects in other semiconductors, as these would expand the technological opportunities available to defect-based quantum systems [1]. We discuss these efforts, which make use of techniques from both computational materials science and experimental quantum physics, focusing on explorations of the 4H polytype of silicon carbide (4H-SiC). In particular, we present recent experimental results that identify several defect spin states in 4H-SiC that function as analogs to the diamond NV. Using optical and microwave techniques similar to those used with diamond NV qubits, the spins of these defects can be optically addressed and coherently controlled in the time domain at temperatures ranging from 20 -- 300 K. Additionally, these defects are optically active near telecom wavelengths, inhabit a host material for which there already exist industrial scale crystal growth and advanced microfabrication techniques, and possess desirable spin coherence properties comparable to those of the diamond NV. This makes them promising candidates for various photonic, spintronic, and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies [2]. \\ \\ {[1]} J. R. Weber*, W. F. Koehl*, J. B. Varley*, A. Janotti, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, \emph{Proc.~Natl~Acad.~Sci.~USA} \textbf{107}, 8513 (2010).\\ {[2]} W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom, \emph{Nature} \textbf{479}, 84 (2011); A. Dzurak, \emph{Nature} \textbf{479}, 47 (2011).   

Quantum Transport in Dirac Materials

Over the past few years, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic structures. Among these, graphene and topological insulators have taken center stage due to their relativistic-like electron dynamics and their potential applications in nanotechnology. In this talk I will briefly review the properties of graphene and topological insulators and discuss some of our recent quantum electronic transport experiments in these systems.   

GHz - THz plasmonic circuits using low dimensional electronic systems

Nature offers a broad variety of plasma systems consisting of electrons unbound from atoms, e.g.; astrophysical plasmas in intergalactic, interstellar, and stellar media; the Earth's ionosphere; and solid-state plasma, the free electrons in metals and semiconductors, only to name a few. A key feature of many plasma systems is collective motions of electrons; as the electron density profile is perturbed from equilibrium, Coulomb restoring forces (and sometimes quantum pressure in dense plasma) arise to power these collective motions, usually in the form of bulk electron density oscillations or electron density waves. Solid-state plasmas are particularly interesting, as the fabrication technologies available for solid-state materials allow us to alter the boundaries and interfaces of the plasma media in various ways to engineer the collective motion. A notable example is the surface plasmons, which have been a source of many breakthroughs in photonics. I will talk about a set of our recent developments where the plasmons are brought down to the electronics-regime (GHz$\sim $THz) and manipulated to produce a range of functionalities, while offering unique advantages to electronics over their purely electromagnetic counterparts. (Co-workers) William Andress (Harvard), Hosang Yoon (Harvard), Kitty Yeung (Harvard), Ling Qin (Harvard), Ken West (Princeton), and Loren Pfeiffer (Princeton).