Recently new materials have emerged in which the electron dynamics are described by the Dirac equation in two dimensions. An example is graphene, the two-dimensional honeycomb lattice of carbon atoms that is the basic building block of graphite. In graphene, the electrons obey a massless Dirac equation, with the role of the relativistic electron spin played by a spinor (“pseudospin”) composed of the two ? orbitals in the unit cell. Single layer molybdenite (MoS2) has a massive Dirac equation and is a direct-bandgap semiconductor while retaining the chiral properties of the spin-1/2 pseudospin. Three-dimensional topological insulators such as Bi2Se3 are insulating in their interiors, but exhibit metallic surface states with a massless Dirac structure similar to graphene, but with the real quantum mechanical spin as the Dirac spinor. Professor Michael Fuhrer’s group is studying these materials experimentally in order to understand how their unusual band structures determine their electronic and optical properties. The experimental research involves:
* Electronic transport measurements on microfabricated devices[1-3,5-10,12]. Semiconductor micro- and nano-fabrication tools (at Monash and at the Melbourne Centre for Nanofabrication) are used to create electronic devices with controlled geometry. Cryogenic electronic measurements of resistivity, Hall effect, etc. are used to understand scattering by disorder and phonons, quantum transport (weak localization or anti-localization, quantum Hall effects), etc.
* Scanning-probe microscopy[4,7,11]. Scanning-probe microscopy techniques, such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are used to understand the atomic structure and electronic properties of two-dimensional materials. By coupling scanned-probe techniques with micro-fabricated devices, new information can be gained using techniques such as Kelvin probe microscopy (to measure local potentials in current-carrying devices) or scanned-gate microscopy (to measure the local sensitivity to a tip acting as a gate to induce charge in a device).
* Surface modification[1,3,5]. Two-dimensional Dirac materials are atomically confined at surfaces and interact strongly with their environments. Ultra-high vacuum surface science techniques are used to controllably modify the properties of two-dimensional materials, introducing charged impurities, point defects, modifying the dielectric constant, adding magnetic interactions, and changing the dopant density. Coupled with electronic transport experiments and scanned probe experiments surface modification allows insight into the relationship between atomic structure and electronic properties of these materials.
* Optical spectroscopy and optoelectronics[9]. Dirac semiconductors such as MoS2 have direct bandgaps, and chiral optical excitation can be used to excite spin and pseudospin polarizations. Additionally, two-dimensional materials have strong and tunable electron-electron interactions because the dielectric properties are determined by the surrounding media, leading to large excitonic effects. Optical spectroscopy can be used to study these effects in Dirac semiconductors.
A range of projects involving these experimental techniques are available for Honours students; the specific project can be tailored to match the skills and interests of the student.
References:
[1] J. H. Chen, C. Jang, S. Adam, M. S. Fuhrer, E. D. Williams, and M. Ishigami, “Charged Impurity Scattering in Graphene,” Nature Physics 4, 377 (2008). [2] J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, “Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2,” Nature Nanotechnology 3, 206 – 209 (2008). [3] C. Jang, S. Adam, J.-H. Chen, E. D. Williams, S. Das Sarma, M. S. Fuhrer, “Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering,” Physical Review Letters 101, 146805 (2008). [4] William G. Cullen, Mahito Yamamoto, Kristen M. Burson, Jianhao Chen, Chaun Jang, Liang Li, Michael S. Fuhrer, Ellen D. Williams, “High-fidelity conformation of graphene to SiO2 topographic features,” Physical Review Letters 105, 215504 (2010). [5] Jian-Hao Chen, W. G. Cullen, E. D. Williams, and M. S. Fuhrer, “Tunable Kondo Effect in Graphene with Defects,” Nature Physics 7, 535 (2011). [6] Sungjae Cho and Michael S. Fuhrer, “Massless and massive particle-in-a-box states in single-and bi-layer graphene,” Nano Research 4, 385 (2011). [7] A. E. Curtin, M. S. Fuhrer, J. L. Tedesco, R. L. Myers-Ward, C. R. Eddy, Jr., and D. K. Gaskill, “Kelvin probe microscopy and electronic transport in graphene on SiC(0001) in the minimum conductivity regime,” Applied Physics Letters 98, 243111 (2011). [8] Dohun Kim, Sungjae Cho, Nicholas P. Butch, Paul Syers, Kevin Kirshenbaum, Shaffique Adam, Johnpierre Paglione, Michael S. Fuhrer, “Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3,” Nature Physics 8, 460 (2012). [9] J. Yan, M.-H. Kim, J.A. Elle, A.B. Sushkov, G.S. Jenkins, H.M. Milchberg, M.S. Fuhrer, and H.D. Drew, “Dual-gated bilayer graphene hot electron bolometer,” Nature Nanotechnology 7, 472 (2012). [10] Sungjae Cho, Dohun Kim, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, and Michael S. Fuhrer, “Topological insulator quantum dot with tunable barriers,” Nano Letters 12, 469 (2012). [11] Mahito Yamamoto, Olivier Pierre-Louis, Jia Huang, Michael S. Fuhrer, T. L. Einstein, William G. Cullen, “Princess and the Pea at the nanoscale: Wrinkling and unbinding of graphene on nanoparticles,” Physical Review X 2, 041018 (2012). [12] Dohun Kim, Qiuzi Li, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, S. Das Sarma, Michael S. Fuhrer, “Intrinsic Electron-Phonon Resistivity in Bi2Se3 in the Topological Regime,” Physical Review Letters 109, 166801 (2012).Supervisor: Prof. Michael Fuhrer
See https://www.monash.edu/science/schools/physics/honours/honours-project to apply.