Academic Appointments

2018-19 Courses

All Publications

  • Designer Dirac fermions and topological phases in molecular graphene NATURE Gomes, K. K., Mar, W., Ko, W., Guinea, F., Manoharan, H. C. 2012; 483 (7389): 306-310


    The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system-molecular graphene-assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p-n and p-n-p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields, wherein Dirac electrons react to 'pseudo' electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.

    View details for DOI 10.1038/nature10941

    View details for Web of Science ID 000301481800043

    View details for PubMedID 22422264

  • Laser-Synthesized Epitaxial Graphene ACS NANO Lee, S., Toney, M. F., Ko, W., Randel, J. C., Jung, H. J., Munakata, K., Lu, J., Geballe, T. H., Beasley, M. R., Sinclair, R., Manoharan, H. C., Salleo, A. 2010; 4 (12): 7524-7530


    Owing to its unique electronic properties, graphene has recently attracted wide attention in both the condensed matter physics and microelectronic device communities. Despite intense interest in this material, an industrially scalable graphene synthesis process remains elusive. Here, we demonstrate a high-throughput, low-temperature, spatially controlled and scalable epitaxial graphene (EG) synthesis technique based on laser-induced surface decomposition of the Si-rich face of a SiC single-crystal. We confirm the formation of EG on SiC as a result of excimer laser irradiation by using reflection high-energy electron diffraction (RHEED), Raman spectroscopy, synchrotron-based X-ray diffraction, transmission electron microscopy (TEM), and scanning tunneling microscopy (STM). Laser fluence controls the thickness of the graphene film down to a single monolayer. Laser-synthesized graphene does not display some of the structural characteristics observed in EG grown by conventional thermal decomposition on SiC (0001), such as Bernal stacking and surface reconstruction of the underlying SiC surface.

    View details for DOI 10.1021/nn101796e

    View details for Web of Science ID 000285449100060

    View details for PubMedID 21121692

  • Detection and Cloaking of Molecular Objects in Coherent Nanostructures Using Inelastic Electron Tunneling Spectroscopy NANO LETTERS Fransson, J., Manoharan, H. C., Balatsky, A. V. 2010; 10 (5): 1600-1604


    We address quantum invisibility in the context of electronics in nanoscale quantum structures. We make use of the freedom of design that quantum corrals provide and show that quantum mechanical objects can be hidden inside the corral, with respect to inelastic electron scattering spectroscopy in combination with scanning tunneling microscopy, and we propose a design strategy. A simple illustration of the invisibility is given in terms of an elliptic quantum corral containing a molecule, with a local vibrational mode, at one of the foci. Our work has implications to quantum information technology and presents new tools for nonlocal quantum detection and distinguishing between different molecules.

    View details for DOI 10.1021/nl903991a

    View details for Web of Science ID 000277444900013

    View details for PubMedID 20402523

  • Theory of Fano resonances in graphene: The influence of orbital and structural symmetries on STM spectra PHYSICAL REVIEW B Wehling, T. O., Dahal, H. P., Lichtenstein, A. I., Katsnelson, M. I., Manoharan, H. C., Balatsky, A. V. 2010; 81 (8)
  • Topological Insulator Nanowires and Nanoribbons NANO LETTERS Kong, D., Randel, J. C., Peng, H., Cha, J. J., Meister, S., Lai, K., Chen, Y., Shen, Z., Manoharan, H. C., Cui, Y. 2010; 10 (1): 329-333


    Recent theoretical calculations and photoemission spectroscopy measurements on the bulk Bi(2)Se(3) material show that it is a three-dimensional topological insulator possessing conductive surface states with nondegenerate spins, attractive for dissipationless electronics and spintronics applications. Nanoscale topological insulator materials have a large surface-to-volume ratio that can manifest the conductive surface states and are promising candidates for devices. Here we report the synthesis and characterization of high quality single crystalline Bi(2)Se(3) nanomaterials with a variety of morphologies. The synthesis of Bi(2)Se(3) nanowires and nanoribbons employs Au-catalyzed vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces, are formed by stacking nanoplatelets along the axial direction of the wires. Nanoribbons are grown along [1120] direction with a rectangular cross-section and have diverse morphologies, including quasi-one-dimensional, sheetlike, zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on nanoribbons show atomically smooth surfaces with approximately 1 nm step edges, indicating single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb atomic lattice, suggesting that the STM tip couples not only to the top Se atomic layer, but also to the Bi atomic layer underneath, which opens up the possibility to investigate the contribution of different atomic orbitals to the topological surface states. Transport measurements of a single nanoribbon device (four terminal resistance and Hall resistance) show great promise for nanoribbons as candidates to study topological surface states.

    View details for DOI 10.1021/nl903663a

    View details for Web of Science ID 000273428700055

    View details for PubMedID 20030392

  • Quantum holographic encoding in a two-dimensional electron gas NATURE NANOTECHNOLOGY Moon, C. R., Mattos, L. S., Foster, B. K., Zeltzer, G., Manoharan, H. C. 2009; 4 (3): 167-172


    The ability of the scanning tunnelling microscope to manipulate single atoms and molecules has allowed a single bit of information to be represented by a single atom or molecule. Although such information densities remain far beyond the reach of real-world devices, it has been assumed that the finite spacing between atoms in condensed-matter systems sets a rigid upper limit on information density. Here, we show that it is possible to exceed this limit with a holographic method that is based on electron wavefunctions rather than free-space optical waves. Scanning tunnelling microscopy and holograms comprised of individually manipulated molecules are used to create and detect electronically projected objects with features as small as approximately 0.3 nm, and to achieve information densities in excess of 20 bits nm-2. Our electronic quantum encoding scheme involves placing tens of bits of information into a single fermionic state.

    View details for DOI 10.1038/NNANO.2008.415

    View details for Web of Science ID 000264318500014

    View details for PubMedID 19265846

  • Surface structure of cleaved (001) USb2 single crystal PHILOSOPHICAL MAGAZINE Chen, S. P., Hawley, M., Van Stockum, P. B., Manoharan, H. C., Bauer, E. D. 2009; 89 (22-24): 1881-1891
  • Single-atom gating of quantum-state superpositions NATURE PHYSICS Moon, C. R., Lutz, C. P., Manoharan, H. C. 2008; 4 (6): 454-458

    View details for DOI 10.1038/nphys930

    View details for Web of Science ID 000256613000011

  • Quantum phase extraction in isospectral electronic nanostructures SCIENCE Moon, C. R., Mattos, L. S., Foster, B. K., Zeltzer, G., Ko, W., Manoharan, H. C. 2008; 319 (5864): 782-787


    Quantum phase is not directly observable and is usually determined by interferometric methods. We present a method to map complete electron wave functions, including internal quantum phase information, from measured single-state probability densities. We harness the mathematical discovery of drum-like manifolds bearing different shapes but identical resonances, and construct quantum isospectral nanostructures with matching electronic structure but divergent physical structure. Quantum measurement (scanning tunneling microscopy) of these "quantum drums"-degenerate two-dimensional electron states on the copper(111) surface confined by individually positioned carbon monoxide molecules-reveals that isospectrality provides an extra topological degree of freedom enabling robust quantum state transplantation and phase extraction.

    View details for DOI 10.1126/science.1151490

    View details for Web of Science ID 000252963000053

    View details for PubMedID 18258909

  • Scanning optical homodyne detection of high-frequency picoscale resonances in cantilever and tuning fork sensors APPLIED PHYSICS LETTERS Zeltzer, G., Randel, J. C., Gupta, A. K., Bashir, R., Song, S., Manoharan, H. C. 2007; 91 (17)

    View details for DOI 10.1063/1.2803774

    View details for Web of Science ID 000250468200103

  • Information transport and computation in nanometre-scale structures Meeting on Organizing Atoms Eigler, D. M., Lutz, C. P., Crommie, M. F., Manoharan, H. C., Heinrich, A. J., Gupta, J. A. ROYAL SOC. 2004: 1135–47


    We discuss two examples of novel information-transport and processing mechanisms in nanometre-scale structures. The local modulation and detection of a quantum state can be used for information transport at the nanometre length-scale, an effect we call a 'quantum mirage'. We demonstrate that, unlike conventional electronic information transport using wires, the quantum mirage can be used to pass multiple channels of information through the same volume of a solid. We discuss a new class of nanometre-scale structures called 'molecule cascades', and show how they may be used to implement a general-purpose binary-logic computer in which all of the circuitry is at the nanometre length-scale.

    View details for Web of Science ID 000222032900002

    View details for PubMedID 15306466

  • Magnetism at the spatial limit 4th Conference on Physical Phenomena at High Magnetic Fields Manoharan, H. WORLD SCIENTIFIC PUBL CO PTE LTD. 2002: 3272–72
  • Low-field magnetoresistance in GaAs two-dimensional holes PHYSICAL REVIEW B Papadakis, S. J., De Poortere, E. P., Manoharan, H. C., Yau, J. B., Shayegan, M., Lyon, S. A. 2002; 65 (24)