Jiun-Haw Chu

Experimental realization of a three-dimensional topological insulator, Bi2Te3.

Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi{sub 2}Te{sub 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. Our results establish that Bi{sub 2}Te{sub 3} is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi{sub 2}Te{sub 3} also points to promising potential for high-temperature spintronics applications.Topological Insulators Topological insulators are a recently discovered state of matter, in which the bulk is an insulator while the surface is metallic with counterpropagating spin states. The surface states are protected by the topology, or structure, of the Fermi surface in the bulk gap and are described by a Dirac cone showing linear dispersion behavior meeting at the Dirac point. Chen et al. (p. 178, published online 11 June) provide a comprehensive photoemission study on Bi2Te3 showing that it too falls into the category of topological band insulators. Moreover, there is just a single surface state with a single Dirac point in the photoemission spectrum. The identification of a material with a single Dirac point removes the ambiguity arising from multiple surface states and provides an ideal test-bed to probe the physics of these exotic new materials. Bi2Te3 is identified as a three-dimensional topological insulator with a single metallic surface state. Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi2Te3 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. Our results establish that Bi2Te3 is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi2Te3 also points to promising potential for high-temperature spintronics applications.

Two-dimensional surface state in the quantum limit of a topological insulator

The edges of graphene-based systems possess unusual electronic properties, originating from the non-trivial topological structure associated with the pseudospinorial character of the electron wavefunctions. These properties, which have no analogue for electrons described by the Schrödinger equation in conventional systems, have led to the prediction of many striking phenomena, such as gate-tunable ferromagnetism and valley-selective transport1, 2, 3. In most cases, however, the predicted phenomena are not expected to survive the strong structural and chemical disorder that unavoidably affects the edges of real graphene devices. Here, we present a theoretical investigation of the intrinsic low-energy states at the edges of electrostatically gapped bilayer graphene, and find that the contribution of edge modes to the linear conductance of realistic devices remains sizable even for highly imperfect edges. This contribution may dominate over that of the bulk for sufficiently clean devices, such as those based on suspended bilayer graphene samples. Our results illustrate the robustness of those phenomena whose origin is [...] LI, Jian, et al. Topological origin of subgap conductance in insulating bilayer graphene. Nature Physics, 2011, vol. 7, no. 1, p. 38-42 DOI : 10.1038/nphys1822

In-Plane Resistivity Anisotropy in an Underdoped Iron Arsenide Superconductor

Jiun-Haw Chu, 2 James G. Analytis, 2 Kristiaan De Greve, Peter L. McMahon, Zahirul Islam, Yoshihisa Yamamoto, 5 and Ian R. Fisher 2 Department of Applied Physics and Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA Stanford Institute of Energy and Materials Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park 94025,California 94305, USA E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA The Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA National Institute of Informatics, Hitotsubashi 2-1-2, Chiyoda-ku, Tokyo 101-8403, Japan (Dated: February 18, 2010)De-Twinning a Superconductor Insight into the mechanism of electrical transport in a solid can often be gained by measuring its resistivity along different spatial directions. However, iron-based superconductors form numerous twin boundaries where two different orientations of a crystal meet, and so the measured resistivity along any in-plane direction will be averaged over these orientations. Chu et al. (p. 824) were able to “de-twin” the compound Ba(Fe1−xCox)2As2, enabling unambiguous measurements of its normal-state resistivity along the in-plane lattice axes. Differences were observed in the resistivity values along the two axes, which suggests that the breaking of the symmetry of the lattice and electron subsystems occur simultaneously. Electronic ordering coincides with a lattice structural transition in an exotic superconductor. High-temperature superconductivity often emerges in the proximity of a symmetry-breaking ground state. For superconducting iron arsenides, in addition to the antiferromagnetic ground state, a small structural distortion breaks the crystal’s C4 rotational symmetry in the underdoped part of the phase diagram. We reveal that the representative iron arsenide Ba(Fe1−xCox)2As2 develops a large electronic anisotropy at this transition via measurements of the in-plane resistivity of detwinned single crystals, with the resistivity along the shorter b axis ρb being greater than ρa. The anisotropy reaches a maximum value of ~2 for compositions in the neighborhood of the beginning of the superconducting dome. For temperatures well above the structural transition, uniaxial stress induces a resistivity anisotropy, indicating a substantial nematic susceptibility.

Bulk Fermi surface coexistence with Dirac surface state in Bi2Se3: A comparison of photoemission and Shubnikov-de Haas measurements

Shubnikov-de Haas (SdH) oscillations and angle-resolved photoemission spectroscopy (ARPES) are used to probe the Fermi surface of single crystals of

Room-temperature antiferromagnetic memory resistor.

{\text{Bi}}_{2}{\text{Se}}_{3}

Symmetry-breaking orbital anisotropy observed for detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition

. We find that SdH and ARPES probes quantitatively agree on measurements of the effective mass and bulk band dispersion. In high carrier density samples, the two probes also agree in the exact position of the Fermi level

Electronic structure of the iron-based superconductor LaOFeP

{E}_{F}

Determination of the phase diagram of the electron doped superconductor Ba(Fe(1-x)Co(x))(2)As(2)

, but for lower carrier density samples discrepancies emerge in the position of

Divergent Nematic Susceptibility in an Iron Arsenide Superconductor

{E}_{F}

Evidence for a Nodal-Line Superconducting State in LaFePO

. In particular, SdH reveals a bulk three-dimensional Fermi surface for samples with carrier densities as low as