Cui-Zu Chang

Band structure engineering in (Bi 1− x Sb x ) 2 Te 3 ternary topological insulators

Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces with Dirac-like band structure. The most challenging problem faced by current investigations of these materials is to establish the existence of significant bulk conduction. Here we show how the band structure of topological insulators can be engineered by molecular beam epitaxy growth of (Bi(1-x)Sb(x))(2)Te(3) ternary compounds. The topological surface states are shown to exist over the entire composition range of (Bi(1-x)Sb(x))(2)Te(3), indicating the robustness of bulk Z(2) topology. Most remarkably, the band engineering leads to ideal TIs with truly insulating bulk and tunable surface states across the Dirac point that behaves like one-quarter of graphene. This work demonstrates a new route to achieving intrinsic quantum transport of the topological surface states and designing conceptually new topologically insulating devices based on well-established semiconductor technology.

Crossover between Weak Antilocalization and Weak Localization in a Magnetically Doped Topological Insulator

We report transport studies on magnetically doped Bi(2)Se(3) topological insulator ultrathin films grown by molecular beam epitaxy. The magnetotransport behavior exhibits a systematic crossover between weak antilocalization and weak localization with the change of magnetic impurity concentration, temperature, and magnetic field. We show that the localization property is closely related to the magnetization of the sample, and the complex crossover is due to the transformation of Bi(2)Se(3) from a topological insulator to a topologically trivial dilute magnetic semiconductor driven by magnetic impurities. This work demonstrates an effective way to manipulate the quantum transport properties of the topological insulators by breaking time-reversal symmetry.

Electron interaction-driven insulating ground state in Bi2Se3 topological insulators in the two-dimensional limit

We report a transport study of ultrathin Bi

Evidence for electron-electron interaction in topological insulator thin films

{}_{2}

Topological insulator Bi2Se3 thin films grown on double-layer graphene by molecular beam epitaxy

Se

Topology-Driven Magnetic Quantum Phase Transition in Topological Insulators

{}_{3}

Thin films of magnetically doped topological insulator with carrier-independent long-range ferromagnetic order.

topological insulators with thickness from one quintuple layer to six quintuple layers grown on sapphire by molecular beam epitaxy. At low temperatures, the film resistance increases logarithmically with decreasing temperature, revealing an insulating ground state. The insulating behavior becomes more pronounced in thinner films. The sharp increase of resistance with magnetic field, however, indicates the existence of weak antilocalization originated from the topological protection. We show that this unusual insulating ground state in the two-dimensional limit of topological insulators is induced by the combined effect of strong electron interaction and topological delocalization.

Weak antilocalization and conductance fluctuation in a submicrometer-sized wire of epitaxial Bi2Se3

We consider in our work single crystal thin films of Bi(2)Se(3), grown by molecular beam epitaxy, both with and without Pb doping. Angle-resolved photoemission data demonstrate topological surface states with a Fermi level lying inside the bulk band gap in the Pb-doped films. Transport data show weak localization behavior, as expected for a thin film in the two-dimensional limit (when the thickness is smaller than the inelastic mean free path), but a detailed analysis within the standard theoretical framework of diffusive transport shows that the temperature and magnetic field dependences of resistance cannot be reconciled in a theory that neglects inter-electron interactions. We demonstrate that an excellent account of quantum corrections to conductivity is achieved when both disorder and interaction are taken into account. These results clearly demonstrate that it is crucial to include electron-electron interaction for a comprehensive understanding of diffusive transport in topological insulators. While both the ordinary bulk and the topological surface states presumably participate in transport, our analysis does not allow a clear separation of the two contributions.

Erratum: Crossover of the three-dimensional topological insulator Bi 2Se3 to the two-dimensional limit (Nature Physics (2010) 6 (584-588))

Atomically flat thin films of topological insulator Bi2Se3 have been grown on double-layer graphene formed on 6H–SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2×1011/cm2, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.Atomically flat thin films of topological insulator Bi{sub 2}Se{sub 3} have been grown on double-layer graphene formed on 6H-SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi{sub 2}Se{sub 3} films. The as-grown films without doping exhibit a low defect density of 1.0{+-}0.2x10{sup 11}/cm{sup 2}, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.

Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction

Simultaneous topological and magnetic quantum phase transitions are observed in thin films of Bi2(SexTe1-x)3 doped with chromium Topological insulators owe their exotic properties to the peculiarities of their band structure, and one can induce a transition between a topologically trivial and nontrivial material by chemical doping. Now, J. Zhang et al. (p. 1582) have gone a step further—simultaneously observing that a magnetic quantum transition as the ratio of Se and Te is varied in Bi2(SexTe1-x)3 thin films grown by molecular beam epitaxy and doped with magnetic Cr. Photoemission and transport experiments, as well as density functional calculations, imply that the topological transition induces magnetism The breaking of time reversal symmetry in topological insulators may create previously unknown quantum effects. We observed a magnetic quantum phase transition in Cr-doped Bi2(SexTe1-x)3 topological insulator films grown by means of molecular beam epitaxy. Across the critical point, a topological quantum phase transition is revealed through both angle-resolved photoemission measurements and density functional theory calculations. We present strong evidence that the bulk band topology is the fundamental driving force for the magnetic quantum phase transition. The tunable topological and magnetic properties in this system are well suited for realizing the exotic topological quantum phenomena in magnetic topological insulators.