Ke He

Experimental Observation of the Quantum Anomalous Hall Effect in a Magnetic Topological Insulator

Quantized and Anomalous The Hall effect, an electromagnetic phenomenon with a straightforward explanation, has many exotic counterparts, including a quantized version occurring independently of the presence of external magnetic fields. Inspired by a theoretical prediction of the quantum anomalous Hall (QAH) effect in magnetically doped topological insulator thin films, Chang et al. (p. 167, published online 14 March; see the Perspective by Oh) prepared thin films of the compound Cr0.15(Bi0.1Sb0.9)1.85Te3, with Cr as the magnetic dopant. They observed a plateau in the Hall resistance as a function of the gating voltage without any applied magnetic fields, signifying the achievement of the QAH state. An elusive effect emerges in thin films of a bismuth-antimony-telluride topological insulator doped with magnetic chromium. [Also see Perspective by Oh] The quantized version of the anomalous Hall effect has been predicted to occur in magnetic topological insulators, but the experimental realization has been challenging. Here, we report the observation of the quantum anomalous Hall (QAH) effect in thin films of chromium-doped (Bi,Sb)2Te3, a magnetic topological insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance reaches the predicted quantized value of h/e2, accompanied by a considerable drop in the longitudinal resistance. Under a strong magnetic field, the longitudinal resistance vanishes, whereas the Hall resistance remains at the quantized value. The realization of the QAH effect may lead to the development of low-power-consumption electronics.

Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO3

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Se-etched SrTiO3 (001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.

Experimental Demonstration of Topological Surface States Protected by Time-Reversal Symmetry

We report direct imaging of standing waves of the nontrivial surface states of topological insulator Bi2Te3 using a scanning tunneling microscope. The interference fringes are caused by the scattering of the topological states off Ag impurities and step edges on the Bi2Te3(111) surface. By studying the voltage-dependent standing wave patterns, we determine the energy dispersion E(k), which confirms the Dirac cone structure of the topological states. We further show that, very different from the conventional surface states, backscattering of the topological states by nonmagnetic impurities is completely suppressed. The absence of backscattering is a spectacular manifestation of the time-reversal symmetry, which offers a direct proof of the topological nature of the surface states.

Landau quantization of topological surface states in Bi2Se3.

1 Department of Physics, Tsinghua University, Beijing 100084, China 2 Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 3 Microsoft Research, Station Q, University of California, Santa Barbara, CA 93106, USA 4 Department of Physics, Stanford University, Stanford CA 94305, USA 5 Physikalisches Institut, Universität Würzburg, D-97074 Würzburg, Germany * These authors contributed equally to this work. ¶ To whom correspondence should addressed. Email: xc@mail.tsinghua.edu.cn, xcma@aphy.iphy.ac.cn

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.

Intrinsic Topological Insulator Bi2Te3 Thin Films on Si and Their Thickness Limit

High-quality Bi2Te3 films can be grown on Si by the state-of-art molecular beam epitaxy technique. In situ angle-resolved photo-emission spectroscopy measurement reveals that the as-grown films are intrinsic topological insulators and the single-Dirac-cone surface state develops at a thickness of two quintuple layers. The work opens a new avenue for engineering of topological materials based on well-developed Si 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.

Direct observation of nodes and twofold symmetry in FeSe superconductor.

Scanning tunneling spectroscopy suggests an orbital ordering mechanism for electron pairing in an iron-based superconductor. We investigated the electron-pairing mechanism in an iron-based superconductor, iron selenide (FeSe), using scanning tunneling microscopy and spectroscopy. Tunneling conductance spectra of stoichiometric FeSe crystalline films in their superconducting state revealed evidence for a gap function with nodal lines. Electron pairing with twofold symmetry was demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms, and Se vacancies. The twofold pairing symmetry was further supported by the observation of striped electronic nanostructures in the slightly Se-doped samples. The anisotropy can be explained in terms of the orbital-dependent reconstruction of electronic structure in FeSe.

Phase separation and magnetic order in K-doped iron selenide superconductor

The discovery that potassium-doped iron selenide undergoes phase separation into a defect-free superconducting phase and an iron-vacancy-ordered insulating phase resolves many questions about the unusual behaviour of this iron-based superconductor.

Topological Insulator Thin Films of Bi2Te3 with Controlled Electronic Structure

Topological insulator thin films of Bi2Te3 with controlled electronic structure can be grown by regulating the molecular beam epitaxy (MBE) growth kinetics without any extrinsic doping. N- to p-type conversion results from the change in the concentrations of Te-Bi donors and Bi-Te acceptors. This represents a step toward controlling topological surface states, with potential applications in devices.