Xucun Ma

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.

Phase diagram and electronic indication of high-temperature superconductivity at 65 K in single-layer FeSe films

Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cuand Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at 65±5 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications. 1 ar X iv :1 20 7. 68 23 v1 [ co nd -m at .s up rco n] 3 0 Ju l 2 01 2 In high temperature cuprate superconductors, superconductivity is realized by doping the parent Mott insulator with charge carriers to suppress the antiferromagnetic state[1]. In the process, the physical property experiences a dramatic change from antiferromagnetic insulator, to a superconductor and eventually to a non-superconducting normal metal. In the superconducting region, the transition temperature Tc can be tuned by the carrier concentration, initially going up with the increasing doping, reaching a maximum at an optimal doping, and then going down with further doping[1]. Such a rich evolution with doping not only provides a handle to tune the physical properties in a dramatic way, but also provides clues and constraints in understanding the origin of the high-Tc superconductivity. The same is true for the Fe-based superconductors where superconductivity is achieved by doping the parent magnetic compounds which are nevertheless metallic[2, 3]. Again, the superconducting transition temperature can be tuned over a wide doping range with an maximum Tc at the optimal doping. Understanding such a rich evolution is also a prerequisite in understanding the origin of high temperature superconductivity in the Fe-based superconductors. The latest discovery of high temperature superconductivity signature in the single-layer FeSe films[4, 5] is significant in a couple of respects. First, it may exhibit a high Tc that breaks the Tc record (∼55 K) in the Fe-based superconductors kept so far since 2008[6– 11]. Second, the discovery of such a high-Tc in the single-layer FeSe film is surprising when considering that its bulk counterpart has a Tc only at 8 K[9] although it can be enhanced to 36.7 K under high pressure[12]. Third, it provides an ideal system to investigate the origin of high temperature superconductivity. On the one hand, this system consists of a single-layer FeSe film that has a simple crystal structure and strictly two-dimensionality; its simple electronic structure may provide key insights on the high Tc superconductivity mechanism in the Fe-based compounds[5]. On the other hand, the unique properties of this system may involve the interface between the single-layer FeSe film and the SrTiO3 substrate that provides an opportunity to investigate the role of interface in generating high-Tc superconductivity[4]. Like in cuprates and other Fe-based superconductors, it is important to explore whether one can tune the single-layer FeSe system to vary its physical properties and superconductivity by changing the charge carrier concentration. In this paper, we report a wide range tunability of the electronic structure and physical properties that is realized in the single-The recent discovery of possible high-temperature superconductivity in single-layer FeSe films has generated significant experimental and theoretical interest. In both the cuprate and the iron-based high-temperature superconductors, superconductivity is induced by doping charge carriers into the parent compound to suppress the antiferromagnetic state. It is therefore important to establish whether the superconductivity observed in the single-layer sheets of FeSe--the essential building blocks of the Fe-based superconductors--is realized by undergoing a similar transition. Here we report the phase diagram for an FeSe monolayer grown on a SrTiO3 substrate, by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase (S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5 K. The wide tunability of the system across different phases makes the FeSe monolayer ideal for investigating not only the physics of superconductivity, but also for studying novel quantum phenomena more generally.

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.

Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T(c)) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T(c) superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T(c)~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T(c) superconductivity in iron-based superconductors.

Polymerized carbon nanobells and their field-emission properties

Aligned nitrogen-containing carbon nanofibers consisting of polymerized “nanobells” have been grown on a large scale using microwave plasma-assisted chemical-vapor deposition with a mixture of methane and nitrogen. A greater part of the fiber surface consists of open ends of the graphitic sheets. A side-emission mechanism is proposed. A low-threshold field of 1.0 V/μm and a high-emission current density of 200 mA/cm2 for an applied field of 5–6 V/μm were achieved, implying that the materials have a high potential for future application as electron field emitters, especially in flat-panel displays.

The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films

All Set for Majoranas When put in the proximity of a superconductor, topological insulators (TIs) are expected to support Majorana fermions, exotic particles that are their own antiparticles. For this to be realized, the interface between the TI and superconductor layers has to be atomically sharp but electronically transparent. Wang et al. (p. 52, published online 15 March) fabricated this heterostructure by growing a film of the TI material Bi2Se3 on the superconductor NbSe2 covered with a Bi bilayer. Scanning tunneling spectroscopy revealed a superconducting gap on the TI surface of the heterostructure with varying thickness of the Bi2Se3 film. This coexistence of superconductivity and topological order should now allow observation of exotic phenomena such as Majorana fermions. A thin layer of a topological insulator grown on the surface of a superconductor is shown to acquire a superconducting gap. Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi2Se3) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi2Se3 surface in the regime of Bi2Se3 film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.