Session P27: Invited Session: Transport Studies of Topological Insulators

Towards the quantum anomalous Hall effect in HgMnTe

Although there are plenty of theoretical and experimental studies of the anomalous Hall effect in the ferromagnetic materials, the quantum version, namely ``quantum anomalous Hall effect'', has never been observed in the realistic materials. In this work, we report the experimental evidence of the quantum anomalous Hall effect in the Mn doped HgTe quantum wells. We observe a long quantized Hall plateau from 0.2 T to $>$ 25 T with the Hall conductance e$^{2}$/h in the p-doped regime. By carefully analyzing the gate voltage and temperature dependence of the experimental data, we find the long plateau origins from the fact that the two zero Landau levels in HgMnTe doped system have the same slope, which is exactly the required condition for the quantum anomalous Hall effect. Theoretical \textbf{k}$\cdot$\textbf{p} calculation is carefully compared with the experimental data to identify the influence from the magnetic impurities. A HgMnTe-ferromagnet hybrid structure is proposed for the possible future device applications.   

Quantum Spin Hall Insulator Made of InAs/GaSb

Quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, topological properties of these systems hold a great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, so far only the semiconductor systems of HgTe/CdTe and, more recently, inverted InAs/GaSb, have shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have helical property, with carrier spin tied to the carrier direction of motion, and protected from back-scattering by time-reversal symmetry. Here we present low temperature transport measurements of inverted InAs/GaSb quantum wells, showing strong evidence for the existence of proposed helical edge channels. Edge modes persist in spite of conductive bulk, which is of non-trivial origin but highly tunable via electrostatic gates, and show only a weak magnetic field dependence. This is a direct consequence of a gap opening away from the zone center leading to effective decoupling of edge to bulk states due to the Fermi velocity mismatch. Low Schottky barrier of this material system and good interface to superconductors allows us to further probe topological properties of helical channels in Andreev reflection measurements and opens a promising route in realization of exotic Majorana modes.   

Transport Studies of Topological Insulators and Superconductors

A topological state of matter is characterized by a topological feature of the quantum-mechanical wavefunction in the Hilbert space. In 3D topological insulators (TIs), a non-trivial Z$_2$ topology of the bulk valence band leads to the emergence of Dirac fermions on the surface. Similarly, in 3D topological superconductors (TSCs), a non-trivial winding number of the superconducting wavefunction leads to the appearance of Majorana fermions on the surface. The Dirac or Majorana fermions in those topological states of matter are of fundamental interest, but there have been significant materials problems that hindered their experimental studies so far: In the case of TIs, most of the materials identified as such are poor insulators in the bulk, making it difficult to probe the peculiar surface transport properties; in the case of TSCs, no concrete example has yet been discovered. In this talk, I will present our recent contributions to address these issues. For TIs, we discovered that the chalcogen-ordered tetradymite TI material Bi$_2$Te$_2$Se presents a high bulk resistivity, allowing one to observe clear surface quantum oscillations [Z. Ren {\it et al.}, PRB {\bf 82}, 241306(R) (2010)]; more recently, we discovered that the bulk-insulating nature can be further improved in the solid-solution system Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$, making it possible to achieve the surface-dominated transport in a bulk crystal and observe both Dirac holes and electrons via Shubnikov-de Haas oscillations [A. A. Taskin {\it et al.}, PRL {\bf 107}, 016801 (2011)]. For TSCs, we developed a new synthesis technique for a candidate TSC, Cu$_x$Bi$_2$Se$_3$, to obtain single-crystal samples with a high shielding fraction [M. Kriener {\it et al.}, PRL {\bf 106}, 127004 (2011); PRB {\bf 84}, 054513 (2011)]. Using these crystals, we have recently succeeded in observing the surface Andreev bound state which gives evidence for an unconventional superconductivity; since the unconventional superconductivity in Cu$_x$Bi$_2$Se$_3$ can only be topological (thanks to the simple and peculiar band structure), one can conclude with confidence that this material is the first concrete example of a TSC [S. Sasaki {\it et al.}, PRL {\bf 107} (2011)]. Work in collaboration with A. A. Taskin, Z. Ren, S. Sasaki, and K. Segawa.   

Quantum anomalous Hall effect in the shin film of magnetic topological insulators and semimetals

The great interests on Hall effects come with their quantization under certain conditions.By now all five types of the Hall effects have been discovered, and the only remaining one is the quantized anomalous Hall effect, which is the quantized Hall effect without external magnetic field and the formation of Landau levels. In the present talk, I will summarize two possible ways proposed by our group to reach such an effect, which are thin films of magnetically doped topological insulators and topological semimetals. I will mainly focused on the latter proposal, which is important in the following sense. First the proposal is based on the stoichiometric material, which is very good for obtaining large mobility. Second, the exchange coupling energy between the magnetization and the valence electrons is of the order of eV, which makes QAHE more easy to be realized.