We demonstrate theoretically that interface engineering can drive Germanium, one of the most commonly-used semiconductors, into topological insulating phase. Utilizing giant electric fields generated by charge accumulation at GaAs/Ge/GaAs opposite semiconductor interfaces and band folding, the new design can reduce the sizable gap in Ge and induce large spin-orbit interaction, which lead to a topological insulator transition. Our work provides a new method on realizing TI in commonly-used semiconductors and suggests a promising approach to integrate it in well developed semiconductor electronic devices.PACS numbers: 71.70. Ej, 75.76.+j, 72.25.Mk Time-reversal invariant topological insulators (TIs) have aroused intensive interests in the past years, with tantalizing properties such as insulating bulk, robust metallic edge or surface modes and exotic topological excitations, and potential applications ranging from spintronics to quantum computation. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Despite these successful progresses, the topological insulator materials are still limited in the narrow gap materials containing heavy atoms, e.g., HgTe, [4,5] Bi 2 X 3 (X=Se, Te,...) [8][9][10][11], transition metal oxide heterostructure [16] and Heusler compounds [12,17]. These materials are often very different from conventional semiconductor materials in structures and properties and are hard to be integrated in current electronics devices that are based on well developed semiconductor fabrication technologies.Although there are theoretical predicts about realizing TI states in graphene, [3,18,19] the main obstacle is the weak intrinsic spin-orbit interaction (SOI) of carbon atoms. Here, instead of searching new TI materials with exotic structures and chemical elements, we take a totally different route: driving the commonly used semiconductors into TI states by using the intrinsic electric field and the strains. The difficulty of this approach lies in the fact that most of the commonly used semiconductors, such as Si, Ge, GaAs and many others usually possess sizable band gaps and do not have strong enough SOI. Furthermore, group IV elements such as Si and Ge have indirect band gap, posing extra difficulty in realizing TI. Inspired by recent theoretical works that the normal insulator [20] can be driven into a TI by an external electric field, our approach is to impose huge electric field by deliberately designed heterostructures. Recently, an interesting way of realizing topological insulating phase in a p-type GaAs quantum well by two-dimentional superimposed potentials with hexagonal symmetry was proposed.[21] Different to that work, our approach rely completely on the material engineering at the atomic level.Since commonly-used semiconductors, e.g., Si, Ge, GaAs, posses sizable bandgap ranging from 0.8eV to 1.4eV, a huge electric field is required to closing bandgap and even invert the conduction and valence bands. Such huge electric field can not be generated utilizing the gate technique. However, recent ...

We present a general theory about electron orbital motions in topological insulators. An in-plane electric field drives spin-up and spin-down electrons bending to opposite directions, the skipping orbital motions, a counterpart of the integer quantum Hall effect, are formed near the boundary of the sample. The accompanying Zitterbewegung can be found and controlled by tuning external electric fields. Ultrafast flipping electron spin leads to a quantum side jump in topological insulator, and a snake orbit motion in two-dimensional electron gas with spin-orbit interactions. This feature provides us a new way to control electron orbital motion by manipulating electron spin.PACS numbers: 71.70. Ej, 75.76.+j, 72.25.Mk The time-reversal invariant topological insulator (TI) is a new state of quantum matter possessing insulating bulk and metallic edge or surface states, which shows a linear massless Dirac dispersion [1, 2]. TIs are distinguished from a normal band insulator by a nontrivial topological invariant Z 2 characterizing its band structure. The quantum spin Hall effect was proposed in graphene [3] and HgTe quantum wells [4] . The existence of edge and surface states was confirmed by the recent experiment in HgTe quantum wells [5] and the angle-resolved photoemission spectroscopy experiments [6,7]. Due to its unique band structure, TI is a good testbed for observing relativistic effects, predicted by the Dirac equation. For instance, the Klein's paradox and Zitterbewegung (ZB).So far, the most previous works in the rapid growing field of TI focused on exploring new TIs and its transport and magnetic properties. Relatively, electron dynamics in TIs is unexplored. In this work we show that the quantum spin Hall effect and quantum anomalous Hall effect [8] can be understood from anomalous electron orbital motions in TIs. These anomalous electron orbital motions in topological insulators give naturally a clear picture about the origin of the edge states, a counterpart of the skipping orbital motion in the integer quantum Hall effect. By applying a series of magnetic field pulses to flip electron spin quickly, a quantum side-jump behavior and a snake-orbit motion can be found for electrons in TIs and normal 2DEG with the spin-orbit interactions (SOIs), respectively. The trembling motion, i.e., the ZB, can be controlled by changing an in-plane electric field and the initial momentum of the electron wavepacket.We consider the single-particle Hamiltonian of electron at low-energy regime in the presence of a uniform electric field Ewhere σ i (i = 1, 2, 3) is the Pauli matrix, ǫ(k) is the kinetic energy. The different forms of the d i can be used to describe the various important systems: 1) the twodimensional electron gas with the Rashba and Dresselhaus SOIs with d [11,12].In the absence of a uniform electric field, the electron position operator y H (t) evolving with the time t can be obtained as y H (t) = e iHt/ ye −iHt/ ,

The exploration of intriguing topological quantum physics in stanene has attracted enormous interest but is challenged by lacking desirable material samples. The successful fabrication of monolayer stanene on PbTe(111) films with low-temperature molecular beam epitaxy and thorough characterizations of its atomic and electronic structures are reported here. In situ angle-resolved photoemission spectroscopy together with first-principles calculations identify two hole bands of p xy orbital with a spin-orbit coupling induced band splitting and meanwhile reveal an automatic passivation of p z orbital of stanene. Importantly, material properties are tuned by substrate engineering, realizing a decorated stanene sample with truly insulating bulk on Sr-doped PbTe. This finding paves a road for studies of stanene-based topological quantum effects and electronics.

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