The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two dimensional electron layer under a strong magnetic field. 1,2 The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon as the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism.

The quantum spin Hall (QSH) state is a topologically nontrivial state of quantum matter which preserves time-reversal symmetry; it has an energy gap in the bulk, but topologically robust gapless states at the edge. Recently, this novel effect has been predicted and observed in HgTe quantum wells and in this Letter we predict a similar effect arising in Type-II semiconductor quantum wells made from InAs/GaSb/AlSb. The quantum well exhibits an "inverted" phase similar to HgTe/CdTe quantum wells, which is a QSH state when the Fermi level lies inside the gap. Due to the asymmetric structure of this quantum well, the effects of inversion symmetry breaking are essential. Remarkably, the topological quantum phase transition between the conventional insulating state and the quantum spin Hall state can be continuously tuned by the gate voltage, enabling quantitative investigation of this novel phase transition.

Following the recent observation of the quantum spin Hall (QSH) effect in HgTe quantum wells, an important issue is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator as a function of temperature in the presence of a magnetic impurity. At high temperatures, Kondo and/or two-particle scattering give rise to a logarithmic temperature dependence. At low temperatures, for weak Coulomb interactions in the edge liquid, the conductance is restored to unitarity with unusual power laws characteristic of a "local helical liquid," while for strong interactions, transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event.

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.

Three-dimensional (3D) topological insulators (TIs) are unusual quantum materials that host conducting helical Dirac states on their surfaces, which are protected by time reversal symmetry (TRS), but are electrically insulating in the bulk [1,2]. TI is distinct from a trivial insulator by its unique electromagnetic response, described by the so-called  term shown below in addition to the ordinary Maxwell terms [3][4][5][6]. Here E and B are the conventional electric and magnetic fields inside an insulator, e is electron charge, and θ is the dimensionless pseudo-scalar parameter describing the insulator. For a trivial insulator, θ=0, while for a TI, θ=. When TRS is preserved, θ is either 0 or , reflecting its topological nature. This  term is related to the axion electrodynamics in particle physics [7]. Since the EB  term can be rewritten as a total derivative, its effect manifests on the surface states. A half-integer quantum Hall effect on the TI surface occurs once the surface Dirac Fermions acquire a mass, i.e. the surface state is gapped by magnetism. Such half-integer quantum Hall effect on TI surface can lead to a variety of exotic phenomena such as the quantum anomalous Hall (QAH) effect [3,[8][9][10][11][12][13][14][15], the quantized magneto-optical effect [3,16,17], the topological magnetoelectric (TME) effect [3][4][5][6]18], and the image magnetic monopole [19]. The QAH and quantized magneto-optical effects have been experimentally demonstrated in pure or magnetic TI films [10][11][12][13][20][21][22]. The TME effect refers to the quantized response of electric 3 polarization to applied magnetic fields and vice versa. The realization of the TME effect requires the following three conditions: (i) the TI film should be in the 3D regime; (ii) all the surfaces are gapped with the chemical potential lying within the gaps; (iii) the interior of the TI maintains TRS or inversion symmetry to maintain  in the bulk. A material system allowing for the realization of TME effect is known as an axion insulator [3][4][5].Recently, two papers reported the possible realization of the axion insulator. (Figs. 3k and 3m). We note that the zero  yx plateau is absent in either uniformly doped or the Cr modulation doped QAH samples [23,26,31,32]. The 3-5-3 SH2 was magnetically trained first by an upward sweep up to  0 H=1.5T before being swept downward. When  0 H=-0.01T, the MFM contrast is uniform (red), indicating that both top V-and bottom Cr-doped TI layers have upward magnetization (Fig. 3c). At  0 H=-0.05T, some reversed magnetic domains (green regions in Fig. 3d) appear, presumably in the 'softer' Cr-doped TI layer. As  0 H is swept further, the green regions expand and fill up the whole scan area at  0 H=-0.09T, indicating the uniform antiparallel magnetization alignment over the entire 3-5-3 SH2. When  0 H is further swept toward H c1 , new reversed magnetic domains (blue regions in Fig. 3g) nucleate at different locations, presumably in the 'harder' V-doped TI layer. Downward parallel magnetiza...

The generally accepted view that spin polarization in non-magnetic solids is induced by the asymmetry of the global crystal space group has limited the search for spintronics materials mainly to non-centrosymmetric materials. In recent times it has been suggested that spin polarization originates fundamentally from local atomic site asymmetries and therefore centrosymmetric materials may exhibit previously overlooked spin polarizations. Here, by using spin- and angle-resolved photoemission spectroscopy, we report the observation of helical spin texture in monolayer, centrosymmetric and semiconducting PtSe2 film without the characteristic spin splitting in conventional Rashba effect (R-1). First-principles calculations and effective analytical model analysis suggest local dipole induced Rashba effect (R-2) with spin-layer locking: opposite spins are degenerate in energy, while spatially separated in the top and bottom Se layers. These results not only enrich our understanding of the spin polarization physics but also may find applications in electrically tunable spintronics.

A conserved guanine-rich sequence could be a new target for anti–hepatitis C virus drug development.

Owing to the chirality of Weyl nodes, the Weyl systems can support one-way chiral zero modes under a strong magnetic field, which leads to nonconservation of chiral currents—the so-called chiral anomaly. Although promising for robust transport of optical information, the zero chiral bulk modes have not been observed in photonics. Here we design an inhomogeneous Weyl metamaterial in which a gauge field is generated for the Weyl nodes by engineering the individual unit cells. We experimentally confirm the presence of the gauge field and observe the zero-order chiral Landau level with one-way propagation. Without breaking the time-reversal symmetry, our system provides a route for designing an artificial magnetic field in three-dimensional photonic Weyl systems and may have potential for device applications in photonics.