Intrinsic magnetic topological insulator (TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remains elusive experimentally so far. Here, we report the experimental realization of high-quality thin films of an intrinsic magnetic TI-MnBi2Te4-by alternate growth of a Bi2Te3 quintuple-layer and a MnTe

A single atomic slice of α -tin-stanene-has been predicted to host the quantum spin Hall effect at room temperature, offering an ideal platform to study low-dimensional and topological physics. Although recent research has focused on monolayer stanene, the quantum size effect in few-layer stanene could profoundly change material properties, but remains unexplored. By exploring the layer degree of freedom, we discover superconductivity in few-layer stanene down to a bilayer grown on PbTe, while bulk α -tin is not superconductive. Through substrate engineering, we further realize a transition from a single-band to a two-band superconductor with a doubling of the transition temperature. In situ angleresolved photoemission spectroscopy (ARPES) together with first-principles calculations elucidate the corresponding band structure. The theory also indicates the existence of a topologically non-trivial band. Our experimental findings open up novel strategies for constructing two-dimensional topological superconductors.Confining superconductivity to a two-dimensional (2D) plane engenders a variety of quantum phenomena 1,2 . Of late, the realization of highly crystalline and atomically thin superconductors has triggered a flurry of discoveries, including the Griffiths singularity behavior 3 and a quantum metallic phase 4,5 , as well as an extremely large critical magnetic field in the plane 6,7 . One strategy for achieving 2D superconductors is to epitaxially grow superconductive single elements, such as Pb, In and Ga, for just one or two atomic layers 3,8,9 . Among the single elements, tin (Sn) is the very material in which the Meissner effect was first discovered 10 , but realizing ultrathin Sn in the superconductive β -phase, known as white tin 11 , remains challenging. The epitaxially grown Sn in the ultrathin limit tends to fall instead in the α -phase 12 , whose bulk is semi-metallic and non-superconductive.Recently, however, intensive research has been devoted to investigate the thinnest possible slice of α -tin (111) ) is the focus of current research. On the other hand, few-layer stanene is expected to show significant thicknessdependent properties due to the strong quantum confinement 20 , but its exploration is still lacking.In this Letter, by going from monolayer to few-layer stanene, surprisingly, we discover superconductivity. We report the stable superconducting properties of uncapped few-layer stanene films on PbTe (111)/Bi 2 Te 3 substrates. The superconducting transition temperature (T c ) can be effectively enhanced by varying the thickness of the PbTe buffer layer. Concomitantly with a doubling of T c , we observe a single-band to two-band transition, which is further elucidated by photoemission spectroscopy and theoretical calculations. The calculated band structure further indicates the existence of inverted bands in our system. Our results therefore underscore the potential of an in-plane integration of 2Dtopological insulator and superconductor-of the same material. The heterostructure, vertically...

We report transport studies of Mn-doped Bi_{2}Te_{3} topological insulator (TI) films with an accurately controlled thickness grown by molecular beam epitaxy. We find that films thicker than five quintuple layers (QLs) exhibit the usual anomalous Hall effect for magnetic TIs. When the thickness is reduced to four QLs, however, characteristic features associated with the topological Hall effect (THE) emerge. More surprisingly, the THE vanishes again when the film thickness is further reduced to three QLs. Theoretical calculations demonstrate that the coupling between the top and bottom surface states at the dimensional crossover regime stabilizes the magnetic Skyrmion structure that is responsible for the THE.

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SnSe fi lms thicker than 20 nm go back to the GeS structure. We checked the surface morphology of a 20 nm fi lm with STM to see how the transition from rock-salt to GeS structures takes place. As shown in Figure 4 d, there are islands distributed on the surface with the height of 11.3 Å, obviously in GeS structure. RHEED and ARPES results show that most part of the fi lm is still of rock-salt SnSe. With more SnSe deposited, the fi lm surface is gradually covered by the GeS-type islands. Therefore, above 20 nm, GeS-type SnSe forms on the top of the rock-salt SnSe fi lm instead of changing the fi lm into the GeS structure. The epitaxial rock-salt SnSe fi lms possess some unique properties of great importance. Since SnSe is electron-doped and SnTe is usually hole-doped, by mixing them into SnTe x Se 1− x , we can control the carrier density and type by Te/Se ratio. Considering that both rock-salt-type SnSe and SnTe are TCIs with similar bulk gap, [ 2,21 ] the topological property and bulk gap will be little changed by the alloying, a remarkable advantage over Pb 1− x Sn x Te(Se). The relatively weak spinorbit coupling may bring rock-salt SnSe different physical properties from other TI and TCI materials [ 1,19 ] for example longer spin relaxation time, which is important for spintronic applications. As a metastable phase, the lattice structure of rock-salt SnSe should be more sensitive to perturbations, a superior property for TCI-based fi eld-effect devices. It is also very helpful and interesting to study the thermoelectric properties of rock-salt-type Adv. Mater. 2015, 27, 4150-4154 www.advmat.de www.MaterialsViews.com Figure 4. The structural change of rock-salt SnSe fi lms above 20 nm. a) RHEED pattern of a nominal 70 nm SnSe fi lm along the [ 112 ] direction. b) ARPES spectra of the 70 nm SnSe fi lm around Γ point. c) STM image of the 70 nm SnSe fi lms (130 nm × 130 nm). The inset shows the line profi le along the black line. d) STM image of a nominal 20 nm SnSe fi lm (150 nm × 150 nm). The inset shows the line profi le along the black line.Figure 3. The energy band structure of a 16 nm SnSe(111) fi lm. The bandmap around Γ point obtained by a) ARPES and b) fi rst-principles calculations. The bandmap around M point obtained by c) ARPES and d) fi rst-principles calculations. The bandmap close to E F around Γ point obtained by e) ARPES and f) fi rst-principles calculations. The bandmap close to E F around M point obtained by g) ARPES and h) fi rst-principles calculations. The red dashed lines in (a-h) indicate E F . i) Real space partial charge distributions of the states of Dirac points at Γ point and M point. 4154 wileyonlinelibrary.com