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/e(2), 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.

or X.C.M. (xcma@iphy.ac.cn).Searching for superconducting materials with high transition temperature (T C ) is one of the most exciting and challenging fields in physics and materials science.Although superconductivity has been discovered for more than 100 years, the copper oxides are so far the only materials with T C above 77 K, the liquid nitrogen boiling point 1,2 . Here we report an interface engineering method for dramatically raising the T C of superconducting films. We find that one unit-cell (UC) thick films of FeSe grown on SrTiO 3 (STO) substrates by molecular beam epitaxy (MBE) show signatures of superconducting transition above 50 K by transport measurement. A superconducting gap as large as 20 meV of the 1 UC films observed by scanning tunneling microcopy (STM) suggests that the superconductivity could occur above 77 K. The occurrence of superconductivity is further supported by the presence of superconducting vortices under magnetic field. Our work not only demonstrates a powerful way for finding new superconductors and for raising T C , but also provides a well-defined platform for systematic study of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.

Abstract:Phosphorene, a single atomic layer of black phosphorus, has recently emerged as a new twodimensional (2D) material that holds promise for electronic and photonic technology. Here we experimentally demonstrate that the electronic structure of few-layer phosphorene varies significantly with the number of layers, in good agreement with theoretical predictions. The interband optical transitions cover a wide, technologically important spectrum range from visible to mid-infrared. In addition, we observe strong photoluminescence in few-layer phosphorene at energies that match well with the absorption edge, indicating they are direct bandgap semiconductors. The strongly layer-dependent electronic structure of phosphorene, in combination with its high electrical mobility, gives it distinct advantages over other twodimensional materials in electronic and opto-electronic applications.Page 3 of 17! ! Atomically thin 2D crystals have emerged as a new class of materials with unique material properties that are potentially important for electronic and photonic technologies [1][2][3][4][5][6][7][8][9][10] . Various 2D crystals have been uncovered, ranging from metallic (and superconducting) NbSe 2 and semimetallic graphene to semiconducting MoS 2 and insulating hexagonal boron nitride (hBN).The energy bandgap, a defining characteristic of an electronic material, varies correspondingly from 0 (in metals and graphene) to 5.8 eV (in hBN) in these 2D crystals. Despite the rich variety currently available, 2D materials with a bandgap in the range from 0.3 eV to 1.5 eV are notably missing 11 . Such a bandgap corresponds to a spectral range from mid-infrared to near-infrared that is important for optoelectronic technologies such as telecommunication and solar energy harvesting. It is therefore desirable to have 2D materials with a bandgap that falls in this range, and in particular, matches that of the technologically important silicon (bandgap = 1.1 eV) and III-V semiconductors like InGaAs, without compromising sample mobility 12 .Monolayer and few-layer phosphorene are predicted to bridge the much needed bandgap range from 0.3 to 2 eV (Refs. 13-17). Inside monolayer phosphorene, each phosphorus atom is covalently bonded with three adjacent phosphorus atoms to form a puckered honeycomb structure 18 . The three near sp 3 bonds together with the lone-pair orbital take up all five valence electrons of phosphorus, so monolayer phosphorene is a semiconductor with a predicted direct optical bandgap of ~ 1.5 eV at the Γ point of the Brillouin zone. The bandgap in few-layer phosphorene can be strongly modified by interlayer interactions, which leads to a bandgap that decreases with phosphorene film thickness, eventually reaching 0.3 eV in the bulk limit.Experimental observations of layer-dependent band structure in phosphorene, on the other hand, have been rather limited. Previously, photoluminescence (PL) spectroscopy has been used to probe the bandgap of monolayer and few-layer phosphorene 8,[19][20][21][22] . Such studies, howeve...

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 the existence of signifi cant 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 behave 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 wellestablished semiconductor technology.

Development of new, high quality functional materials has been at the forefront of condensed matter research. The recent advent of two-dimensional black phosphorus has greatly enriched the material base of two-dimensional electron systems.Significant progress has been made to achieve high mobility black phosphorus twodimensional electron gas (2DEG) since the development of the first black phosphorus field-effect transistors (FETs) [1][2][3][4] . Here, we reach a milestone in developing high quality black phosphorus 2DEG -the observation of integer quantum Hall (QH) effect. We achieve high quality by embedding the black phosphorus 2DEG in a van der Waals heterostructure close to a graphite back gate; the graphite gate screens the impurity potential in the 2DEG, and brings the carrier Hall mobility up to 6000. The exceptional mobility enabled us, for the first time, to observe QH effect, and to gain important information on the energetics of the spin-split Landau levels in black phosphorus. Our results set the stage for further study on quantum transport and device application in the ultrahigh mobility regime.Quantum Hall effect, the emergence of quantized Hall resistance in 2DEG sample when subjected to low temperatures and strong magnetic fields, has had a lasting impact in modern condensed matter research. The exact, universal quantization regardless of detailed sample geometry and impurity configuration has enabled the establishment of a metrological resistance standard, and served as the basis for an independent determination of the fine structure constant 5 . Even though the exact quantization of the Hall resistance relies on certain amount of impurities 6 , the observation of QH effect, paradoxically, requires high-purity, low-defect specimens. Because of the stringent requirement on the ). In this work, we achieved high Hall mobility in black phosphorus FETs that is significantly higher than previous record value. This is accomplished by constructing a van der Waals heterostructure with the few-layer black phosphorus sandwiched between two hBN flakes (Fig. 1a,b) and placed on graphite back gate. The top hBN protects the black phosphorus flakes from sample degradation in air. More importantly, the thin bottom hBN (thickness ~ 25 nm) allows the electrons in the graphite to screen the impurity potential at the black phosphorus-hBN interface, where the 2DEG resides. The high mobility enable us, for the first time, to observe the QH effect in black Page 4 of 15 phosphorus 2DEG. Black phosphorus thus joins the selected few materials 5,7,8,11 to become the only 2D atomic crystal apart from graphene 9,10 having requisite material quality to show QH effect.We constructed the van der Waals heterostructure using the dry-transfer technique described in ref. 31. We first cleaved graphite and h-BN flakes onto SiO2/Si wafers, and black phosphorus flake onto poly-propylene carbon (PPC) film. The black phosphorus flake on the PPC film was then used to pick up the h-BN flake on the SiO2/Si wafer. Finally, the black phosphor...