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.

Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cu-and Fe-based high temperature superconductors.In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO 3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (T c ) at 65±5 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-T c , make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications.

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention . How to further increase the superconducting transition temperature ( T c ) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T c superconductivity in a single-layer FeSe is therefore both surprising and signifi cant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T c ~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T c superconductivity in iron-based superconductors.

Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

The quantum anomalous Hall (QAH) effect, which has been realized in magnetic topological insulators (TIs), is the key to applications of dissipationless quantum Hall edge states in electronic devices. However, investigations and utilizations of the QAH effect are limited by the ultralow temperatures needed to reach full quantization-usually below 100 mK in either Cr- or V-doped (Bi,Sb) Te of the two experimentally confirmed QAH materials. Here it is shown that by codoping Cr and V magnetic elements in (Bi,Sb) Te TI, the temperature of the QAH effect can be significantly increased such that full quantization is achieved at 300 mK, and zero-field Hall resistance of 0.97 h/e is observed at 1.5 K. A systematic transport study of the codoped (Bi,Sb) Te films with varied Cr/V ratios reveals that magnetic codoping improves the homogeneity of ferromagnetism and modulates the surface band structure. This work demonstrates magnetic codoping to be an effective strategy for achieving high-temperature QAH effect in TIs.

Quantum anomalous Hall (QAH) effect in magnetic topological insulator (TI) is a novel transport phenomenon in which theThe realization of QAH effect requires that a two-dimensional (2D) material must be FM, topological, and insulating simultaneously 9 . Magnetically doped TIs have been proposed 1, 2, 10-12 and experimentally proved 3-6 to be an ideal material system for fulfilling these stringent requirements. For a 3D TI, the inverted bulk band structure ensures topologically protected metallic surface states (SSs), which become 2D when the film is sufficiently thin 13 . The spontaneous FM order induced by magnetic doping not only leads to the anomalous Hall effect, but also opens an energy gap at the Dirac point. When the Fermi level (E F ) lies within this gap, the only remaining conduction channel is the quasione-dimensional chiral edge state, which gives rise to quantized Hall resistance and vanishing longitudinal resistance at zero magnetic field 3, 14 . Up to date, the QAH effect has been observed in Cr or V doped (Bi,Sb) 2 Te 3 TI thin films with accurately controlled chemical composition and thickness grown by molecular beam epitaxy (MBE) 3-6 .The MBE-grown QAH insulator film studied here has a chemical formula Fig. 1a is a schematic drawing of the transport device, which is similar to that reported previously 3 .The film is manually scratched into a Hall bar geometry, and the SrTiO 3 substrate is used as the bottom gate oxide due to its large dielectric constant at low temperature. The Cr concentration, hence the density of local moment, is higher than that in the sample where the QAH effect was originally discovered 3 . As a result, the FM order forms at a higher Curie temperature T C = 24 K as determined by the temperature dependent anomalous Hall effect (supplementary Fig. S1). Another important consequence of higher Cr doping is that the sample becomes more disordered, which is crucial to the physics that will be discussed in this work.We first demonstrate the existence of QAH effect in this sample. Fig. 1b displays the gate voltage (V g ) dependence of the Hall resistance  yx (blue curve) and longitudinal resistance  xx (red curve) measured at T = 10 mK in a strong magnetic field B = 12 T applied perpendicular to the film. The  yx exhibits a plateau for -10 V < V g < 10 V with its maximum value close to 99.1% of the quantum resistance h/e 2 ~ 25.8 k. In the same V g range  xx shows a pronounced dip with its minimum value close to 0.1 h/e 2 . To show that the apparent Hall quantization in Fig. 1b is due to the QAH effect rather than conventional QH effect in high magnetic field, in Fig. 1c we display the field dependence of  yx measured at V g = -5 V, when  xx reaches a minimum in Fig. 1b. The Hall trace shows an abrupt jump at zero magnetic field, characteristic of the anomalous Hall effect.With increasing magnetic field, the  yx value increases gradually and approaches h/e 2 at 12 T. The  xx shown in Fig. 1d exhibits two sharp peaks at the coercive field H C , and decreases rapidly on b...

Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi1−xSbx)2Te3 films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually positive magnetoconductivity emerges when the electron system becomes strongly localized. This works reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.The concept of Anderson localization has profoundly influenced our understanding of electron conductivity [1]. While examples for disorder driven metal-insulator transition are abundant in three-dimensions (3D), the question of whether Anderson transitions exist in 2D has posed a lot of theoretical and experimental challenges. Scaling theory proposed by Abrahams et al. predicts that there are no truly metallic states in non-interacting 2D electron systems [2,3]. It was, however, discovered later that extended electron states may exist when electronelectron (e-e) interaction, spin-orbit coupling (SOC) or magnetic field comes in to play [4,5]. The 3D topological insulators (TIs) discovered in recent years [6,7] provide novel types of 2D electron systems that are of particular interest for study of the localization-delocalization problem. The Dirac surface states of 3D TIs are believed to be topologically protected from localization due to its special symmetry class [8][9][10][11][12]. Moreover, when a 3D TI thin film is sufficiently thin, the hybridization between the top and the bottom surface states opens an energy gap near the Dirac point [13], and it is suggested theoretically that the hybridization gap would drive the electron system to topologically different phase, such as a quantum spin Hall insulator or a trivial band insulator [14][15][16]. Even though a lot of work has been carried out on electron transport properties of TI thin films [17][18][19][20], the fate of such electron systems under the condition of strong disorder (or in other words, whether Anderson localization could take place), still remains unclear.In this work, we have studied electron transport in a large number of highly gate-tunable TI thin films with various thicknesses and chemical compositions. We found that only in ultrathin TI films in which surface hybridization and disorder effects are significant, hopping transport, a hallmark of strong localization [21][22][23], can be observed. The observed temperature and magnetic field dependences of conductivity suggest that electron transport can be driven from the diffusive transport governed by we...