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.

Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1-unit cell (UC) limit. The ferroelectric transition temperature T(c) of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

The charge states of single molecular magnetic chains were manipulated with a scanning tunneling microscope and identified by spin-flip inelastic tunneling spectroscopy. We show that the charged and neutral states have different spin structures and therefore exhibit different features associated with the spin-flip processes in tunneling spectra. The experiment demonstrates a general approach for detecting the charge states at the nanometer scale in a more straightforward manner than using indirect information.

KEYWORDS: NbSe 2 , transition-metal dichalcogenides, macro-size monolayer film, ultralow temperature and high magnetic field electrical transport, Ising superconductivity, quantum phase transition 2 ABSTRACT Two-dimensional (2D) transition metal dichalcogenides (TMDs) have a range of unique physics properties and could be used in the development of electronics, photonics, spintronics and quantum computing devices. The mechanical exfoliation technique of micro-size TMD flakes has attracted particular interest due to its simplicity and cost effectiveness. However, for most applications, large area and high quality films are preferred. Furthermore, when the thickness of crystalline films is down to the 2D limit (monolayer), exotic properties can be expected due to the quantum confinement and symmetry breaking. In this paper, we have successfully prepared macro-size atomically flat monolayer NbSe 2 films on bilayer graphene terminated surface of 6H-SiC(0001) substrates by molecular beam epitaxy (MBE) method. The films exhibit an onset superconducting critical transition temperature (T c onset ) above 6 K, 2 times higher than that of mechanical exfoliated NbSe 2 flakes. Simultaneously, the transport measurements at high magnetic fields reveal that the parallel characteristic field B c// is at least 4.5 times higher than the paramagnetic limiting field, consistent with Zeeman-protected Ising superconductivity mechanism. Besides, by ultralow temperature electrical transport measurements, the monolayer NbSe 2 film shows the signature of quantum Griffiths singularity when approaching the zero-temperature quantum critical point. TEXTQuasi-2D superconductors such as ultrathin films with thickness down to monolayer [1][2][3][4][5][6][7] 10, 11,13 , the coexistence of charge density wave (CDW) and the superconducting phase was observed down to the monolayer limit but the T c of monolayer NbSe 2 got significantly suppressed (less than 3.1 K) compared with its bulk value (7.2 K).Superconductor-insulator (metal) transition (SIT/SMT), a paradigm of quantum phase transition, is an important topic in condensed matter physics. In the 2D limit regime, the orbital effect is restricted in parallel magnetic field. Calculations show that in anisotropic superconductors, the FFLO state might lead to an enhancement of the upper critical field between 1.5 and 2.5 times of the Pauli paramagnetic limit field 32,35 . In perpendicular magnetic field cases, the characteristic field B c (0) of NbSe 2 film is estimated to be 2.87 T, smaller than the Pauli paramagnetic limit field (8.21 T). Besides, the 8 Maki parameter α⊥ = 0.44 is smaller than 1.8. In parallel magnetic field cases, the characteristic field B c// (0) ~ 37.22 T is at least 4.5 times of Pauli paramagnetic limit field, which exceeds the theoretical predictions of 1.5 ~ 2.5 times 32,35 . Therefore, the chance of the existence of FFLO state in monolayer NbSe 2 is little.The sample 3 (with T c onset ~ 6 K, Figure S5 For SIT, the sample critical resistance on phase transition is th...

Topological insulators are a new class of material 1,2 , that exhibit robust gapless surface states protected by time-reversal symmetry 3,4 . The interplay of such symmetry-protected topological surface states and symmetry-broken states (for example, superconductivity) provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions 5 , and Majorana zero modes 6 that may find application in faulttolerant quantum computation 7,8 . Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization 1,2 . Here, by growing high-quality topological insulator Bi 2 Se 3 films on a d-wave superconductor Bi 2 Sr 2 CaCu 2 O 8+δ using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi 2 Se 3 films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi 2 Sr 2 CaCu 2 O 8+δ , the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.The search for exotic quantum phenomena and new functionalities has been among the most tremendous driving forces for the fields of condensed-matter physics and materials science. Majorana zero modes, that is, Majorana fermions that are their own antiparticles and occur at exactly zero energy, are particularly fascinating not only because of their intriguing physics obeying robust non-Abelian statistics, but also owing to their potential application as building blocks for topological quantum computers 7,8 . Although significant progress has been made recently in one-dimensional semiconductor quantum wires coupled with conventional superconductors 9-12 , decisive evidence of Majorana zero modes has been lacking and many puzzles remain 13 . Topological insulators, whose hallmark is time-reversal-symmetryprotected surface states, may offer less restrictive experimental conditions for realizing Majorana zero modes 1,2 . Theoretically, Majorana zero modes are predicted to occur in vortex cores of three-dimensional topological insulators when they are in close proximity to conventional s-wave superconductors 6 ; however,

The high carrier mobility of graphene is key to its applications, and understanding the factors that limit mobility is essential for future devices. Yet, despite significant progress, mobilities in excess of the 2×10(5) cm(2) V(-1) s(-1) demonstrated in free-standing graphene films have not been duplicated in conventional graphene devices fabricated on substrates. Understanding the origins of this degradation is perhaps the main challenge facing graphene device research. Experiments that probe carrier scattering in devices are often indirect, relying on the predictions of a specific model for scattering, such as random charged impurities in the substrate. Here, we describe model-independent, atomic-scale transport measurements that show that scattering at two key defects--surface steps and changes in layer thickness--seriously degrades transport in epitaxial graphene films on SiC. These measurements demonstrate the strong impact of atomic-scale substrate features on graphene performance.

Tunneling spectra for individual atoms and dimers of Mn and Cr adsorbed on superconducting Pb thin films were measured by a low temperature scanning tunneling microscope. Multiple-resonance structures within the superconducting gap on the adsorbates were resolved and interpreted as the magnetic impurity-induced bound states associated with different scattering channels. The experiment demonstrates a spectroscopic approach to characterizing the spin states of magnetic structures and exploring the competition between superconductivity and magnetism at the nanometer scale.