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.

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Heterostructure based interface engineering has been proved an effective method for finding new superconducting systems and raising superconductivity transition temperature (T C ) 1-7 . In previous work on one unit-cell (UC) thick FeSe films on SrTiO 3 (STO) substrate, a superconducting-like energy gap as large as 20 meV 8 , was revealed by in situ scanning tunneling microscopy/spectroscopy (STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of 65 ± 5 K 9-11 . If this transition is indeed the superconducting transition, then the 1-UC FeSe represents the thinnest high T C superconductor discovered so far. However, up to date direct transport measurement of the 1-UC FeSe films has not been reported, mainly because growth of large scale 1-UC FeSe films ischallenging and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we successfully prepared 1-UC FeSe films on insulating STO substrates with non-superconducting FeTe protection layers. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset T C above 40 K and a extremely large critical current density J C ~ 1.7×10 6 A/cm 2 at 2 K. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.The FeSe films and FeTe protection layer are grown by molecular beam epitaxy (MBE) (see Methods).

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,

Large-scale hydrophilic Fe3O4 nanoparticles (NPs) were prepared in the presence of citrate and sodium nitrate via a facile method. The Fe3O4 NPs are quite stable and can be freely dispersed in water. The as-prepared magnetic nanoparticle solution can be stable for more than 1 month. The mean diameter of the Fe3O4 NPs can be controlled in the range of ∼20 to ∼40 nm in mean diameter. The NPs show superparamagnetic properties with a relatively high saturation magnetization moment 58 emu/mg at room temperature. Furthermore, a possible formation mechanism is proposed to explain why the magnetic nanoparticles are very well soluble in water.

Monodisperse Au, Ag, and Au3Pd nanoparticles (NPs) with narrow size distribution are prepared by direct reaction of the related metal salt with oleylamine in toluene. Oleylamine serves as both a reducing agent and a surfactant in the synthesis. The sizes and shape of these NPs are tuned by reaction temperatures. The hydrophobic oleylamine-coated NPs can be made water soluble by replacing oleylamine with 3-mercaptopropionic acid. Both surface plasmonic resonance (SPR) and surface enhanced Raman scattering (SERS) observed from the Au and Ag NPs are found to be NP size- and surface-dependent.

Silica coated magnetite (Fe3O4@SiO2) core-shell nanoparticles (NPs) with controlled silica shell thicknesses were prepared by a modified Stöber method using 20 nm hydrophilic Fe3O4 NPs as seeds. The core-shell NPs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED), and UV-Vis adsorption spectra (UV-Vis). The results imply that NPs consist of a crystalline magnetite core and an amorphous silica shell. The silica shell thickness can be controlled from 12.5 nm to 45 nm by varying the experimental parameters. The reaction time, the ratio of TEOS/Fe3O4, and the concentration of hydrophilic Fe3O4 seeds were found to be very influential in the control of silica shell thickness. These well-dispersed core-shell Fe3O4@SiO2 NPs show superparamagnetic properties at room temperature.

We elucidate the existing controversies in the newly discovered K-doped iron selenide (K x Fe 2y Se 2-z ) superconductors. The stoichiometric KFe 2 Se 2 with √2×√2 charge ordering was identified as the parent compound of K x Fe 2-y Se 2-z superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe 2 Se 2 by either Se vacancies or interacting with the anti-ferromagnetic K 2 Fe 4 Se 5 compound. Totally four phases were found to exist in K x Fe 2-y Se 2-z : parent compound KFe 2 Se 2 , superconducting KFe 2 Se 2 with √2×√5 charge ordering, superconducting KFe 2 Se 2-z with Se vacancies and insulating K 2 Fe 4 Se 5 with √5×√5 Fe vacancy order. The phase separation takes place at the mesoscopic scale under standard molecular beam epitaxy condition.