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.
MCM-41/ZSM-5 composites were prepared using a dual templating method through a process of two-step crystallization. Mesoporous MCM-41 was first synthesized using the self-assembling of surfactant cetyltrimethylammonium bromide and subsequently the amorphous wall of MCM-41 was recrystallized with a structure-directing agent tetrapropylammonium bromide, which was introduced into the MCM-41 wall through a pretreatment process. A solid to solid-phase transformation mechanism was presented for the recrystallization of MCM-41 framework. Two kinds of stable MCM-41/ZSM-5 composites can be synthesized during the course of recrystallization. Crystallized mesoporous MCM-41 containing only short-range ordered ZSM-5 structure was first synthesized in the early stage of the recrystallization. With the increase of recrystallization time, some discrete micron-sized ZSM-5 crystals were produced and firmly attached to the loose aggregates of crystallized MCM-41, and another kind of MCM-41/ZSM-5 composite containing interconnected mesopore and micropore was therefore obtained. Because of improved acidity and a 2-fold pore system, both MCM-41/ZSM-5 composites are more advantageous than amorphous MCM-41 and a mechanical mixture of MCM-41 and ZSM-5 in acid catalysis.
We propose a general theory on the standing waves (quasiparticle interference pattern) caused by the scattering of surface states off step edges in topological insulators, in which the extremal points on the constant energy contour of surface band play the dominant role. Experimentally we image the interference patterns on both Bi2Te3 and Bi2Se3 films by measuring the local density of states using a scanning tunneling microscope. The observed decay indices of the standing waves agree excellently with the theoretical prediction: In Bi2Se3, only a single decay index of −3/2 exists; while in Bi2Te3 with strongly warped surface band, it varies from −3/2 to −1/2 and finally to −1 as the energy increases. The −1/2 decay indicates that the suppression of backscattering due to time-reversal symmetry does not necessarily lead to a spatial decay rate faster than that in the conventional two-dimensional electron system. Our formalism can also explain the characteristic scattering wave vectors of the standing wave caused by non-magnetic impurities on Bi2Te3. PACS numbers: 73.20.-r 68.37.Ef 73.43.Cd 72.10.FkThe discovery of topological insulators (TIs) has attracted a great deal of attention [1][2][3][4][5][6][7][8][9][10][11][12]. The threedimensional TIs are characterized by the gapped bulk states and gapless surface states (SSs), which are protected by time-reversal symmetry (TRS) and consist of an odd number of spin-helical Dirac cones. Exotic effects such as Majorana fermions [13,14] and magnetic monopole [15] are predicted to exist as results of the topological SSs.The low-temperature scanning tunneling microscope (STM) and spectroscopy (STS) provide a direct way to study the SSs through probing the local density of states (LDOS) oscillations in the vicinity of impurities or step edges [16]. The quasiparticle interference (QPI) patterns induced by non-magnetic impurities on the surface of Bi x Sb 1−x [17] and Bi 2 Te 3 [18], together with subsequent theoretical analysis [19][20][21][22][23][24], demonstrated the absence of backscattering for the topological SSs. Meanwhile, the LDOS oscillations of SSs near step edges on Bi 2 Te 3 showed a power-law decay with index −1 in a certain energy range [25], compared to −1/2 for the conventional two-dimensional electron system (2DES) [16]. The faster decay of QPI once again indicates the suppression of backscattering in TIs.Despite the intensive investigation, a complete understanding of QPI on the surface of TIs remain elusive partially due to the warping effect of the Dirac cone [8,26]. The warping effect of the SSs results from not only the anisotropic surface band dispersion, but also the coupling between the surface and bulk bands. In this Letter, we present a general formalism to account for the complex scattering geometry. We propose the interference patterns are dominated by the extremal points on the constant energy contour (CEC) of 2D electron band. In ap-plying the theory to Bi 2 Te 3 with strong warping effect, we show that the decay index varies from −3/2 to −1/2 an...
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