Gate Tunable Supercurrent in Josephson Junctions Based on Bi_2Te_3 Topological Insulator Thin Films

We report transport measurements on Josephson junctions consisting of Bi_2Te_3 topological insulator(TI) thin films contacted by superconducting Nb electrodes.For a device with junction length L=134 nm,the critical supercurrent I_c can be modulated by an electrical gate which tunes the carrier type and density of the TI film.I_c can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state.In the p-type regime the Josephson current can be well described by a short ballistic junction model.In the n-type regime the junction is ballistic at 0.7 K T 3.8 K while for T 0.7 K the diffusive bulk modes emerge and contribute a larger I_c than the ballistic model.We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p-n junctions.Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.     

High-Quality Bi_2Te_3 Single Crystalline Films on Flexible Substrates and Bendable Photodetectors

Recently,great efforts have been made in the fabrication of arbitrary warped devices to satisfy the requirement of wearable and lightweight electronic products.Direct growth of high crystalline quality films on flexible substrates is the most desirable method to fabricate flexible devices owing to the advantage of simple and compatible preparation technology with current semiconductor devices,while it is a very challenging work,and usually amorphous,polycrystalline or discontinuous single crystalline films are achieved.Here we demonstrate the direct growth of high-quality Bi_2 Te_3 single crystalline films on flexible polyimide substrates by the modified hot wall epitaxy technique.Experimental results reveal that adjacent crystallites are coherently coalesced to form a continuous film,although amounts of disoriented crystallites are generated due to fast growth rate.By inserting a quartz filter into the growth tube,the number density of disoriented crystallites is effectively reduced owing to the improved spiral interaction.Furthermore,flexible Bi_2 Te_3 photoconductors are fabricated and exhibit strong near-infrared photoconductive response under different degrees of bending,which also confirms the obtained flexible 61 ms suitable for electronic applications.     

Effects of Thickness and Polarization Field on the Photovoltaic Properties of BiFeO3 Thin Films

We study the effects of film thickness on lattice parameters, direct band gap and photovoltaic outputs in the sol-gel derived BiFeO3 thin films. With the change of the film thickness, the great transitions will take place in the preferred orientation and lattice parameters. Furthermore, the photovoltaic outputs are significantly dependent on the film thickness. The results show that the open circuit voltage gradually increases and the short circuit current reciprocally decreases with the increase of film thickness. In particular, we demonstrate for the first time that there are tunable photovoltaic outputs with external electric field polarization switching in the polycrystalline BiFeO3 film, which is critical for the future device applications based on the photovoltaic properties of BiFeO3 films.     

厚度对未掺杂ZnO薄膜光谱性能的影响

X射线衍射光谱、拉曼光谱和紫外可见透射光谱技术是薄膜材料检测的重要技术手段.通过对薄膜材料光谱性能的分析,可以获得薄膜材料的物相、晶体结构和透光性能等信息.为了解厚度对未掺杂ZnO薄膜的X射线衍射光谱、拉曼光谱和紫外可见透射光谱性能的影响,利用溶胶-凝胶法在石英衬底上旋涂制备了不同厚度的未掺杂ZnO薄膜样品,并对薄膜样...     

Transport properties of doped Bi_2Se_3 and Bi_2Te_3 topological insulators and heterostructures

In this review article, the recent experimental and theoretical research progress in Bi_2Se_3-and Bi_2Te_3-based topological insulators is presented, with a focus on the transport properties and modulation of the transport properties by doping with nonmagnetic and magnetic elements. The electrical transport properties are discussed for a few different types of topological insulator heterostructures, such as heterostructures formed by Bi_2Se_3-and Bi_2Te_3-based binary/ternary/quaternary compounds and superconductors, nonmagnetic and magnetic metals, or semiconductors.     

Effects of post-annealing on crystalline and transport properties of Bi_2Te_3 thin films

A well-established method is highly desirable for growing topological insulator thin films with low carrier density on a wafer-level scale. Here, we present a simple, scalable method based on magnetron sputtering to obtain high-quality Bi_2 Te_3 films with the carrier density down to 4.0 × 10~(13) cm~(-2). In contrast to the most-used method of high substrate temperature growth, we firstly sputtered Bi_2 Te_3 thin films at room temperature and then applied post-annealing. It enables the growth of highly-oriented Bi_2 Te_3 thin films with larger grain size and smoother interface. The results of electrical transport show that it has a lower carrier density as well as a larger coherent length(～ 228 nm, 2 K). Our studies pave the way toward large-scale, cost-effective production of Bi_2 Te_3 thin films to be integrated with other materials in wafer-level scale for electronic and spintronic applications.     

高压合成AgSbTe2-Sb2Te3的热电性能研究

 利用高压方法,合成了富Sb₂Te₃的AgSbTe₂热电材料(AgSbTe₂)_1-x(Sb₂Te₃)_x (0≤x≤0.3),并对其结构和热电性质进行了研究。结果表明：(AgSbTe₂)_1-x(Sb₂Te₃)_x样品为近单相的AgSbTe₂材料;随着制备压力和Sb₂Te₃掺杂量的增加,(AgSbTe₂)_1-x(Sb₂Te₃)_x的电阻率大幅降低;Seebeck系数在高压作用下变小,而少量掺杂Sb₂Te₃却能提高Seebeck系数;在高压和微量掺杂Sb₂Te₃的共同作用下,AgSbTe₂的功率因子得到了提高;2.0 GPa高压下,制备的Ag_0.9Sb_1.1Te_2.1的品质因子达到0.466,接近Bi₂Te₃的品质因子。     

Fundamental and progress of Bi_2Te_3-based thermoelectric materials

Thermoelectric materials,enabling the directing conversion between heat and electricity,are one of the promising candidates for overcoming environmental pollution and the upcoming energy shortage caused by the over-consumption of fossil fuels.Bi_2Te_3-based alloys are the classical thermoelectric materials working near room temperature.Due to the intensive theoretical investigations and experimental demonstrations,significant progress has been achieved to enhance the thermoelectric performance of Bi_2Te_3-based thermoelectric materials.In this review,we first explored the fundamentals of thermoelectric effect and derived the equations for thermoelectric properties.On this basis,we studied the effect of material parameters on thermoelectric properties.Then,we analyzed the features of Bi_2Te_3-based thermoelectric materials,including the lattice defects,anisotropic behavior and the strong bipolar conduction at relatively high temperature.Then we accordingly summarized the strategies for enhancing the thermoelectric performance,including point defect engineering,texture alignment,and band gap enlargement.Moreover,we highlighted the progress in decreasing thermal conductivity using nanostructures fabricated by solution grown method,ball milling,and melt spinning.Lastly,we employed modeling analysis to uncover the principles of anisotropy behavior and the achieved enhancement in Bi_2Te_3,which will enlighten the enhancement of thermoelectric performance in broader materials     

膜层厚度对蓝宝石衬底上生长的ITO薄膜性质的影响

采用磁控溅射的方法在蓝宝石衬底上制备了氧化铟锡(ITO)透明氧化物薄膜;研究了不同厚度薄膜的结构、光学和电学特性。经X射线衍射(XRD)测量,发现在蓝宝石衬底上生长的ITO薄膜呈现了较高的(222)择优取向;随着膜层厚度的增加,该衍射峰对应的2θ衍射角逐渐向大角度方向移动,同时该衍射峰的半峰全宽逐渐减小,平均晶粒尺寸增大。 经光学透射光谱测量,发现随着膜层厚度的增加,光学透过率逐渐减小。膜层厚度为0.2 μm时,可见光透过率超过80%,当膜层厚度为0.8 μm时,可见光透过率下降到60%。电学测量结果表明,随着膜层厚度的增加,薄膜电阻率逐渐减小。膜层厚度为0.2 μm时,电阻率为9×10^-4 Ω·cm, 膜层厚度为0.8 μm时,电阻率为5.5×10^-4 Ω·cm。     

Influence of vacuum degree on growth of Bi_2Te_3 single crystal

Bi2Te3single crystals were prepared by the solid-state reaction method. The effect of the vacuum on the growth of Bi2Te3 single crystals was studied with varying the oxygen content by controlling the air pressure in the silica tube. High quality Bi2Te3 single crystals have been obtained and there is no influence on the growth by an extremely small amount of oxygen in a high vacuum at 1.0 × 10-3Pa. As the air pressure is increased at 1.0 × 10-2Pa, oxygen only mainly impacts on the growth of the surface for the prepared samples. Micron-sized rod-like structure and flower-like clusters are observed on the surface. For the samples prepared at 1.0 × 10-1Pa, x-ray diffraction data show that the yellow part on the surface is Bi2 Te O5, while the Bi2Te3 single crystal is still the major phase as the inside part. More interestingly, various crystal morphologies are observed by scanning electron microscope for Bi2Te3 near the boundary between Bi2Te3 and Bi2Te O5.Possible growth mechanisms for Bi2Te3 with different morphologies are discussed in detail.     

Effects of Annealing Temperature on Structural and Optical Properties of ZnO Thin Films

The effects of annealing temperature on the structural and optical properties of ZnO films grown on Si （100） substrates by sol-gel spin-coating are investigated. The structural and optical properties are characterized by x-ray diffraction, scanning electron microscopy and photoluminescence spectra. X-ray diffraction analysis shows the crystal quality of ZnO films becomes better after annealing at high temperature. The grain size increases with the temperature increasing. It is found that the tensile stress in the plane of ZnO films first increases and then decreases with the annealing temperature increasing, reaching the maximum value of 1.8 GPa at 700℃. PL spectra of ZnO films annealed at various temperatures consists of a near band edge emission around 380 nm and visible emissions due to the electronic defects, which are related to deep level emissions, such as oxide antisite （OZn）, interstitial oxygen （Oi）, interstitial zinc （Zni） and zinc vacancy （VZn^-）, which are generated during annealing process. The evolution of defects is analyzed by PL spectra based on the energy of the electronic transitions.     

Yb-doped passively mode-locked fiber laser with Bi_2Te_3-deposited

In this study we present an all-normal-dispersion Yb-doped fiber laser passively mode-locked with topological insulator(Bi2Te3) saturable absorber. The saturable absorber device is fabricated by depositing Bi2Te3 on a tapered fiber through using pulsed laser deposition(PLD) technology, which can give rise to less non-saturable losses than most of the solution processing methods. Owing to the long interaction length, Bi2Te3 is not exposed to high optical power, which allows the saturable absorber device to work in a high power regime. The modulation depth of this kind of saturable absorber is measured to be 10%. By combining the saturable absorber device with Yb-doped fiber laser, a mode-locked pulse operating at a repetition rate of 19.8 MHz is achieved. The 3-d B spectral width and pulse duration are measured to be 1.245 nm and317 ps, respectively.     

Temperature and Thickness Effects on Electrical Properties of InP Films Deposited by Spray Pyrolysis

InP film samples are prepared by spray pyrolysis technique using aqueous solutions of InCl₃ and Na₂HPO₄, which are atomized with compressed air as carrier gas onto glass substrates at 500°C with different thicknesses of the films. The structural properties of the samples are determined by x-ray diffraction (XRD). It is found that the crystal structure of the InP films is polycrystalline hexagonal. The orientations of all the obtained films are along the c-axis perpendicular to the substrate. The electrical measurements of the samples are obtained by dc four-probe technique on rectangular-shape samples. The effects of temperature on the electrical properties of the InP films are studied in detail.     

Molecular beam epitaxy of superconducting PdTe_2 films on topological insulator Bi_2Te_3

Majorana fermions have been observed in topological insulator/s-wave superconductor heterostructures. To manipulate Majorana fermions, superconducting materials should be deposited on the surfaces of topological insulators. In this study, highquality superconducting PdTe_2 films are deposited on the topological insulator Bi_2Te_3 surface using molecular beam epitaxy. The surface topography and electronic properties of PdTe_2/Bi_2Te_3 heterostructures are investigated via in situ scanning tunneling microscopy/spectroscopy. Under Te-rich conditions, the Pd atoms presumably form PdTe_2 film on Bi_2Te_3 surface rather than diffuse into Bi_2Te_3. The superconductivity of the PdTe_2/Bi_2Te_3 heterostructure is detected at a transition temperature of ～1.4 K using the two-coil mutual inductance technique. This study proposes a method for fabricating superconducting materials on topological insulator surfaces at low doping levels, paving ways for designing nanodevices that can manipulate Majorana fermions.     

硫化铋微晶掺杂薄膜的制备及光学性能的研究

用溶胶－凝胶法能够有效地制备含０．５％的具有量子尺寸效应的Ｂｉ２Ｓ３微晶掺杂硅薄膜。薄膜的室温透射光谱发现，在５００℃热处理时，随着热处理时间的延长，薄膜的特征透射谱谷会发生向长波方向的移动，作者认为这样的移动于量子尺寸效应。本文还报道了在ＹＡＧ强激光的作用下能够观察到薄膜样品所产生的倍频信号，并讨论了产生倍频的原因。     

Structural and Thermoelectric Properties of Optically Transparent Thin Films Based on Single-Walled Carbon Nanotubes

Physics of the Solid State - Thin films have been produced via a spray method from commercially available single-walled carbon nanotubes (SWCNTs). A SWCNT film thickness has ranged from ~10 to ~80...     

应变对La_(1-x)Sr_xMnO_3薄膜的物理性质的影响(英文)

Both tensile strain and compressive strain effects on the properties of La_(1-x)Sr_xMnO_3 (x=0.1) films were investigated.The films on SrTiO_3 (100) display 'unusual' tensile strain~([1]), which supports a ferromagnetic metallic behavior when film thickness is larger than 10nm.The films on NdGaO_3 (100) presenting compressive strain, on the other hand, demonstrate strongly enhanced insulating behavior.Inordertoobtainunambiguousresults,allfilmsampleswerepreparedusingthesamedepositionconditi…     

Spatially resolved gap closing in single Josephson junctions constructed on Bi_2Te_3 surface

Full gap closing is a prerequisite for hosting Majorana zero modes in Josephson junctions on the surface of topological insulators.Previously,we have observed direct experimental evidence of gap closing in Josephson junctions constructed on Bi_2Te_3 surface.In this paper we report further investigations on the position dependence of gap closing as a function of magnetic flux in single Josephson junctions constructed on Bi_2Te_3 surface.     

Cu掺杂Ga2O3薄膜的光学性能

采用射频磁控溅射和N2气氛退火处理制备了多晶Ga2O3薄膜和Cu掺杂Ga2O3薄膜.用X射线衍射仪、紫外-可见分光光度计、荧光光谱仪对Ga2O3薄膜和Cu掺杂Ga2O3薄膜的结构和光学性能进行了表征.结果表明,Cu掺杂后Ga2O3薄膜的结晶质量变差,透过率明显降低,吸收率增加,光学带隙减小.本征Ga2O3薄膜在紫外、蓝光和绿光出现了发光带,Cu掺杂后紫外和蓝光发射增强,且在475nm处出现了一个新的发光峰.     

Topological superconductivity in a Bi_2Te_3/NbSe_2 heterostructure:A review

Topological superconductors carry globally protected gapless boundary excitations, which are robust under local perturbations, and thus exhibit both fundamental and applicational importance. An unconventional pairing with p-wave symmetry, such as a nontrivial topology in the superconductor's wavefunction, is required to generate a non-zero Berry phase. Until now, the Bi_2 Te_3/2 H–NbSe_2 heterostructure has proven to be a practical way to realize a topological superconductor in real materials. This complex system, where odd numbers of spin-momentum locked Dirac cone surface states on the topological insulator Bi_2 Te_3 become superconducting, induced by the proximate effect from the underlying s-wave superconductor 2 H–NbSe_2, realizes an effective two-dimensional(2 D) spinless p_x + i p_y topological superconductor. In this review article, we summarize the recent experimental progress of the successful synthesis of Bi_2 Te_3/2 H–NbSe_2 heterostructures using molecular beam epitaxy, determining the thickness limit of the heterostructure, detecting the long thought Majorana quasiparticle inside a magnetic vortex core state by means of scanning tunneling microscopy and demonstration of the unique spatial and spin properties of a Majorana zero mode.