退火对NiCo薄膜各向异性磁电阻(AMR)的影响

用磁控溅射法制备了以NiFeCr和Ta分别为缓冲层的两种NiCo薄膜样品,在200、300、400℃温度下对两种样品退火.结果表明,不同的退火温度导致样品AMR值不同,在退火200℃时样品AMR值最大,超过200℃退火磁电阻急剧下降.XRD和摇摆曲线的结果都表明,退火后晶粒的平均晶粒尺寸都会长大,晶内缺陷减少.振动样品磁强计(VSM)测量的结果显示,退火后样品的磁矩有所减小,矫顽力增大,表明缓冲层与NiCo磁薄膜之间有扩散现象.     

Atomically smooth ultrathin films of topological insulator Sb2Te3

The growth and characterization of single-crystalline thin films of topological insulators (TIs) is an important step towards their possible applications. Using in situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we show that moderately thick Sb₂Te₃ films grown layer-by-layer by molecular beam epitaxy (MBE) on Si(111) are atomically smooth, single-crystalline, and intrinsically insulating. Furthermore, these films were found to exhibit a robust TI electronic structure with their Fermi energy lying within the energy gap of the bulk that intersects only the Dirac cone of the surface states. Depositing Cs in situ moves the Fermi energy of the Sb₂Te₃ films without changing the electronic band structure, as predicted by theory. We found that the TI behavior is preserved in Sb2Te3 films down to five quintuple layers (QLs).      

Liquid-gated ambipolar transport in ultrathin films of a topological insulator Bi2Te3

Using ionic-liquid (IL) gating in electric-double-layer transistors (EDLTs), we investigate field-effect electrical transport properties of ultrathin epitaxial films of a topological insulator (TI), Bi(2)Te(3). Because of their extreme thinness, the Bi(2)Te(3) films show a band gap opening and resulting semiconducting transport properties. Near room temperature, an obvious ambipolar transistor operation with an ON-OFF ratio close to 10(3) was observed in the transfer characteristics of liquid-gated EDLTs and further confirmed by a sign change of the Hall coefficients. Modulation of the electronic states and a phase transition from a semiconducting conduction (dR(xx)/dT < 0) to a metallic transport (dR(xx)/dT > 0) were observed in the temperature-dependent resistance of the ultrathin Bi(2)Te(3) channel, demonstrating that the liquid gating is an effective way to modulate the electronic states of TIs.     

Anomalous magnetoresistance in polycrystalline high-purity tin wires

The magnetoresistance of polycrystalline high-purity tin wires has been measured in the temperature range 3.8–4.2 K in fields up to 20 kOe. In the case of transverse magnetic field, we observe deviations from Kohler's rule of a few percent, which differ from the generally observed deviations. On the contrary, the deviations in the longitudinal case agree with those generally observed. The origin of the anomalous deviations in the transverse case (also observed by J. F. Kos in indium above 7 K)is not yet understood, although the influence of the size effect seems evident.Work supported by the Fonds National Suisse de la Recherche Scientifique.     

Experimental evidence of the thickness- and electric-field-dependent topological phase transitions in topological crystalline insulator SnTe(111) thin films

Nano Research - Using in situ angle-resolved photoemission spectroscopy, we systematically studied molecular-beam-epitaxy-grown topological crystalline insulator SnTe(111) thin films with varied...     

Manipulating surface states in topological insulator nanoribbons

Topological insulators display unique properties, such as the quantum spin Hall effect, because time-reversal symmetry allows charges and spins to propagate along the edge or surface of the topological insulator without scattering. However, the direct manipulation of these edge/surface states is difficult because they are significantly outnumbered by bulk carriers. Here, we report experimental evidence for the modulation of these surface states by using a gate voltage to control quantum oscillations in Bi(2)Te(3) nanoribbons. Surface conduction can be significantly enhanced by the gate voltage, with the mobility and Fermi velocity reaching values as high as ~5,800?cm(2)?V(-1)?s(-1) and ~3.7?×?10(5)?m?s(-1), respectively, with up to ~51% of the total conductance being due to the surface states. We also report the first observation of h/2e periodic oscillations, suggesting the presence of time-reversed paths with the same relative zero phase at the interference point. The high surface conduction and ability to manipulate the surface states demonstrated here could lead to new applications in nanoelectronics and spintronics.     

Thickness-dependent magneto-transport of Bi2Se3/SiO2 topological insulator thin films

Topological insulators are immensely investigated for their surface states related properties as these materials can be used for various spintronics, quantum computing, and optoelectronics applications. In this perspective, different thicknesses of bismuth selenide thin films are deposited on the 250 nm SiO₂ substrate with the help of thermal deposition. The motive of this study is to investigate the surface and bulk-related behaviour with different thicknesses. The deposited films are characterized through GI-XRD (grazing incidence X-ray diffractometer) and Raman spectroscopy, which ensure the impurity-less deposition. Further, the transport properties are investigated, using Hikami–Larkin–Nagaoka (HLN) model in low field regime (up to 1 Tesla). Also, a quadratic term in field and a constant term added in HLN model to investigate the occurrence of quantum scattering and defects in thin films and this shows thickness dependence of weak anti-localization effect (WAL) in the present Bi₂Se₃/SiO₂ thin films.

     

In situ Raman spectroscopy of topological insulator Bi2Te3 films with varying thickness

Topological insulators （TIs） are a new state of quantum matter with a band gap in bulk and conducting surface states. In this work, the Raman spectra of topological insulator Bi2Te3 films prepared by molecular beam epitaxy （MBE） have been measured by an in situ ultrahigh vacuum （UHV）-MBE-Raman spectroscopy system. When the thickness of Bi2Te3 films decreases from 40 quintuple-layers （QL） to 1 QL, the spectral characteristics of some Raman modes appearing in bulk Bi2Te3 vary and a new vibrational mode appears, which has not been reported in previous studies and might be related to quantum size effects and symmetry breaking. In addition, an obvious change was observed at 3 QL when a Dirac cone formed. These results offer some new information about the novel quantum states of TIs.     

Ni81Fe19各向异性磁电阻薄膜的工艺和微结构的研究

在不同本底真空和工作气压下制备了Ta／Ni81Fe19薄膜，并进行了磁性测量和原子力显微镜分析。结果表明较高的本底真空和较低的工作气压下制备的薄膜有较大的各向异性磁电阻值△R／R。其原因是高本底真空和低工作气压导致大晶粒尺寸和低粗糙度。进而降低电子的散射．减小电阻R，增大各向异性磁电阻△R／R。在不同真空度下剞备了(Ni81Fel9)64Cr36／Ni81Fe19薄膜，与以Ta为衬底的薄膜相比，具有更大的各向异性磁电阻值，达到了3．2％，这是国内目前报道的最大值，但它受真空度的影响更大。     

Structure and topological transport in Pb-doping topological crystalline insulator SnTe(001) film

Topological crystalline insulator(TCI) as a new type of topological materials has attracted extensive research interests for its tunable topological properties. Due its symmetry topological protection essence,the structure investigation provides a solid basement for tuning its topological transport properties.On Sr TiO_3(111) substrate, the Sn Te film was found to be epitaxial growth only along [001] while not[111] direction. The detailed structural study was performed and a structural model was proposed to elucidate epitaxial growth of the Sn Te(001) film. The transport properties of Sn Te(001) film were further investigated and a typical weak anti-localization effect was observed. By Pb-doping into Sn Te, the bulk carriers were inhibited and its topological surface states were strengthened to induce the enhanced surface transport contribution. With tunable multiple transport channels from the even Dirac cones, the TCI Sn Te film systems will have the potential application in future spintronics devices.     

基片表面粗糙度对坡莫合金AMR以及磁性能的影响

利用霍尔离子源轰击Si基片获得了不同表面粗糙度的基片,然后利用磁控溅射方法制备了一系列Ta(5nm)/Ni 80Fe 20(12nm)/Ta(2nm)薄膜样品,重点研究了基片表面粗糙度对薄膜结构和各向异性磁阻效应的影响。采用AFM测量基片的表面粗糙度,采用四探针法测量薄膜的各向异性磁阻效应。研究结果表明,随着基片表面粗糙度的增加,坡莫合金的AMR值显著降低,且ΔH显著增加。     

Control over topological insulator photocurrents with light polarization

Three-dimensional topological insulators represent a new quantum phase of matter with spin-polarized surface states that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission and tunnelling spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions and spin-polarized electrical currents. However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here, we show that illuminating the topological insulator Bi(2)Se(3) with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators and lead to novel opto-spintronic devices.     

Josephson supercurrent through a topological insulator surface state

The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. An essential step in the hunt for this emergent particle is the unequivocal observation of supercurrent in a topological phase. Here, direct evidence for Josephson supercurrents in superconductor (Nb)-topological insulator (Bi(2)Te(3))-superconductor electron-beam fabricated junctions is provided by the observation of clear Shapiro steps under microwave irradiation, and a Fraunhofer-type dependence of the critical current on magnetic field. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. This surface state is attributed to mediate the ballistic Josephson current despite the fact that the normal state transport is dominated by diffusive bulk conductivity. The lateral Nb-Bi(2)Te(3)-Nb junctions hence provide prospects for the realization of devices supporting Majorana fermions.     

Giant magnetoresistance in electrodeposited Co-Ag granular films

We report on the electrodeposition of Co-Ag granular films with GMR from a chloride-based electrolyte. Two different electrochemical techniques (chronoamperometry and pulse plating) were used to prepare the films. Both electrochemical and TEM analysis have revealed the heterogeneity of the as-deposited samples. GMR values up to 7% at room temperature were measured. The longitudinal (LMR) and transverse (TMR) magnetoresistance were practically indistinguishable indicative of real granular systems. An appropriate numerical analysis of the magnetoresistance curves showed high superparamagnetic contribution to the total MR.     

Scattering from topological insulator circular cylinder buried in a semi-infinite medium

The problem of electromagnetic scattering of a plane wave from a topological insulator (TI) cylinder buried beneath a flat interface is undertaken with the method of analysis based on spectral plane wave representation of fields. Saddle point method is employed to evaluate the involved spectral integrals serving as basis functions in the representations of scattered-reflected and scattered-transmitted fields. The distinctions between the scattering pattern for time reversal symmetry TI cylinder and that of time reversal symmetry broken TI cylinder are pronounced. Results are validated through the comparison with the previously available results.     

Tunneling anisotropic magnetoresistance in Co/AlOx/Au tunnel junctions

We observe spin-valve-like effects in nanoscaled thermally evaporated Co/AlOx/Au tunnel junctions. The tunneling magnetoresistance is anisotropic and depends on the relative orientation of the magnetization direction of the Co electrode with respect to the current direction. We attribute this effect to a two-step magnetization reversal and an anisotropic density of states resulting from spin-orbit interaction. The results of this study points to future applications of novel spintronics devices involving only one ferromagnetic layer.     

Oscillations in longitudinal magnetoresistance in single-crystal films of bismuth

Theoretically, it has been shown by Erukhimov and Tavger that, when the magnetic and electric fields are in the same direction and parallel to the film surface, a change in the film thickness or the magnetic field leads to oscillations in conductivity for experimentally attainable conditions. We report small amplitude oscillations in the longitudinal magnetoresistance of single-crystal bismuth films in weak magnetic fields at liquid nitrogen temperature, due to the influence of the magnetic field on size quantization.