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.     

Femtosecond Tm-Ho co-doped fiber laser using a bulk-structured Bi_2Se_3 topological insulator

We experimentally demonstrate a femtosecond mode-locked thulium-holmium(Tm-Ho) co-doped fiber laser incorporating a saturable absorber(SA) based on a bulk-structured bismuth selenide(Bi_2Se_3) topological insulator(TI). The SA was prepared by depositing a mechanically exfoliated Bi_2Se_3 TI layer onto a side-polished optical fiber platform. Unlike high-quality nano-structured Bi_2Se_3 TI-based SA, bulk-structured Bi_2Se_3 with non-negligible oxidation was used as a saturable absorption material for this experimental demonstration due to its easy fabrication process. The saturation power and modulation depth of the prepared SA were measured to be ~ 28.6 W and ~13.4%, respectively. By incorporating the prepared SA into a Tm-Ho co-doped fiber ring cavity, stable soliton pulses with a temporal width of ~ 853 fs could be generated at 1912.12 nm. The 3-dB bandwidth of the mode-locked pulse was measured to be ~4.87 nm. This experimental demonstration reaffirms that Bi_2Se_3 is a superb base material for mid-infrared passive mode-locking even under oxidation.     

Dual-wavelength Bi_2Se_3-based passively Q-switching Nd~(3+)-doped glass all-fiber laser

We demonstrate a dual-wavelength passively Q-switched Nd~(3+)-doped glass fiber laser using a few-layer topological insulator Bi_2Se_3 as a saturable absorber(SA) for the first time, to the best of our knowledge. The laser resonator is a simple and compact linear cavity using two fiber end-facet mirrors. The SA is fabricated by Bi_2Se_3/polyvinyl alcohol composite film. By inserting the SA into the laser cavity, a stable Q-switching operation is achieved with the shortest pulse width and maximum pulse repetition rate of 601 ns and 205.2 kHz,respectively. The maximum average output power and maximum pulse energy obtained are about 6.6 mW and 38.8 nJ, respectively.     

Experimental Realization of an Intrinsic Magnetic Topological Insulator

An intrinsic magnetic topological insulator(TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi_2Te_4, by alternate growth of a Bi_2Te_3 quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.     

Electronic states and spin-filter effect in three-dimensional topological insulator Bi_2Se_3 nanoribbons

We study the electronic band structure, density distribution, and transport of a Bi_2Se_3 nanoribbon. We find that the density distribution of the surface states is dependent on not only the shape and size of the transverse cross section of the nanoribbon, but also the energy of the electron. We demonstrate that a transverse electric field can eliminate the coupling between surface states on the walls of the nanoribbon, remove the gap of the surface states, and restore the quantum spin Hall effects. In addition, we study the spin-dependent transport property of the surface states transmitting from top and bottom surfaces(x-y plane) to the side surfaces(z-x plane) of a Bi_2Se_3 nanoribbon. We find that transverse electric fields can open two surface channels for spin-up and-down Dirac electrons, and then switch off one channel for the spin-up Dirac electron. Our results may provide a simple way for the design of a spin filter based on topological insulator nanostructures.     

Improved thermoelectric performance in p-type Bi_(0.48)Sb_(1.52)Te_3 bulk material by adding MnSb_2Se_4

Bismuth telluride(Bi_2Te_3) based alloys, such as p-type Bi_(0.5)Sb_(1.5)Te_3, have been leading candidates for near room temperature thermoelectric applications. In this study, Bi_(0.48)Sb_(1.52)Te_3 bulk materials with MnSb_2Se_4 were prepared using high-energy ball milling and spark plasma sintering(SPS) process. The addition of MnSb_2Se_4 to Bi_(0.48)Sb_(1.52)Te_3 increased the hole concentration while slightly decreasing the Seebeck coefficient, thus optimising the electrical transport properties of the bulk material. In addition, the second phases of MnSb_2Se_4 and Bi_(0.48)Sb_(1.52)Te_3 were observed in the Bi_(0.48)Sb_(1.52)Te_3 matrix. The nanoparticles in the semi-coherent second phase of MnSb_2Se_4 behaved as scattering centres for phonons,yielding a reduction in the lattice thermal conductivity. Substantial enhancement of the figure of merit, ZT, has been achieved for Bi_(0.48)Sb_(1.52)Te_3 by adding an Mn_(0.8)Cu_(0.2)Sb_2Se_4(2mol%) sample, for a wide range of temperatures, with a peak value of 1.43 at 375 K, corresponding to ~40% improvement over its Bi_(0.48)Sb_(1.52)Te_3 counterpart. Such enhancement of the thermoelectric(TE) performance of p-type Bi_2Te_3 based materials is believed to be advantageous for practical applications.     

Site Preference of Se and Te in Bi_2Se_(3-x)Te_x Thin Films

The ternary topological insulators Bi_2Se_(3-x)Te_x have attracted a great deal of attention due to their exotic physical and chemical properties.While most of the studies focus on the properties of these ternary TIs,limited research was performed to investigate the dynamic atomic stack of its crystal structure.We prepared highquality Bi_2Se_(3-x)Te_x thin films on Ga As(111)B substrates using molecular beam epitaxy,characterized with Raman spectroscopy,x-ray diffraction and photoelectron spectroscopy.It is found that when Se is replaced by Te,the preferred substituting sites are the middle layer at 0x1,and this is also valid for Se substituting Te at 2x3.In the middle region,the substituting atoms prefer to go to the first and the fifth layer.     

Low-Frequency Noise in Gate Tunable Topological Insulator Nanowire Field Emission Transistor near the Dirac Point

Low-frequency flicker noise is usually associated with material defects or imperfection of fabrication procedure.Up to now,there is only very limited knowledge about flicker noise of the topological insulator,whose topologically protected conducting surface is theoretically immune to back scattering.To suppress the bulk conductivity we synthesize antimony doped Bi_2Se_3 nanowires and conduct transport measurements at cryogenic temperatures.The low-frequency current noise measurement shows that the noise amplitude at the high-drain current regime can be described by Hooge's empirical relationship,while the noise level is significantly lower than that predicted by Hooge's model near the Dirac point.Furthermore,different frequency responses of noise power spectrum density for specific drain currents at the low drain current regime indicate the complex origin of noise sources of topological insulator.     

Point-contact tunneling spectroscopy between a Nb tip and an ideal topological insulator Sn-doped Bi_(1.1)Sb_(0.9)Te_2S

The lightly Sn-doped Bi_(1.1)Sb_(0.9)Te_2S is a good material to investigate the pure topological surface state because the bulk bands are far away from the Fermi level. By measuring point-contact tunneling spectra on the topological insulator Bi_(1.08)Sn_(0.02)Sb_(0.9)Te_2S samples with a superconducting Nb tip, we observed the suppression of differential conductance near zero bias, instead of the enhancement due to Andreev reflection on the spectra. The fitting to the measured spectrum results in a superconducting gap of more than 4 meV, and this value is much larger than the superconducting gap of the bulk Nb. The gaped feature exists at temperatures even above the critical temperature of bulk Nb, and is visible when the magnetic field is as large as 9 T at 3 K. We argue that such behaviors may be related to the pressure induced superconductivity by the tip in the junction area, or just some novel phenomena arising from the junction between an s-wave superconductor and an ideal topological insulator.     

Thin-film magnetization dynamics on the surface of a topological insulator

We theoretically study the magnetization dynamics of a thin ferromagnetic film exchange coupled with a surface of a strong three-dimensional topological insulator. We focus on the role of electronic zero modes imprinted by domain walls (DWs) or other topological textures in the magnetic film. Thermodynamically reciprocal hydrodynamic equations of motion are derived for the DW responding to electronic spin torques, on the one hand, and fictitious electromotive forces in the electronic chiral mode fomented by the DW, on the other. An experimental realization illustrating this physics is proposed based on a ferromagnetic strip, which cuts the topological insulator surface into two gapless regions. In the presence of a ferromagnetic DW, a chiral mode transverse to the magnetic strip acts as a dissipative interconnect, which is itself a dynamic object that controls (and, inversely, responds to) the magnetization dynamics.     

Bi(110)薄膜在NbSe_2衬底上的扫描隧道显微镜研究

二维拓扑绝缘体因其特殊的能带结构带来的新奇物理性质,成为近年来凝聚态物理的研究热点.尤其是在引入超导电性之后,二维拓扑绝缘体中可能存在马约拉纳费米子(Majorana fermion),因此在量子计算方面具有重大应用前景.在Bi(111)薄膜被证实为二维拓扑绝缘体之后, Bi(110)薄膜引起了广泛关注,然而其拓扑性质还存在争议.本文利用分子束外延技术在室温低生长速率环境下成功制备出了高质量的单晶Bi(110)薄膜.通过扫描隧道显微镜测量发现,薄膜以约8个原子层厚度为分界,从双层生长转变为单层生长模式.结合隧道谱测量发现,在NbSe_2衬底上生长的Bi(110)薄膜因为近邻效应而具有明显的超导性质,但并未显示出拓扑边缘态的存在.此外,对薄膜中特殊的量子阱态现象也进行了讨论.     

在预刻蚀的衬底上通过分子束外延直接生长出拓扑绝缘体薄膜的微器件

在利用光刻将拓扑绝缘体外延薄膜加工成微米尺寸结构的过程中,所用的各种化学物质会导致薄膜质量的下降.在实验中,通过在钛酸锶衬底上预先光刻出Hall bar形状的凸平台并以此为模板进行拓扑绝缘体(Bi x Sb1-x)2Te3薄膜的分子束外延生长,直接获得了薄膜的Hall bar微器件,从而避免了光刻过程对材料质量的影响.原子力显微镜和输运测量结果均显示该微器件保持了(Bi x Sb1-x)2Te3外延薄膜原有的性质.这种新的微器件制备方法有助于在拓扑绝缘体中实现各种新奇的量子效应,并可推广于其他外延生长的低维系统.     

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.     

Influence of the thickness on structural,magnetic and electrical properties of La0.7Ca0.3MnO3 thin film

We have studied the effect of thickness on the structural, magnetic and electrical properties of La_0.7Ca_0.3MnO₃ thin films prepared by pulsed laser deposition method using X-ray diffraction, electrical transport, magneto-transport and dc magnetization. X-ray diffraction pattern reflects that all films have c-axis epitaxial growth on LaAlO₃ substrate. The decrease in out-of-plane cell parameter specifies a progressive relaxation of in the plane compressive strain as the film thickness is increases. From the dc magnetization measurements, it is observed that ferromagnetic to paramagnetic transition temperature increases with increase in the film thickness. Magneto-resistance and temperature coefficient of resistance increases with film thickness and have maximum value near its metal to insulator transition temperature.     

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering

Three-dimensional(3 D) topological insulators(TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure.Rapid,low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs.Here we show that wafer-sized Bi_2 Te_3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO_2/Si substrates by magnetron cosputtering.The SiO_2/Si substrates enable us to electrically tune(Bi_(1-x)Sb_x)_2 Te_3 and Cr-doped(Bi_(1-x)Sb_x)_2 Te_3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states,such as the quantum anomalous Hall effect(QAHE).This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.     

Influence of Magnetization on the Spin Pumping Efficiency in a Ferromagnet–Normal Metal Bilayer Structure

The problem of spin current generation and transformation into electric signals in thin-film ferromagnet/nonmagnetic metal bilayer structures is investigated. This direction is of considerable scientific interest and promising for applications in spintronics. An LSMO/Pt structure consisting of an epitaxial film of ferromagnetic manganite La2/3Sr1/3O3 grown on a single-crystal NdGaO3 substrate and coated with a platinum film has been studied experimentally. The spin current was generated by the spin pumping method upon the excitation of a ferromagnetic resonance in the ferromagnetic layer and was detected by the electric voltage USP arising in the nonmagnetic metal layer due to the inverse spin Hall effect. Owing to its relatively low Curie temperature (~350 K), using LSMO allowed the influence of ferromagnetic-layer magnetization on the spin current generation to be studied in detail in the temperature range 100–350 K. In this case, the influence of the shape of the ferromagnetic resonance line, which is the convolution of homogeneous (Lorentzian) spin packets and inhomogeneous Gaussian broadening (Voigt model), was consistently taken into account. As a result of our analysis of all the parameters defining USP, we have obtained the temperature dependence of the mixed spin conductance, which has turned out to be approximately proportional to the ferromagnet magnetization squared. This result is compared with existing theoretical models.     

Impurity effect on weak antilocalization in the topological insulator Bi2Te3

We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.     

La,V共掺杂的Bi_4Ti_3O_(12)铁电薄膜的溶胶-凝胶法制备及性能测试

采用溶胶-凝胶(sol-gel)工艺在Pt/TiO2/SiO2/p-Si(100)衬底上制备出Bi4Ti3O12(BIT)和Bi3.25La0.75Ti2.97V0.03O12(BLTV)铁电薄膜,研究了La,V共掺杂对BIT薄膜的晶体结构和电学性能的影响.BIT薄膜为c轴择优取向,BLTV薄膜为随机取向,拉曼光谱分析表明V掺杂降低了TiO6(或VO6)八面体的对称性,也增强了Ti—O键(或V—O键)杂化.BLTV薄膜的剩余极化Pr为25.4μC/cm2,远大于BIT薄膜的9.2μC/cm2,表现出良好的铁电性能.疲劳、漏电流测试显示BLTV薄膜具有优良的抗疲劳特性和漏电流特性,表明La,V共掺杂能有效地降低薄膜中的氧空位.     

拓扑绝缘体(Bi0.5Sb0.5)2Te3薄膜中的线性磁阻

本文报道了拓扑绝缘体(Bi_0.5Sb_0.5)₂Te₃薄膜中线性磁阻问题的系统性研究工作. 此体系中, 线性磁阻在很宽的温度和磁场范围内出现: 磁场高达18 T时磁阻仍没有饱和趋势, 并且当温度不高于50 K时, 线性磁阻的大小对温度的变化不敏感. 栅压调控化学势可明显改变线性磁阻的大小. 当化学势接近狄拉克点时, 线性磁阻最为显著. 这些结果说明电荷分布的不均匀性是引起该材料线性磁阻的根源.     

Ultrashort optical pulse in a thin film of a topological insulator

We consider the propagation of ultrashort optical pulses in a thin film of a topological insulator within the framework of an effective long-wave Hamiltonian for low-temperature media. The electromagnetic field is taken as classical Maxwellian. We reveal the dependence on the maximum amplitude of ultrashort optical pulses.