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.     

Negative Magnetoresistance in Antiferromagnetic Topological Insulator EuSn_2As_2

The measurements of magnetization,longitudinal and Hall resistivities are carried out on the intrinsic antiferromagnetic(AFM) topological insulator ElSn_2 As_2.It is confirmed that our ElSn_2 As_2 crystal is a heavily hole doping A-type AFM metal with the Neel temperature T_N=24 K,with a metamagnetic transition from an AFM to a ferromagnetic(FIM) phase occurring at a certain critical magnetic Held for the different Held orientations.Meanwhile,we also find that the carrier concentration does not change with the evolution of magnetic order,indicating that the weak interaction between the localized magnetic moments from Eu~(2+)4f~7 orbits and the electronic states near the Fermi level.Although the quantum anomalous Hall effect(AHE) is not observed in our crystals,it is found that a relatively large negative magnetoresistance (-13%) emerges in the AFM phase,and exhibits an exponential dependence upon magnetic Held,whose microscopic origin is waiting to be clarified in future research.     

Making Axion Dynamical in Non-Centrosymmetric Magnetic Topological Insulators

“Axion”was predicted as a hypothetical elementary particle to resolve the strong conjugation-parity problem in particle physics,^[1]and it is also an attractive candidate for the as-yet-unobserved dark matter in cosmology.^[2]While the detection of axions still remains elusive in particle physics and cosmology,it was recently proposed that the elegant physics of axions,known“axion electrodynamics”,^[3]can emerge in certain condensed matter systems,particularly topological insulator materials,^[4]in which a variety of exotic physical phenomena(e.g.topological magnetoelectric effect)have been theoretically predicted.     

Pressure-Induced Topological and Structural Phase Transitions in an Antiferromagnetic Topological Insulator

Recently,natural van der Waals heterostructures of(MnBi₂ Te₄)_m(Bi₂ Te₃)_n have been theoretically predicted and experimentally shown to host tunable magnetic properties and topologically nontrivial surface states.We systematically investigate both the structural and electronic responses of MnBi₂ Te₄ and MnBi₄ Te₇ to external pressure.In addition to the suppression of antiferromagnetic ord...     

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.     

Magnetic Proximity Effect in an Antiferromagnetic Insulator/Topological Insulator Heterostructure with Sharp Interface

We report an experimental study of electron transport properties of MnSe/(Bi,Sb)_2Te_3 heterostructures,in which MnSe is an antiferromagnetic insulator,and(Bi,Sb)_2Te_3 is a three-dimensional topological insulator(TI).Strong magnetic proximity effect is manifested in the measurements of the Hall effect and longitudinal resistances.Our analysis shows that the gate voltage can substantially modify the anomalous Hall conductance,which exceeds 0.1 e~2/h at temperature T=1.6 K and magnetic field μ_0H=5 T,even though only the top TI surface is in proximity to MnSe.This work suggests that heterostructures based on antiferromagnetic insulators provide a promising platform for investigating a wide range of topological spintronic phenomena.     

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.     

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.     

Atomic-Ordering-Induced Quantum Phase Transition between Topological Crystalline Insulator and Z_2 Topological Insulator

Topological phase transition in a single material usually refers to transitions between a trivial band insulator and a topological Dirac phase, and the transition may also occur between different classes of topological Dirac phases.It is a fundamental challenge to realize quantum transition between Z_2 nontrivial topological insulator(TI) and topological crystalline insulator(TCI) in one material because Z_2 TI and TCI have different requirements on the number of band inversions. The Z_2 TIs must have an odd number of band inversions over all the time-reversal invariant momenta, whereas the newly discovered TCIs, as a distinct class of the topological Dirac materials protected by the underlying crystalline symmetry, owns an even number of band inversions. Taking PbSnTe_2 alloy as an example, here we demonstrate that the atomic-ordering is an effective way to tune the symmetry of the alloy so that we can electrically switch between TCI phase and Z_2 TI phase in a single material. Our results suggest that the atomic-ordering provides a new platform towards the realization of reversibly switching between different topological phases to explore novel applications.     

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     

Anisotropy Properties of Mn_2P Single Crystals with Antiferromagnetic Transition

Single crystals of hexagonal structure Mn₂P are synthesized by Sn flux for the first time. Transport and magnetic properties have been performed on the single crystals, which is an antiferromagnet with Neel temperature 103 K.Obvious anisotropy of resistivity is observed below the Neel temperature, which is manifested by metallic behavior with a current along the C-axis and semiconducting behavior with a current along the α-axis. The negative slope of temperature-dependent resistivity ...     

Long-Time Magnetic Relaxation in Antiferromagnetic Topological Material EuCd_2As_2

Magnetic topological materials have attracted much attention due to the correlation between topology and magnetism. Recent studies suggest that EuCd_2As_2 is an antiferromagnetic topological material. Here by carrying out thorough magnetic, electrical and thermodynamic property measurements, we discover a long-time relaxation of the magnetic susceptibility in EuCd_2As_2. The(001) in-plane magnetic susceptibility at 5 K is found to continuously increase up to ~10% over the time of ~14 hours. The magnetic relaxation is anisotropic and strongly depends on the temperature and the applied magnetic field. These results will stimulate further theoretical and experimental studies to understand the origin of the relaxation process and its effect on the electronic structure and physical properties of the magnetic topological materials.     

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.     

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.     

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.     

Floquet Topological Insulator in the BHZ Model with the Polarized Optical Field

Topological phase of newly found matter has aroused wide interests, especially related with the external periodical modulating. With the help of the Floquet theory, we investigate the possibility of externally manipulating the topological property in a HgTe/CdTe quantum well system with the polarized optical field. We give the phase diagram, showing that by modulating the parameters of the polarized optical field, especially the phase, the topological phase transition can be realized in the QW and lead to the so-called Floquet topological insulator. When the optical field is weak, the driven QSH state of QW is robust with the optical field. However, when the optical field is relatively larger, the group velocity of edge states and the gap between the bulk states exhibit certain oscillations. The implications of our results are discussed.     

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.     

Modulation of the Dirac Point Band Gap in the Antiferromagnetic Topological Insulator MnBi2Te4 due to the Surface Potential Gradient Change

Journal of Experimental and Theoretical Physics - The possibility of significant modulation of the band gap open at the Dirac point (DP) in the range from 15 to 55 meV for different samples of the...     

Detection of Magnetic Gap in Topological Surface States of MnBi_2Te_4

Recently, intrinsic antiferromagnetic topological insulator MnBi_2Te_4 has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect, axion insulator state, and other quantum phases. An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi_2Te_4.However, the experimental observations of the surface gap remain controversial. Here, we report the observation of the surface gap via the point contact tunneling spectroscopy. In agreement with theoretical calculations, the gap size is around 50 me V, which vanishes as the sample becomes paramagnetic with increasing temperature.The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field, substantiating the surface ferromagnetism. Furthermore, the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy.Combining these results, the magnetism-induced gap in topological surface states of MnBi_2Te_4 is revealed.     

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.