Observation of π Berry phase in quantum oscillations of three‐dimensional Fermi surface in topological insulator Bi2Se3

We report the quantum transport studies on Bi₂Se₃ single crystal with bulk carrier concentration of ～10¹⁹ cm^–3. The Bi₂Se₃ crystal exhibits metallic character, and at low temperatures, the field dependence of resistivity shows clear Shubnikov–de Haas (SdH) oscillations above 6 T. The analysis of these oscillations through Lifshitz–Kosevich theory reveals a non‐trivial π Berry phase coming from three‐dimensional (3D) Fermi surface, which is a strong signature of Dirac fermions with three‐dimensional dispersion. The large Dingle temperature and non zero slope of Williamson–Hall plot suggest the presence of enhanced local strain field in our system which possibly transforms the regions of topological insulator to 3D Dirac fermion metal state. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca doping the Bi₂Se₃ topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppresses the bulk conductivity. Non-metallic Bi₂Se₃ crystals are obtained. On the other hand, the Bi₂Se₃ topological insulator can also be induced to become a bulk superconductor, with T_c∼3.8 K, by copper intercalation in the van der Waals gaps between the Bi₂Se₃ layers. Likewise, an as-grown crystal of metallic Bi₂Te₃ can be turned into a non-metallic crystal by slight variation in the Te content. The Bi₂Te₃ topological insulator shows small amounts of superconductivity with T_c∼5.5 K when reacted with Pd to form materials of the type Pd_zBi₂Te₃.     

One-pot synthesis in liquid paraffin of ternary alloy CdSexS1−x nanocrystals with fluorescence emission covering entire visible region

We report the synthesis and characterization of layer-structure Bi₂Se₃ nanomaterials. Bi₂Se₃ nanomaterial has attracted many researchers, because it has a unique three-dimensional topological insulator property characterized by a metallic surface which coexists with an insulating interior. This can be achieved by having large surface-to-volume ratio in the nanomaterial. We synthesized highly single-crystalline topological insulator Bi₂Se₃ nanomaterials with various morphologies, including straight nanowires, zig-zag nanowires, and nanobelts, by adjusting experimental parameters such as the growth temperature, pressure, and carrier gas flow rate. The results show that the width and length of Bi₂Se₃ nanowires increase significantly with increasing values of each parameter. Furthermore, we studied the growth mechanism of individual morphologies based on the layered structure of Bi₂Se₃.     

Topological insulator nanostructures: Materials synthesis, Raman spectroscopy, and transport properties

Nanostructured topological insulator materials such as ultrathin films, nanoplates, nanowires, and nanoribbons are attracting much attention for fundamental research as well as potential applications in low-energy dissipation electronics, spintronics, thermoelectrics, magnetoelectrics, and quantum computing due to their extremely large surface-to-volume ratios and exotic metallic edge/surface states. Layered Bi₂Se₃ and Bi₂Te₃ serve as reference topological insulator materials with a large nontrivial bulk gap up to 0.3 eV (equivalent to 3600 K) and simple single-Dirac-cone surface states. In this mini-review, we present an overview of recent advances in nanostructured topological insulator Bi₂Se₃ and Bi₃Te₃ from the viewpoints of controlled synthesis and physical properties. We summarize our recent achievements in the vapor-phase synthesis and structural characterization of nanostructured topological insulator Bi₂Se₃ and Bi₂Te₃, such as nanoribbons and ultrathin nanoplates.We also demonstrate the evolution of Raman spectra with the number of few-layer topological insulators, as well as the transport measurements that have succeeded in accessing the surface conductance and surface state manipulations in the device of topological insulator nanostructures.     

Nanoimprinted substrates for high-yield production of topological insulator nanoribbons

We present a growth process mediated by nanoimprinted nanostructures specifically for producing bismuth selenide (Bi₂Se₃) topological insulator nanoribbons with a high yield. In this process, topological insulator nanostructures are grown on nanoimprinted gratings by using a nanoparticle-catalyzed vapor–liquid–solid mechanism. In comparison with the growth processes performed on flat and randomly rough substrates, such a nanograting-mediated growth method produces topological insulator nanoribbons with a higher yield (～15?000 nanoribbons/mm²), a narrower average ribbon width (w _avg<60 nm), and a higher uniformity in ribbon width (σ<30 nm); effectively suppresses the formation of other unwanted morphologies; and also results in the axial growth of nanoribbons along specific in-plane directions relative to pre-structured gratings. Such technical merits of nanograting-mediated growth are attributed to the preferential nucleation of Bi₂Se₃ crystal seeds and the concomitant pinning of catalytic nanoparticles at ordered grating edges. Finally, Aharonov–Bohm interference oscillations in the magnetoresistance were observed and demonstrated the coherent transport of electrons through topological surface states of Bi₂Se₃ nanoribbons. This growth process in combination with large-area nanoimprint lithography could serve as an important foundation for nanomanufacturing topological insulator nanoribbons with controllable feature size, large-area uniformity, and ordering, suitable for applications in future low-dissipation nanoelectronics.     

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi₂Se₃ thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi₂Se₃ grew on amorphous Al₂O₃ even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi₂Se₃ channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.     

Bound states induced by a ferromagnetic delta-layer inserted into a three-dimensional topological insulator

We report on theoretical study of the bound electron states induced by a ferromagnetic delta-layer embedded into a narrow-band-gap semiconductor of the Bi₂Se₃-type which is a three-dimensional topological insulator with large spin-orbit coupling. We make use of an effective Hamiltonian taking into account the inverted band structure of the semiconductor host at the ?? point and describe the properties of the in-gap bound states: energy spectrum, characteristic length and spin polarization. We highlight a role of these states for a magnetic proximity effect in digital magnetic heterostructures based on the Bi₂Se₃-type semiconductors.     

The heterostructure and electrical properties of Sb2Se3/Bi2Se3 grown by molecular beam epitaxy

We report the growth and characterization of Sb₂Se₃/Bi₂Se₃ bilayer films fabricated by molecular beam epitaxy. High quality heterostructures are obtained as evidenced from the X-ray diffraction (XRD), atomic force microscopy and high-resolution transmission electron microscopic (HRTEM) analysis. Interestingly, Sb₂Se₃ grows as a (120) hexacrystal film in orthorhombic phase on rhombohedral Bi₂Se₃ (0001) plane, as verified by the out-of-plane and in-plane XRD scans. The cross-sectional TEM studies indicate a sharp interface between Sb₂Se₃ and Bi₂Se₃, which is important for the protection of surface states Bi₂Se₃. The ultraviolet photoelectron spectroscopy indicates that the Fermi level located 0.95 eV above the valence band maximum in Sb₂Se₃. The insulating nature of Sb₂Se₃ is confirmed by the nonlinear current-voltage curve via the vertical junction electrical measurement. By four point probe measurements, we confirm the charge transfer effect from Sb₂Se₃ into Bi₂Se₃, and such effect can be reduced in the Sb₂Se₃/(Bi_0.7Sb_0.3)₂Se₃ bilayer. This work opens a new avenue for the synthesis of multilayers consisting of topological insulators and ordinary insulator, which is important for harvesting of the multiple surface states for advanced electronic and spintronic applications.     

Specific features of the electronic,spin, and atomic structures of a topological insulator Bi<Subscript>2</Subscript>Te<Subscript>2.4</Subscript>Se<Subscript>0.6</Subscript>

The specific features of the electronic and spin structures of a triple topological insulator Bi₂Te_2.4Se_0.6, which is characterized by high-efficiency thermoelectric properties, have been studied with the use of angular- and spin-resolved photoelectron spectroscopy and compared with theoretical calculations in the framework of the density functional theory. It has been shown that the Fermi level for Bi₂Te_2.4Se_0.6 falls outside the band gap and traverses the topological surface state (the Dirac cone). Theoretical calculations of the electronic structure of the surface have demonstrated that the character of distribution of Se atoms on the Te–Se sublattice practically does not influence the dispersion of the surface topological electronic state. The spin structure of this state is characterized by helical spin polarization. Analysis of the Bi₂Te_2.4Se_0.6 surface by scanning tunnel microscopy has revealed atomic smoothness of the surface of a sample cleaved in an ultrahigh vacuum, with a lattice constant of ~4.23 Å. Stability of the Dirac cone of the Bi₂Te_2.4Se_0.6 compound to deposition of a Pt monolayer on the surface is shown.     

Topological insulator as an optical modulator for pulsed solid‐state lasers

Topological insulators are states of quantum matter that have narrow topological nontrivial energy gaps and a large third‐order nonlinear optical response. The optical absorption of topological insulators can become saturated under strong excitation. In this work, with Bi₂Se₃ as an example, it was demonstrated that a topological insulator can modulate the operation of a bulk solid‐state laser by taking advantage of its saturable absorption. The result suggests that topological insulators are potentially attractive as broadband pulsed modulators for the generation of short and ultrashort pulses in bulk solid‐state lasers, in addition to other promising applications in physics and computing.     

Magnetic and transport properties of Mn-doped Bi2Se3 and Sb2Se3

We report on the single crystal growth and thermoelectric and magnetic properties of Mn-doped Bi₂Se₃ and Sb₂Se₃ single crystals prepared by the temperature gradient solidification method. The composition and crystal structure were determined using electron probe microanalysis and θ–2θ powder X-ray diffraction studies, respectively. The lattice constants of several percent Mn-doped Bi₂Se₃ and Sb₂Se₃ were slightly smaller than those of the undoped sample due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi₂Se₃ and Sb₂Se₃ showed spin-glass and paramagnetic properties, respectively.     

Superconductivity of topological matters induced via pressure

By applying pressure on the topological insulator Bi₂Te₃ single crystal, superconducting phase was found without a crystal structure phase transition. The new superconducting phase is under the pressure range of 3 GPa to 6 GPa. The high pressure Hall effect measurements indicated that the superconductivity caused by bulk hole pockets. The high pressure structure investigations with synchrotron X-ray diffraction indicated that the superconducting phase is of similar structure to that of ambient phase structure with only slight change with lattice parameter and internal atomic position. The topological band structures indicate the superconducting phase under high pressure remained topologically nontrivial. The results suggested that topological superconductivity can be realized in Bi₂Te₃ due to the proximity effect between superconducting bulk states and Diractype surface states. We also discussed the possibility that the bulk state could be a topological superconductor.     

Bismuth telluride topological insulator nanosheet saturable absorbers for q‐switched mode‐locked Tm:ZBLAN waveguide lasers

Nanosheets of bismuth telluride (Bi₂Te₃), a topological insulator material that exhibits broadband saturable absorption due to its non‐trivial Dirac‐cone like energy structure, are utilized to generate short pulses from Tm:ZBLAN waveguide lasers. By depositing multiple layers of a carefully prepared Bi₂Te₃ solution onto a glass substrate, the modulation depth and the saturation intensity of the fabricated devices can be controlled and optimized. This approach enables the realization of saturable absorbers that feature a modulation depth of 13% and a saturation intensity of 997 kW/cm². For the first time to our knowledge, Q‐switched mode‐locked operation of a linearly polarized mid‐IR ZBLAN waveguide chip laser was realized in an extended cavity configuration using the topological insulator Bi₂Te₃. The maximum average output power of the laser is 16.3 mW and the Q‐switched and mode‐locked repetition rates are 44 kHz and 436 MHz, respectively.     

Efficient step-mediated intercalation of silver atoms deposited on the Bi2Se3 surface

The intercalation of silver atoms into the van der Waals gap of the prototypical three-dimensional topological insulator Bi₂Se₃ is studied by means of ab initio total-energy calculations. Two possible intercalation mechanisms are examined: penetration from the terrace under the step and penetration via interstitials and/or vacancies of the surface quintuple layer block. It is shown that the former mechanism is strongly preferred over the latter one due to significant energy gain appearing at the step. According to performed estimations, the room temperature diffusion length of silver atoms reaches ten microns within a couple of minutes both on the surface and within the van der Waals gap, which essentially exceeds a typical distance between steps. These results shed light on the mechanism of intercalation of metal atoms deposited on the Bi₂Se₃ surface.     

Three- and two-dimensional topological insulators in Pb2Sb2Te5, Pb2Bi2Te5, and Pb2Bi2Se5 layered compounds

The electronic structure of ternary compounds Pb₂Sb₂Te₅, Pb₂Bi₂Te₅, and Pb₂Bi₂Se₅, which have a layered structure that consists of nine-layer atomic blocks separated by van der Waals gaps, has been theoretically studied. It has been shown that all studied compounds are three-dimensional topological insulators. The possibility of the existence of a two-dimensional topological insulator has been found in ultrathin (0001) Pb₂Sb₂Te₅ and Pb₂Bi₂Te₅ films. Oscillations of the ℤ₂ topological invariant with an increase in the film thickness have been observed in the latter compound.     

<Superscript>77</Superscript>Se Low-Temperature NMR in the Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystalline Topological Insulator

⁷⁷Se nuclear magnetic resonance (NMR) measurements in the Bi₂Se₃ topological insulator single crystal were carried out at temperatures 15.8, 88, and 293 K. Bismuth selenide single crystalline plate was studied in the orientation when the crystallographic c-axis was parallel to the external magnetic field B₀. We observed two component NMR spectra at the three temperatures. It was shown that the NMR spectrum almost did not move with decreasing temperature and the density of charge carriers did not follow the thermal activation law.     

First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

On the origin of two-dimensional electron gas states at the surface of topological insulators

The ab initio calculations of the electronic structure in the bulk and at the (0001) surface of narrow-band Bi₂Se₃, Sb₂Te₃, Sb₂STe₃, and Sb₂SeTe₂ semiconductors have been performed. It has been shown that ternary compounds Sb₂STe₂ and Sb₂SeTe₂, as well as the previously known compounds Bi₂Se₃ and Sb₂Te₃, are three-dimensional topological insulators. The influence of the subsurface van der Waals gap expansion on the surface electronic structure of these compounds has been analyzed. It has been shown that this expansion leads to the formation of new (trivial) surface states, namely a parabolic state in the conduction band and an M-shaped state in the valence band. These results explain the phenomena discovered recently in photoemission experiments and reveal the nature of new states that are caused by the adsorption of atoms on the surfaces of the layered topological insulators.     

Quantum interference effects in Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> topological insulator nanowires with variable cross-section lengths

Stimulated by the recent realization of three dimensional topological insulator nanowireinterferometers, a theoretical analysis of quantum interference effects on the low energyspectrum of Bi₂Se₃ nanowires is presented. The electronic properties areanalyzed in nanowires with circular, square and rectangular cross-sections starting from acontinuum three dimensional model with particular emphasis on magnetic and geometricaleffects. The theoretical study is based on numerically exact diagonalizations of thediscretized model for all the geometries. In the case of the cylindrical wire, anapproximate analytical solution of the continuum model is also discussed. Although amagnetic field corresponding to half quantum flux is expected to close the band gapinduced by Berry phase, in all the studied geometries with finite area cross-sections, thegap closes for magnetic fields typically larger than those expected. Furthermore,unexpectedly, due to geometrical quantum interference effects, for a rectangular wire witha sufficiently large aspect ratio and smaller side ranging from 50 Å and 100 Å, the gapcloses for a specific finite area cross-section without the application of a magneticfield.     

Carrier transport in Bi2Se3 topological insulator slab

Electron transport in Bi₂Se₃ topological insulator slabs is investigated in the thermal activation regime (>50 K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi₂Se₃ slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments.