Topological Insulator Films Research Articles

Observation of Quantum Anomalous Hall Effect and Exchange Interaction in Topological Insulator/Antiferromagnet Heterostructure.

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall effect in a magnetically-doped topological insulator grown on the antiferromagnetic insulator Cr2 O3 . The exchange coupling between the two materials is investigated using field-cooling-dependent magnetometry and polarized neutron reflectometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr2 O3 and the magnetic topological insulator, manifested as an exchange bias when the sample is field-cooled under an out-of-plane magnetic field, and an exchange spring-like magnetic depth profile when the system is magnetized within the film plane. These results identify antiferromagnetic insulators as suitable candidates for the manipulation of magnetic and topological order in topological insulator films.

Europium Doping Impact on the Properties of MBE Grown Bi2Te3 Thin Film.

The impact of europium doping on the electronic and structural properties of the topological insulator Bi2Te3 is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi2Te3 matrix remains divalent and substitutes bismuth in a Bi2Te3 matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi2Te3 topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.

Gapped Dirac cones and spin texture in thin film topological insulator

The protected surface states of topological insulators (TIs) form gapless Dirac cones corresponding non-degenerate eigenstates with helical spin polarisation. The presence of a warping term deforms the isotropic cone of the most simple model into snowflake Fermi surfaces as in Bi2Se3 and Bi2Te3. Their features have been identified in STM quasiparticle interference (QPI) experiments on isolated surfaces. Here we investigate the QPI spectrum for the TI thin-film geometry with finite tunnelling between the surface states. This leads to a dramatic change of spectrum due to gapping and a change in spin texture that should leave distinct signatures in the QPI pattern. We consider both normal and magnetic exchange scattering from the surface impurities and obtain the scattering t-matrix in Born approximation as well as the general closed solution. We show the expected systematic variation of QPI snowflake intensity features by varying film thickness and study, in particular, the influence on backscattering processes. We predict the variation of the QPI spectrum for Bi2Se3 thin films using the observed gap dependence from ARPES results.

Structurally and chemically compatible BiInSe3 substrate for topological insulator thin films

Quality of epitaxial films strongly depends on their structural and chemical match with the substrates: the more closely they match, the better the film quality is. Topological insulators (TI) such as Bi2Se3 thin films are of no exception. However, there do not exist commercial substrates that match with TI films both structurally and chemically, at the level commonly available for other electronic materials. Here, we introduce BiInSe3 bulk crystal as the best substrate for Bi2Se3 thin films. These films exhibit superior surface morphology, lower defect density and higher Hall mobility than those on other substrates, due to structural and chemical match provided by the BiInSe3 substrate. BiInSe3 substrate could accelerate the advance of TI research and applications.

Parasitic conduction channels in topological insulator thin films

Thin films of topological insulators (TI) usually exhibit multiple parallel conduction channels for the transport of electrical current. Beside the topologically protected surface states (TSS), parallel channels may exist, namely the interior of the not-ideally insulating TI film, the interface layer to the substrate, and the substrate itself. To be able to take advantage of the auspicious transport properties of the TSS, the influence of the parasitic parallel channels on the total current transport has to be minimized. Because the conductivity of the interior (bulk) of the thin TI film is difficult to access by measurements, we propose here an approach for calculating the mobile charge carrier concentration in the TI film. To this end, we calculate the near-surface band bending using parameters obtained experimentally from surface-sensitive measurements, namely (gate-dependent) four-point resistance measurements and angle-resolved photoelectron spectroscopy (ARPES). While in most cases another parameter in the calculations, i.e. the concentration of unintentional dopants inside the thin TI film, is unknown, it turns out that in the thin-film limit the band bending is largely independent of the dopant concentration in the film. Thus, a well-founded estimate of the total mobile charge carrier concentration and the conductivity of the interior of the thin TI film proves possible. Since the interface and substrate conductivities can be measured by a four-probe conductance measurement prior to the deposition of the TI film, the total contribution of all parasitic channels, and therefore also the contribution of the vitally important TSS, can be determined reliably.

Spin polarization in nanojunctions of quantum anomalous Hall insulators

We theoretically investigate spin polarized conductance through nanojunctions made by magnetic domains in a thin films of three dimensional ferromagnetic topological insulator (TI) as a quantum anomalous Hall insulator. Writing of magnetic domains is now feasible in recent experiments. Ferromagnetism induced in such materials can control topological phase of the band structure and as consequence, configuration of the states in each band. By means of band analysis, it is demonstrated that in topologically non-trivial bands with non-zero Chern number, there exists a change in spin dominant states among Brillouin zone while spin polarization of trivial bands shows dominant of one spin type. Indeed, there is an interplay between topological invariant of the band spectrum and contribution of the spin states in conductance. We show that in-plane magnetization or structure inversion asymmetry result in a k-dependent parity configuration of surface states in ultrathin film topological insulator. In the conduction and valence bands, a nearly perfect spin polarized conductance is found in a nanojunction composed of anti-parallel magnetic domains which is mediated by a layer under application of in-plane magnetization or perpendicular applied bias. This spin polarized region is extendable with the magnetization. To the sake of completeness, proposed polarization in conductance was also supported by interpretation based on spin and parity local density of states along the interfaces of magnetic domain walls. Analysis of the spin-parity states sheds light to the quantum transport through magnetically doped TI thin-film’s nanojunctions with different mass terms.

Anisotropic topological magnetoelectric effect in axion insulators

Three-dimensional topological insulators or axion insulators exhibit the topological magnetoelectric effect, which is isotropic with a universal coefficient of proportionality quantized in units of $e^2/2h$. Here we study the finite-size effect of topological magnetoelectric effect, and find the magnetoelectric coefficients are anisotropic, namely $\alpha_{xx}\neq\alpha_{zz}$. Both of them are shown to converge to a quantized value when the thickness of topological insulator film $d$ increases reaching the three-dimensional bulk limit. The nonzero value of $(\alpha_{xx}-\alpha_{zz})\propto1/d$ could be measured by using the gyrotropic or nonreciprocal birefringence of terahertz light. The unique $1/d$ dependence on film thickness of the rotation angle of optical principle axes is the manifestation of topological magnetoelectric effect, which may also serve as a smoking gun signature for axion insulators.

Electrically Tunable Wafer-Sized Three-Dimensional Topological Insulator Thin Films Grown by Magnetron Sputtering**Supported by the National Key R&D Plan Program of China (Grant No. 2017YFF0206104), the National Key Scientific Research Projects of China (Grant No. 2015CB921502), the National Natural Science Foundation of China (Grant Nos. 61574169 and 51871018), Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, the Opening Project

Three-dimensional (3D) 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 Bi2Te3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO2/Si substrates by magnetron cosputtering. The SiO2/Si substrates enable us to electrically tune (Bi1–xSbx)2Te3 and Cr-doped (Bi1–xSbx)2Te3 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.

Precise resistance measurement of quantum anomalous Hall effect in magnetic heterostructure film of topological insulator

The accuracy of Hall resistance in the quantum anomalous Hall effect has been studied at zero magnetic field using Crx(Bi,Sb)2−x Te3-based magnetic heterostructure films of topological insulators. The measured deviation of the Hall resistance from its theoretical value h/e2 was less than 2 ppm when the source drain current was 30 nA. This result has verified that the quantization of the Hall resistance is very accurate in the magnetic heterostructure films and in the previously reported uniformly doped films.

Crystalline Bi2Se3 topological insulator films prepared by dc magnetron sputtering

Topological insulator thin films of Bi2Se3 have been deposited on SrTiO3 (111) substrate by dc magnetron sputtering and annealed in Se environment. X-ray Diffraction (XRD) measurement shows (00l) (l = 3n, where n is an integer) type of reflections over 2θ range of 10–80° without any other peaks indicating that the films are of single crystalline quality along the [001] growth direction confirming that highly c-axis-preferred orientated Bi2Se3 thin films can be obtained by relatively simpler and easily scalable dc magnetron sputtering technique. The results are also confirmed by Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDXS), Photo-electron Spectroscopy (PES) and X-ray absorption spectroscopy (XAS) measurements.

Dynamical density response and collective modes of topological-insulator ultrathin films

We analytically calculate the intra- and inter-surface dynamical density-density linear responses of ultra-thin topological insulator films with finite tunneling between their top and bottom surfaces in both metallic and insulating regimes. Employing the random phase approximation we investigate the dispersions of in-phase and out-of-phase collective density modes of this system in the metallic regime. We find that in contrast to the bilayers of the conventional two-dimensional electron gas, where finite tunneling gaps out the out-of-phase mode, in topological insulator thin films, this mode remains linear at long wavelengths. Depending on different system parameters, the velocity of out-of-phase mode can be tuned to be larger or substantially smaller than the Fermi velocity of electrons on the isolated surfaces of the topological insulator. Finite tunneling generally reduces the energy of collective modes, making them more confined in space.

Josephson Effect and Charge Distribution in Thin Bi2Te3 Topological Insulators

Thin layers of topological insulator materials are quasi-2D systems featuring a complex interplay between quantum confinement and topological band structure. To understand the role of the spatial distribution of carriers in electrical transport, the Josephson effect, magnetotransport, and weak anti-localization are studied in bottom-gated thin Bi2 Te3 topological insulator films. The experimental carrier densities are compared to a model based on the solutions of the self-consistent Schrödinger-Poisson equations and they are in excellent agreement. The modeling allows for a quantitative interpretation of the weak antilocalization correction to the conduction and of the critical current of Josephson junctions with weak links made from such films without any ad hoc assumptions.

Coexistence of Surface and Bulk Ferromagnetism Mimics Skyrmion Hall Effect in a Topological Insulator

Here we report the investigation of the anomalous Hall effect in the magnetically doped topological insulator (V,Bi,Sb)2Te3. We find it contains two contributions of opposite sign. Both components are found to depend differently on carrier density, leading to a sign inversion of the total anomalous Hall effect as a function of applied gate voltage. The two contributions are found to have different magnetization reversal fields, which in combination with a temperature dependent study points towards the coexistence of two ferromagnetic orders in the system. Moreover, we find that the sign of total anomalous Hall response of the system depends on the thickness and magnetic doping density of the magnetic layer. The thickness dependence suggests that the two ferromagnetic components originate from the surface and bulk of the magnetic topological insulator film. We believe that our observations provide insight on the magnetic behavior, and thus will contribute to an eventual understanding of the origin of magnetism in this material class. In addition, our data bears a striking resemblance to anomalous Hall signals often associated with skyrmion contributions. Our analysis provides a straightforward explanation for both the magnetic field dependence of the Hall signal and the observed change in sign without needing to invoke skyrmions, and thus suggest that caution is needed when making claims of effects from skyrmion phases.

Large-scale interlayer rotations and Te grain boundaries in (Bi,Sb)2Te3 thin films

We report the observation of two distinct large-scale defects in ${(\mathrm{Bi},\mathrm{Sb})}_{2}\mathrm{T}{\mathrm{e}}_{3}$ topological insulator (TI) thin films grown by molecular beam epitaxy. Small-angle rotations are detected between quintuple layers of the TI film, extending throughout a grain and beyond, and nm-sized Te formations are discovered that extend along grain boundaries. Density functional theory calculations suggest that the rotational defects can affect the local band structure of the film while preserving spin-momentum locking in the Dirac bands, and that the Te nanostructures at grain boundaries can result in wider-band-gap regions between the grains.

High-performance THz emission: From topological insulator to topological spintronics

Ferromagnet/nonmagnet (FM/NM) heterostructure under the excitation of femtosecond laser has proved to be a potential candidate for high-efficiency terahertz (THz) emission. Topological insulator (TI) is a novel two-dimensional (2D) material with a strong spin-orbital coupling, which endows this material with an extremely large spin-Hall angle. Thus, TI appears to be an attractive alternative to achieving higher-performance spintronic THz emitter when integrated with ferromagnetic material. In this paper, we discuss the ultrafast photocurrent response mechanism in TI film on the basis of the analysis of its crystal and band structures. The discussion of the mechanism reveals a relationship between THz radiation and external conditions, such as crystal orientation, polarized direction and chirality of the laser. Furthermore, we review the spintronic THz emission and manipulation in FM/NM heterostructure. The disclosed relationship between THz radiation and magnetization directions enables an effective control of the THz polarization by optimizing the system, such as by applying twisted magnetic field or fabricating cascade emitters. After integration, the FM/TI heterostructure presents a high efficiency and easy operation in THz radiation. This high-performance topological spintronic THz emitter presents a potential for the achievement of arbitrary polarization-shaping terahertz radiation.

Helicity-dependent photocurrent of the top and bottom Dirac surface states of epitaxial thin films of three-dimensional topological insulators Sb2Te3

Helicity-dependent photocurrent (HDPC) has been observed in epitaxial thin films of the three-dimensional topological insulator ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ with a thickness of 7, 20, and 30 quintuple layers (QLs). By analyzing the incident angle dependence of the HDPC, we successfully extract the photocurrent induced by the circular photon drag effect (CPDE) of even parity from the HDPC. It is found that the circular photogalvanic effect (CPGE) is larger than the odd parity of CPDE. The CPGEs of the top and bottom surface states are successfully distinguished at room temperature. As the thickness of the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ film is increased from 7 to 20 QLs, the dominant contribution of the CPGE current switches from the top surface states to the bottom surface states under the front illumination of a 1064-nm laser. Additionally, it is found that the CPGE currents of the 20- and 30-QL samples reverse sign when the temperature is increased from 77 to 300 K, which may be attributed to the HDPC current induced by the combined action of the inverse spin Hall effect and the vertical thermoelectric effect overwhelming the CPGE current at low temperature in the samples. The insights we obtained are significant for engineering optospintronic devices based on three-dimensional topological insulators with a strong thermoelectric effect.

Formation of the Magnetic Order in Three-Dimensional Topological Insulators for the Quantum Anomalous Hall Effect (Scientific Summary)

The topical problem of realization of the proper transverse conductivity in magnetic heterostructures based on topological insulators has been discussed. Our recent theoretical studies developing a universal approach to analytically simulate the quantum anomalous Hall effect at various ways for introducing a magnetic order into a topological insulator film have been briefly reviewed. Our model involves a consistent description of electronic states of the system, which determine the quantum anomalous Hall effect, with the reduction of the dimension from three-dimensional bulk through two-dimensional interface to one-dimensional edge. This makes it possible to emphasize the particular role of physical boundaries such as interfaces and side faces for spin-dependent transport in heterostructures. A number of new results recently obtained within the proposed approach have been reported. The data of recent experiments with new magnetic materials and structures have been discussed in terms of our theoretical results.

Tunable chiral and helical edge state transport in a magnetic topological insulator bilayer

The quantum anomalous Hall effect in magnetic topological insulators provides an ideal platform for building topological phases and devices. Here we construct a bilayer structure consisting of two magnetic topological insulator films with drastically different coercive field. In the intermediate field regime, the two quantum anomalous insulators have opposite spin orientation and counter-propagating edge states, thus realizing a synthetic quantum spin Hall phase. Multi-terminal transport measurements show that a moderate magnetic field can tune the system between chiral and helical edge state transport regimes. The interlayer transport realizes quantized spin-biased resistance, and the coupling between the two edges can be tuned by an epitaxially grown spacer layer in a sandwich structure. The tunable chiral and helical edge states in the quantum anomalous Hall bilayer may find unique applications in spintronics and topological quantum computation.

Terahertz Faraday and Kerr rotation spectroscopy of Bi1−xSbx films in high magnetic fields up to 30 tesla

We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of $\text{Bi}_{1-x}\text{Sb}_{x}$, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition $x$ increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed mini-coil magnet, we conducted measurements in magnetic fields up to 30~T, observing distinctly different behaviors between semimetallic ($x < 0.07$) and topological insulator ($x > 0.07$) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blue-shifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields ($>$20~T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi$_{1-x}$Sb$_x$.

Tuning topology in thin films of topological insulators by strain gradients

We theoretically show that the coupling of inhomogeneous strains to the Dirac fermions of three-dimensional topological insulators (3DTI) in thin film geometries results in the occurrence of phase transitions between topologically distinct insulating phases. By means of minimal k dot p models for strong 3DTI in the Bi 2 Se 3 materials class, we find that in thin films of stoichiometric materials a strain-gradient induced structure inversion asymmetry drives a phase transition from a quantum spin-Hall phase to a topologically trivial insulating phase. Interestingly, in alloys with strongly reduced bulk band gaps strain gradients have an opposite effect and promote a topologically non-trivial phase from a parent normal band insulator. These strain-gradient assisted switchings between topologically distinct phases are expected to yield a flexomagnetic coupling in magnetic topological insulator thin films.