Topological Insulator Thin Films Research Articles

Surface step states and Majorana end states in profiled topological-insulator thin films

The protected helical surface states in thin films of topological insulators (TI) are subject to inter-surface hybridisation. This leads to gap opening and spin texture changes as witnessed in photoemission and quasiparticle interference investigations. Theoretical studies show that universally the hybridisation energy exhibits exponential decay as well as sign oscillations as a function of film thickness, depending on the effective band parameters of the material. When a step is introduced in the TI film e.g. by profiling the substrate such that the hybridisation has different signs on both sides of the step, 1D bound states appear within the hybridisation gap which decay exponentially with distance from the step. The step bound states have linear dispersion and inherit the helical spin locking from the surface states and are therefore non-degenerate. When the substrate becomes an s-wave superconductor Majorana zero modes located at the step ends are created inside the superconducting gap. The proposed scenario involves just a suitably stepped interface of superconductor and TI and therefore may be a most simple device being able to host Majorana zero modes.

In-plane plasmon coupling in topological insulator Bi2Se3 thin films

The surface states of the 3D topological insulator (TI), Bi2Se3, are known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz spectral range. In TI thin films, the DPPs excited on the top and bottom surfaces couple, leading to an acoustic mode and an optical plasmon mode. Vertical coupling in these materials is, therefore, reasonably well-understood, but in-plane coupling among localized TI DPPs has yet to be investigated. In this paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that they exhibit dipole–dipole type coupling. The coupling becomes negligible when the lattice constant is greater than approximately 2.8 times the stripe width, which is comparable to results for in-plane coupling of localized plasmons excited on metallic nanoparticles or graphene plasmon polaritons. This understanding could be leveraged for the creation of TI-based metasurfaces.

Nonlocal sidewall response and deviation from exact quantization of the topological magnetoelectric effect in axion-insulator thin films

Topological insulator (TI) thin films with surface magnetism are expected to exhibit a quantized anomalous Hall effect (QAHE) when the magnetizations on the top and bottom surfaces are parallel, and a quantized topological magnetoelectric (QTME) response when the magnetizations have opposing orientations (axion insulator phase) and the films are sufficiently thick. We present a unified picture of both effects that associates deviations from exact quantization of the QTME caused by finite thickness with non-locality in the side-wall current response function. Using realistic tight-binding model calculations, we show that in $Bi_2Se_3$ TI thin films deviations from quantization in the axion insulator-phase are reduced in size when the exchange coupling of tight-binding model basis states to the local magnetization near the surface is strengthened. Stronger exchange coupling also reduces the effect of potential disorder, which is unimportant for the QAHE but detrimental for the QTME, which requires that the Fermi energy lie inside the gap at all positions.

The dependence of surface morphology on the growth temperature of the Pb0.7Sn0.3Te/Si(111) topological insulator thin films

The possibility of epitaxial growth of Pb0.7Sn0.3Te crystalline topological insulator films on the Si(111) surface was shown and epitaxial relations were found. It was shown that, depending on the growth temperature, it is possible to control not only the character of the morphology, but also, to a significant extent, the smoothness of the epitaxial layer surface, which is extremely important for further transport measurements of the films. Analysis of the grown films surface morphology made it possible to establish the average value of the height and lateral size of the terraces and islands forming Pb0.7Sn0.3Te surface.

Front Cover: Lifting the Spin‐Momentum Locking in Ultra‐Thin Topological Insulator Films (Adv. Quantum Technol. 11/2021)

In article number 2100083, Arthur Leis, Bert Voigtländer and co-workers demonstrate nanoscale four-probe measurements on atomically thin topological insulator films. For this purpose, four individual scanning tunneling microscope tips are directly contacting individual terraces on the sample surface to quantify the thickness-dependent film conductivity. This enables the authors to detect a breakdown of spin-momentum locking in the topological surface state as a function of the film thickness. In the few-layer limit, this effect can realize helical edge modes along step edges with potential applications in spintronics and quantum computing.

Optimization of the Growth of the Van der Waals Materials Bi2Se3 and (Bi0.5In0.5)2Se3 by Molecular Beam Epitaxy

The naturally existing chalcogenide Bi2Se3 is topologically nontrivial due to the band inversion caused by strong spin-orbit coupling inside the bulk of the material. The surface states are spin polarized, protected by the time-inversion symmetry, and thus robust to the scattering caused by non-magnetic defects. A high purity topological insulator thin film can be easily grown via molecular beam epitaxy (MBE) on various substrates to enable novel electronics, optics, and spintronics applications. However, the unique surface state properties have historically been limited by the film quality, which is evaluated by crystallinity, surface morphology, and transport data. Here we propose and investigate different MBE growth strategies to improve the quality of Bi2Se3 thin films grown by MBE. In addition, growths of topological trivial insulator (Bi0.5In0.5)2Se3 (BIS) are also investigated. BIS is often used as a buffer layer or separation layer for topological insulator heterostructures. Based on the surface passivation status, we have classified the substrates into two categories, self-passivated or unpassivated, and determine the optimal growth mechanisms on the representative sapphire and GaAs, respectively. Growth temperature is a crucial control parameter for the van der Waals epitaxy for both types of substrates. For Bi2Se3 on GaAs, the surface passivation status determines the dominant growth mechanism.

Quantized Goos-Hänchen shifts on the surface of hybridized topological insulator thin films

We study the Goos-Hänchen (GH) shift from the surface of a hybridized topological insulator (TI) thin film in the presence of a static magnetic field. We impinge a circularly polarized Gaussian beam on TI thin film substrate and calculate the lateral and angular GH shifts under partial reflection conditions. We show that magnetic field modulated giant positive and negative quantized spatial and angular beam shifts can be obtained by tuning the angle of incidence in the vicinity of Brewster angle and frequency in the terahertz regime. We also investigate the GH shifts in different topological phases of the TI thin film by driving the system through several topological quantum phase transitions (TQPTs), i.e., from topologically nontrivial to a semi metallic state and further to a band insulating state by manipulating the interplay between Zeeman and hybridization interactions. Furthermore, we investigate the effects of chemical potential on the GH shifts. Our results show that the sign of these shifts changes from negative to positive values as the chemical potential is changed from n-type to p-type. The work in this paper sheds important light on controlling GH shifts in different phases of a TI.

Anderson localization induced by spin-flip disorder in large-Chern-number quantum anomalous Hall effect

We numerically investigate the localization mechanisms of the quantum anomalous Hall effect (QAHE) with a large Chern number $\mathcal{C}$ in bilayer graphene and magnetic topological insulator thin films doped with nonmagnetic or spin-flip (magnetic) disorder. By calculating the modified Berry curvature in real space, we demonstrate that QAHEs in both systems turn into Anderson insulators when the disorder strength is large enough in the presence of nonmagnetic disorder. However, in the presence of spin-flip disorder, the localization mechanisms in the two systems are completely distinct. For ferromagnetic bilayer graphene with Rashba spin-orbit coupling, the QAHE with $\mathcal{C}=4$ first enters a metallic phase and then turns into an Anderson insulator with the increase of disorder strength, whereas for magnetic topological insulator thin films, the QAHE with $\mathcal{C}=\ensuremath{-}\mathcal{N}$ first enters a metallic phase, then turns into another QAHE with $\mathcal{C}=\ensuremath{-}(\mathcal{N}\ensuremath{-}1)$ as disorder strength increases, and finally turns into an Anderson insulator after $\mathcal{N}\ensuremath{-}1$ cycles between QAHE and metallic phases. The phase transitions in the two systems originate from the exchange of Berry curvature between conduction and valence bands. In the end, we provide a phenomenological picture related to the topological charges to help understand the underlying physical origins of the two different phase-transition mechanisms.

Modulated electronic heat capacity of topological crystalline insulator thin films

In this paper, we theoretically address the strain and electric field effects on the electronic heat capacity of TCI SnTe (001) thin film with the aid of low-energy Dirac theory, the Green's function technique, and the semi-classical Boltzmann approach. We found that the Schottky anomaly in TCI SnTe (001) thin films decreases with both uniaxial and biaxial strains and the electric field with different decreasing rates. Furthermore, the Schottky temperature does not change with strain, while it fluctuates with the electric field. Our results show that before and after a critical low-temperature, the electronic heat capacity behaves differently. Finally, the results confirm that the electronic heat capacity does not change significantly with strain and electric field at high temperatures due to the thermal effects (quantum effects are quite weak at high enough temperatures). The results may help to understand the exotic thermal phenomena in practical applications.

In-plane magnetic field induced helicity dependent photogalvanic effect on the surface states of topological insulators (BixSb1−x)2Te3

A hallmark signature of the three-dimensional (3D) topological insulator (TI) is that the spin-momentum locked massless Dirac fermions populate its surface states, where the carrier spins are locked to their momentum. Here, we report on the magnetic-field induced helicity dependent photogalvanic effect (MHPGE) of 3D TI thin films Bi2Te3 or (BixSb1−x)2Te3 of different thicknesses excited by near-infrared (1064 nm) under an in-plane magnetic field. It is found that the MHPGE current Jcx under the longitudinal geometry, i.e., Jcx∥Bx, is induced by the Larmor procession, while that under the transverse geometry, i.e., Jcx∥By, is mainly introduced by the hexagonal warping, which can be enhanced by the in-plane magnetic field. Our work demonstrates the possibility to tune the spin-polarized photocurrent of the surface states in 3D TIs via a magnetic field, which may be utilized to design new kinds of opto-spintronic devices.

Structural Characterization of Pb0.7Sn0.3Te Crystalline Topological Insulator Thin Films Grown on Si(111)

Structural Characterization of Pb0.7Sn0.3Te Crystalline Topological Insulator Thin Films Grown on Si(111)

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Quantum coherence modulation in bismuth selenide topological insulator thin film by ion irradiation

The robust surface states of bismuth selenide (Bi2Se3) topological insulator (TI) that could potentially be harnessed for quantum computing and spintronics applications are often eclipsed by defect-induced bulk carriers. With the objective of suppressing the electronic transport in the bulk, we explore the effects of ion irradiation-driven isoelectronic doping in thin films of Bi2Se3 with 600 keV antimony (Sb+) ions at 45° oblique incidence. The distribution of Sb+ ions introduced into the system has been mapped using Rutherford backscattering spectroscopy. The heavy Sb+ ion irradiation has been observed to cause a change from parabolic magnetoresistance to linear magnetoresistance behavior in the irradiated films due to mobility fluctuations. Subsequent annealing in selenium environment has been found to reverse the damage produced by the irradiation, but with significant modifications in the quantum coherence signatures. Weak antilocalization behavior has been observed in the weak magnetic field limit and has been analyzed within the purview of the Hikami-Larkin-Nagaoka model. It has been found that the channel-separated Bi2Se3 film becomes completely channel-mixed due to the induced defects, yielding a system with a single coherent channel upon irradiation. In the as-deposited film, the temperature-dependent power law decay of the coherence length has been found to have two different exponents within the measured temperature range. On the other hand, the irradiated-annealed film has a single decay exponent which has been found to tend to − 0.5, indicating that electron-electron interaction is the only de-phasing mechanism involved. This work establishes the effectiveness of ion irradiation in tuning the quantum coherence phenomena in Bi-based TI systems.

Light-induced topological phases in thin films of magnetically doped topological insulators

We study the photon-dressed electronic band structure of topological insulator thin films which could be also doped doped by magnetic impurities in response to an off-resonance time-periodic electromagnetic field. The thin films irradiated by a circularly polarized light undergo phase transition, in a driven system made by and a fascinating feature of distinct phases emerges in the phase diagram depending on the parameters such as frequency, intensity and polarization of the light. As a particular case, quantum anomalous Hall insulator phase is induced purely by the light-induced mass term with no need to any external magnetic field or even magnetization arising from the doped-doping magnetic impurities. Moreover, a novel phase, quantum pseudo-spin Hall insulator, emerges in the phase diagram leading to anisotropic helical edge states with zero total Chern number. We verify these achievements in the phase diagrams are supported by numerical calculations for a nanoribbon of the thin film for which the edge mode behavior is observed at several points on the phase diagram. The emergence of the mentioned topological phases and the edge modes are further confirmed by both calculating the Hall conductivity by means of the Kubo formula and the Chern number of each band. The effect of light parameters on the Landau level fan diagram in the presence of a perpendicular magnetic field indicates various topological phases occurring at higher Chern numbers.

Realization of the Chern-insulator and axion-insulator phases in antiferromagnetic MnTe/Bi2(Se,Te)3/MnTe heterostructures

Breaking time-reversal symmetry in three-dimensional topological insulator thin films can lead to different topological quantum phases, such as the Chern insulator (CI) phase, and the axion insulator (AI) phase. Using first-principles density functional theory methods, we investigate the onset of these two topological phases in a tri-layer heterostructure consisting of a Bi$_2$Se$_3$ (Bi$_2$Te$_3$) TI thin film sandwiched between two antiferromagnetic MnTe layers. We find that an orthogonal exchange field from the MnTe layers, stabilized by a small anisotropy barrier, opens an energy gap of the order of 10 meV at the Dirac point of the TI film. A topological analysis demonstrates that, depending on the relative orientation of the exchange field at the two interfaces, the total Chern number of the system is either ${\cal C} = 1$ or ${\cal C} = 0$, characteristic of the CI and the AI phase, respectively. Non-topological surface states inside the energy-gap region, caused by the interface potential, complicate this identification. Remarkably though, the calculation of the anomalous Hall conductivity shows that such non-topological surface states do not affect the topology-induced transport properties. Given the size of the exchange gap, we estimate that gapless chiral edge states, leading to the quantum anomalous Hall effect, should emerge on the sidewalls of these heterostructures in the CI phase for widths $\ge 200$ nm. We also discuss the possibility of inducing transitions between the CI and the AI phases by means of the spin-orbit torque caused by the spin Hall effect in an adjacent conducting layer.

Theory for linear and nonlinear planar Hall effect in topological insulator thin films

Recently, the linear and nonlinear planar Hall effect (PHE) on the topological insulators (TIs) surface has been extensively studied in experiments. To explain this phenomenon, various microscopic mechanisms are proposed theoretically, and one has to employ different mechanisms to separately understand the linear and nonlinear PHE even for the same system. Here, we study the planar magnetic resistance effect in TI thin film and find that a peculiar anisotropic scattering and a spin valve structure with respect to the PHE can be caused by the tilt and shift of Dirac cones, respectively, which are induced by the combination of spin-momentum locking of surface states and an in-plane magnetic field. The tilt and shift effects can act as the origin of both the linear and nonlinear PHE by distorting the spin texture of surface states or forming the spin polarization. These two mechanisms interplay and dominate, respectively, in strong coupling (thin TI) and decoupling (thick TI) between bottom and top surfaces. For thick TI film, we show that both the linear and nonlinear PHEs induced by the tilt effect can recover the results observed in recent experiments. Our theory provides a perspective to understand the origin of both linear and nonlinear PHE observed in recent experiments.

Disorder effects in topological insulator thin films

Thin films of topological insulators (TI) attract large attention because of expected topological effects from the inter-surface hybridization of Dirac points. However, these effects may be depleted by unexpectedly large energy smearing $\Gamma$ of surface Dirac points by the random potential of abundant Coulomb impurities. We show that in a typical TI film with large dielectric constant $\sim 50$ sandwiched between two low dielectric constant layers, the Rytova-Chaplik-Entin-Keldysh modification of the Coulomb potential of a charge impurity allows a larger number of the film impurities to contribute to $\Gamma$. As a result, $\Gamma$ is large and independent of the TI film thickness $d$ for $d > 5$ nm. In thinner films $\Gamma$ grows with decreasing $d$ due to reduction of screening by the hybridization gap. We study the surface conductivity away from the neutrality point and at the neutrality point. In the latter case, we find the maximum TI film thickness at which the hybridization gap is still able to make a TI film insulating and allow observation of the quantum spin Hall effect, $d_{\max} \sim 7$ nm.

Influence of post-deposition annealing on the transport properties of sputtered Bi2Se3 thin films

The fabrication of chalcogenide-based topological insulator (TI) thin films with low defects and vacancies is an important research problem. In the past decade, Bi2Se3 is one of the most investigated TI materials. The physical properties of Bi2Se3 are dictated by selenium vacancies irrespective of the growth method. Controlling the number of vacancies is very challenging, but it can be influenced by substrate and growth temperature. In this work, we demonstrate that post-deposition annealing temperature is also effective in controlling the carrier concentration of Bi2Se3 thin films. Ultra-thin Bi2Se3 films were fabricated on quartz substrates using magnetron sputtering at room temperature, and their resistivity, bulk carrier concentration, and bulk mobility are measured as a function of the post-deposition annealing temperature under vacuum conditions ranging from 180–350 °C. We find that the carrier concentration can vary by an order of magnitude, and we obtained values as low as ~1 × 1019 cm−3 between 200-250 °C which compares very well with some of the best literature reports on films made by molecular beam epitaxy. Room temperature bulk mobility values in excess of 100 cm2/Vs were recorded for the optimally annealed samples. Energy dispersive x-ray spectroscopy measurements showed that selenium-rich Bi2Se3 films have the highest carrier concentration values. X-ray diffraction measurements showed that the best crystallographic properties are obtained for stoichiometric Bi2Se3. Our results indicate that higher mobility values may be possible if better crystal structure and lower resistivity are obtained in selenium-rich films.

Effects of post-annealing on crystalline and transport properties of Bi2Te3 thin films* *Project supported by the National Natural Science Foundation of China (Grant Nos. 52072030, 52071025, and 51871018), the Beijing Outstanding Young Scientists Projects (Grant No. BJJWZYJH01201910005018), Beijing Natural Science Foundation, China (Grant No. Z180014), the Science and Technology Innovation Team Program of Foshan (Grant No. FSOAA-KJ919-4402-0087), and Beijing Laboratory

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 Bi2Te3 films with the carrier density down to 4.0 × 1013 cm−2. In contrast to the most-used method of high substrate temperature growth, we firstly sputtered Bi2Te3 thin films at room temperature and then applied post-annealing. It enables the growth of highly-oriented Bi2Te3 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 Bi2Te3 thin films to be integrated with other materials in wafer-level scale for electronic and spintronic applications.

Gate Tunable Supercurrent in Josephson Junctions Based on Bi2Te3 Topological Insulator Thin FilmsSupported by the Basic Science Center Project of the National Natural Science Foundation of China (Grant No. 51788104), and the National Key R&D Program of China (Grant No. 2017YFA0302900).

We report transport measurements on Josephson junctions consisting of Bi2Te3 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.