Topological Insulator Thin Films Research Articles

Quantum Anomalous Parity Hall Effect in Magnetically Disordered Topological Insulator Films.

In magnetically doped thin-film topological insulators, aligning the magnetic moments generates a quantum anomalous Hall phase supporting a single chiral edge state. We show that as the system demagnetizes, disorder from randomly oriented magnetic moments can produce a "quantum anomalous parity Hall" phase with helical edge modes protected by a unitary reflection symmetry. We further show that introducing superconductivity, combined with selective breaking of reflection symmetry by a gate, allows for creation and manipulation of Majorana zero modes via purely electrical means and at zero applied magnetic field.

Spin and charge transport of electron on a mesoscopic ring of topological insulator thin film in uniform magnetic field

Spin dynamics and persistent spin and charge currents of an electron on a mesoscopic ring of topological insulator (TI) thin film in a uniform magnetic field are investigated. We find that the circular symmetry of TI in the magnetic field leads to a shift of the valence band maxima and conduction band minima in the energy spectrum from the charge neutrality point depending on the strength of the magnetic field, in addition to the bandgap induced by the hybridization and Zeeman energies. The numerical analysis of the dynamical equations obtained from the Heisenberg equation of motion shows that the tangential, radial, and longitudinal components of the electron’s spin exhibit periodic oscillations. Interestingly, the longitudinal component of spin polarized current vanishes due to the inversion symmetry along the cylindrical axis, whereas its x- and y-components oscillate with a finite phase shift. The persistent charge current on the ring of the TI thin film changes sign from positive to negative approaching maximal saturated values at large magnetic fluxes. Moreover, we investigate the effect of dephasing on persistent currents when the ring is coupled to an electron reservoir. Strikingly, both charge and spin persistent currents dissipate significantly with increasing the coupling parameter.

Size Effect in the Electrical Conductivity of Thin Films of Topological Insulator Bi2Se3

The electrical resistivity of thin films of topological insulator (TI) Bi2Se3 10 to 75 nm thick is measured in the temperature range of 4.2 to 300 K. A size effect is observed in the electrical conductivity of the Bi2Se3 films; i.e., there is a linear dependence of a film’s conductivity on its inverse thickness. It is assumed that a similar effect can be observed in other TIs and in systems with nonuniform distributions of current over the cross section of a sample.

Structure and basal twinning of topological insulator Bi2Se3 grown by MBE onto crystalline Y3Fe5O12

Whereas thin films of topological insulators grown by molecular beam epitaxy often display regular, triangular features, $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ films grown onto yttrium iron garnet (YIG) display much greater disorder. Here, we present observations of various types of disorder present in these films using atomic force microscopy and scanning transmission electron microscopy. The investigation reveals the presence of an amorphous metal oxide layer between the substrate and the film, which appears to smooth out the nanometer-scale undulations in the YIG surface. It also shows the existence of quasiordered arrays of heavy atoms in some interfacial regions, as well as rotations and tilting between adjacent grains and basal twinning at various heights in the film. Using density functional theory, we explore the impact of these prominent basal twins on the electronic structure of the film.

The growth of Bi2Te3 topological insulator films: Physical vapor transport vs molecular beam epitaxy

Bi2Te3 topological insulator thin films have been obtained by several techniques. For future applications in spintronics or quantum computing high quality homogenous films with few defects and impurities are required. Molecular beam epitaxy has been widely used for the growth of these materials, however, this technique operates in ultra-high vacuum conditions which significantly increases its cost. In this work, the use of the physical vapor transport technique is explored as a low cost alternative. A comparison of samples obtained by both methods allowed to conclude that they do not have significant differences regarding the structural and morphological properties in spite of the film thickness difference. Moreover, ARPES measurements indicate bulk insulating behavior with Dirac-like, conducting surface states for all samples. This demonstrates the feasibility of the physical vapor transport technique as a simpler and cost-effective alternative to molecular beam epitaxy, for the growth of Bi2Te3.

Absence of strong localization at low conductivity in the topological surface state of low-disorder Sb2Te3

We present low-temperature transport measurements of a gate-tunable thin film topological insulator system that features high mobility and low carrier density. Upon gate tuning to a regime around the charge neutrality point, we infer an absence of strong localization even at conductivities well below $e^2/h$, where two dimensional electron systems should conventionally scale to an insulating state. Oddly, in this regime the localization coherence peak lacks conventional temperature broadening, though its tails do change dramatically with temperature. Using a model with electron-impurity scattering, we extract values for the disorder potential and the hybridization of the top and bottom surface states.

Observation of Interfacial Antiferromagnetic Coupling between Magnetic Topological Insulator and Antiferromagnetic Insulator.

Inducing magnetic orders in a topological insulator (TI) to break its time reversal symmetry has been predicted to reveal many exotic topological quantum phenomena. The manipulation of magnetic orders in a TI layer can play a key role in harnessing these quantum phenomena toward technological applications. Here we fabricated a thin magnetic TI film on an antiferromagnetic (AFM) insulator Cr2O3 layer and found that the magnetic moments of the magnetic TI layer and the surface spins of the Cr2O3 layers favor interfacial AFM coupling. Field cooling studies show a crossover from negative to positive exchange bias clarifying the competition between the interfacial AFM coupling energy and the Zeeman energy in the AFM insulator layer. The interfacial exchange coupling also enhances the Curie temperature of the magnetic TI layer. The unique interfacial AFM alignment in magnetic TI on AFM insulator heterostructures opens a new route toward manipulating the interplay between topological states and magnetic orders in spin-engineered heterostructures, facilitating the exploration of proof-of-concept TI-based spintronic and electronic devices with multifunctionality and low power consumption.

Interface effects on the magnetic-proximity-induced quantized Hall response in heterostructures based on three-dimensional topological insulators

We report a theoretical study of the spin-dependent transport properties in heterostructures containing a three-dimensional topological insulator (TI) thin film and ferromagnetic normal insulator (FMNI) slab. Within the framework of a continual approach for the FMNI/TI/FMNI trilayer model, we reveal how the magnetic proximity effect at the TI/FMNI interface can influence an intrinsic Hall response of the system. We predict that the FMNI/TI/FMNI trilayer undergoes a transition into the quantum anomalous Hall phase either from the topologically trivial phase or from the quantum spin Hall phase, which is controlled by tuning the proximity-induced exchange field, the TI film thickness, and the TI band structure parameters. We draw the corresponding phase diagram of the FMNI/TI/FMNI trilayer. Moreover, we argue that the roughness at the TI/FMNI interfaces can cause the decomposition of the TI film into topologically distinct domains, which affects the Hall conductivity. We discuss the specifics of manifestation and complexities of observation of quantized conductivity in realistic TI/FMNI heterostructures.

Signatures of induced superconductivity in AlOx-capped topological heterostructures

In order to access exotic Dirac and Majorana states in (Bi,Sb)-based topological insulators (TIs), the physical surface of those crystals should not be exposed to air. 2–3 nm of in situ deposited Al on top of pristine TI thin films immediately oxidizes after taking the sample to ambient conditions. The native AlOx provides a favorable hard capping, which preserves the topological surface states during ex situ device fabrication. Here, we present a process on how to construct superconductor – topological insulator – superconductor (S-TI-S) junctions from in situ capped thin films comprised of 15 nm Sb2Te3 on top of 6 nm Bi2Te3. The thicknesses of the Sb2Te3 and the Bi2Te3 layer allow us to precisely tune the Fermi level of the upper surface of the Sb2Te3 layer. The challenge is to provide a transparent interface between Sb2Te3 and the superconductive Nb, while assuring an AlOx-capped weak link in between two closely separated Nb electrodes. Low temperature experiments on our junctions provide evidence for charge transport mediated by coherent Andreev states. Magnetic field dependent measurements yielded Fraunhofer-like patterns, whose periodicities are in good agreement with the effective areas of the respective junctions. Transmission electron micrographs of the narrowest junction confirm a crystalline and capped weak link. Our results provide the first reported signatures of induced superconductivity in S-TI-S junctions, which are capped by native AlOx. The presented process allows for accessing S-TI hybrid devices via magnetic flux, while assuring in situ conserved weak links. This makes as-prepared junctions a promising platform for proposed flux-controllable Majorana devices.

Nonvolatile and Reversible Ferroelectric Control of Electronic Properties of Bi2Te3 Topological Insulator Thin Films Grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals.

Single-phase (00 l)-oriented Bi2Te3 topological insulator thin films have been deposited on (111)-oriented ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) single-crystal substrates. Taking advantage of the nonvolatile polarization charges induced by the polarization direction switching of PMN-PT substrates at room temperature, the carrier density, Fermi level, magnetoconductance, conductance channel, phase coherence length, and quantum corrections to the conductance can be in situ modulated in a reversible and nonvolatile manner. Specifically, upon the polarization switching from the positively poled Pr+ state (i.e., polarization direction points to the film) to the negatively poled Pr- (i.e., polarization direction points to the bottom electrode) state, both the electron carrier density and the Fermi wave vector decrease significantly, reflecting a shift of the Fermi level toward the Dirac point. The polarization switching from Pr+ to Pr- also results in significant increase of the conductance channel α from -0.15 to -0.3 and a decrease of the phase coherence length from 200 to 80 nm at T = 2 K as well as a reduction of the electron-electron interaction. All these results demonstrate that electric-voltage control of physical properties using PMN-PT as both substrates and gating materials provides a simple and a straightforward approach to realize reversible and nonvolatile tuning of electronic properties of topological thin films and may be further extended to study carrier density-related quantum transport properties of other quantum matter.

Robustness of topological states in Bi2Se3 thin film grown by Pulsed Laser Deposition on (0 0 1)-oriented SrTiO3 perovskite

We report on the reproducible surface topological electron states in Bi2Se3 topological insulator thin films when epitaxially grown by Pulsed Laser Deposition (PLD) on (001)-oriented SrTiO3 (STO) perovskite substrates. Bi2Se3 has been reproducibly grown with single (001)-orientation and low surface roughness as controlled by ex-situ X-ray diffraction and in situ scanning tunnel microscopy and low-energy electron diffraction. Finally, in situ synchrotron radiation angle-resolved photo-emission spectroscopy measurements show a single Dirac cone and Dirac point at EB∼0.38 eV located in the center of the Brillouin zone likewise found from exfoliated single-crystals. These results demonstrate that the topological surface electron properties of PLD-grown Bi2Se3 thin films grown on (001)-oriented STO substrates open new perspectives for applications of multi-layered materials based on oxide perovskites.

Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films.

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all-optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi2 Se3 /ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi2 Se3 or Co, a giant terahertz emission is observed in the heterostructure that originates from spin-to-charge conversion, in which the topological surface states play a crucial role. A 0.12 ps timescale is identified that sets a technological speed limit of spin-to-charge conversion processes in topological insulators. In addition, it is shown that the spin-to-charge conversion efficiency is temperature independent in Bi2 Se3 as expected from the nature of the surface states, paving the way for designing next-generation high-speed optospintronic devices based on topological insulators at room temperature.

Optical control of spin-polarized photocurrent in topological insulator thin films

Dirac electrons in topological insulators (TIs) provide one possible avenue to achieve control of photocurrents and spin currents without the need to apply external fields by utilizing characteristic spin-momentum locking. However, for TI crystals with electrodes it is actually difficult to characterize the net flow of spin-polarized photocurrents because of the coexistence of surface carriers and bulk carriers generated by optical excitations. We demonstrate here that the net flow directions of spin-polarized photocurrents in TI polycrystalline thin films without electrodes can be precisely and intentionally controlled by the polarization of the excitation pulse alone, which is characterized by performing time-domain terahertz (THz) wave measurements and time-resolved magneto-optical Kerr rotation measurements that are non-contact methods. We show that the amplitudes of s-polarized THz waves radiated from photocurrents under right- and left-circularly polarized excitations are inverted relative to one another. Moreover, we observe the inversion of time-resolved magneto-optical Kerr rotation signals between the two excitations. Our results will open the way as innovative methods to control spin-polarized electrons in optoelectronic and spintronic TI devices without the need to apply external fields.

Strongly Enhanced Thermoelectric Performance over a Wide Temperature Range in Topological Insulator Thin Films

Thermoelectric (TE) devices have been attracting increasing attention because of their ability to convert heat directly to electricity. To date, improving the TE figure of merit remains the key cha...

Impurity Effects at Surfaces of a Photon-Dressed Bi2Se3 Thin Film**Supported by the National Key R&D Program of China under Grant No 2017YFA0303203, and the National Natural Science Foundation of China under Grant Nos 11574217 and 11474149.

We investigate the impurity effects on surfaces of a thin film topological insulator, applied by an off-resonant circular polarized light. It is found that the off-resonant driving induces a quantized total Hall conductivity, when the driving strength is larger than a critical value and the Fermi level lies in the band gap, indicating that our system is converted into the topological phase. We also find that with the increasing disorder strength, the Dirac masses of top and bottom surfaces are renormalized and then fixed to half of their initial values, respectively, which will shrink the widths of the half-integer plateau of anomalous Hall conductivities.

In situ disentangling surface state transport channels of a topological insulator thin film by gating

In the thin film limit, the surface state of a three-dimensional topological insulator gives rise to two parallel conduction channels at the top and bottom surface of the film, which are difficult to disentangle in transport experiments. Here, we present gate-dependent multi-tip scanning tunneling microscope transport measurements combined with photoemission experiments all performed in situ on pristine BiSbTe3 thin films. To analyze the data, we develop a generic transport model including quantum capacitance effects. This approach allows us to quantify the gate-dependent conductivities, charge carrier concentrations, and mobilities for all relevant transport channels of three-dimensional topological insulator thin films (i.e., the two topological surface state channels, as well as the interior of the film). For the present sample, we find that the conductivity in the bottom surface state channel is minimized below a gate voltage of Vgate = −34 V and the top surface state channel dominates the transport through the film.

Spin accumulation in topological insulator thin films—influence of bulk and topological surface states

Current-induced spin torques in topological insulators (TIs) include both field-like torques as well as damping-like torques. In many experimental spin torque measurements, the Fermi energies are such that both of the topological surface states (TSSs) and the bulk states are occupied. In this work, we calculate the electric field-induced spin accumulation that leads to these torques in TI thin film systems, and account for the contributions of both of the TSSs as well as bulk states in the presence of interfacial band bending and disorder scattering. We analyze how the band structure of a TI thin film is affected by the film thickness, magnetization coupling, and band bending. The variation of the band structure due to these factors in turn affects the spin accumulation along both the in-plane and out-of-plane directions, and hence the induced spin torques. We show that the in-plane spin accumulation is influenced more by the TSS, whereas the bulk bands have a significant effect on the out-of-plane spin accumulation. The predicted variation of spin accumulation with sample thickness, and doping, substrate, and ferromagnetic material choices, mirrors the widely varying spin torque measurements in experiments.

Effect of high-frequency pumping on thin-film topological insulators

Thin-film topological insulators (TIs) with the thickness below 10 nm are characterized by the presence of a gap in the spectrum of topologically protected surface states. This unusual behavior, also known as anomalous finite size effects, is associated with the hybridization between the states propagating along the opposite boundaries of thin-slab TI. In this work we consider a bismuth-based TI and show how an intense high-frequency linearly polarized light can be used for the tuning of the value of a gap. We also theoretically predict the effect of band inversion in the spectrum of the surface states under external electromagnetic pumping.

Topological Transitions Induced by Antiferromagnetism in a Thin-Film Topological Insulator

Magnetism in topological insulators (TIs) opens a topologically nontrivial exchange band gap, providing an exciting platform for manipulating the topological order through an external magnetic field. Here, we show that the surface of an antiferromagnetic thin film can magnetize the top and the bottom TI surface states through interfacial couplings. During the magnetization reversal, intermediate spin configurations are ascribed from unsynchronized magnetic switchings. This unsynchronized switching develops antisymmetric magnetoresistance spikes during magnetization reversals, which might originate from a series of topological transitions. With the high Néel ordering temperature provided by the antiferromagnetic layers, the signature of the induced topological transition persists up to ∼90 K.

Part-per-million quantization and current-induced breakdown of the quantum anomalous Hall effect.

In the quantum anomalous Hall effect, quantized Hall resistance and vanishing longitudinal resistivity are predicted to result from the presence of dissipationless, chiral edge states and an insulating two-dimensional bulk, without requiring an external magnetic field. Here, we explore the potential of this effect in magnetic topological insulator thin films for metrological applications. Using a cryogenic current comparator system, we measure quantization of the Hall resistance to within one part per million and, at lower current bias, longitudinal resistivity under 10 mΩ at zero magnetic field. Increasing the current density past a critical value leads to a breakdown of the quantized, low-dissipation state, which we attribute to electron heating in bulk current flow. We further investigate the prebreakdown regime by measuring transport dependence on temperature, current, and geometry, and find evidence for bulk dissipation, including thermal activation and possible variable-range hopping.