Topological Insulator Thin Films Research Articles

Electric field tuning of the optical absorbance of topological insulator thin films

In view of electro-optic modulators and switches operating in the terahertz frequency range, we study the absorbance of a topological insulator (TI) thin film subjected to a static electric field. Adopting the Hamiltonian for the three-dimensional ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ class of TIs including terms to second order in the electron wave vector and to first order in the electric field, we present an effective model for a TI thin film. We demonstrate the distinct influences of in-plane and out-of-plane external electric fields, the in-plane electric field-induced dichroism, and the electric field and chemical potential tuning of the absorbance edge. Under the influence of an out-of-plane electric field, a TI film with a thickness of about 2 nm exhibits a considerable absorbance at the absorbance edge. Of prime practical importance, the absorbance edge can be shifted about $0.3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ with the application of moderate electric fields about $0.4\phantom{\rule{0.28em}{0ex}}\mathrm{V}{\mathrm{nm}}^{\ensuremath{-}1}$ or with chemical potential variations about $0.3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$.

Quantum anomalous Hall effect in perfectly compensated collinear antiferromagnetic thin films

We show that the quantum anomalous Hall effect almost always occurs in magnetic topological insulator thin films whenever the top and bottom surface layer magnetizations are parallel at some temperature, independent of the interior layer magnetization configuration. Using this criterion, we identify structures that have a quantum anomalous Hall effect even though they have collinear magnetic structures with no net magnetization, and discuss strategies for realizing these interesting magnetic states experimentally.

Visible to near-infrared broadband photodetector employing thin film topological insulator heterojunction (p-TlBiSe2/n-Si) diode

The work presents a study of quasi-2D thin topological insulator film of TlBiSe2 and its heterojunction TlBiSe2/Glass and TlBiSe2/Si by thermal coating. The vibrational modes were identified in both the heterojunctions using Raman spectroscopy with the identification of the surface phonon mode (SPM) in the TlBiSe2/Si heterojunction. Ultrafast dynamics were studied for the relaxation dynamics of the charge carriers in these heterojunctions. The study identified the interface phenomenon in the form of charge states (CT) in TlBiSe2/Si p-n heterojunction. The p-TlBiSe2/n-Si heterojunction showed good p-n diode characteristics with a rectification ratio under the dark. Its optical response was studied by varying optical power (2.37–3.78 µW) and wavelength (500–1900 nm). It showed excellent photoresponse in the visible to near-infrared (NIR) range which peaks at 900 nm wavelength in both forward as well reverse bias. The explanation of its optical and electrical performance was modeled in terms of band structure, surface states, and interface phenomenon. The study is important in terms of the implementation of next-generation topological insulator-based photodetectors.

Spin transport properties in ferromagnet/superconductor junctions on topological insulator

The spin-dependent Andreev reflection is investigated theoretically by analyzing the electronic transport in a thin-film topological insulator (TI) ferromagnet/superconductor (FM/SC) junction. The tunneling conductance and shot noise are calculated based on the Dirac–Bogoliubov–de Gennes equation and Blonder–Tinkham–Klapwijk theory. It is found that the magnetic gap in ferromagnet can enhance the Andreev retro-reflection but suppress the specular Andreev reflection. The gate potential applied to the electrode on top of superconductor can enhance the two types of reflections. There is a transition between the two types of reflections at which both the tunneling conductance and differential shot noise become zero. These results provide a method to realize and detect experimentally the intra-band specular Andreev reflection in thin film TI-based FM/SC structures.

Enhanced and stable spin Hall conductivity in a disordered time-reversal and inversion symmetry broken topological insulator thin film

We consider a disordered topological insulator thin film placed on the top of a ferromagnetic insulator with a perpendicular exchange field M and subjected to a perpendicular electric field. The presence of ferromagnetic insulator causes that bottom surface states of the topological insulator thin film become spin polarized and the electric field provides a potential difference V between the two surface states, resulting in breaking of time-reversal and inversion symmetry in the system. Using Kubo formalism and employing the first Born approximation as well as the self-consistent Born approximation, we calculate the spin Hall conductivity. We find that for small values of V, a large spin conductivity can be generated through large values of M away from the charge neutrality point. But for large values of V, the spin conductivity can be promoted even with small values of M around the charge neutrality point. The effect of vertex corrections and the stability of the obtained large spin conductivity against disorders are also examined.

Effects of Crystalline Disorder on Interfacial and Magnetic Properties of Sputtered Topological Insulator/Ferromagnet Heterostructures

Thin films of Topological insulators (TIs) coupled with ferromagnets (FMs) are excellent candidates for energy-efficient spintronics devices. Here, the effect of crystalline structural disorder of TI on interfacial and magnetic properties of sputter-deposited TI/FM, Bi2Te3/Ni80Fe20, heterostructures is reported. Ni and a smaller amount of Fe from Py was found to diffuse across the interface and react with Bi2Te3. For highly crystalline c-axis oriented Bi2Te3 films, a giant enhancement in Gilbert damping is observed, accompanied by an effective out-of-plane magnetic anisotropy and enhanced damping-like spin-orbit torque (DL-SOT), possibly due to the topological surface states (TSS) of Bi2Te3. Furthermore, a spontaneous exchange bias is observed in hysteresis loop measurements at low temperatures. This is because of an antiferromagnetic topological interfacial layer formed by reaction of the diffused Ni with Bi2Te3 which couples with the FM, Ni80Fe20. For increasing disorder of Bi2Te3, a significant weakening of exchange interaction in the AFM interfacial layer is found. These experimental results Abstract length is one paragraph.

Surface plasmon-phonon-magnon polariton in a topological insulator-antiferromagnetic bilayer structure

We present a robust technique for computationally studying surface polariton modes in hybrid materials. We use a semiclassical model that allows us to understand the physics behind the interactions between collective excitations of the hybrid system and develop a scattering and transfer matrix method that imposes the proper boundary conditions to solve Maxwell's equations and derive a general equation describing the surface polariton in a heterostructure consisting of N constituent materials. We apply this method to a test structure composed of a topological insulator (TI) and an antiferromagnetic material (AFM) to study the resulting surface Dirac plasmon-phonon-magnon polariton (DPPMP). We find that interactions between the excitations of the two constituents result in the formation of hybridized modes and the emergence of avoided-crossing points in the dispersion relations for the DPPMP. For the specific case of a $\mathrm{Bi}_{2}\mathrm{Se}_{3}$ TI material combined with a 3D AFM such as NiO, $\mathrm{MnF}_{2}$, or $\mathrm{FeF}_{2}$, the polariton branch with low frequency below 2 THz redshifts upon increasing the thickness of TI thin film, which leads to an upper bound on the thickness of the TI layer that will allow an observable signature of strong coupling and the emergence of hybridized states. We also find that the strength of the coupling between the TI and the AFM, which is parametrized by the amplitude of the avoided-crossing splitting between the two polariton branches at the magnon resonance frequency, depends on the magnitude of the magnetic dipole and the linewidth of the magnon in the AFM material as well as on the Fermi energy of Dirac plasmon in the TI. Finally, we show that materials with extremely high quality, i.e., low scattering loss rate, are essential to achieve an experimentally observable strong coupling between a TI and 3D AFM material. The overall analysis identifies the material properties that are necessary to achieve experimentally observable strong coupling for the interaction between THz excitations in a TI/AFM heterostructure and can thereby guide experimental efforts.

Magnetotransport of thin film topological insulators in proximity with helical spin density wave

The magnetoconductivity of an ultrathin film of topological insulator in proximity with helical spin density wave is calculated. Pronounced Weiss oscillations periodic in the inverse magnetic field appear for upper and bottom surfaces. With finite Zeeman coupling and hybridization across the surfaces Weiss oscillations for the top surface are more pronounced than the bottom surface. However in the absence of Zeeman coupling both the surfaces show same magnitude of oscillations.

Correlated disorder induced anomalous transport in magnetically doped topological insulators

We examine the transport properties of magnetically doped topological insulator (TI) thin films subject to correlated non-magnetic disorder. For the disorder we choose a quasi-periodic potential with a random phase. We restrict the disorder to a central region, which is coupled to two leads in a clean quantum spin Hall insulator (QSHI) state and concentrate on different orientations of the quasi-periodicity in the two-dimensional central region. In the case of a diagonally oriented or purely longitudinal quasi-periodicity we find different topological Anderson insulator (TAI) phases, with a quantum anomalous Hall insulator (QAHI), a quantum spin Chern insulator (QSCI), or a QSHI phase being realized before the Anderson insulation takes over at large disorder strength. Quantized transport from extended bulk states is found for diagonal quasi-periodicity in addition to the above TAI phases that are also observed for the case of uncorrelated disorder. For a purely transverse orientation of the quasi-periodicity the emerging QSHI and QSCI phases persist to arbitrarily strong disorder potential. These topological phase transitions (except to the Anderson insulator phase), can be understood from a self consistent Born approximation.

Controlling the hybridization gap and transport in a thin-film topological insulator: Effect of strain, and electric and magnetic field

In a thin-film topological insulator (TI), the edge states on two surfaces may couple by quantum tunneling, opening a gap known as the hybridization gap. Controlling the hybridization gap and transport has a variety of potential uses in photodetection and energy-harvesting applications. In this paper, we report the effect of strain, and electric and magnetic field, on the hybridization gap and transport in a thin ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ film, investigated within the tight-binding theoretical framework. We demonstrate that vertical compression decreases the hybridization gap, as does tensile in-plane strain. Applying an electric field breaks the inversion symmetry and leads to a Rashba-like spin splitting proportional to the electric field, hence closing and reopening the gap. The influence of a magnetic field on thin-film TI is also discussed, starting from the role of an out-of-plane magnetic field on quantum Hall states. We further demonstrate that the hybridization gap can be controlled by an in-plane magnetic field, and that by applying a sufficiently strong field a quantum phase transition from an insulator to a semimetal can be achieved.

Magneto-optical absorption properties of topological insulator thin films

We theoretically study the magneto-optical absorption coefficients (MOACs) and the refractive index changes (RICs) due to both intra- and inter-band transitions in topological insulator (TI) thin films. The interplay between Zeeman energy and hybridization contribution leads to a transition between the normal insulator phase and the TI phase. The difference in the optical response in these two phases as well as at the phase transition point has been analyzed. The influence of the electron density, magnetic field, and temperature on the MOACs and RICs in both intra- and inter-band transitions is investigated. Our results show that the electron density affects directly the threshold energy. At a finite temperature, the thermal excitation causes the triggering of some new transitions which do not appear at T = 0 K. Evidence of the half-peak feature of the first inter-band transition is also found in TI thin films.

Surface‐Driven Evolution of the Anomalous Hall Effect in Magnetic Topological Insulator MnBi2Te4 Thin Films

AbstractUnderstanding the effects of the interfacial modification to the functional properties of magnetic topological insulator thin films is crucial for developing novel technological applications from spintronics to quantum computing. Here, a large electronic and magnetic response is reported to be induced in the intrinsic magnetic topological insulator MnBi2Te4 by controlling the propagation of surface oxidation. It is shown that the formation of the surface oxide layer is confined to the top 1–2 unit cells but drives large changes in the overall magnetic response. Specifically, a dramatic reversal of the sign of the anomalous Hall effect is observed to be driven by finite thickness magnetism, which indicates that the film splits into distinct magnetic layers each with a unique electronic signature. These data reveal a delicate dependence of the overall magnetic and electronic response of MnBi2Te4 on the stoichiometry of the top layers. This study suggests that perturbations resulting from surface oxidation may play a non‐trivial role in the stabilization of the quantum anomalous Hall effect in this system and that understanding targeted modifications to the surface may open new routes for engineering novel topological and magnetic responses in this fascinating material.

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

Thin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films.

Particle–hole asymmetry and quantum confinement effects on the magneto-optical response of topological insulator thin-films

Intrinsically broken symmetries in the bulk of topological insulators (TIs) are manifested in their surface states. Despite particle–hole asymmetry in TIs, it has often been assumed that their surface states are characterized by a particle–hole symmetric Dirac energy dispersion. In this work, we demonstrate that the effect of particle–hole asymmetry is essential to correctly describe the energy spectrum and the magneto-optical response in TIs thin-films. In thin-films of TIs with a substantial degree of particle–hole symmetry breaking, such as Sb2Te3, the longitudinal optical conductivity displays absorption peaks arising from optical transitions between bulk and surface Landau levels for low photon energies. The transition energies between the bulk and surface Landau levels exhibit clearly discernable signatures from those between surface Landau levels due to their distinct magnetic field dependence. Bulk contributions to the magneto-optical conductivity in a TI thin-film are enhanced via one type of doping while being suppressed by the other. This asymmetric dependence on the type of doping aids in revealing the particle–hole asymmetry in TI thin-films.

Thermopower and magnetotransport properties of Bi100−xSbx topological insulator thin films prepared by flash evaporation

We have measured the temperature dependence of resistance R(T), thermopower S(T), magnetoresistance (MR) and the Hall effect (HE) of Bi80Sb20, Bi85Sb15 and Bi90Sb10 topological insulator thin films. Samples were prepared by sequential flash-evaporation at room temperature and annealing at T = 350 K. The R(T) of the three investigated samples show metallic-like behavior at temperatures less than T = 75 K, while at higher temperatures, R(T) curves show a semiconducting-like behavior. The thermopower S(T) of the three investigated samples is negative in the entire temperature range measured in this work, with a linear behavior from 5 K up to ≈100 K. The magnetoresistance of all samples is positive with a small temperature dependence. The highest MR(B = 7 T) was observed in Bi85Sb15 with a ≈600% and ≈125% change at 5 K and 300 K, respectively. Clear evidence of weak antilocalization contribution to the MR was observed only in sample Bi85Sb15 at temperatures T < 75 K. Quantum oscillations in the MR originating from the Fermi surface, which has a clear two-dimensional character, were observed in sample Bi85Sb15 up to ≈21 K. Carrier mobility information of sample Bi85Sb15 was extracted from low field HE data, showing a remarkably high value of μ ≈ 2.8 × 104 cm2/Vs at 5 K, with a small decrease for increasing temperature.

Lamellae preparation for atomic-resolution STEM imaging from ion-beam-sensitive topological insulator crystals

Good specimen quality is a key factor in achieving successful scanning transmission electron microscope analysis. Thin and damage-free specimens are prerequisites for obtaining atomic-resolution imaging. Topological insulator single crystals and thin films in the chalcogenide family such as Sb2Te3 are sensitive to electron and ion beams. It is, therefore, challenging to prepare a lamella suitable for high-resolution imaging from these topological insulator materials using standard focused ion-beam instruments. We have developed a modified method to fabricate thin focused ion-beam (FIB) lamellae with minimal ion-beam damage and artifacts. The technique described in the current study enables the reliable preparation of high-quality transmission electron microscope (TEM) specimens necessary for studying ultra-thin surface regions. We have successfully demonstrated that the careful selection of FIB milling parameters at each stage minimizes the damage layer without the need for post-treatment.

Surface states-dependent giant quantized photonic spin Hall effect in a magnetic topological insulator thin film

We theoretically investigate the in-plane and transverse photonic spin Hall effect (PSHE) in a magnetic topological ultra-thin film (MTF) in the terahertz regime in the presence of an external magnetic field. We take into account the hybridization between the top and bottom surface states (SSs) and demonstrate that both the in-plane and transverse PSHE are quantized due to the Landau levels (LLs) quantization of the magneto-optical (MO) conductivities. The interplay between the effective energy induced by the hybridization interaction and the Zeeman energy plays an important role as it results in topological and normal insulating phases. By manipulating the interplay between these two interaction energies we drive the system through several topological quantum phase transitions (TQPTs). We show that the PSHE can be greatly enhanced and the magnitude of the giant PSHE in MTF is several hundreds times of the incident gaussian beam wavelength. Furthermore, we show that the PSHE is sensitive to the SSs of the Dirac fermions which is due to its topological character. This work sheds important light both on the PSHE and on the MO transport properties in MTFs in the presence of an external magnetic field which harbor great potentials in the development of new-generation photonics spin-Hall and optoelectronic devices.

Visualization of ferromagnetic domains in vanadium-doped topological insulator thin films and heterostructures

Visualization of ferromagnetic domains in vanadium-doped topological insulator thin films and heterostructures

Thickness-Driven Quantum Anomalous Hall Phase Transition in Magnetic Topological Insulator Thin Films.

The quantized version of the anomalous Hall effect realized in magnetic topological insulators (MTIs) has great potential for the development of topological quantum physics and low-power electronic/spintronic applications. Here we report the thickness-tailored quantum anomalous Hall (QAH) effect in Cr-doped (Bi,Sb)2Te3 thin films by tuning the system across the two-dimensional (2D) limit. In addition to the Chern number-related QAH phase transition, we also demonstrate that the induced hybridization gap plays an indispensable role in determining the ground magnetic state of the MTIs; namely, the spontaneous magnetization owing to considerable Van Vleck spin susceptibility guarantees the zero-field QAH state with unitary scaling law in thick samples, while the quantization of the Hall conductance can only be achieved with the assistance of external magnetic fields in ultrathin films. The modulation of topology and magnetism through structural engineering may provide useful guidance for the pursuit of other QAH-based phase diagrams and functionalities.

Magneto-optical properties in thin film topological insulators with quadratic momentum term

In this work we investigate the influence of quadratic in momentum term (Schrodinger term) on magneto-transport properties of thin film topological insulators. The Schrodinger term modifies the Dirac cones into an hourglass shape which results in inter and intraband Landau levels crossings. Breaking of the particle–hole symmetry in Landau level spectrum in the presence of k 2 term leads to asymmetrical density of states profile. We calculate collisional and Hall conductivity for mixed Dirac–Schrodinger system in linear response regime and show oscillatory behavior in collisional conductivity, while Zeeman and hybridization terms provide a doubly split peak structure in collisional conductivity for the case m/m e → ∞. We calculate Hall conductivity analytically and show that for mixed system filling factor is not symmetric about Fermi energy unlike symmetric plateaus for pure Dirac case.