Properties Of Topological Insulators Research Articles

Exchange bias in heterostructures combining magnetic topological insulator MnBi2Te4 and metallic ferromagnet Fe3GeTe2

Introducing magnetism into topological insulators enables exotic phenomena such as quantum anomalous Hall effect. By fabricating van der Waals (vdW) heterostructures using layered magnetic materials, we can not only induce a gap in the non-magnetic topological surface states through magnetic proximity but also further manipulate the magnetic properties of magnetic topological insulators. However, the scarcity of 2D ferromagnetic insulator materials limits the fabrication of such heterostructures. Here, we demonstrate the vdW heterostructure devices comprising metal ferromagnetic Fe3GeTe2 nanoflakes and few-layer antiferromagnetic topological insulator MnBi2Te4 separated by an insulating hexagonal-boron nitride spacer. These devices exhibit significant exchange bias with the exchange bias field of over 100 mT under certain conditions. Our results prove that besides magnetic insulators, metallic magnets can also effectively adjust the magnetic properties of topological insulators, thereby inspiring diverse configurations of the heterostructures between topological insulators and magnetic materials.

Magnetic Dirac semimetal state of (Mn,Ge)Bi2Te4

The ability to finely tune the properties of magnetic topological insulators (TIs) is crucial for quantum electronics. We studied solid solutions with a general formula GexMn1-xBi2Te4 between two isostructural Z2 TIs, magnetic MnBi2Te4 and nonmagnetic GeBi2Te4 with Z2 invariants of 1;000 and 1;001, respectively. We observed linear x-dependent magnetic properties, composition-independent pairwise exchange interactions, and topological phase transitions (TPTs) between topologically nontrivial phases and the semimetal state. The TPTs are driven purely by the variation of orbital contributions. By tracing the x-dependent Bi 6p contribution to the states near the fundamental gap, the effective spin-orbit coupling variation is extracted. The gapless state observed at x = 0.42 closely resembles a Dirac semimetal above the Néel temperature and shows a magnetic gap below, which is clearly visible in raw photoemission data. The observed behavior demonstrates an ability to precisely control topological and magnetic properties of TIs.

The synthesis and tunable topological properties of Sm-doped (Bi, Sb)2Te2S crystal

Topological insulators exhibit Dirac points in their topological surface states, but often such Dirac points are far from the Fermi level and sometimes even reside outside the bulk band gap. Here, we report the synthesis of a new topological insulator material Sm-doped (Bi, Sb)2Te2S, which has its surface tunable Dirac point exposed within the bulk gap and close to the Fermi level at a minimal distance of 73 meV with a large tunability of 157 meV, as evidenced by angle-resolved photoemission spectroscopy. The compensation doping by Sm creates a nearly insulating bulk and triggers an interesting phenomenon of all-configuration negative magnetoresistance. This signals a nearly compensated state where the bulk transport is mainly via percolation through charge puddles. Our work offers a new material platform suitable for regulating the position of surface Dirac points and exploring unique physical properties of topological insulators.

High‐Index and Low‐Loss Topological Insulators for Mid‐Infrared Nanophotonics: Bismuth and Antimony Chalcogenides

AbstractTopological insulators generally have dielectric bulk and conductive surface states. Consequently, some of these materials are shown to support polaritonic modes at visible and THz frequencies. At the same time, the optical properties of topological insulators in the mid‐infrared (mid‐IR) remain poorly investigated. Here, near‐field imaging is employed to probe the mid‐IR response from the exfoliated flakes of bismuth (Bi)/selenide (Se)/telluride (Te)/antimony (Sb) crystals with varying stoichiometry – Bi2Se3, Bi2Te2Se, and Bi1.5Sb0.5Te1.7Se1.3 – in pristine form as well as covered by thin flakes of hexagonal boron nitride (hBN) is employed. Contrary to theoretical expectations, all three materials exhibit a dielectric response with a high refractive index and with a loss below the experimental detection limit. Particularly, the near‐field mapping of propagating phonon‐polaritons in hBN demonstrates that all three van der Waals crystals with different stoichiometry act as a practically lossless dielectric substrate with an ultra‐high refractive index of up to 7.5 in Bi2Te2Se. Such a unique dielectric crystal will be of great advantage for numerous nanophotonic applications in the mid‐IR.

Effect of localization–delocalization transition on thermoelectric properties of Bi2Te2Se topological insulator

The thermal transport properties of Bi2Te2Se topological insulators show a range of complex features. Large bulk resistivities coexisting with prominent Shubnikov–de Haas quantum oscillations and proximity to metallic states mark this p-band system as an unconventional topological quantum material. Here, using the density functional plus dynamical mean-field theory method, we show how localization–delocalization transition underpins the T-dependence of thermoelectric responses from room down to low temperatures. We present the implications of our many-particle analysis to resistivity, Seebeck coefficient, thermal conductivity, and Lorenz number and propose that related broadband systems close to electronic transitions could be of use in thermoelectrics.

Investigation of thermoelectric and magnetotransport properties of single crystalline Bi2Se3 topological insulator

The realization of remarkable thermoelectric (TE) properties in a novel single-crystalline quantum material is a topic of prime interest in the field of thermoelectricity. It necessitates a proper understanding of transport properties under magnetic field and magnetic properties at low field. We report polarized Raman spectroscopic study, TE properties, and magneto-resistance (MR) along with magnetic characterization of single-crystalline Bi2Se3. Polarized Raman spectrum confirms the strong polarization effect of A1g1 and A1g2 phonon modes, which verifies the anisotropic nature of the Bi2Se3 single crystal. Magnetization measurement along the in-plane direction of single crystal divulges a cusp-like paramagnetic response in susceptibility plot, indicating the presence of topological surface states (TSSs) in the material. In-depth MR studies performed in different configurations also confirm the presence of anisotropy in the single-crystalline Bi2Se3 sample. A sharp rise in MR value near zero magnetic field and low-temperature regime manifests a weak anti-localization (WAL) effect, depicting the quantum origin of the conductivity behavior at low temperature. Moreover, in-plane magneto-conductivity data at low-temperature (up to 5 K) and low-field region (≤15 kOe) confirm the dominance of the WAL effect (due to TSS) with a negligible bulk contribution. Quantum oscillation (SdH) in magneto-transport data also exhibits the signature of TSS. Additionally, an exceptional TE power factor of ∼950 μW m−1 K−2 at 300 K is achieved, which is one of the highest values reported for pristine Bi2Se3. Our findings pave the way for designing single crystals, which give dual advantages of being a good TE material along with a topological insulator bearing potential application.

Axially lattice-matched wurtzite/rock-salt GaAs/Pb1−xSnxTe nanowires

We investigate the full and half-shells of Pb1−xSnxTe topological crystalline insulator deposited by molecular beam epitaxy on the sidewalls of wurtzite GaAs nanowires (NWs). Due to the distinct orientation of the IV–VI shell with respect to the III–V core the lattice mismatch between both materials along the nanowire axis is less than 4%. The Pb1−xSnxTe solid solution is chosen due to the topological crystalline insulator properties above some critical concentrations of Sn (x ≥ 0.36). The IV–VI shells are grown with different compositions spanning from binary SnTe, through Pb1−xSnxTe with decreasing x value down to binary PbTe (x = 0). The samples are analysed by scanning transmission electron microscopy, which reveals the presence of (110) or (100) oriented binary PbTe and (100) Pb1−xSnxTe on the sidewalls of wurtzite GaAs NWs.

Electronic and Optical Properties of Mn-Doped HgSe Topological Insulator for Spintronic Devices

HgSe is a Mercury chalcogenide material of the HgX family (where X = S, Se, Te) which crystallises in the zincblende crystal phase. The electronic band structure of HgSe is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. This paper reports ab-initio calculations of the structural, electronic, magnetic, and optical properties of zincblende Mercury selenide (HgSe) doped with manganese (Mn) in the inter sites HgMnxSe, with x = 0, 0.058, and 0.117, using the framework of spin-polarized density functional theory (SP-DFT). The aim of our investigation is to discuss the different properties of this doped material in order to improve the promising new domain of spintronics with topological systems. Both the GGA + U + mBJ approach and spin–orbit coupling (SOC) are used for band structure calculations and density of states. The results show a nontrivial topological semimetal order for HgSe and a ferromagnetic topological and metallic behaviour for HgMnxSe. The frequency response of optical properties shows interesting characteristics. Furthermore, the variation with concentration x of the critical point for each of the optical parameters is similar to that of the inverted band gap.

The effect of charged particle irradiation on the transport properties of bismuth chalcogenide topological insulators: a brief review.

Topological insulators (TIs) offer a novel platform for achieving exciting applications, such as low-power electronics, spintronics, and quantum computation. Hence, the spin-momentum locked and topologically nontrivial surface state of TIs is highly coveted by the research and development industry. Particle irradiation in TIs is a fast-growing field of research owing to the industrial scalability of the particle irradiation technique. Unfortunately, real three-dimensional TI materials, such as bismuth selenide, invariably host a significant population of charged native defects, which cause the ideally insulating bulk to behave like a metal, masking the relatively weak signatures of metallic topological surface states. Particle irradiation has emerged as an effective technique for Fermi energy tuning to achieve an insulating bulk in TI along with other popularly practiced methods, such as substitution doping and electrical gating. Irradiation methods have been used for many years to enhance the thermoelectric properties of bismuth chalcogenides, predominantly by increasing carrier density. In contrast, uncovering the surface states in bismuth-based TI requires the suppression of carrier density via particle irradiation. Hence, the literature on the effect of irradiation on bismuth chalcogenides extends widely to both ends of the spectrum (thermoelectric and topological properties). This review attempts to collate the available literature on particle irradiation-driven Fermi energy tuning and the modification of topological surface states in TI. Recent studies on particle irradiation in TI have focused on precise local modifications in the TI system to induce magnetic topological ordering and surface selective topological superconductivity. Promising proposals for TI-integrated circuits have also been put forth. The eclectic range of irradiation-based studies on TI has been reviewed in this manuscript.

Strain-tunable electronic anisotropy of the AlSb double-layer honeycomb structure.

Two-dimensional (2D) materials show promising applications in nanoelectronic devices due to their excellent physical and chemical properties, large specific surface area, and good flexibility. 2D AlSb, a representative of a new class of two-dimensional materials with a double-layer honeycomb (DLHC) structure was recently obtained in experiments and was reported to be a direct band gap semiconductor. Strain engineering is an effective way of tuning the properties of 2D materials. Here, by first-principles calculations, the strain effects on the electronic structure of AlSb were investigated. An interesting inversion of band order near the bottom of the conduction band can be induced by applying uniaxial strain, which will introduce a large electronic anisotropy. In addition, under tensile strain along the armchair direction larger than 5%, band inversion occurs, indicating possible topological insulator properties of AlSb. Small carrier effective mass and strain tunable electronic anisotropy pave the way for the application of AlSb in future nanoelectronic devices.

Doping of n-type Bi2Se3 single crystal with Fe, Ru, Os, and Mo

Doping is one of the most suitable methods for tuning the electronic properties of topological insulators and a promising approach for band gap opening in surface states. In this study, we developed a reliable method for preparing high-quality single crystal substrates comprising Bi2Se3 doped with VIIIB and VIB column elements. We combined experimental photoelectron spectroscopy (X-ray photoelectron spectroscopy, angle resolved photoelectron emission spectroscopy, and ultraviolet photoelectron spectroscopy) and theoretical (ab initio) methods to analyze the electronic properties and chemical states of atoms in the substitutional position and native defects in the topological insulator, which can be achieved using the free melt crystallization method for sample growth. The relationship between the position of the Dirac cone and valence band maximum was explored and discussed.

A comparative study of magnetic (Mn) and non-magnetic (In) elements-doped Bi2Se3 topological insulator: Structural, magnetic, transport, and weak anti-localization property exploration

The present comparative study explores the effect of magnetic (Mn) and non-magnetic (In) doping on structural, magnetic, and quantum-transport properties of Bi2Se3 topological insulator. Un-doped as well as Mn, In-doped Bi2Se3 samples have been synthesized using the solid-state reaction method, confirmed by HR-TEM results. The XRD pattern along with Rietveld refinement reveals that, both Mn and In-dopants substitute the Bi-ion together with slightly interstitially incorporation in host Bi2Se3. It is also found that In-doping is more preferable for Bi-substitution than Mn, which is well supported through our XPS analysis. The study of magnetism indicates the diamagnetic nature of un-doped Bi2Se3 gradually transformed into ferromagnetism for high Mn-doping, whereas In-doped Bi2Se3 shows a mixing of diamagnetism and paramagnetism. Electrical-transport establishes that In-doped Bi2Se3 preserves the hostʼs metallic nature; however, the arrest of metallic response is observed through the evidence of Kondo effect below 185 K for high Mn-doping. Interestingly, the Mn-doping creates a transformation of hostʼs weak anti-localization (WAL) towards weak localization (WL), and produces a significant reduction in the phase coherence length (lφ) of host Bi2Se3, verified by the magneto-conductance analysis. Whereas, In-doping shows relatively minor deviation in hostʼs WAL effect for higher doping limit.

An investigation into the distinctive properties of LiAuS and NaAuS topological insulators

Topological insulators (TIs) have garnered significant interest in condensed matter physics and materials science due to their unique properties. Recently, Half-Heusler compounds have been predicted to exhibit topological insulator behavior based on electronic structure. We conducted a study on potential alternatives for optoelectronic, photorefractive, and photovoltaic devices. Using density-functional-theory (DFT) calculations, a systematic comparison of the structural, electronic, and optical properties of LiAuS and NaAuS is performed. The geometrically optimized structures of cubic LiAuS and NaAuS were compared with theoretical values. The electronic band structure revealed a non-trivial zero band gap at the G-point, and the influence of different orbitals on the conduction band (CB) and valence band (VB) was analyzed. The optical properties including absorption, reflectivity, index of refraction, extinction coefficient, conductivity, and dielectric function were examined over the energy spectrum ranging from 0 to 30 eV. LiAuS displays a notable absorption peak at 7.95 eV, whereas NaAuS exhibits a peak at 6.79 eV. The reflectivity peak, on the other hand, is higher at 19.79 eV and 8.29 eV for LiAuS, and at 6.74 eV for NaAuS. The reflective indices for LiAuS and NaAuS are determined to be 3.5 and 7.03, respectively. It is observed that LiAuS and NaAuS possess elevated optical conductivity at 6.26 eV and 6.14 eV, respectively. The conductivities of LiAuS and NaAuS in terms of their visual properties have been claimed to be 8.0(1/fs) and 6.84(1/fs), individually. In view of its high dielectric constant, LiAuS exhibits significant absorption peaks at 0.91 eV, 4.63 eV, and 13.63 eV, whereas NaAuS displays peaks at 0.07 eV, 4.45 eV, and 13.12 eV. Owing to their optical characteristics, LiAuS and NaAuS possess the potential for implementation in photorefractive and optoelectronic devices.

Tuning the Fermi surface of PrSb to enhance the interplay between superconductivity and magnetism by Sn substitution

The nature of Fermi surface in view of shapes and density of the nearest energy bands plays important role in tuning the physical and chemical properties of heavy fermions, Weyl metals, topological insulators, and superconductors. The interplay between superconductivity, magnetism, and fermionic nature of materials can be enhanced by modifying the shape of Fermi surface. The modification may be accomplished by tuning the surface topology, such as flatness and thickness, of electronic energy bands around the Fermi surface. Herein, we investigate the interplay between the magnetism, superconductivity, and heavy Fermionic nature of PrSb by tuning the shape of the Fermi surface by Sn substitution via PrSnxSb1-x (x = 0.0,0.25,0.50,0.75 &1.0) substitution scheme. For the theoretical designing of the materials and computation of the targeted properties, we implement density functional theory using the generalized–gradient approximation extended to the Perdew-Bruke-Ernzerhof correction to account for the exchange–correlation. The modification in the thickness and flatness of energy bands and density of states around the Fermi surface of PrSb compounds by changing the Sn/Pb substitution ratio, leads to simultaneous enhancement of the interplay between magnetism, superconductivity, and fermionic nature of the compounds.

Electronic and spin transport in Bismuthene with magnetic impurities

Topological insulators have remained as candidates for future electronic devices since their first experimental realization in the past decade. The existence of topologically protected edge states could be exploited to generate a robust platform and develop quantum computers. In this work we explore the role of magnetic impurities in the transport properties of topological insulators, in particular, we study the effect on the zigzag bismuthene nanoribbon edge states conductivity. By means of realistic abinitio calculations we simulate the interaction between magnetic adatoms and topological insulators, furthermore, our main goal is to obtain the transport properties for large samples as it would be possible to localize edge states at large scales.

Experimental investigation of the solid phase equilibria at 300 K in the SnBi2Te4-PbBi2Te4-Bi2Te3 system

The phase equilibria of the SnBi2Te4-PbBi2Te4-Bi2Te3 system were experimentally studied using differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. According to the experimental results, the isothermal section of the system at 300 K were constructed and 4 single-phase and 3 two-phase regions were identified. It was shown that along with previously confirmed SnBi2Te4–PbBi2Te4 and SnBi4Te7–PbBi4Te7 sections, SnBi6Te10–PbBi6Te10 section forms continuous series of solid solutions with a tetradymite-type layered structure. Lattice parameters of solid solutions were determined by full-profile Rietveld refinements and results show that both a and c parameters increase linearly with the Pb concentration according to Vegard's law. This study can help elucidate the phase equilibria of the SnTe-PbTe-Bi2Te3 pseudo-ternary system which provides important information for the design of new tetradymite-type layered phases with topological insulator and thermoelectric properties.

Growth of Bi2Te3 topological insulator ultra-thin layers via molecular beam epitaxy on GaAs (100)

Ultra-thin layers (<8 nm) of a Bi2Te3 topological insulator have been grown on GaAs (100) substrates using molecular beam epitaxy. The growth was performed from a single Bi2Te3 effusion cell and one source of extra tellurium. Optical and structural characterizations were carried out through Raman spectroscopy, x-ray diffraction, atomic force microscopy, and scanning electron microscopy. The topological insulator properties were also investigated by angle-resolved photoelectron spectroscopy. A layer of 5 nm showed Dirac cone-like linear electronic band dispersion, indicating the signature of a topological insulator with the Dirac point having large binding energy relative to the Fermi level as expected for ultra-thin films. Topological insulator properties were also investigated at the initial growth stage where deposition follows an islandlike growth mode. Our results can contribute to the development of practical chalcogenide-based thin-film spintronics devices.

Near-field observation of mid-infrared edge modes in topological photonic crystals

Topological photonic crystals inherit the unique properties of topological insulators, including topologically protected energy transfer and unidirectional propagation, which offer an excellent platform for exploring exotic physics and developing photonic devices. However, topological photonic crystals possessing mid-infrared edge modes that have potential applications in infrared imaging, biosensing, thermal radiation energy transfer, etc., are seldom brought into focus. In this work, we study the topological properties of a photonic crystal slab (PCS) consisting of silicon square veins in the mid-infrared, which is intended to mimic the two-dimensional Su–Schrieffer–Heeger model. By interfacing topologically trivial and nontrivial PCSs, mid-infrared edge modes can appear at domain wall, according to the principle of bulk-edge correspondence. It is also demonstrated high-efficiency mid-infrared light transport can be achieved by these edge modes. In addition, adjusting the vertical offset near the interface can manipulate the bandwidth for various applications and turns the connected PCS structure to a photonic realization of Rice–Mele model. We further fabricate the PCS and provide an experimental observation of transverse-electric-like edge modes in mid-infrared by using the scattering-type scanning near-field optical microscope. Additionally, we integrate it with phase change material of nanoscale thickness, Ge2Sb2Te5, to realize an ultrafast and switchable topological waveguide with zero static power. This work not only enriches the fundamental understanding of topological physics in mid-infrared optical settings, but also shows promising prospects in compact devices for energy transfer and information processing for light sources in these wavelengths, for instance, thermal radiation.

Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene

The emergence of phosphorene has generated significant interest in 2D group VA nanomaterials. Among this group, bismuthene exhibits layer-dependent direct bandgaps, high carrier mobility, and topological insulator properties because of its unique structure and ultrathin nature, distinguishing it as a promising candidate for photonic applications. Particularly, its outstanding stability in air makes bismuthene more advantageous than phosphorene for practical applications. Here, we provide a comprehensive review of recent advances regarding 2D bismuth by focusing on the aspects of methods of synthesis and photonic applications. First, the structure and fundamental properties of bismuthene are described, referring to its crystallinity and band structures, as well as to its nonlinear optical properties. Subsequently, the common synthesis methods for 2D bismuth are summarized, including both top-down and bottom-up approaches. Then, potential photonic applications based on 2D bismuth, involving nonlinear photonic devices, photocatalyst, and photodetectors, are illustrated. The performance, mechanisms, and features of the devices are discussed. Finally, the review is summarized and some challenges and future outlooks in this field are addressed.

Topological insulators in relativistic quantum mechanics

By employing supersymmetric quantum mechanics, the analog of relativistic quantum mechanics for topological insulators is considered. The procedure determines the general structure of the spin–orbit coupling even though it constrains the coupling constant. Through analogies with nonelativistic quantum mechanics, we construct relativistic Hamiltonians that share properties of topological insulators. As an application of our results, we study in detail the nonabelian extension of the Dirac–Landau oscillator and relativistic scattering with a spin–orbit coupling term.