Topological Insulator Materials Research Articles

Double ferromagnetic barriers resonant tunneling diodes (RTD) based on the surface of a topological insulator

Abstract Resonant tunneling diodes (RTDs), which are based on double barrier quantum well structures, are typically achieved by combining different materials with varying band gap sizes. However, this approach often poses challenges such as material mismatching and dislocations. In this study, we present a novel resonant tunneling diode scheme utilizing the unique properties of topological insulator materials. Specifically, we exploit the gap opening in the band structure of the topological insulator by employing perpendicular magnetization. In this proposed RTD platform, the barrier regions are formed from a ferromagnetic topological insulator through the proximity effect. By adjusting the thickness and spacing of the ferromagnetic barriers, a well region with confined states emerges between the barrier regions. Theoretical analysis reveals that by tuning the back gate voltage, the I-V characteristics exhibit two significant behaviors: negative differential resistance (NDR) and step-like behavior for Fermi energy values of EF=-3 and EF=3, respectively. Furthermore, we observe an increase in the peak-to-valley ratio (PVR) with higher magnetization values. Notably, the PVR reaches a value of 7.13 for a magnetization value of m=9. Additionally, we investigate the influence of the well width and barrier thickness on the transport properties of the device.

Application of femtosecond mode-locked SnTe thin films and generation of bound-state solitons

In the realm of ultrafast laser technology, the exploration of two-dimensional materials as saturable absorbers (SA) has garnered significant research interest. Our research investigates the characteristics of SnTe thin films, a topological crystalline insulator material, as a potential saturable absorber for ultrafast lasers. Using the molecular beam epitaxy (MBE) technique, we analyze the films’ morphology and composition through atomic force microscopy (AFM) and successfully deposit SnTe epilayers on Au(111)/mica substrates. Through the utilization of SnTe-SA, an erbium-doped fiber laser is fabricated, demonstrating a pulse output with a width of 276 fs and a center wavelength of 1560 nm, highlighting the potential of SnTe films in manufacturing ultrafast optical devices. Additionally, tightly bound solitons with a soliton interval of 1.01 ps are observed, contributing to the exploration of soliton nonlinear dynamics.

Transition metals of Pt and Pd on the surface of topological insulator Bi2Se3.

Transition metal catalysts supported on topological insulators are predicted to show improved catalytic properties due to the presence of topological surface states, which may float up to the catalysts and provide robust electron transfer. However, experimental studies of surface structures and corresponding catalytic properties of transition metal/topological insulator heterostructures have not been demonstrated so far. Here, we report the structures, chemical states, and adsorption behaviors of two conventional transition metal catalysts, Pt and Pd, on the surface of Bi2Se3, a common topological insulator material. We reveal that Pt forms nanoparticles on the Bi2Se3 surface. Moreover, the interaction between Pt and surface Se is observed. Furthermore, thermal dosing of O2 onto the Pt/Bi2Se3 heterostructure leads to no oxygen adsorption. Detailed scanning tunneling microscopy study indicates that Pt transforms into PtSe2 after the thermal process, thus preventing O2 from adsorption. For another transition metal Pd, it exhibits approximate layer-island growth on Bi2Se3, and Pd-Se interaction is also observed. Our work provides significant insights into the behaviors of transition metals on top of a common topological insulator material and will assist in the future design of catalysts built with topological materials.

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.

Enhancement of spin to charge conversion efficiency at the topological surface state by inserting normal metal spacer layer in the topological insulator based heterostructure

In this study, we report efficient spin to charge conversion (SCC) in the topological insulator (TI) based heterostructure (BiSbTe1.5Se1.5/Cu/Ni80Fe20) by using spin-pumping technique, where BiSbTe1.5Se1.5 is the TI and Ni80Fe20 is the ferromagnetic (FM) layer. The SCC, characterized by inverse Edelstein effect length (λIEE) in the TI material, gets altered with an intervening Copper (Cu) layer, and it depends on the interlayer thickness. The introduction of Cu layer at the interface of TI and FM metal provides a new degree of freedom for tuning the SCC efficiency of the topological surface states. The significant enhancement of the measured spin-pumping voltage and the increased linewidth of ferromagnetic resonance absorption spectra due to the insertion of Cu layer at the interface indicate a reduction in spin memory loss at the interface that resulted from the presence of exchange coupling between the surface states of TI and the local moments of FM metal. The temperature dependence (from 8 to 300 K) of the evaluated λIEE data for all the trilayer systems, TI/Cu/FM with different Cu thicknesses, confirms the effect of exchange coupling between the TI and FM layer on the SCC efficiency of the topological surface state. This study offers promising ways for designing more efficient spin-charge conversion devices of the TI-based heterostructure by controlling the interface effects.

Pressure-driven dome-shaped superconductivity in topological insulator GeBi2Te4

The discovery of new superconductors based on topological insulators always captures special attention due to their unique structural and electronic properties. High pressure is an effective way to regulate the lattice as well as electronic states in the topological insulators, thus altering their electronic properties. Herein, we report the structural and electrical transport properties of the topological insulator GeBi2Te4 by using high-pressure techniques. The synchrotron x-ray diffraction revealed that GeBi2Te4 underwent two structural phase transitions from R-3m (phase I) to C2/m (phase II) and then into Im-3m (phase III). Superconductivity was observed at 6.6 GPa to be associated with the first structural phase transition. The superconducting transition temperature Tc reached a maximum value of 8.4 K, accompanied by the RH sign changing from negative to positive at 14.6 GPa, then gradually decreased with increasing pressure in phase III, showing a dome-shaped phase diagram. The present results provide a platform for understanding the interplay between the crystal structure and superconductivity by the regulation of pressure in the topological insulator materials.

Study on Bulk-Surface Transport Separation and Dielectric Polarization of Topological Insulator Bi1.2Sb0.8Te0.4Se2.6.

This study successfully fabricated the quaternary topological insulator thin films of Bi1.2Sb0.8Te0.4Se2.6 (BSTS) with a thickness of 25 nm, improving the intrinsic defects in binary topological materials through doping methods and achieving the separation of transport characteristics between the bulk and surface of topological insulator materials by utilizing a comprehensive Physical Properties Measurement System (PPMS) and Terahertz Time-Domain Spectroscopy (THz-TDS) to extract electronic transport information for both bulk and surface states. Additionally, the dielectric polarization behavior of BSTS in the low-frequency (10-107 Hz) and high-frequency (0.5-2.0 THz) ranges was investigated. These research findings provide crucial experimental groundwork and theoretical guidance for the development of novel low-energy electronic devices, spintronic devices, and quantum computing technology based on topological insulators.

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.

Brief discussion on quantum anomaly Hall effect: From theory to application

The investigation of topological insulator materials plays a crucial role in the exploration of the quantum anomalous Hall effect. A topological insulator is a distinct type of insulator characterized by its band structure with non-trivial topological features. Topological insulators are characterized by the occurrence of topological phase transitions in the electron energy bands at the Fermi level, which can be attributed to the combined effects of spin-orbit coupling and an external magnetic field. These transitions give rise to the emergence of Hall conductive boundary states, facilitating the manifestation of quantum Hall conductance even in the absence of magnetic fields. The quantum anomalous Hall effect exhibits promising prospects for various applications. For instance, it can serve as a viable means of current transmission in low-power electronic devices, or alternatively, as a medium for constructing qubits in topological quantum computing systems. Furthermore, the utilization of the quantum anomalous Hall effect extends to the development of magnetic sensors with superior performance characteristics and the creation of energy-efficient spintronic devices. This work endeavors to conduct a comprehensive examination and evaluation of the theory and practical implementation of the quantum Anomaly Hall effect by the analysis and review of relevant literature. In addition, it intends to provide potential avenues for future applications in this field.

High-performance photothermal effect in MOCVD grown topological insulator Sb2Te3 nanograting

Photothermal energy has been widely used in high-tech applications, such as heating/cooling systems, bio-imaging, bio-sensing, and medical therapies. However, conventional photothermal materials have narrow photo-absorption bandwidth and low photothermal conversion efficiency. Innovative materials that can more efficiently harvest photothermal energy are highly demanded. Topological insulator materials with excellent optical properties hold great potential in photo-absorption and photothermal conversion. This work investigated and engineered photo-absorption and photothermal effect in Sb2Te3 topological insulator nanograting. The TI material was grown by metal-organic chemical vapor deposition to exploit the benefits of the process, yielding high material quality and large deposition areas. Through a meticulous process encompassing material synthesis, engineering, and characterization, highly absorptive Sb2Te3 topological insulator nanograting and efficient photothermal conversion have been achieved. This research contributes to the advancement of the fundamental knowledge of light–matter interaction and photothermal effects in topological insulator materials. The outcomes of this study can benefit the development of efficient photothermal materials for high-performance nano-energy and biomedical technologies.

Orietation-controlled synthesis and Raman study of 2D SnTe

Tin telluride (SnTe), as a narrow bandgap semiconductor material, has great potential for developing photodetectors with wide spectra and ultra-fast response. At the same time, it is also an important topological crystal insulator material, with different topological surface states on several common surfaces. Here, we introduce different Sn sources and control the growth of regular SnTe nanosheets along the (100) and (111) planes through the atmospheric pressure chemical vapor deposition method. It has been proven through various characterizations that the synthesized SnTe is a high-quality single crystal. In addition, the angular resolved Raman spectra of SnTe nanosheets grown on different crystal planes are first demonstrated. The experimental results showed that square SnTe nanosheets grown along the (100) plane exhibit in-plane anisotropy. At the same time, we use micro-nanofabrication technology to manufacture SnTe-based field effect transistors and photodetectors to explore their electrical and optoelectronic properties. It has been confirmed that transistors based on grown SnTe nanosheets exhibit p-type semiconductor characteristics and have a high response to infrared light. This work provides a new approach for the controllable synthesis of SnTe and adds new content to the research of SnTe-based infrared detectors.

Gapless linear dispersion in Bi2Se3 nanoparticles for high-performance broadband photodetectors

Topological insulators have demonstrated novel optoelectronic properties owing to their gapless linear dispersion and robust surface states. In this study, we implemented high-performance broadband photodetectors based on mixed-dimensional heterostructure of the topological insulator Bi2Se3 nanoparticles (NPs) with the semiconductor WSe2. Prominently, the Bi2Se3 NPs/WSe2 structure exhibited photoresponsivity of 22.17 A/W in the visible (Vis) wavelengths, and 5 A/W in the near-infrared (NIR) wavelengths and a fast response time of 40 μs. In comparison with typical semiconducting PbS NPs/WSe2 structure, we confirmed that the enhancement in the photoresponsivity, response time, and broadband photoresponse of the Bi2Se3 NPs/WSe2 structure can be attributed to the gapless behavior in surface states of topological insulator materials. Numerical simulations also confirmed a dramatic increase in the interfacial electric field by two orders of magnitude, resulting from the surface states in topological insulator NPs. The observed experimental results and the proposed mechanism pave the way for the emergence of efficient optoelectronic devices based on topological insulator materials.

Edge-induced excitations in Bi[formula omitted]Te[formula omitted] from spatially-resolved electron energy-gain spectroscopy

Among the many potential applications of topological insulator materials, their broad potential for the development of novel tunable plasmonics at THz and mid-infrared frequencies for quantum computing, terahertz detectors, and spintronic devices is particularly attractive. The required understanding of the intricate relationship between nanoscale crystal structure and the properties of the resulting plasmonic resonances remains, however, elusive for these materials. Specifically, edge- and surface-induced plasmonic resonances, and other collective excitations, are often buried beneath the continuum of electronic transitions, making it difficult to isolate and interpret these signals using techniques such as electron energy-loss spectroscopy (EELS). Here we focus on the experimentally clean energy-gain EELS region to characterise collective excitations in the topologically insulating material Bi2Te3 and correlate them with the underlying crystalline structure with nanoscale resolution. We identify with high significance the presence of a distinct energy-gain peak around −0.8eV, with spatially-resolved maps revealing that its intensity is markedly enhanced at the edge regions of the specimen. Our findings illustrate the reach of energy-gain EELS analyses to accurately map collective excitations in quantum materials, a key asset in the quest towards new tunable plasmonic devices.

Photo-electrochemical effects in topological insulator Sb2Te3 thin films

Topological insulators have attracted increased attention owing to their fascinating properties and important applications, such as spintronic devices, quantum computers, and optoelectronic devices. In this study, we demonstrated the photo-electrochemical properties of a typical topological insulator material, Sb2Te3 thin film. Sb2Te3 thin films were grown on a glass substrate using atomic layer deposition subsequently, samples or thin films were characterized using Raman spectroscopy, scanning electron microscopy, X-ray diffraction, and atomic force microscopy. The as-fabricated Sb2Te3 thin films have continuous and coarse surfaces with high crystallization. The photo-electrochemical effects of Sb2Te3 electrodes were observed. We show that the Sb2Te3 films have potential in photocatalytic water splitting application.

Sensitivity of actuation dynamics of Casimir oscillators on finite temperature with topological insulator materials: Response of repulsive vs attractive interactions

We explore the sensitivity of repulsive and attractive Casimir interactions on thermal fluctuations in topological insulator materials, and the associated effects on non-linear actuation towards chaotic motion. For system at low temperature and applied magnetization in the same direction, by increasing the temperature the sensitivity of the Casimir force to the magnetization, at large separations, weakens. Moreover, by increasing the temperature the repulsive force vanishes for weak magnetization. Furthermore, for conservative systems the stable actuation area for high temperature does not show strong sensitivity to changes of the magnetization and the magnetoelectric effect. For non-conservative system with weak magnetization in the same direction leading to attractive Casimir forces, increment of the temperature increases the possibility for occurrence of chaotic motion.

Low-damage electron beam lithography for nanostructures on Bi2Te3-class topological insulator thin films

Nanostructured topological insulators (TIs) have the potential to impact a wide array of condensed matter physics topics, ranging from Majorana physics to spintronics. However, the most common TI materials, the Bi2Se3 family, are easily damaged during nanofabrication of devices. In this paper, we show that electron beam lithography performed with a 30 or 50 kV accelerating voltage—common for nanopatterning in academic facilities—damages both nonmagnetic TIs and their magnetically doped counterparts at unacceptable levels. We additionally demonstrate that electron beam lithography with a 10 kV accelerating voltage produces minimal damage detectable through low-temperature electronic transport. Although reduced accelerating voltages present challenges in creating fine features, we show that with careful choice of processing parameters, particularly the resist, 100 nm features are reliably achievable.

Quantum Topological Neuristors for Advanced Neuromorphic Intelligent Systems.

Neuromorphic artificial intelligence systems are the future of ultrahigh performance computing clusters to overcome complex scientific and economical challenges. Despite their importance, the advancement in quantum neuromorphic systems is slow without specific device design. To elucidate biomimicking mammalian brain synapses, a new class of quantum topological neuristors (QTN) with ultralow energy consumption (pJ) and higher switching speed (µs) is introduced. Bioinspired neural network characteristics of QTNs are the effects of edge state transport and tunable energy gap in the quantum topological insulator (QTI) materials. With augmented device and QTI material design, top notch neuromorphic behavior with effective learning-relearning-forgetting stages is demonstrated. Critically, to emulate the real-time neuromorphic efficiency, training of the QTNs is demonstrated with simple hand gesture game by interfacing them with artificial neural networks to perform decision-making operations. Strategically, the QTNs prove the possession of incomparable potential to realize next-gen neuromorphic computing for the development of intelligent machines and humanoids.

Transport measurement of fractional charges in topological models

The static topological fractional charge (TFC) in condensed matter systems is related to the band topology and thus has potential applications in topological quantum computation. However, the experimental measurement of these TFCs in electronic systems is quite challenging. We propose an electronic transport measurement scheme in which both the charge amount and the spatial distribution of the TFC can be extracted from the differential conductance through a quantum dot coupled to the topological system being measured. For one-dimensional Su–Schrieffer–Heeger (SSH) model, both the e/2 charge of the TFC and its distribution can be verified. As for the disorder effect, it is shown that the Anderson disorder, which breaks certain symmetry related to the TFC, is significant in higher-dimensional systems while having little effect on the one-dimensional SSH chain. Nonetheless, our measurement scheme can still work well for specific higher-order topological insulator materials, for instance, the 2e/3 TFC in the breathing kagome model could be confirmed even in the presence of disorder effect. These conclusions about spatial dimension and disorder effect are quite universal, which also applies to other topological systems such as topological classic wave system.

Phonon-mediated strong coupling between a three-dimensional topological insulator and a two-dimensional antiferromagnetic material

We numerically study strong coupling between terahertz excitations in a hybrid material consisting of a three-dimensional (3D) topological insulator (TI) and a quasi-two-dimensional (2D) van der Waals antiferromagnet. We find that the interaction between a surface Dirac plasmon polariton in the 3D TI and a magnon polariton in the 2D antiferromagnet is mediated by the phonon coupling in the 3D TI material and can result in emergence of a new hybridized mode, namely, a surface Dirac plasmon-phonon-magnon polariton. We numerically study the dependence of the strong coupling on a variety of structural parameters of the 3D-TI/2D-antiferromagnetic (AFM) hybrid material. Our results reveal that the strength of the coupling depends primarily on the anisotropy constant of the 2D AFM material, as well as on its thickness, and reaches a maximum when the AFM layer is sufficiently thick to be considered a half-infinite slab. We show that the extremely large anisotropy constant reported for certain 2D van der Waals antiferromagnets results in a coupling strength that should be experimentally observable even in the presence of realistic scattering losses.

Femtosecond Fiber Laser Based on BiSbTeSe2 Quaternary Material Saturable Absorber

Topological insulator materials, including Bi2Te3, Sb2Te3, Sb2Te3, and Bi2Se3, have attracted some attention due to their narrow band gaps, high carrier mobility, wide spectral absorption ranges and other characteristics. We report a new multi-compound topological insulator material BiSbTeSe2 that, compared with the traditional topological insulator composed of two elements, can integrate the physical advantages of each element, helpful to build an experimental platform with rich physical properties. The nonlinear optical characteristics of the quaternary material BiSbTeSe2 is obtained in the erbium-doped fiber laser. Using the BiSbTeSe2 as a saturable absorber material, the passive Q-switched and mode-locked fiber lasers are achieved. The pulse duration and signal-to-noise ratio (SNR) of the Q-switched fiber laser are 854 ns and 70 dB, respectively. Meanwhile, the pulse duration and SNR of the mode-locked fiber laser are 259 fs and 87.75 dB, respectively. This work proves that the BiSbTeSe2 has a considerable application prospect as a saturable absorber in fiber lasers, and provides a new reference for selection of high-performance saturable absorber materials.