Search

We review recent progress in the electronic structure study of intrinsic magnetic topological insulators (MnBi2Te4) · (Bi2Te3)n (⁠|$n\ = \ 0,\ 1,\ 2,\ 3$|⁠) family. Specifically, we focus on the ubiquitously (nearly) gapless behavior of the topological Dirac surface state observed by photoemission spectroscopy, even though a large Dirac gap is expected because of surface ferromagnetic order. The dichotomy between experiment and theory concerning this gap behavior is perhaps the most critical and puzzling question in this frontier. We discuss various proposals accounting for the lack of magnetic effect on the topological Dirac surface state, which are mainly categorized into two pictures, magnetic reconfiguration and topological surface state redistribution. Band engineering towards opening a magnetic gap of topological surface states provides great opportunities to realize quantized topological transport and axion electrodynamics at higher temperatures. [ABSTRACT FROM AUTHOR]

One of the approaches to manipulate MnBi 2 Te 4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn 1 − x Ge x Bi 2 Te 4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te p z and Bi p z contributions at the Γ -point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn 1 − x Ge x Bi 2 Te 4 /MnBi 2 Te 4 , it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation. [ABSTRACT FROM AUTHOR]

Topological states of matter possess bulk electronic structures categorized by topological invariants and edge/surface states due to the bulk-boundary correspondence. Topological materials hold great potential in the development of dissipationless spintronics, information storage and quantum computation, particularly if combined with magnetic order intrinsically or extrinsically. Here, we review the recent progress in the exploration of intrinsic magnetic topological materials, including but not limited to magnetic topological insulators, magnetic topological metals, and magnetic Weyl semimetals. We pay special attention to their characteristic band features such as the gap of topological surface state, gapped Dirac cone induced by magnetization (either bulk or surface), Weyl nodal point/line and Fermi arc, as well as the exotic transport responses resulting from such band features. We conclude with a brief envision for experimental explorations of new physics or effects by incorporating other orders in intrinsic magnetic topological materials. [ABSTRACT FROM AUTHOR]

Despite ongoing research, the rational design of non-trivial topological semimetal surface states for the selective photocatalytic CO 2 conversion into valuable products remains full of challenges. Herein, we present the synthesis of lattice-matched 1 T-OsCoTe 2 for the photoreduction upgrading of CO 2 to tri-carbon (C 3) alkane (propane, C 3 H 8) by the integration of experimental work and theory prediction/calculation. Experimental studies suggested a high electron-based selectivity of 71.2 % for C 3 H 8 and an internal quantum efficiency of 54.6 % at 380 nm. In-situ X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy demonstrated that Co and Os atoms coordinated with Te atoms enable an efficient Os―Te―Co electron transfer to activate the generation of *CH 3 , *CHOCO and *CH 2 OCOCO. Density functional theory calculations further confirmed Os―Te―Co electron bridging on the improved CO 2 conversion kinetics. To our knowledge, this is the first report suggesting the role of Os atoms in accelerating the photocatalytic CO 2 conversion activity of the topological semimetal 1 T-OsCoTe 2. [Display omitted] • The non-trivial TSSs may have stabilized critical *C 2+ intermediates. • Os―Te―Co electron bridging enhances the adsorption of *CHOCO and *CH 2 OCOCO. • A high electron-based selectivity of 71.2 % for C 3 H 8. [ABSTRACT FROM AUTHOR]

Three-dimensional topological insulators feature unconventional two-dimensional surface states, the carriers in which are helical Dirac fermions and protected from backscattering. Thus, they exhibit novel electronic response upon illuminate ultrashort and intense laser light. We briefly reviewed recent studies on ultrafast phenomena from the surface of the topological insulators driven by laser pulse ranging from visible to THz frequency. Ultrafast dynamics of Dirac fermions can be excited by helical photons and driven by strong light field. Many unique nonlinear behaviors have been demonstrated, such as the excitation of helicity-dependent photocurrent, the formation of Floquet-Bloch bands, lightwave-driven Dirac currents and the generation of optical high-harmonic emission. This review aimed at understanding the microscopic mechanism of the ultrafast charge and spin dynamics in topological surface states and its prospects for coherent manipulation of Dirac fermions by laser light.

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states (TSS) with ultrahigh efficiency. Here, efficient SOT switching of a hard MTI, V-doped (Bi,Sb) 2 Te 3 (VBST), with a large coercive field that can prevent the influence of an external magnetic field, is demonstrated. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems, which makes the Hall channel a good readout and eliminates the need to fabricate complicated magnetic tunnel junction (MTJ) structures. The SOT switching current density can be reduced to 2.8 × 10 5  A   cm -2 , indicating its high efficiency. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to (1.56 ± 0.12) × 10 -6  T   A -1  cm 2 and the interfacial charge-to-spin conversion efficiency to 3.9 ± 0.3 nm -1 . The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices., (© 2024 Wiley‐VCH GmbH.)

Topological materials carrying topological surface states (TSSs) have extraordinary carrier mobility and robustness, which provide a new platform for searching for efficient hydrogen evolution reaction (HER) electrocatalysts. However, the majority of these TSSs originate from the sp band of topological quantum catalysts rather than the d band. Here, based on the density functional theory calculation, it is reported a topological semimetal Pd 3 Sn carrying TSSs mainly derived from d orbital and proposed that optimizing surface state electrons of Pd 3 Sn by introduction heteroatoms (Ni) can promote hybridization between hydrogen atoms and electrons, thereby reducing the Gibbs free energy (ΔG H ) of adsorbed hydrogen and improving its HER performance. Moreover, this is well verified by electrocatalytic experiment results, the Ni-doped Pd 3 Sn (Ni 0.1 Pd 2.9 Sn) show much lower overpotential (-29 mV vs RHE) and Tafel slope (17 mV dec -1 ) than Pd 3 Sn (-39 mV vs RHE, 25 mV dec -1 ) at a current density of 10 mA cm -2 . Significantly, the Ni 0.1 Pd 2.9 Sn nanoparticles exhibit excellent stability for HER. The electrocatalytic activity of Ni 0.1 Pd 2.9 Sn nanoparticles is superior to that of commercial Pt. This work provides an accurate guide for manipulating surface state electrons to improve the HER performance of catalysts., (© 2024 Wiley‐VCH GmbH.)

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1−xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1−xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.

Majorana zero modes (MZMs) are Majorana-fermion-like quasiparticles existing in crystals or hybrid platforms with topologically non-trivial electronic structures. They obey non-Abelian braiding statistics and are considered promising to realize topological quantum computing. Discovery of MZM in the vortices of the iron-based superconductors (IBSs) has recently fueled the Majorana research in a way which not only removes the material barrier requiring construction of complicated hybrid artificial structures, but also enables observation of pure MZMs under higher temperatures. So far, MZMs have been observed in iron-based superconductors including FeTe0.55Se0.45, (Li0.84Fe0.16)OHFeSe, CaKFe4As4, and LiFeAs. In this topical review, we present an overview of the recent STM studies on the MZMs in IBSs. We start with the observation of MZMs in the vortices in FeTe0.55Se0.45 and discuss the pros and cons of FeTe0.55Se0.45 compared with other platforms. We then review the following up discovery of MZMs in vortices of CaKFe4As4, impurity-assisted vortices of LiFeAs, and quantum anomalous vortices in FeTe0.55Se0.45, illustrating the pathway of the developments of MZM research in IBSs. Finally, we give perspective on future experimental works in this field. [ABSTRACT FROM AUTHOR]

Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)2Te3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac Fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.

Significant efforts have been put in the past to minimize the bulk conduction to probe the exotic surface states of topological insulators (TIs), nevertheless, the surface transport is usually hindered by the presence of bulk conduction. The identification and separation of the contribution of topological surface states (TSS) by electrical transport is important for understanding the unique properties of TIs and developing new electronic devices based on these materials. In this report we have studied Shubnikov-de Haas (SdH) oscillations, magneto-resistance (MR) and hall resistivity under high magnetic field in (Bi 0. 45 Sb 0. 60) 2 Te 3 single crystal grown by modified Bridgman method. Our results show the existence of SdH oscillations which arise from TSS. The obtained Fermi wave vector and Fermi velocity of surface charge carriers show a good agreement with those found by angle-resolved photoemission spectroscopy experiments. Two band model fit to hall resistivity confirms the high mobility of surface bands and low mobility of bulk bands. Band bending effect is investigated which exhibits a depletion of carriers from near by bulk towards the surface states resulting in an upward band bending. A large non-saturating MR (∼ 347)% is observed as a result of multichannel quantum coherent transport mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

Topological materials is one of the hottest topics in condensed matter physics because of its exotic properties such as robust metallic boundary states, Fermi arcs, and the spin‐momentum‐locking helicity. The topologically protected conducting boundary states spanning the whole bandgap are expected to serve as robust and wide‐range‐energy transition states facilitating catalytic reactions. Recently, some topological materials have been found to be high‐performance catalysts, which might open an emerging research field. Herein, an overview of topological materials is given and then recent progress in topological material catalysts (TMCs) is presented. As it is a new field, more detailed and accurate mechanisms behind the high performance of TMCs are urgently needed. Combining theoretical and experimental studies is a promising way to resolve these puzzles. Heterostructures, dopants, and defects have the chance to tune the catalytic activity of TMCs while retaining topological surface states (TSSs). Also, more TMCs are needed to be discovered, and more catalytic reactions are to be investigated for TMCs in the future. [ABSTRACT FROM AUTHOR]

Spin‐to‐charge conversion (SCC) involving topological surface states (TSS) is one of the most promising routes for highly efficient spintronic devices for terahertz (THz) emission. Here, the THz generation generally occurs mainly via SCC consisting in efficient dynamical spin injection into spin‐locked TSS. In this work, sizable THz emission from a nanometric thick topological insulator (TI)/ferromagnetic junction—SnBi2Te4/Co—specifically designed to avoid bulk band crossing with the TSS at the Fermi level, unlike its parent material Bi2Te3 is demonstrated. THz emission time domain spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by investigating the TI thickness and angular dependence of the THz emission. This work illustrates THz emission TDS as a powerful tool alongside angular resolved photoemission spectroscopy (ARPES) methods to investigate the interfacial spintronic properties of TI/ferromagnet bilayers. [ABSTRACT FROM AUTHOR]

Topological materials is one of the hottest topics in condensed matter physics because of its exotic properties such as robust metallic boundary states, Fermi arcs, and the spin‐momentum‐locking helicity. The topologically protected conducting boundary states spanning the whole bandgap are expected to serve as robust and wide‐range‐energy transition states facilitating catalytic reactions. Recently, some topological materials have been found to be high‐performance catalysts, which might open an emerging research field. Herein, an overview of topological materials is given and then recent progress in topological material catalysts (TMCs) is presented. As it is a new field, more detailed and accurate mechanisms behind the high performance of TMCs are urgently needed. Combining theoretical and experimental studies is a promising way to resolve these puzzles. Heterostructures, dopants, and defects have the chance to tune the catalytic activity of TMCs while retaining topological surface states (TSSs). Also, more TMCs are needed to be discovered, and more catalytic reactions are to be investigated for TMCs in the future.

We unravel the interplay of topological properties and the layered (anti)ferromagnetic ordering in EuSn2P2, using spin and chemical selective electron and X-ray spectroscopies supported by firstprinciple calculations. We reveal the presence of in-plane longrange ferromagnetic order triggering topological invariants and resulting in the multiple protection of topological Dirac states. We provide clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn2P2 as a prime candidate axion insulator for topological antiferromagnetic spintronics applications. [ABSTRACT FROM AUTHOR]

We unravel the interplay of topological properties and the layered (anti)ferromagnetic ordering in EuSn2P2, using spin and chemical selective electron and X-ray spectroscopies supported by firstprinciple calculations. We reveal the presence of in-plane longrange ferromagnetic order triggering topological invariants and resulting in the multiple protection of topological Dirac states. We provide clear evidence that layer-dependent spin-momentum locking coexists with ferromagnetism in this material, a cohabitation that promotes EuSn2P2 as a prime candidate axion insulator for topological antiferromagnetic spintronics applications. [ABSTRACT FROM AUTHOR]

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature scanning tunneling microscopy/spectroscopy observation of interaction between Rashba states and topological surface states, which manifests local electronic structure along step edges controllable by the layer thickness of thin films. The first-principles theoretical calculation elucidates the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. Furthermore, the Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi 2 Se 3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling the quantum states.

Topological insulators (TIs) with large spin Hall effect have potential applications for ultralow power consumption spintronic devices. However, the present methods for fabricating TIs mainly rely on epitaxial methods, which limits their mass production. Herein, perpendicular magnetized Pt/Co/(Bi1–xSbx)2Te3 (BST) heterostructures are prepared to measure the spin–orbit torque efficiency of BST fabricated by the conventional magnetron sputtering method, which shows large‐scale wafer production ability. The Fermi levels and the topological surface states (TSSs) are modulated by the composition of Sb in BST. The effective damping‐like spin Hall angle (θ SHeff) is independent of the thickness of TIs. When the Fermi level is near the Dirac point, θ SHeff for these non‐epitaxial heterostructures reaches its maximum value around 0.77, which is much larger than that of traditional ferromagnetic/heavy metal (FM/HM) heterostructures and quite close to that of the epitaxial TIs. This indicates that the TSS has the dominant contribution to θ SHeff. The large θ SHeff results in the small critical current density (Jc ≈ 105 A cm−2) for fully switching the magnetization of Co, compared with FM/HM heterostructures with Jc around 107 A cm−2. This work paves the way for potential large‐scale spintronic devices using low energy consumption non‐epitaxial TIs. [ABSTRACT FROM AUTHOR]

Optical chirality is an effective means in screening molecules and their enantiomers in bioengineering, and recently has garnered attention as an implementation of qubits in quantum information processing. The conventional detection of circularly polarized light (CPL) is based on phase retardation and polarization separation using multiple optical components. An intrinsic solid‐state chirality detection device would be favorable for easier integration and implementation. Optical spin injection to the spin‐momentum‐locked topological surface states of topological insulators (TIs) by circularly polarized light leads to a directional DC photocurrent and hence possible circular polarization detection. However, this DC photocurrent is also accompanied by other photo‐responses. Here, a photodetection strategy using a TI transistor which senses CPL without the use of any additional components is demonstrated, it achieves a uniform response over the entire device with a sensitivity ≈5.6%. The Stokes parameters can also be extracted by arithmetic operation of photocurrents obtained with different bias and gate for a complete characterization of a polarized light beam. Therefore, this method enables chirality detection and Stokes parameter analysis using a single device. The proposed miniaturized intrinsic chirality detectors facilitate polarimetry sensing in applications from circular dichroism spectroscopy to biomedical diagnosis. [ABSTRACT FROM AUTHOR]

This article reports damping enhancement in a ferromagnetic NiFe thin film due to an adjacent α‐Sn thin film. Ferromagnetic resonance studies show that an α‐Sn film separated from a NiFe film by an ultrathin Ag spacer can cause an extra damping in the NiFe film that is three times bigger than the intrinsic damping of the NiFe film. Such an extra damping is absent in structures where the α‐Sn film interfaces directly with a NiFe film, or is replaced by a β‐Sn film. The data suggest that the extra damping is associated with topologically nontrivial surface states in the topological Dirac semimetal phase of the α‐Sn film. This work suggests that, like topological insulators, topological Dirac semimetal α‐Sn may have promising applications in spintronics. [ABSTRACT FROM AUTHOR]

Spontaneous symmetry breaking has been a paradigm to describe the phase transitions in condensed matter physics. In addition to the continuous electromagnetic gauge symmetry, an unconventional superconductor can break discrete symmetries simultaneously, such as time reversal and lattice rotational symmetry. In this work we report a characteristic in-plane 2-fold behaviour of the resistive upper critical field and point-contact spectra on the superconducting semimetal PbTaSe 2 with topological nodal-rings, despite its hexagonal lattice symmetry (or D 3 h in bulk while C 3 v on surface, to be precise). The 2-fold behaviour persists up to its surface upper critical field H c 2 R even though bulk superconductivity has been suppressed at its bulk upper critical field H c 2 HC ≪ H c 2 R , signaling its probable surface-only electronic nematicity. In addition, we do not observe any lattice rotational symmetry breaking signal from field-angle-dependent specific heat within the resolution. It is worth noting that such surface-only electronic nematicity is in sharp contrast to the observation in the topological superconductor candidate, Cu x Bi 2 Se 3 , where the nematicity occurs in various bulk measurements. In combination with theory, superconducting nematicity is likely to emerge from the topological surface states of PbTaSe 2 , rather than the proximity effect. The issue of time reversal symmetry breaking is also addressed. Thus, our results on PbTaSe 2 shed new light on possible routes to realize nematic superconductivity with nontrivial topology. [ABSTRACT FROM AUTHOR]

The study of topological semimetals hosting spin-1 Weyl points (WPs) beyond Dirac points and WPs has attracted a great deal of attention. However, a spin-1 Weyl semimetal that exclusively possesses spin-1 WPs in a clean frequency window without being shadowed by any other nodal points is yet to be discovered. This study reports a spin-1 Weyl semimetal in a phononic crystal. Its spin-1 WPs are touched by two linear dispersions and an additional flat band and carry monopole charges (−2,0,2) or (2,0,−2) for the three bands from the bottom to the top. They result in double Fermi arcs, which occur between the first and second bands, as well as between the second and third bands. Further robust propagation is observed against the multiple joints and topological negative refraction of the acoustic surface arc wave. The results of this study create the basis for the exploration of the unusual properties of spin-1 Weyl physics on a macroscopic scale. [ABSTRACT FROM AUTHOR]

Recently, the topological insulators (TIs) with topological surface states (TSS) exhibiting large spin–orbit torques (SOTs) in topological insulators/ferromagnetic (TIs/FM) systems have been a hot spot research in spintronics. Herein, the effective spin Hall angles (θSHeff) in perpendicularly magnetized Pt/Co/Bi2Se3 stacks with different thicknesses of the Bi2Se3 layers are investigated by the harmonic Hall resistance measurements. The results show that the θSHeff for the TIs/FM heterostructures is substantially larger than the previously reported heavy metal/ferromagnetic (HM/FM). Moreover, θSHeff keeps constant with the value around 0.35 in Pt/Co/Bi2Se3 stacks with the variation of the thickness of Bi2Se3 from 3.0 to 15.0 nm, indicating that the SOTs are mainly derived from the contribution of the TSS of Bi2Se3 rather than the interfacial Rashba effect and/or bulk spin Hall effect. The results confirm that TIs with unique spintronic texture can play a crucial role in spintronics applications. [ABSTRACT FROM AUTHOR]

• Co 2 MnGa sample exhibits both high catalytic activity and high stability for HER. • We discover a linear relationship between the ΔG H* and the areas of TSSs. • The density of TSSs at Fermi energy level can be modulated by external factors. • Transitioning topological catalysis from nonmagnetic to magnetic categories. Topological catalysis has become a major topic in electrochemical catalysis due to its outstanding performance in electrochemical reactions. Recently, a universal catalytic descriptor, known as topological surface states (TSS), has been proposed to describe catalytic performance in several nonmagnetic topological catalysts [Mater. Today 67, 23 (2023)]. However, the universality of this descriptor still lacks experimental evidence and has not been verified in magnetic topological catalysts yet. In this study, we investigate the catalytic performance and topological properties of the magnetic material Co 2 MnGa, which is a representative topological nodal chain semimetal with both theoretical and experimental validation. Our experimental findings demonstrate that Co 2 MnGa exhibits excellent catalytic performance in the hydrogen evolution reaction (HER). Our research reveals the presence of TSS from the nodal chain state is responsible for the high catalytic activity in Co 2 MnGa. Further, a direct relationship between the strength of TSS and Gibbs free energy can be built in the system. This comprehensive study has expanded our understanding of topological catalysts by exploring their material properties and corresponding topological mechanisms, extending our knowledge from nonmagnetic to magnetic categories. [ABSTRACT FROM AUTHOR]

It was confirmed that the photocurrent value was significantly improved by measuring the photoresponse using TI Bi 2 Se 3 -carrier transfer layer and Au electrode in the same WSe 2 to exclude light absorption rate and device-specific diversity. Factors enabling efficient charge separation include the formation of a few hybridization states at the interface and the presence of a TSS that facilitates effective carrier injection, not relying solely on the bulk state. [Display omitted] • Bi 2 Se 3 enhances photoresponsivity, emphasizing the influence of bulk state and TSS in photocurrent.. • Bi 2 Se 3 thickness alters electron-hole pair separation efficiency, influenced by the topological surface state. • Closer interfaces result in the formation of a hybridization state, serving as a site for electron-hole recombination. • TSS in carrier dynamics implies future progress in interfacial technology for next-gen optoelectronic devices. Carrier scattering in photodetectors employing 2D dichalcogenide as an absorber impedes carrier transport. As scattering at the interface between the semiconductor and the electrode restricts carrier transport, it is essential to develop alternative electrode materials to achieve high photoresponsivity. A carrier can effectively move through a topologically protected Dirac surface state without scattering: i.e., alternatives such as topological insulators (TI) containing the surface state can effectively reduce scattering at the interface between the semiconductor and the electrode. In this stucy, TI Bi 2 Se 3 with a well-ordered interfacial structure was used as a carrier transport layer (CTL) to effectively employ the topological surface state (TSS) for improving WSe 2 photoresponsivity. At 520 nm, the Bi 2 Se 3 TI-CTL photodetector has a photoresponsivity of up to 11.47 AW−1, 100 times higher than the photodevice using only Au electrodes. The photoresponsivity and time-resolved photoluminescence as a function of thickness indicate that TSS contributes to improved carrier separation with high carrier transfer efficiency, thereby affecting photocurrent generation. The fast carrier transfer through TSS helps efficiently separate electron-hole pairs and suppress recombination at the interface, generating ultra-fast and high-efficiency photocurrents in the broadband region (450–1550 nm). Therefore, TSS based on carrier dynamics can lead to further applications of next-generation optoelectronic devices. [ABSTRACT FROM AUTHOR]

We report the electrical and magnetic transport behavior in vertical Cu-doped Bi 2 Se 3 nanoplate films prepared by the chemical vapor deposition method. In vertical Cu-doped Bi 2 Se 3 nanoplate films, the topological surface states are tuned by both the large surface-to-bulk ratio and the Cu doping. Due to their high specific surface area, the magnetoresistance of the vertical undoped Bi 2 Se 3 nanoplate film exhibits a weak antilocalization effect, and it indicates that the topological surface state properties are greatly enhanced. In vertical Cu-doped Bi 2 Se 3 nanoplate films, the electron doping is inhibited, and the carrier type is changed from n-type to p-type. The observed linear magnetoresistance is attributed to have a quantum origin from the topological surface states. When the Cu concentration reaches 1.73 at.% in vertical Bi 2 Se 3 nanoplate film, the linear magnetoresistance can be maintained up to 100 K. Meanwhile, these vertical nanoplate films exhibit the 3D magnetotransport property. Thus, using the same material system with a broad range of carrier density and type, our work shows the transition from a weak-antilocalization to linear magnetoresistance in nanostructured topological insulator Bi 2 Se 3 films with an unusual morphology where the nanoplates are vertically aligned to increase the surface area. [Display omitted] • The magnetoresistance exhibits a WAL characteristic in our film with high specific surface area. • The linear magnetoresistance characteristic can be maintained up to 100 K at most. • The carrier type is changed from n-type to p-type in vertical Cu-doped Bi 2 Se 3 nanoplate films. • The TSSs are enhanced greatly by both the large surface-to-bulk ratio and the Cu doping. [ABSTRACT FROM AUTHOR]

The remarkable spin-charge interconversion ability of transition metal dichalcogenides (TMDs) makes them promising candidates for spintronic applications. Nevertheless, their potential as spintronic terahertz (THz) emitters (STEs) remains constrained mainly due to their sizable resistivity and low spin Hall conductivity (SHC), which consequently result in modest THz emission. In this work, the TMD PtTe 2 , a type-II Dirac semimetal is effectively utilized to develop efficient STEs. This high efficiency primarily results from the large SHC of PtTe 2 , stemming from its low resistivity and significant spin-to-charge conversion efficiency, attributed to surface states and the local Rashba effect in addition to the inverse spin Hall effect. Remarkably, the peak THz emission from PtTe 2 /Co-STE exceeds that of Pt/Co-STE by ∼15% and is nearly double that of a similarly thick Pt/Co-STE. The efficient THz emission in the PtTe 2 /Co heterostructure opens new possibilities for utilizing the semimetal TMDs for developing THz emitters.

The confinement of electrons in one-dimensional (1D) space highlights the prominence of the role of electron interactions or correlations, leading to a variety of fascinating physical phenomena. The quasi-1D electron states can exhibit a unique spin texture under spin-orbit interaction (SOI) and thus could generate a robust spin current by forbidden electron backscattering. Direct detection of such 1D spin or SOI information, however, is challenging due to complicated techniques. Here, we identify an anomalous planar Hall effect (APHE) in the magnetotransport of quasi-1D van der Waals (vdW) topological materials as exemplified by Bi 4 Br 4 , which arises from the quantum interference correction of 1D weak antilocalization (WAL) to the ordinary planar Hall effect and demonstrates a deviation from the usual sine and cosine curves. The occurrence of 1D WAL is correlated to the line-shape Fermi surface and persistent spin texture of (100) topological surface states of Bi 4 Br 4 , as revealed by both our angle-resolved photoemission spectroscopy and first-principles calculations. By generalizing the observation of APHE to other non-vdW bulk materials, this work provides a possible characteristic of magnetotransport for identifying the spin/SOI information and quantum interference behavior of 1D states in 3D topological material.

This investigation provides a comprehensive exploration into the intricate interplay between topological surface states (TSS) and catalytic performance in two-dimensional (2D) materials, with specific emphasis on monolayer Cu 2 Se. Leveraging the unique characteristics of nodal loop semimetals (NLSMs), we delve into the precise influence of TSS on catalytic activity, particularly in the domain of CO 2 electrochemical reduction. Our findings illuminate the central role played by these TSS, arising from the underlying NLSM framework, in sculpting catalytic efficiency. The length of these surface states emerges as a critical determinant of surface density of states (DOSs), a fundamental factor governing catalytic behavior. Extension of these surface states correlates with heightened surface DOSs, yielding lower Gibbs free energies and consequently enhancing catalytic performance, particularly in the electrochemical reduction of CO 2 . Moreover, we underscore the profound importance of preserving symmetries that protect the nodal loop. The disruption of these symmetries is found to result in a significant degradation of catalytic efficacy, underscoring the paramount significance of topological features in facilitating catalytic processes. Therefore, this study not only elucidates the fundamental role of TSS in dictating the catalytic performance of topological 2D materials but also paves the way for harnessing these unique attributes to drive sustainable and highly efficient catalysis across a diverse spectrum of chemical processes., (© 2024 IOP Publishing Ltd.)

Abstract The central challenge faced in hydrogen evolution reaction is the low reaction efficiency when using non-precious materials as potential new types of electrocatalysts. Here, we report an elegant synergistic effect of local bonding and topological surface states (TSSs) for optimally enhanced hydrogen evolution reaction by invoking topological insulator heterostructures purely composed of abundant elements. Using first-principles calculations within density functional theory, we demonstrate that the three-dimensional topological insulator of Bi 2 Se 3 covered with a single layer of ZnSe can function as an ideal platform for maximal hydrogen evolution reaction with optimal hydrogen adsorption free energy. Such a highly desirable functionality is attributed to the TSSs of Bi 2 Se 3 serving as an electron bath in enhancing the hydrogen adsorption strength, which would be too weak on top of the ZnSe overlayer without considering topological effects, but too strong upon increasing the ZnSe overlayer thickness. We also demonstrate precise tunability of the vertical location of the TSSs via the ZnSe overlayer thickness, which can be exploited for other functionalities of such topological insulator heterostructures. The present study provides an important linkage between the topological insulators as a new class of quantum matter and catalytic materials for clean energy. Graphical abstract fx1 Highlights • Synergistic effect for optimally enhanced HER is discovered via ab initial studies. • Maximal HER can take place on the platform of one monolayer ZnSe on Bi 2 Se 3. • Synergy is due to local bonding on ZnSe and topological surface states of Bi 2 Se 3. • The vertical location of topological surface states is tuned by the ZnSe thickness. [ABSTRACT FROM AUTHOR]

As topological insulators (TIs) are becoming increasingly intriguing, the community is exploring transformative applications that require interfacing TIs with other materials such as ferromagnets or superconductors. Herein, we report on the manifestations of superconducting electrons carried by topological surface states (TSS) in Bi 2 Se 3 films. As key signatures of TSS-carried Cooper pairs, we uncover the hysteresis of magnetoresistance (MR) and the switching behavior of anisotropic magnetoresistance (AMR). For in-plane fields perpendicular to the injected current, AMR shows negative switching (resistance drop) when the contacts become superconducting, which is consistent with a cooperative Zeeman effect enabled by the spin-momentum locking of TSS. The MR and AMR behaviors are robust, occurring reliably in multiple samples, from different sources, and with different defect concentrations. Our findings can guide novel developments in superconductor/TI quantum devices relying on supercurrent detection as well as lead to more refined transport signatures of Majorana zero-modes in the future.

Topological insulators are novel electronic materials, where time reversal symmetry protects the spin‐polarized surface states from backscattering. Although, the bulk offers a trivial semiconducting response to incident light, but the metallic surface states exhibit interesting electrical response towards the incident radiation, such as polarization dependent surface photocurrent and topological phase transitions. Here, we study the temperature dependent near‐infrared photoresponse in bismuth selenide (Bi2Se3) nano‐flakes. A very good photo‐sensitivity to near‐infrared 800 nm wavelength is observed for the temperature range 300–2 K. Voltage responsivity at 2 K (1.481 × 1010 VW−1) is estimated to be four‐order greater than at 300 K (2.095 × 106 VW−1) and is comparatively much higher than the previously reported values of voltage responsivity in other materials. Interestingly, we also find the temperature dependent existence of both positive and negative photoresponse in our device. An anomalous photoresponse reversal is observed at 2 K, where resistance of the nano‐flake increases under light illumination. We propose that this phenomenon is due to the topological surface states, which have dominant transport contribution at very low temperatures. These observations establish the fact that topological insulators can be further engineered to develop a dual state photo‐switch, by manipulating the bulk and surface state conduction contribution. [ABSTRACT FROM AUTHOR]

Abstract: Topological Kondo insulators (TKIs) are a new class of topological materials in which topological surface states dominate the transport properties at low temperatures. They are also an ideal platform for studying the interplay between strong electron correlations and topological order. Here, hysteretic magnetoresistance (MR) is observed in TKI SmB6 thin nanowires at temperatures up to 8 K, revealing the strong magnetism at the surface of SmB6. It is also found that such MR anomaly exhibits an intriguing finite size effect and only appears in nanowires with diameter smaller than 58 nm. These nontrivial phenomena are discussed in terms of the latest Kondo breakdown model, which incorporates the RKKY magnetic interaction mediated by surface states with the strong electron correlation in SmB6. It would provide new insight into the nature of TKI surface states. Additionally, a non‐monotonically temperature dependent positive magnetoresistance is observed at intermediate temperatures, suggesting the possible impurity‐band conduction in SmB6, other than the surface state transport at low temperatures and the bulk‐band transport at high temperatures. [ABSTRACT FROM AUTHOR]

Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)2Te3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac Fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators. [ABSTRACT FROM AUTHOR]

Magneto-transport results obtained for the strained 100 nm thick Hg 1 − x Cd x Te (x=0.135) layer grown by MBE on the CdTe/GaAs substrate are interpreted by the 8×8 kp model with the in-plane tensile strain. The dispersion relation for the investigated structure proves that the Dirac point is located in the gap caused by the strain. It is also shown that the fan of the Landau Levels (LL's) energy calculated for topological protected surface states for the studied HgCdTe alloy corresponds to the fan of the LL's calculated using the graphen-like Hamiltonian which gives excellent agreement with the experimental data for velocity on the Fermi level equal to v f ≈ 0.85×10 6 m/s. That characterized strained Hg 1 − x Cd x Te layers (0.13 < x < 0.14) are a perfect Topological Insulator with good perspectives of further applications. [ABSTRACT FROM AUTHOR]

We present low-temperature magnetotransport measurements on selectively grown Sb 2 Te 3 -based topological insulator ring structures. These devices display clear Aharonov-Bohm oscillations in the conductance originating from phase-coherent transport around the ring. The temperature dependence of the oscillation amplitude indicates that the Aharonov-Bohm oscillations originate from ballistic transport along the ring arms. We attribute these oscillations to the topological surface states. Further insight into the phase coherence is gained by comparing with similar Aharonov-Bohm-type oscillations in topological insulator nanoribbons exposed to an axial magnetic field. Here, quasi-ballistic phase-coherent transport is confirmed for closed-loop topological surface states in the transverse direction enclosing the nanoribbon. In contrast, the appearance of universal conductance fluctuations indicates phase-coherent transport in the diffusive regime, which is attributed to bulk carrier transport. Thus, it appears that even in the presence of diffusive p -type charge carriers in Aharonov-Bohm ring structures, phase-coherent quasi-ballistic transport of topological surface states is maintained over long distances.

We have analyzed different factors responsible for changes in the Dirac gap in MnBi 2 Te 4 and the routes that determine the possibilities of the purposeful gap modulation. It was shown that upon changing the surface van der Waals interval and surface spin–orbit coupling strength the topological surface states localization shifts between the surface septuple layers with opposite magnetizations, which leads to a nonmonotonic change in the Dirac gap size. The minimum in the Dirac gap corresponds to the point of changing the sign of the emerging exchange field. Moreover, we have shown that the Dirac gap can be effectively modulated by replacing magnetic Mn atoms in the surface layer with nonmagnetic ones or Bi and Te atoms with atoms of elements with a lower spin–orbit coupling that makes it possible to create synthetic layered topological systems with purposeful modification of the surface properties. • A spatial redistribution of the topological surface states changes the Dirac gap value. • A surface van der Waals interval of MnBi 2 Te 4 is supposedly slightly compressed. • Purposeful modification of MnBi 2 Te 4 properties by replacing atoms in surface layers. [ABSTRACT FROM AUTHOR]

We report on systematic study of transport properties of a 1000 nm HgTe film. Unlike thinner and strained HgTe films, which are known as high-quality three-dimensional topological insulators, the film under study is much thicker than the limit of pseudomorphic growth of HgTe on a CdTe substrate. Therefore, the 1000 nm HgTe film is expected to be fully relaxed and has the band structure of bulk HgTe, i.e. a zero gap semiconductor. Additionally, the system is characterized by the bands inversion, so that the two-dimensional topological surface states (TSSs) are expected to exist. To check this claim we studied classical and quantum transport response of the system. We demonstrate that by tuning the top-gate voltage one can change the electron-dominating transport to the hole one. The highest electron mobility is found to be more than300×103 cm 2  Vs -1 . The system exhibits Shubnikov-de Haas (SdH) oscillations with a complicated pattern and shows up to five independent frequencies in corresponding Fourier spectra. These Fourier peaks are attributed to the TSSs, Volkov-Pankratov states and spin-degenerate bulk states in the accumulation layer near the gate. The observed peculiarities of the quantum transport are the strong SdH oscillations of the Hall resistance, and the suppressed oscillatory response of the TSSs., (Creative Commons Attribution license.)

We review recent progress in the electronic structure study of intrinsic magnetic topological insulators (MnBi 2 Te 4 ) · (Bi 2 Te 3 ) n ([Formula: see text]) family. Specifically, we focus on the ubiquitously (nearly) gapless behavior of the topological Dirac surface state observed by photoemission spectroscopy, even though a large Dirac gap is expected because of surface ferromagnetic order. The dichotomy between experiment and theory concerning this gap behavior is perhaps the most critical and puzzling question in this frontier. We discuss various proposals accounting for the lack of magnetic effect on the topological Dirac surface state, which are mainly categorized into two pictures, magnetic reconfiguration and topological surface state redistribution. Band engineering towards opening a magnetic gap of topological surface states provides great opportunities to realize quantized topological transport and axion electrodynamics at higher temperatures., (© The Author(s) 2023. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.)

Based on the first-principles calculations, we obtain the superconducting transition temperature (T c) of 21.17 K for NbRu 3 C. For the superconductivity of NbRu 3 C, the strong electron–phonon coupling mainly comes from the coupling between Ru 4 d orbitals and low-energy phonon modes which include triply degenerate T 1 u and doubly degenerate E. Moreover, NbRu 3 C is found to have a topological insulating phase, confirmed by the calculated nontrivial surface states and spin textures. The presence of superconductivity and topological surface states in NbRu 3 C suggests that NbRu 3 C may be a candidate for the topological superconductor. • NbRu 3 C is found to be a phonon-mediated superconductor with T c of 21.17 K. • The electron–phonon interactions and superconductivity of NbRu 3 C are discussed. • Ru-related vibrations strongly couple with electrons of Ru 4 d orbitals. • The topological surface states in the electronic band of NbRu 3 C are studied. [ABSTRACT FROM AUTHOR]

In this paper the results of the numerical calculation obtained for the three-dimensional (3D) strained Hg 1 − x Cd x Te layers for the x -Cd composition from 0.1 to 0.155 and a different mismatch of the lattice constant are presented. For the investigated region of the Cd composition ( x value) the negative energy gap ( E g = Γ 8 − Γ 6 ) in the Hg 1 − x Cd x Te is smaller than in the case of pure HgTe which, as it turns out, has a significant influence on the topological surface states (TSS) and the position of the Dirac point. The numerical calculation based on the finite difference method applied for the 8×8 kp model with the in-plane tensile strain for (001) growth oriented structure shows that the Dirac cone inside the induced insulating band gap for non zero of the Cd composition and a bigger strain caused by the bigger lattice mismatch (than for the 3D HgTe TI) can be obtained. It was also shown how different x -Cd compounds move the Dirac cone from the valence band into the band gap. The presented results show that 75 nm wide 3D Hg 1 − x Cd x Te structures with x ≈ 0.155 and 1.6% lattice mismatch make the system a true topological insulator with the dispersion of the topological surface states similar to those ones obtained for the strained CdTe/HgTe QW. [ABSTRACT FROM AUTHOR]

Ferroelectricity and band topology are two extensively studied yet distinct properties of insulators. Nonetheless, their coexistence has never been observed in a single material. Using first-principles calculations, we demonstrate that a noncentrosymmetric perovskite structure of CsPbI3 allows for the simultaneous presence of ferroelectric and topological orders with appropriate strain engineering. Metallic topological surface states create an intrinsic short-circuit condition, helping stabilize bulk polarization. Exploring diverse structural phases of CsPbI3 under pressure, we identify that the key structural feature for achieving a ferroelectric topological insulator is to suppress PbI6 cage rotation in the perovskite structure, which could be obtained via strain engineering. Ferroelectric control over the density of topological surface states provides a new paradigm for device engineering, such as perfect-focusing Veselago lens and spin-selective electron collimator. Our results suggest that CsPbI3 is a simple model system for ferroelectric topological insulators, enabling future studies exploring the interplay between conventional symmetry-breaking and topological orders and their novel applications in electronics and spintronics. [ABSTRACT FROM AUTHOR]