Robust Surface States Research Articles

Robust gapped surface states and filtering effect in a photonic topological gyroelectromagnetic metamaterial

The recent reported robust surface states in a photonic metamaterial are usually limited to the gapless type. In this work, we propose a photonic topological metamaterial that can support surface states with gaps. The gaps appear when the electromagnetic duality symmetry of the material system is broken. Despite the existence of gaps, the surface waves can still bypass sharp corners robustly. We further demonstrate the filtering effect which can be viewed as a fingerprint of the existence of the gapped surface states. By changing the electromagnetic parameters of the covering materials, the filtering effect with controllable bandwidth can be realized.

Topological Anderson Insulator in Cation-Disordered Cu2ZnSnS4.

We present the first candidate for the realization of a disorder-induced Topological Anderson Insulator in a real material system. High-energy reactive mechanical alloying produces a polymorph of Cu2ZnSnS4 with high cation disorder. Density functional theory calculations show an inverted ordering of bands at the Brillouin zone center for this polymorph, which is in contrast to its ordered phase. Adiabatic continuity arguments establish that this disordered Cu2ZnSnS4 can be connected to the closely related Cu2ZnSnSe4, which was previously predicted to be a 3D topological insulator, while band structure calculations with a slab geometry reveal the presence of robust surface states. This evidence makes a strong case in favor of a novel topological phase. As such, the study opens up a window to understanding and potentially exploiting topological behavior in a rich class of easily-synthesized multinary, disordered compounds.

Exceptional topological insulators

We introduce the exceptional topological insulator (ETI), a non-Hermitian topological state of matter that features exotic non-Hermitian surface states which can only exist within the three-dimensional topological bulk embedding. We show how this phase can evolve from a Weyl semimetal or Hermitian three-dimensional topological insulator close to criticality when quasiparticles acquire a finite lifetime. The ETI does not require any symmetry to be stabilized. It is characterized by a bulk energy point gap, and exhibits robust surface states that cover the bulk gap as a single sheet of complex eigenvalues or with a single exceptional point. The ETI can be induced universally in gapless solid-state systems, thereby setting a paradigm for non-Hermitian topological matter.

Dislocation as a bulk probe of higher-order topological insulators

Topological materials occupy the central stage in the modern condensed matter physics because of their robust metallic edge or surface states protected by the topological invariant, characterizing the electronic band structure in the bulk. Higher-order topological (HOT) states extend this usual bulk-boundary correspondence, so they host the modes localized at lower-dimensional boundaries, such as corners and hinges. Here we theoretically demonstrate that dislocations, ubiquitous defects in crystalline materials, can probe higher-order topology, recently realized in various platforms. We uncover that HOT insulators respond to dislocations through symmetry protected finite-energy in-gap electronic modes, localized at the defect core, which originate from an interplay between the orientation of the HOT mass domain wall and the Burgers vector of the dislocation. As such, these modes become gapless only when the Burgers vector points toward lower-dimensional gapless boundaries. Our findings are consequential for the systematic probing of the extended bulk-boundary correspondence in a broad range of HOT crystals and photonic and phononic or mechanical metamaterials through the bulk topological lattice defects.

Quantum coherence modulation in bismuth selenide topological insulator thin film by ion irradiation

The robust surface states of bismuth selenide (Bi2Se3) topological insulator (TI) that could potentially be harnessed for quantum computing and spintronics applications are often eclipsed by defect-induced bulk carriers. With the objective of suppressing the electronic transport in the bulk, we explore the effects of ion irradiation-driven isoelectronic doping in thin films of Bi2Se3 with 600 keV antimony (Sb+) ions at 45° oblique incidence. The distribution of Sb+ ions introduced into the system has been mapped using Rutherford backscattering spectroscopy. The heavy Sb+ ion irradiation has been observed to cause a change from parabolic magnetoresistance to linear magnetoresistance behavior in the irradiated films due to mobility fluctuations. Subsequent annealing in selenium environment has been found to reverse the damage produced by the irradiation, but with significant modifications in the quantum coherence signatures. Weak antilocalization behavior has been observed in the weak magnetic field limit and has been analyzed within the purview of the Hikami-Larkin-Nagaoka model. It has been found that the channel-separated Bi2Se3 film becomes completely channel-mixed due to the induced defects, yielding a system with a single coherent channel upon irradiation. In the as-deposited film, the temperature-dependent power law decay of the coherence length has been found to have two different exponents within the measured temperature range. On the other hand, the irradiated-annealed film has a single decay exponent which has been found to tend to − 0.5, indicating that electron-electron interaction is the only de-phasing mechanism involved. This work establishes the effectiveness of ion irradiation in tuning the quantum coherence phenomena in Bi-based TI systems.

Robust Surface States and Coherence Phenomena in Magnetically Alloyed SmB_{6}.

Samarium hexaboride is a candidate for the topological Kondo insulator state, in which Kondo coherence is predicted to give rise to an insulating gap spanned by topological surface states. Here we investigate the surface and bulk electronic properties of magnetically alloyed Sm_{1-x}M_{x}B_{6} (M=Ce, Eu), using angle-resolved photoemission spectroscopy and complementary characterization techniques. Remarkably, topologically nontrivial bulk and surface band structures are found to persist in highly modified samples with up to 30% Sm substitution and with an antiferromagnetic ground state in the case of Eu doping. The results are interpreted in terms of a hierarchy of energy scales, in which surface state emergence is linked to the formation of a direct Kondo gap, while low-temperature transport trends depend on the indirect gap.

Pressure induced superconducting state in ideal topological insulator BiSbTe3

Structural, pressure-dependent resistivity, angle resolved photoemission spectroscopy (ARPES), x-ray photoelectron diffraction (XPD) and band structure by DFT calculation have been investigated for BiSbTe3 Topological insulator. It has been demonstrated that the Dirac point of the topological surface state (TSS) located exactly at the Fermi level. Additionally, superconductivity emerges under pressure of 8 GPa with a critical temperature of ∼2.5 K. With further increase of pressure, the superconducting transition temperature (Tc) increases and at 14 GPa it shows the maximum Tc (∼3.3 K). It has also been shown that the surface state remains unchanged under pressure and has been suggested that the origin of the superconductivity is due to the bulk state. The investigation indicates that the BiSbTe3 has robust surface states and becomes superconductor under pressure.

Emergence of −s, −p–d band inversion in zincblende gold iodide topological insulator and its thermoelectric properties

We employ first-principles calculations to investigate the topological states (TS) and thermoelectric (TE) transport properties of three dimensional (3D) gold iodide (AuI) which belongs to the zincblende family. We explore, semi-metal (SM) to topological conductor (TC) and topological insulator (TI) phase transitions. Under pristine conditions, AuI exhibits Dirac SM nature but, under the influence of mild isotropic compressive pressure the system undergoes electronic quantum phase transition driving it into non-trivial topological state. This state exhibits Dresselhaus like band spin splitting leading to a TC state. In order to realize TI state from the SM state, we break the cubic symmetry of the system by introducing a compressive pressure along (001) crystal direction. The non-trivial TI nature of the system is characterized by the emergence of robust surface states and the invariant ν 0 = 1 which indicates a strong TI nature. A novel facet of the phase transition discussed here is, the −s and −p, −d orbital band inversion mechanism which is unconventional as compared to previously explored TI families. This mechanism unravels new path by which TI materials can be predicted. Also, we investigated the lattice and electronic contributions to the TE transport properties. We characterize the TE performance by calculating the figure of merit (zT) and find that, at room temperature (300 K) and for a fixed doping concentration (i.e., n = 1 × 1019 cm−3) the zT is 0.55 and 0.53 for electrons and holes respectively. This is quite remarkable since, higher values of zT are generally predicted at higher temperature scales whereas, zT values as in the present case are desired at room temperatures for various energy applications. The manifestation of non-trivial TS governed by the unconventional band inversion mechanism and the TE properties of AuI make it a unique multi-functional candidate with probable thermoelectric and spintronic applications.

Strong Spin-Orbital Coupling and Robust Surface States in SrirO <sub>3</sub> Films: Confirmation by Increasing Lattice Disorder

Strong Spin-Orbital Coupling and Robust Surface States in SrirO <sub>3</sub> Films: Confirmation by Increasing Lattice Disorder

Recent advancements in the study of intrinsic magnetic topological insulators and magnetic Weyl semimetals

The studies of topological insulators (TI) and topological semimetals have been at frontiers of condensed matter physics and material science. Both classes of materials are characterized by robust surface states created by the topology of the bulk band structures and exhibit exotic transport properties. When magnetism is present in topological materials and breaks the time-reversal symmetry, more exotic quantum phenomena can be generated, e.g., quantum anomalous Hall effect (QAHE), axion insulator, and large intrinsic AHE. In this research update, we briefly summarize the recent research progress in magnetic topological materials, including intrinsic magnetic TI and magnetic Weyl semimetals.

Stoichiometry-Controlled MoxW1-xTe2 Nanowhiskers: A Novel Electrocatalyst for Pt-Free Dye-Sensitized Solar Cells.

Novel polymorphic MoxW1-xTe2-based counter electrodes possess high carrier mobility, phase-dependent lattice distortion, and surface charge density wave to boost the charge-transfer kinetics and electrocatalytic activity in dye-sensitized solar cells (DSSCs). Here, we report the syntheses of stoichiometry-controlled binary and ternary MoxW1-xTe2 nanowhiskers directly on carbon cloth (CC), denoted by MoxW1-xTe2/CC, with an atmospheric chemical vapor deposition technique. The synthesized MoxW1-xTe2/CC samples, including 1T'-MoTe2/CC, Td-WTe2/CC, Td-Mo0.26W0.73Te2.01/CC, and 1T'- & Td-Mo0.66W0.32Te2.02/CC, were then employed as different counter electrodes to study their electrochemical activities and efficiencies in DSSCs. The photovoltaic parameter analysis manifests that MoxW1-xTe2/CCs are more stable than a standard Pt/CC in the I-/I3- electrolyte examined by cyclic voltammetry over 100 cycles. A 1T'- & Td-Mo0.66W0.32Te2.02/CC-based DSSC can achieve a photocurrent density of 16.29 mA cm-2, a maximum incident photon-to-electron conversion efficiency of 90% at 550 nm excitation, and an efficiency of 9.40%, as compared with 8.93% of the Pt/CC counterpart. Moreover, the 1T'- & Td-Mo0.66W0.32Te2.02/CC shows lower charge-transfer resistance (0.62 Ω cm2) than a standard Pt/CC (1.19 Ω cm2) in electrocatalytic reactions. Notably, MoxW1-xTe2 nanowhiskers act as an electron expressway by shortening the path of carrier transportation in the axial direction from a counter electrode to electrolytic ions to enhance the reaction kinetics in DSSCs. This work demonstrates that the nanowhisker-structured 1T'- & Td-Mo0.66W0.32Te2.02/CC with high carrier mobility and robust surface states can serve as a highly efficient counter electrode in DSSCs to replace the conventional Pt counter electrode for electrocatalytic applications.

Heterogeneous catalysis at the surface of topological materials

Intriguing properties are frequently reported in various topologically non-trivial materials. They include robust metallic surface states, high carrier mobility, chiral fermions, and ultralong Fermi arcs. An exciting recent finding is that these properties are strongly related to adsorption and electron transfer in various heterogeneous catalysis reactions, such as hydrogen evolution, oxygen evolution, oxygen reduction, enantiospecific adsorption, and hydrometallation. Thus, we expect that the introduction of non-trivial symmetry-protected topological order will offer important freedom for designing high-performance heterogeneous catalysts. To uncover the contribution of the topologically non-trivial electronic structure to the heterogeneous reactions, in situ techniques are urgently needed to detect the interaction between surface states, topological electrons, and reaction intermediates.

Expediting Hydrogen Evolution through Topological Surface States on Bi2Te3

Recently, the development of efficient and non-noble metal electrocatalysts with excellent durability for the hydrogen evolution reaction (HER) has attracted increasing attention. The exotic and robust metallic surface states of topological insulators (TIs) are theoretically predicted to enhance surface catalytic activity of overlaying catalysts, but no experimental evidence for TIs directly used as electrocatalysts has ever been reported. In this work, we fabricated the TI thin films of Bi2Te3 with different thicknesses using the molecular beam epitaxy method, and found that these thin films exhibit high electrocatalytic activity in HER. The 48 nm Bi2Te3 thin film has the best performance, which is attributed to its largest active area arising from the spiral growth mode of triangular domains as revealed by atomic force microscopy imaging. Importantly, our theoretical calculations reveal that while pure Bi2Te3 is not a good electrocatalyst, the Bi2Te3 thin films with partially oxidized surfaces or Te vacancies have high HER activity. The existence of the corresponding surface oxides on the Bi2Te3 thin films is supported by our X-ray photoelectron spectroscopy data. Particularly, we demonstrate that the topological surface states play a key role in enhancing the HER performance. Our study offers a new direction to design cost-effective electrocatalysts.

Quantum oscillations of robust topological surface states up to 50\u2009K in thick bulk-insulating topological insulator

As personal electronic devices increasingly rely on cloud computing for energy-intensive calculations, the power consumption associated with the information revolution is rapidly becoming an important environmental issue. Several approaches have been proposed to construct electronic devices with low-energy consumption. Among these, the low-dissipation surface states of topological insulators (TIs) are widely employed. To develop TI-based devices, a key factor is the maximum temperature at which the Dirac surface states dominate the transport behavior. Here, we employ Shubnikov-de Haas oscillations (SdH) as a means to study the surface state survival temperature in a high-quality vanadium doped Bi1.08Sn0.02Sb0.9Te2S single crystal system. The temperature and angle dependence of the SdH show that: (1) crystals with different vanadium (V) doping levels are insulating in the 3–300 K region; (2) the SdH oscillations show two-dimensional behavior, indicating that the oscillations arise from the pure surface states; and (3) at 50 K, the V0.04 single crystals (Vx:Bi1.08-xSn0.02Sb0.9Te2S, where x = 0.04) still show clear sign of SdH oscillations, which demonstrate that the surface dominant transport behavior can survive above 50 K. The robust surface states in our V doped single crystal systems provide an ideal platform to study the Dirac fermions and their interaction with other materials above 50 K.

Topological phase transition of Bi2–xInxTe3

Compared with Bi2Se3 with its Dirac point in the bulk gap, the Dirac point of Bi2Te3 is in the valence band, then it is generally considered that it is difficult to observe the topological phase transition course in the Bi2Te3 system via nonmagnetic lighter element doping. Here, we report the phase transition of Bi2–xInxTe3, from a topological insulator to a trivial band insulator observed by angle-resolved photoemission spectroscopy. The robust surface state exists in the range of . At , the surface state completely disappears and only the bulk gap exists, suggesting a sudden gap closure and topological phase transition around .

Selective control of surface spin current in topological pyrite-type OsX2 (X\u2009=\u2009Se, Te) crystals

Topological materials host robust surface states that could form the basis for future electronic devices. As such states have spins that are locked to the momentum, they are of particular interest for spintronic applications. Understanding spin textures of the surface states of topologically nontrivial materials, and being able to manipulate their polarization, is therefore essential if they are to be utilized in future technologies. Here we use first-principles calculations to show that pyrite-type crystals OsX2 (X = Se, Te) are a class of topological materials that can host surface states with spin polarization that can be either in-plane or out-of-plane. We show that the formation of low-energy states with symmetry-protected energy- and direction-dependent spin textures on the (001) surface of these materials is a consequence of a transformation from a topologically trivial to nontrivial state, induced by spin orbit interactions. The unconventional spin textures of these surface states feature an in-plane to out-of-plane spin polarization transition in the momentum space protected by local symmetries. Moreover, the surface spin direction and magnitude can be selectively filtered in specific energy ranges. Our demonstration of a new class of topological materials with controllable spin textures provides a platform for experimentalists to detect and exploit unconventional surface spin textures in future spin-based nanoelectronic devices.

Dirac Nodal Arc Semimetal PtSn4 : An Ideal Platform for Understanding Surface Properties and Catalysis for Hydrogen Evolution.

Conductivity, carrier mobility, and a suitable Gibbs free energy are important criteria that determine the performance of catalysts for a hydrogen evolution reaction (HER). However, it is a challenge to combine these factors into a single compound. Herein, we discover a superior electrocatalyst for a HER in the recently identified Dirac nodal arc semimetal PtSn4. The determined turnover frequency (TOF) for each active site of PtSn4 is 1.54 H2 s−1 at 100 mV. This sets a benchmark for HER catalysis on Pt‐based noble metals and earth‐abundant metal catalysts. We make use of the robust surface states of PtSn4 as their electrons can be transferred to the adsorbed hydrogen atoms in the catalytic process more efficiently. In addition, PtSn4 displays excellent chemical and electrochemical stabilities after long‐term exposure in air and long‐time HER stability tests.

Dirac Nodal Arc Semimetal PtSn4: An Ideal Platform for Understanding Surface Properties and Catalysis for Hydrogen Evolution

AbstractConductivity, carrier mobility, and a suitable Gibbs free energy are important criteria that determine the performance of catalysts for a hydrogen evolution reaction (HER). However, it is a challenge to combine these factors into a single compound. Herein, we discover a superior electrocatalyst for a HER in the recently identified Dirac nodal arc semimetal PtSn4. The determined turnover frequency (TOF) for each active site of PtSn4 is 1.54 H2 s−1 at 100 mV. This sets a benchmark for HER catalysis on Pt‐based noble metals and earth‐abundant metal catalysts. We make use of the robust surface states of PtSn4 as their electrons can be transferred to the adsorbed hydrogen atoms in the catalytic process more efficiently. In addition, PtSn4 displays excellent chemical and electrochemical stabilities after long‐term exposure in air and long‐time HER stability tests.

Mn-doped topological insulators: a review

Topological insulators (TIs) host robust edge or surface states protected by time-reversal symmetry (TRS), which makes them prime candidates for applications in spintronic devices. A promising avenue of research for the development of functional TI devices has involved doping of three-dimensional (3D) TI thin film and bulk materials with magnetic elements. This approach aims to break the TRS and open a surface band gap near the Dirac point. Utilizing this gapped surface state allows for a wide range of novel physical effects to be observed, paving a way for applications in spintronics and quantum computation. This review focuses on the research of 3D TIs doped with manganese (Mn). We summarize major progress in the study of Mn doped chalcogenide TIs, including Bi2Se3, Bi2Te3, and Bi2(Te,Se)3. The transport properties, in particular the anomalous Hall effect, of the Mn-doped Bi2Se3 are discussed in detail. Finally, we conclude with future prospects and challenges in further studies of Mn doped TIs.

Photonic Weyl semimetals in two-dimensional dielectric arrays

Weyl semimetal (WSM) is a novel topological phase of matter that broadens classifications of topological solid-state materials beyond insulators. WSM features robust surface states that connect pairs of Weyl nodes with opposite topological charges in momentum space. In this work, we show that in a dielectric photonic crystal on a square lattice, topological edge states appear owing to nontrivial topological charges of degenerate px and py electromagnetic modes in momentum space, which is similar to WSM. Our results serve to design topological semimetal states in bosonic systems without breaking any symmetry.