Spin-momentum Locked Surface States Research Articles

Observation of spin-momentum locked surface states in amorphous Bi2Se3.

Crystalline symmetries have played a central role in the identification and understanding of quantum materials. Here we investigate whether an amorphous analogue of a well known three-dimensional strong topological insulator has topological properties in the solid state. We show that amorphous Bi2Se3 thin films host a number of two-dimensional surface conduction channels. Our angle-resolved photoemission spectroscopy data are consistent with a dispersive two-dimensional surface state that crosses the bulk gap. Spin-resolved photoemission spectroscopy shows this state has an anti-symmetric spin texture, confirming the existence of spin-momentum locked surface states. We discuss these experimental results in light of theoretical photoemission spectra obtained with an amorphous topological insulator tight-binding model, contrasting it with alternative explanations. The discovery of spin-momentum locked surface states in amorphous materials opens a new avenue to characterize amorphous matter, and triggers the search for an overlooked subset of quantum materials outside of current classification schemes.

Research progress of novel quantum states and charge-spin interconversion in transition metal oxides

For efficient storage and processing of massive data in the information technology era, spintronic device attracts tremendous attention due to its low power consumption and non-volatile feature. Spin source material, which can efficiently generates spin current, is an important constituent of novel spin-orbit torque device. The efficiency of spin current generation in spin source material directly determines the performances of various spintronic devices. In the past two decades, great progress has been made in exploring high-efficient spin source material systems and understanding the relevant physical mechanisms. A wide variety of materials are explored, ranging from traditional heavy metals and semiconductors to topological insulators and two-dimensional (2D) materials. Recently, the material family of transition metal oxides attracts tremendous attention due to its efficient and highly tunable charge-spin conversion intimately related to its emerging novel quantum states and electronic structure. The mechanism of charge-spin conversion generally has two contributions: the bulk spin Hall effect and the spin-momentum locked interface with inversion symmetry breaking. Novel electronic structures such as topological band structures and spin-momentum locked surface states can realize efficient charge-spin conversion. For example, the Weyl points in SrRuO<sub>3</sub> and the topological Dirac nodal line in SrIrO<sub>3</sub> are predicted to give rise to a large Berry curvature and corresponding spin Hall conductance; the topological surface states can generate spin accumulation due to spin-momentum locking; the Rashba states at the oxide interface such as the 2D electron gas in SrTiO<sub>3</sub> and KTaO<sub>3</sub> can generate spin current by Rashba-Edelstein effect. Furthermore, the entanglement of various degrees of freedom, including spin, charge, lattice and orbit in transition metal oxides lead to the electronic structure being highly tunable by various methods including gate voltage, substrate constraint, thickness, interface engineering, etc. Therefore, charge-spin conversion in transition metal oxides is of great significance for both modulating of novel electronic structure in fundamental research and exploring its promising potential in future spintronic devices. In this review, we focus on introducing aspects of exotic electronic structures, spin transport mechanism, charge-spin interconversion characterization, efficiency and manipulation in transition metal oxides, and giving a prospect on the future development trend.

Photothermal Control of Helicity-Dependent Current in Epitaxial Sb2Te2Se Topological Insulator Thin-Films at Ambient Temperature.

Optical control of helicity-dependent photocurrent in topological insulator (TI) Sb2Te2Se has been studied at room temperature on epitaxial thin-films grown by pulsed laser deposition (PLD). Comparison with a theoretical model, which fits the data very well, reveals different contributions to the measured photocurrent. Study of the dependence of photocurrent on the angle of incidence (wave-vector) of the excitation light with respect to the sample normal helps to identify the origin of different components of the photocurrent. Enhancement and inversion of the photocurrent in the presence of the photothermal gradient for light incident on two opposite edges of the sample occur due to selective spin-state excitation with two opposite circularly polarized lights in the presence of the unique spin-momentum locked surface states. These observations render the PLD-grown epitaxial TI thin-films promising for optoelectronic devices such as sensors, switches, and actuators whose response can be controlled by polarization as well as the angle of incidence of light under ambient conditions. The polarization response can also be tuned by the photothermal effect by suitably positioning the incident light beam on the device.

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi2Se3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. By applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (∼6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

Surface Engineering of Antisymmetric Linear Magnetoresistance and Spin-Polarized Surface State Transport in Dirac Semimetals.

Topological materials that possess spin-momentum locked surface states provide an ideal platform to manipulate the quantum spin states by electrical means. However, an antisymmetric magnetoresistance (MR) superimposed on the spin-polarized transport signals is usually observed in the spin potentiometric measurements of topological materials, rendering more power loss and reduced signal-to-noise ratio. Here we reveal the mechanism of surface-bulk interaction for the observed antisymmetric linear MR in the spin transport of Dirac semimetal Cd3As2 nanoplates. The antisymmetric linear MR can be eliminated through sample surface modifications. As a consequence, clean signals of charge current induced spin-polarized transport are observed, robust up to room temperature. The purification of spin signals can be attributed to the isolation of surface and bulk transport channels via forming a charge depletion layer with surface modifications. This surface engineering strategy should be valuable for high-performance spintronic devices on topological materials.

Intrinsic spin Hall effect in topological insulators: A first-principles study

The intrinsic spin Hall conductivity of typical topological insulators Sb$_2$Se$_3$, Sb$_2$Te$_3$, Bi$_2$Se$_3$, and Bi$_2$Te$_3$ in the bulk form, is calculated from first-principles by using density functional theory and the linear response theory in a maximally localized Wannier basis. The results show that there is a finite spin Hall conductivity of 100--200 ($\hbar$/2e)(S/cm) in the vicinity of the Fermi energy. Although the resulting values are an order of magnitude smaller than that of heavy metals, they show a comparable spin Hall angle due to their relatively lower longitudinal conductivity. The spin Hall angle for different compounds are then compared to that of recent experiments on topological-insulator/ferromagnet heterostructures. The comparison suggests that the role of the bulk in generating a spin current and consequently a spin torque in magnetization switching applications is comparable to that of the surface including the spin-momentum locked surface states and the Rashba-Edelstein effect at the interface.

Quasiparticle interference of spin momentum locked surface states at step edges on Re(0001)

Quasiparticle interference patterns formed by a surface state on the Re(0001) surface were investigated using scanning tunneling spectroscopy. The energy dispersion is inferred from Fourier-transformed differential conductivity maps for occupied and unoccupied states. The band dispersion for occupied states agrees with earlier published results obtained by angle-resolved photoemission spectroscopy. An analysis of the phase of interference patterns at step edges reveals a drastic change in the effective energy barrier for backscattering above and below the Fermi level. The attenuation of the interference pattern with increasing distance indicates interband scattering is the dominant scattering mechanism. Step decoration by Ni has a negligible influence on the pattern, excluding spin-flip scattering as a dominant contribution. The one-dimensional Re/Au line interface, however, reverses the scattering barrier behavior, indicating a coupling of surface states.

Manipulating long-lived topological surface photovoltage in bulk-insulating topological insulators Bi2Se3 and Bi2Te3

The appearance of topologically protected spin-momentum locked surface states in topological insulators gives rise to robust room temperature spin currents making them ideal candidates for the realization of spintronic devices. New methods are needed to access and manipulate such currents with timescales that are compatible with modern electronics. Here we reveal that an optically induced long-lived (~10 ns), spin-polarized surface state excitation in topological insulators can be easily tuned in both magnitude and duration. Time-resolved angle-resolved photoemission spectroscopy, together with a quantitative model, reveals the ideal conditions for a surface photovoltage in two different topological insulators. Our model predicts that the reported effects are an intrinsic property of topological insulators, as long as the chemical potential falls within the band gap. This work demonstrates that persistent excited topological surface states are photon-accessible and easily tuned in both magnitude and duration, merging photonics- and spintronics-based devices in the same material.

Room-Temperature Spin-Orbit Torque from Topological Surface States.

Spin-momentum locked surface states in topological insulators (TIs) provide a promising route for achieving high spin-orbit torque (SOT) efficiency beyond the bulk spin-orbit coupling in heavy metals (HMs). However, in previous works, there is a huge discrepancy among the quantitative SOTs from TIs in various systems determined by different methods. Here, we systematically study the SOT in the TI(HM)/Ti/CoFeB/MgO systems by the same method, and make a conclusive assessment of SOT efficiency for TIs and HMs. Our results demonstrate that TIs show more than one order of magnitude higher SOT efficiency than HMs even at room temperature, at the same time the switching current density as low as 5.2×10^{5} A cm^{-2} is achieved with (Bi_{1-x}Sb_{x})_{2}Te_{3}. Furthermore, we investigate the relationship between SOT efficiency and the position of Fermi level in (Bi_{1-x}Sb_{x})_{2}Te_{3}, where the SOT efficiency is significantly enhanced near the Dirac point, with the most insulating bulk and conducting surface states, indicating the dominating SOT contribution from topological surface states. This work unambiguously demonstrates the ultrahigh SOT efficiency from topological surface states.

Dephasing effects in topological insulators

Topological insulators, a class of typical topological materials in both two dimensions and three dimensions, are insulating in bulk and metallic at surface. The spin-momentum locked surface states and peculiar transport properties exhibit promising potential applications on quantum devices, which generate extensive interest in the last decade. Dephasing is the process of the loss of phase coherence, which inevitably exists in a realistic sample. In this review, we focus on recent progress in dephasing effects on the topological insulators. In general, there are two types of dephasing processes: normal dephasing and spin dephasing. In two-dimensional topological insulators, the phenomenologically numerical investigation shows that the longitudinal resistance plateaus is robust against normal dephasing but fragile with spin dephasing. Several microscopic mechanisms of spin dephasing are then discussed. In three-dimensional topological insulators, the helical surface states exhibit a helical spin texture due to the spin-momentum locking mechanism. Thus, normal dephasing has close connection to spin dephasing in this case, and gives rise to anomalous “gap-like” feature. Dephasing effects on properties of helical surface states are investigated.

Inverse Spin Hall Effect in Electron Beam Evaporated Topological Insulator Bi2Se3 Thin Film

Spintronics exploiting pure spin current in ferromagnetic (FM)/heavy metals (HM) is a subject of intense research. Topological insulators having spin momentum locked surface states exhibit high spin–orbit coupling and thus possess a huge potential to replace the HM like Pt, Ta, W, etc. In this context, the spin pumping phenomenon in Bi2Se3/CoFeB bilayers has been investigated. Bi2Se3 thin films are fabricated by electron beam evaporation method on Si (100) substrate. In order to confirm the topological nature of Bi2Se3, low temperature magnetotransport measurement on a 30 nm thick Bi2Se3 film which shows 10% magnetoresistance (MR) at 1.5 K has been performed. A linear increase in MR with applied magnetic field indicates the presence of spin momentum‐locked surface states. A voltage has been measured at room temperature to quantify the spin pumping which is generated via inverse spin Hall effect (ISHE). For the separation of spin rectification effects mainly produced by the FM CoFeB layer, in plane angular dependence of the dc voltage with respect to applied magnetic field has been measured. Our analysis reveals that spin pumping induced ISHE is the dominant contribution in the measured voltage.

Spin Momentum–Locked Surface States in Metamaterials without Topological Transition

AbstractThe photonic analogy of the quantum spin Hall Effect, that is, a photonic topological insulator (PTI), is of great relevance to science and technology in optics based on the promise of scattering‐free surface states. The challenges in realizing such scattering‐free surface states in PTIs and other types of symmetry‐protected topological phases are the result of the exact symmetry needed for creating a pair of time reversal pseudo‐spin states or special boundary conditions, wherein the exact symmetry imposes strict requirements on materials or boundary conditions. Here, it is experimentally demonstrated that scattering‐free edge states can be created with neither the aforementioned exact symmetry requirements for materials nor the topological transitions. This system is constructed by simply placing together regular homogeneous metamaterials, which are characterized by highly different bianisotropies. Of the particular surface states, backward reflection would be deeply suppressed, provided that the related evanescent tail into the bulk regions vanishes shortly and that the pseudo‐spin is not flipped by the scatterers. This work gives an example of constructing scattering‐free surface states in classical systems without strict symmetry protections and may potentially stimulate various novel applications in the future.

Manipulating the Topological Interface by Molecular Adsorbates: Adsorption of Co-Phthalocyanine on Bi2Se3.

Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer.

Quasiparticle scattering from topological crystalline insulator SnTe (001) surface states

Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states; but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultra-low temperature scanning tunneling microscopy and spectroscopy study on topological crystalline insulator SnTe nanoplates grown by molecular beam epitaxy. We observed quasiparticle interference patterns on the SnTe (001) surface that can be interpreted in terms of electron scattering from the four Fermi pockets of the topological crystalline insulator surface states in the first surface Brillouin zone. A quantitative analysis of the energy dispersion of the quasiparticle interference intensity shows two high energy features related to the crossing point beyond the Lifshitz transition when the two neighboring low energy surface bands near the point merge. A comparison between the experimental and computed quasiparticle interference patterns reveals possible spin texture of the surface states.

A topological insulator surface under strong Coulomb, magnetic and disorder perturbations

Three dimensional topological insulators embody a newly discovered state of matter characterized by conducting spin-momentum locked surface states that span the bulk band gap as demonstrated via spin-resolved ARPES measurements . This highly unusual surface environment provides a rich ground for the discovery of novel physical phenomena. Here we present the first controlled study of the topological insulator surfaces under strong Coulomb, magnetic and disorder perturbations. We have used interaction of iron, with a large Coulomb state and significant magnetic moment as a probe to \textit{systematically test the robustness} of the topological surface states of the model topological insulator Bi$_2$Se$_3$. We observe that strong perturbation leads to the creation of odd multiples of Dirac fermions and that magnetic interactions break time reversal symmetry in the presence of band hybridization. We also present a theoretical model to account for the altered surface of Bi$_2$Se$_3$. Taken collectively, these results are a critical guide in manipulating topological surfaces for probing fundamental physics or developing device applications.

Interfacial Dirac Cones from Alternating Topological Invariant Superlattice Structures ofBi2Se3

When the three-dimensional topological insulators Bi2Se3 and Bi2Te3 have an interface with vacuum, i.e., a surface, they show remarkable features such as topologically protected and spin-momentum locked surface states. However, for practical applications, one often requires multiple interfaces or channels rather than a single surface. Here, for the first time, we show that an interfacial and ideal Dirac cone is realized by alternating band and topological insulators. The multichannel Dirac fermions from the superlattice structures open a new way for applications such as thermoelectric and spintronics devices. Indeed, utilizing the interfacial Dirac fermions, we also demonstrate the possible power factor improvement for thermoelectric applications.