Surface States Of Topological Insulators Research Articles

Dirac Semimetal Heterostructures: 3D Cd3 As2 on 2D Graphene.

Dirac semimetal is an emerging class of quantum matters, ranging from 2D category, such as, graphene and surface states of topological insulator to 3D category, for instance, Cd3 As2 and Na3 Bi. As 3D Dirac semimetals typically possess Fermi-arc surface states, the 2D-3D Dirac van der Waals heterostructures should be promising for future electronics. Here, graphene-Cd3 As2 heterostructures are fabricated through direct layer-by-layer stacking. The electronic coupling results in a notable interlayer charge transfer, which enables us to modulate the Fermi level of graphene through Cd3 As2 . A planar graphene p-n-p junction is achieved by selective modification, which demonstrates quantized conductance plateaus. Moreover, compared with the bare graphene device, the graphene-Cd3 As2 hybrid device presents large nonlocal signals near the Dirac point due to the charge transfer from the spin-polarized surface states in the adjacent Cd3 As2 . The results enrich the family of van der Waals heterostructure and should inspire more studies on the application of Dirac/Weyl semimetals in spintronics.

Observation of the possible chiral edge mode in Bi1−xSbx

After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three-dimensional topological insulators. Applying laser pulses to the surface state of Bi1−xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.

Dirac surface state-modulated spin dynamics in a ferrimagnetic insulator at room temperature.

This work demonstrates markedly modified spin dynamics of magnetic insulator (MI) by the spin momentum-locked Dirac surface states of the adjacent topological insulator (TI), which can be harnessed for spintronic applications. As the Bi concentration x is systematically tuned in 5-nm-thick (Bi x Sb1-x )2Te3 TI films, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10-nm-thick Y3Fe5O12 MI films in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi concentration dependence. These extraordinary effects of the Dirac surface states distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer.

Higher-order topological insulators and superconductors protected by inversion symmetry

We study surface states of topological crystalline insulators and superconductors protected by inversion symmetry. These fall into the category of "higher-order" topological insulators and superconductors which possess surface states that propagate along one-dimensional curves (hinges) or are localized at some points (corners) on the surface. We show that the surface states of higher-order topological insulators and superconductors can be thought of as globally irremovable topological defects and provide a complete classification of these inversion-protected phases in any spatial dimension for the ten symmetry classes by means of a layer construction. Furthermore, we discuss possible physical realizations of such states starting with a time-reversal invariant topological insulator (class AII) in three dimensions or a time-reversal invariant topological superconductor (class DIII) in two or three dimensions. The former can be used to build a three-dimensional second-order topological insulator which exhibits one-dimensional chiral or helical modes propagating along opposite edges, whereas the latter enables the construction of three-dimensional third-order or two-dimensional second-order topological superconductors hosting Majorana zero modes localized to two opposite corners. Being protected by inversion, such states are not pinned to a specific pair of edges or corners thus offering the possibility of controlling their location by applying inversion-symmetric perturbations such as magnetic field.

Photogalvanic Effect in Topological Insulator Surface States Investigated by Time-resolved Two Photon Photoemission Spectroscopy

Photogalvanic Effect in Topological Insulator Surface States Investigated by Time-resolved Two Photon Photoemission Spectroscopy

Nonlinear spin susceptibility in topological insulators

We theoretically study the effect of impurity resonances on the indirect exchange interaction between magnetic impurities in the surface states of a three-dimensional topological insulator. The interaction is composed of an isotropic Heisenberg, and anisotropic Ising and Dzyaloshinskii-Moriya contributions. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya contribution. We show that the spin-independent component of the impurity scattering can generate large values of the DM term in comparison with the Heisenberg and Ising terms, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and non-collinear configurations are allowed magnetic configurations of the impurities.

Thermoelectric Properties of Topological Insulators

Many efficient thermoelectric materials belong to the class of topological insulators. Therefore, in recent years considerable efforts have been made to elucidate the influence of nontrivial electronic topology on the thermoelectric properties of thermoelectrics. This review describes the basic transport properties of helical surface states of topological insulators. Their effect on the thermoelectric efficiency of materials is discussed. The main attention is paid to (Bi,Sb)2(Te,Se)3 alloys, which combine the properties of the best thermoelectrics and a model topological insulator.

Modeling all-electrical detection of the inverse Edelstein effect by spin-polarized tunneling in a topological-insulator/ferromagnetic-metal heterostructure

The spin-momentum locking of the surface states in a three-dimensional topological insulator (TI) allows a charge current on the surface of the TI induced by an applied spin current onto the surface, which is known as the inverse Edelstein effect (IEE), that could be achieved either by injecting pure spin current by spin-pumping from a ferromagnetic metal (FM) layer or by injecting spin-polarized charge current by direct tunneling of electrons from the FM to the TI. Here, we present a theory of the observed IEE effect in a TI-FM heterostructure for the spin-polarized tunneling experiments. If an electrical current is passed from the FM to the surface of the TI, because of density-of-states polarization of the FM, an effective imbalance of spin-polarized electrons occurs on the surface of the TI. Due to the spin-momentum helical locking of the surface states in the TI, a difference of transverse charge accumulation appears on the TI surface in a direction orthogonal to the direction of the magnetization of the FM, which is measured as a voltage difference. Here, we derive the two-dimensional transport equations of electrons on the surface of a diffusive TI, coupled to a FM, starting from the quantum kinetic equation, and analytically solve the equations for a rectangular geometry to calculate the voltage difference.

Impact of Ultrathin Pb Films on the Topological Surface and Quantum-Well States of Bi2Se3 and Sb2Te3 Topological Insulators

The effect of an ultrathin Pb film deposited on the surface of Bi2Se3 and Sb2Te3 compounds on the electronic state structure of topological insulators is studied experimentally by the angle-resolved photoemission spectroscopy (ARPES) technique. The following features are revealed: formation of two-dimensional quantum-well states in the near-surface region, an increase in the binding energy of the Dirac cone and the core levels, and a simultaneous electronic states intensity redistribution in the system in photoemission spectra. The results obtained show that topological states may coexist at the interface between studied materials and a superconductor, which seems to be promising for application in quantum computers.

Manipulating the one-dimensional topological edge state of Bi bilayer nanoribbons via magnetic orientation and electric field

Despite the superiority of two-dimensional (2D) topological insulators (TIs) over their three-dimensional (3D) counterparts in various aspects and the essential distinction between them in structural symmetry, the variation of the topological one-dimensional (1D) edge states upon magnetic interaction and their application for spintronic devices have not been sufficiently illuminated. Here, we reveal that 1D edge states of 2D TIs have a unique magnetic response never observed in 2D surface states of 3D TIs, and using this exotic nature we propose a way to utilize the spin-polarized channel for spintronic applications. We investigate the effects of width and magnetic decoration on the 1D topological edge state of Bi bilayer nanoribbons (BNRs). Through the Zak phase, we find that the zero-energy states are enforced at the magnetic domain boundaries in the Cr-decorated BNR and directly examine their robustness using short-range magnetic domain structures. We also demonstrate that 1D edge states of BNRs can be selectively and reversibly controlled by the combination of magnetic reorientation and electric field without compromising their structural integrity. Our work provides a fundamental understanding of 1D topological edge states and shows the opportunity of using these features in spintronic devices.

Electrochemical Biosensor Composed of Silver Ion‐Mediated dsDNA on Au‐Encapsulated Bi2Se3 Nanoparticles for the Detection of H2O2 Released from Breast Cancer Cells

A newly developed electrochemical biosensor composed of a topological insulator (TI) and metallic DNA (mDNA) is fabricated. The bismuth selenide nanoparticle (Bi2 Se3 NP) is synthesized and sandwiched between the gold electrode and another Au-deposited thin layer (Bi2 Se3 @Au). Then, eight-silver-ion mediated double-stranded DNA (mDNA) is immobilized onto the substrate (Bi2 Se3 @Au-mDNA) for the further detection of hydrogen peroxide. The Bi2 Se3 NP acts as the electrochemical-signal booster, while unprecedentedly its encapsulation by the Au thin layer keeps the TI surface states protected, improves its electrochemical-signal stability and provides an excellent platform for the subsequent covalent immobilization of the mDNA through Au-thiol interaction. Electrochemical results show that the fabricated biosensor represents much higher Ag+ redox current (≈10 times) than those electrodes prepared without Bi2 Se3 @Au. The characterization of the Bi2 Se3 @Au-mDNA film is confirmed by atomic force microscopy, scanning tunneling microscopy, and cyclic voltammetry. The proposed biosensor shows a dynamic range of 00.10 × 10-6 m to 27.30 × 10-6 m, very low detection limit (10 × 10-9 m), unique current response (1.6 s), sound H2 O2 recovery in serum, and substantial capability to classify two breast cancer subtypes (MCF-7 and MDA-MB-231) based on their difference in the H2 O2 generation, offering potential applications in the biomedicine and pharmacology fields.

Origin and evolution of surface spin current in topological insulators

The Dirac surface states of topological insulators offer a unique possibility for creating spin polarized charge currents due to the spin-momentum locking. Here we demonstrate that the control over the bulk and surface contribution is crucial to maximize the charge-to-spin conversion efficiency. We observe an enhancement of the spin signal due to surface-dominated spin polarization while freezing out the bulk conductivity in semiconducting ${\mathrm{Bi}}_{1.5}{\mathrm{Sb}}_{0.5}{\mathrm{Te}}_{1.7}{\mathrm{Se}}_{1.3}$ below $100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Detailed measurements up to room temperature exhibit a strong reduction of the magnetoresistance signal between $2\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}100\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which we attribute to the thermal excitation of bulk carriers and to the electron-phonon coupling in the surface states. The presence and dominance of this effect up to room temperature is promising for spintronic science and technology.

Ultrafast nonlinear optical response of Dirac fermions in graphene

The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we investigate the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations.

Rf Quantum Capacitance of the Topological Insulator Bi2Se3 in the Bulk Depleted Regime for Field-Effect Transistors

The massless surface states of topological insulators (TIs) are of great interest for tomorrow's electronics. However, excessive defect doping of bulk bands significantly hinders the studies that would enable applications of TIs, in particular as efficient channel materials for high-frequency transistors. The authors take an important step toward resolving this problem, by realizing a Bi${}_{2}$Se${}_{3}$-based capacitive device operating at radio frequencies. Judicious choice of growth technique, substrate, and gate dielectric allow the depletion of bulk carriers and quantitative measurement of the rf quantum capacitance of the topological surface states.

High surface conductivity of Fermi-arc electrons in Weyl semimetals

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semi--quantized anomalous Hall effect and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi arcs surface states. Here we analyze electron--phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight binding model with the presence of quenched surface vacancies using the Coherent Potential Approximation (CPA) and Kubo--Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is a factor of 50 larger of a comparable set up with surface states of TI. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material.

Edelstein effect in Weyl semimetals

Using semiclassical Boltzmann transport theory, we predict a current-induced spin polarization in Weyl semimetals, similar to the Edelstein effect of surface states in Rashba systems or in topological insulators. The theory is applied to the Weyl semimetal TaAs, for which we estimate the magnitude of the effect. The main contribution comes from the topological surface states, i.e., the Fermi arcs, which provide an enormous current-induced spin polarization enhanced by at least one order of magnitude in comparison to Rashba systems and the surface states of topological insulators.

Excitonic instability in optically pumped three-dimensional Dirac materials

Recently it was suggested that transient excitonic instability can be realized in optically-pumped two-dimensional (2D) Dirac materials (DMs), such as graphene and topological insulator surface states. Here we discuss the possibility of achieving a transient excitonic condensate in optically-pumped three-dimensional (3D) DMs, such as Dirac and Weyl semimetals, described by non-equilibrium chemical potentials for photoexcited electrons and holes. Similar to the equilibrium case with long-range interactions, we find that for pumped 3D DMs with screened Coulomb potential two possible excitonic phases exist, an excitonic insulator phase and the charge density wave phase originating from intranodal and internodal interactions, respectively. In the pumped case, the critical coupling for excitonic instability vanishes; therefore, the two phases coexist for arbitrarily weak coupling strengths. The excitonic gap in the charge density wave phase is always the largest one. The competition between screening effects and the increase of the density of states with optical pumping results in a reach phase diagram for the transient excitonic condensate. Based on the static theory of screening, we find that under certain conditions for the value of the dimensionless coupling constant screening in 3D DMs can be weaker than in 2D DMs. Furthermore, we identify the signatures of the transient excitonic condensate that could be probed by scanning tunneling spectroscopy, photoemission and optical conductivity measurements. Finally, we provide estimates of critical temperatures and excitonic gaps for existing and hypothetical 3D DMs.

Bilinear magnetoelectric resistance as a probe of three-dimensional spin texture in topological surface states

Surface states of three-dimensional topological insulators exhibit the phenomenon of spin-momentum locking, whereby the orientation of an electron spin is determined by its momentum. Probing the spin texture of these states is of critical importance for the realization of topological insulator devices, however the main technique available so far is the spin- and angle-resolved photoemission spectroscopy. Here we reveal a close link between the spin texture and a new kind of magneto-resistance, which depends on the relative orientation of the current with respect to the magnetic field as well as the crystallographic axes, and scales linearly with both the applied electric and magnetic fields. This bilinear magneto-electric resistance can be used to map the spin texture of topological surface states by simple transport measurements. For a prototypical Bi2Se3 single layer, we can map both the in-plane and the out-of-plane components of the spin texture - the latter arising from hexagonal warping. Theoretical calculations suggest that the bilinear magneto-electric resistance originates from the conversion of a non-equilibrium spin current into a charge current under the application of the external magnetic field.

Dynamics of Electronic States and Magnetoabsorption in 3D Topological Insulators in a Quantizing Magnetic Field

Quantum states have been calculated analytically; the dynamics of a wave packet in a magnetic field has been investigated, and the optical absorption coefficient has been calculated for surface states in 3D topological insulators of the Bi2Te3 family. We have detected a qualitative effect of the hexagonal warping of the spectrum on the structure of wavefunctions at the Landau levels, its manifestation in the features of the wave packet dynamics in a quantizing magnetic field, as well as in the frequency dependence of the optical absorption coefficient, in which new peaks that are absent in the isotropic model of the spectrum appear depending on the polarization of the incident wave. The effects considered here can be manifested in the optical and transport experiments with topological insulators, which makes it possible to determine the parameters of their band structure.

Creation of Spin-Triplet Cooper Pairs in the Absence of Magnetic Ordering.

In superconducting spintronics, it is essential to generate spin-triplet Cooper pairs on demand. Up to now, proposals to do so concentrate on hybrid structures in which a superconductor (SC) is combined with a magnetically ordered material (or an external magnetic field). We, instead, identify a novel way to create and isolate spin-triplet Cooper pairs in the absence of any magnetic ordering. This achievement is only possible because we drive a system with strong spin-orbit interaction-the Dirac surface states of a strong topological insulator (TI)-out of equilibrium. In particular, we consider a bipolar TI-SC-TI junction, where the electrochemical potentials in the outer leads differ in their overall sign. As a result, we find that nonlocal singlet pairing across the junction is completely suppressed for any excitation energy. Hence, this junction acts as a perfect spin-triplet filter across the SC, generating equal-spin Cooper pairs via crossed Andreev reflection.