Controllable magnetic doping of the surface state of a topological insulator

Fermi-Arc Metals

We predict a novel metallic state of matter that emerges in a Weyl-semimetal superstructure with spatially varying Weyl-node positions. In the new state, the Weyl nodes are stretched into extended, anisotropic Fermi surfaces, which can be understood as being built from Fermi arclike states. This "Fermi-arc metal" exhibits the chiral anomaly of the parental Weyl semimetal. However, unlike in the parental Weyl semimetal, in the Fermi-arc metal the "ultraquantum state," in which the anomalous chiral Landau level is the only state at the Fermi energy, is already reached for a finite energy window at zero magnetic field. The dominance of the ultraquantum state implies a universal low-field ballistic magnetoconductance and the absence of quantum oscillations, making the Fermi surface "invisible" to de Haas-van Alphen and Shubnikov-de Haas effects, although it signifies its presence in other response properties.

Influence of Doping on the Topological Surface States of Crystalline Bi2Se3 Topological Insulators

We present STM/STS, ARPES and magnetotransport studies of the surface topography and electronic structure of pristine Bi₂Se₃ in comparison to Bi_1.96Mg_0.04Se₃ and Bi_1.98Fe_0.02Se₃. The topography images reveal a large number of complex, triangle-shaped defects at the surface. The local electronic structure of both the defected and non-defected regions is examined by STS. The defect-related states shift together with the Dirac point observed in the undefected area, suggesting that the local electronic structure at the defects is influenced by doping in the same way as the electronic structure of the undefected surface. Additional information about the electronic structure of the samples is provided by ARPES, which reveals the dependence of the bulk and surface electronic bands on doping, including such parameters as the Fermi wave vector. The subtle changes of the surface electronic structure by doping are verified with magneto-transport measurements at low temperatures (200 mK) allowing the detection of Shubnikov-de Haas (SdH) quantum oscillations.

Europium Doping Impact on the Properties of MBE Grown Bi2Te3 Thin Film

The impact of europium doping on the electronic and structural properties of the topological insulator Bi₂Te₃ is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi₂Te₃ matrix remains divalent and substitutes bismuth in a Bi₂Te₃ matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi₂Te₃ topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.

Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures

Controlling interfacial interactions in magnetic/topological insulator heterostructures is a major challenge for the emergence of novel spin-dependent electronic phenomena. As for any rational design of heterostructures that rely on proximity effects, one should ideally retain the overall properties of each component while tuning interactions at the interface. However, in most inorganic interfaces, interactions are too strong, consequently perturbing, and even quenching, both the magnetic moment and the topological surface states at each side of the interface. Here, we show that these properties can be preserved using ligand chemistry to tune the interaction of magnetic ions with the surface states. By depositing Co-based porphyrin and phthalocyanine monolayers on the surface of Bi₂Te₃ thin films, robust interfaces are formed that preserve undoped topological surface states as well as the pristine magnetic moment of the divalent Co ions. The selected ligands allow us to tune the interfacial hybridization within this weak interaction regime. These results, which are in stark contrast with the observed suppression of the surface state at the first quintuple layer of Bi₂Se₃ induced by the interaction with Co phthalocyanines, demonstrate the capability of planar metal-organic molecules to span interactions from the strong to the weak limit.

Robust Gapless Surface State against Surface Magnetic Impurities on (Bi_{0.5}Sb_{0.5})_{2}Te_{3} Evidenced by In Situ Magnetotransport Measurements

Despite extensive experimental and theoretical efforts, the important issue of the effects of surface magnetic impurities on the topological surface state of a topological insulator (TI) remains unresolved. We elucidate the effects of Cr impurities on epitaxial thin films of (Bi_{0.5}Sb_{0.5})_{2}Te_{3}: Cr adatoms are incrementally deposited onto the TI held in ultrahigh vacuum at low temperatures, and in situ magnetoconductivity and Hall effect measurements are performed at each increment with electrostatic gating. In the experimentally identified surface transport regime, the measured minimum electron density shows a nonmonotonic evolution with the Cr density (n_{Cr}): it first increases and then decreases with n_{Cr}. This unusual behavior is ascribed to the dual roles of the Cr as ionized impurities and electron donors, having competing effects of enhancing and decreasing the electronic inhomogeneities in the surface state at low and high n_{Cr}, respectively. The magnetoconductivity is obtained for different n_{Cr} on one and the same sample, which yields clear evidence that the weak antilocalization effect persists and the surface state remains gapless up to the highest n_{Cr}, contrary to the expectation that the deposited Cr should break the time-reversal symmetry and induce a gap opening at the Dirac point.

Topological Phase Control of Surface States in Bi2Se3 via Spin-Orbit Coupling Modulation through Interface Engineering between HfO2-X

The direct control of topological surface states in topological insulators is an important prerequisite for the application of these materials. Conventional attempts to utilize magnetic doping, mechanical tuning, structural engineering, external bias, and external magnetic fields suffer from a lack of reversible switching and have limited tunability. We demonstrate the direct control of topological phases in a bismuth selenide (Bi₂Se₃) topological insulator in 3 nm molecular beam epitaxy-grown films through the hybridization of the topological surface states with the hafnium (Hf) d-orbitals in the topmost layer of an underlying oxygen-deficient hafnium oxide (HfO₂) substrate. The higher angular momentum of the d-orbitals of Hf is hybridized strongly by topological insulators, thereby enhancing the spin-orbit coupling and perturbing the topological surface states asymmetry in Bi₂Se₃. As the oxygen defect is cured or generated reversibly by external electric fields, our research facilitates the complete electrical control of the topological phases of topological insulators by controlling the defect density in the adjacent transition metal oxide. In addition, this mechanism can be applied in other related topological materials such as Weyl and Dirac semimetals in future endeavors to facilitate practical applications in unit-element devices for quantum computing and quantum communication.

Device Applications of Synthetic Topological Insulator Nanostructures

This review briefly describes the development of synthetic topological insulator materials in the application of advanced electronic devices. As a new class of quantum matter, topological insulators with insulating bulk and conducting surface states have attracted attention in more and more research fields other than condensed matter physics due to their intrinsic physical properties, which provides an excellent basis for novel nanoelectronic, optoelectronic, and spintronic device applications. In comparison to the mechanically exfoliated samples, the newly emerging topological insulator nanostructures prepared with various synthetical approaches are more intriguing because the conduction contribution of the surface states can be significantly enhanced due to the larger surface-to-volume ratio, better manifesting the unique properties of the gapless surface states. So far, these synthetic topological insulator nanostructures have been implemented in different electrically accessible device platforms via electrical, magnetic and optical characterizations for material investigations and device applications, which will be introduced in this review.

TCNQ Physisorption on the Topological Insulator Bi2 Se3

Topological insulators are promising candidates for spintronic applications due to their topologically protected, spin-momentum locked and gapless surface states. The breaking of the time-reversal symmetry after the introduction of magnetic impurities, such as 3d transition metal atoms embedded in two-dimensional molecular networks, could lead to several phenomena interesting for device fabrication. The first step towards the fabrication of metal-organic coordination networks on the surface of a topological insulator is to investigate the adsorption of the pure molecular layer, which is the aim of this study. Here, the effect of the deposition of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on the surface of a prototypical topological insulator, bismuth selenide (Bi₂ Se₃ ), is investigated. Scanning tunneling microscope images at low-temperature reveal the formation of a highly ordered two-dimensional molecular network. The essentially unperturbed electronic structure of the topological insulator observed by photoemission spectroscopy measurements demonstrates a negligible charge transfer between the molecular layer and the substrate. Density functional theory calculations confirm the picture of a weakly interacting adsorbed molecular layer. These results reveal significant potential of TCNQ for the realization of metal-organic coordination networks on the topological insulator surface.

Heterostructured ferromagnet-topological insulator with dual-phase magnetic properties

The introduction of ferromagnetism at the surface of a topological insulator (TI) produces fascinating spin-charge phenomena. It has been assumed that these fascinating effects are associated with a homogeneous ferromagnetic (FM) layer possessing a single type of magnetic phase. However, we obtained phase separation within the FM layer of a Ni80Fe20/Bi2Se3 heterostructure. This phase separation was caused by the diffusion of Ni into Bi2Se3, forming a ternary magnetic phase of Ni:Bi2Se3. The inward diffusion of Ni led to the formation of an FeSe phase outward, transforming the original Ni80Fe20/Bi2Se3 into a sandwich structure comprising FeSe/Ni:Bi2Se3/Bi2Se3 with dual-phase magnetic characteristics similar to that driven by the proximity effect. Such a phenomenon might have been overlooked in previous studies with a strong focus on the proximity effect. X-ray magnetic spectroscopy revealed that FeSe and Ni:Bi2Se3 possess horizontal and perpendicular magnetic anisotropy, respectively. The overall magnetic order of the heterostructure can be easily tuned by adjusting the thickness of the Bi2Se3 as it compromises the magnetic orders of the two magnetic phases. This discovery is essential to the quantification of spin-charge phenomena in similar material combinations where the FM layer is composed of multiple elements.

Overgrowth of Bi2Te3 nanoislands on Fe-based epitaxial ferromagnetic layers

Bi₂Te₃ is deposited by hot wall epitaxy in an attempt to form nanosheets on epitaxially-grown ferromagnetic layers of Fe, Fe₃Si and Co₂FeSi.

Local optical control of ferromagnetism and chemical potential in a topological insulator

Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate reconfigurable micron-scale optical control of both magnetization (which breaks time-reversal symmetry) and chemical potential in ferromagnetic thin films of Cr-(Bi,Sb)2Te3 grown on SrTiO3 By optically modulating the coercivity of the films, we write and erase arbitrary patterns in their remanent magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO3 substrates, we control the local chemical potential of the films. This optical gating effect allows us to write and erase p-n junctions in the films, which we study with photocurrent microscopy. Both effects are persistent and may be patterned and imaged independently on a few-micron scale. Dynamic optical control over both magnetization and chemical potential of a TI may be useful in efforts to understand and control the edge states predicted at magnetic domain walls in quantum anomalous Hall insulators.

Chemical Disorder in Topological Insulators: A Route to Magnetism Tolerant Topological Surface States

We show that the chemical inhomogeneity in ternary three-dimensional topological insulators preserves the topological spin texture of their surface states against a net surface magnetization. The spin texture is that of a Dirac cone with helical spin structure in the reciprocal space, which gives rise to spin-polarized and dissipation-less charge currents. Thanks to the nontrivial topology of the bulk electronic structure, this spin texture is robust against most types of surface defects. However, magnetic perturbations break the time-reversal symmetry, enabling magnetic scattering and loss of spin coherence of the charge carriers. This intrinsic incompatibility precludes the design of magnetoelectronic devices based on the coupling between magnetic materials and topological surface states. We demonstrate that the magnetization coming from individual Co atoms deposited on the surface can disrupt the spin coherence of the carriers in the archetypal topological insulator Bi₂Te₃, while in Bi₂Se₂Te the spin texture remains unperturbed. This is concluded from the observation of elastic backscattering events in quasiparticle interference patterns obtained by scanning tunneling spectroscopy. The mechanism responsible for the protection is investigated by energy resolved spectroscopy and ab initio calculations, and it is ascribed to the distorted adsorption geometry of localized magnetic moments due to Se-Te disorder, which suppresses the Co hybridization with the surface states.

Interfacial reactions at Fe/topological insulator spin contacts

The authors study the composition and abruptness of the interfacial layers that form during deposition and patterning of a ferromagnet, Fe on a topological insulator (TI), Bi₂Se₃, Bi₂Te₃, and SiO_x/Bi₂Te₃. Such structures are potentially useful for spintronics. Cross-sectional transmission electron microscopy, including interfacial elemental mapping, confirms that Fe reacts with Bi₂Se₃ near room temperature, forming an abrupt 5 nm thick FeSe_0.92 single crystalline binary phase, predominantly (001) oriented, with lattice fringe spacing of 0.55 nm. In contrast, Fe/Bi₂Te₃ forms a polycrystalline Fe/TI interfacial alloy that can be prevented by the addition of an evaporated SiO_x separating Fe from the TI.

Effect of impurity resonant states on optical and thermoelectric properties on the surface of a topological insulator

We investigate the thermoelectric effect on a topological insulator surface with particular interest in impurity-induced resonant states. To clarify the role of the resonant states, we calculate the dc and ac conductivities and the thermoelectric coefficients along the longitudinal direction within the full Born approximation. It is found that at low temperatures, the impurity resonant state with strong energy de-pendence can lead to a zero-energy peak in the dc conductivity, whose height is sensitively dependent on the strength of scattering potential, and even can reverse the sign of the thermopower, implying the switching from n- to p-type carriers. Also, we exhibit the thermoelectric signatures for the filling process of a magnetic band gap by the resonant state. We further study the impurity effect on the dynamic optical conductivity, and find that the resonant state also generates an optical conductivity peak at the absorption edge for the interband transition. These results provide new perspectives for understanding the doping effect on topological insulator materials.

Probing Single Vacancies in Black Phosphorus at the Atomic Level

Utilizing a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and electronic structure calculations, we characterize the structural and electronic properties of single atomic vacancies within several monolayers of the surface of black phosphorus. We illustrate, with experimental analysis and tight-binding calculations, that we can depth profile these vacancies and assign them to specific sublattices within the unit cell. Measurements reveal that the single vacancies exhibit strongly anisotropic and highly delocalized charge density, laterally extended up to 20 atomic unit cells. The vacancies are then studied with STS, which reveals in-gap resonance states near the valence band edge and a strong p-doping of the bulk black phosphorus crystal. Finally, quasiparticle interference generated near these vacancies enables the direct visualization of the anisotropic band structure of black phosphorus.

Structural and electronic properties of ultrathin FeSe films grown on Bi2Se3(0 0 0 1) studied by STM/STS

We report scanning tunnelling microscopy and spectroscopy (STM/STS) studies on one and two unit cell (UC) high FeSe thin films grown on Bi₂Se₃(0 0 0 1). In our thin films, we find the tetragonal phase of FeSe and dumb-bell shaped defects oriented along Se-Se bond directions. In addition, we observe striped moiré patterns with a periodicity of (7.3  ±  0.1) nm generated by the mismatch between the FeSe lattice and the Bi₂Se₃ lattice. We could not find any signature of a superconducting gap in the tunneling spectra measured on the surface of one and two UC thick islands of FeSe down to 6.5 K. The spectra rather show an asymmetric behavior across and a finite density of states at the Fermi level (E _F) resembling those taken in the normal state of bulk FeSe.

Interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator

The discovery of high-temperature superconductivity in Fe-based compounds triggered numerous investigations on the interplay between superconductivity and magnetism, and on the enhancement of transition temperatures through interface effects. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture remains elusive because of the lack of atomically resolved data. Here we present spin-polarized scanning tunnelling spectroscopy of ultrathin FeTe_1-xSe_x (x=0, 0.5) films on bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level, indicating superconducting correlations up to T_c∼6 K for one unit cell FeTe grown on Bi₂Te₃, in contrast to the non-superconducting bulk FeTe. The gap spatially coexists with bi-collinear AFM order. This finding opens perspectives for theoretical studies of competing orders in Fe-based superconductors and for experimental investigations of exotic phases in superconducting layers on topological insulators.

Observation of hidden atomic order at the interface between Fe and topological insulator Bi2Te3

The first compelling evidence of unique atomic order at the ferromagnet Fe/topological insulator Bi₂Te₃ interface.

Tunable Magnetic Anisotropy from Higher-Harmonics Exchange Scattering on the Surface of a Topological Insulator

We show that higher-harmonics exchange scattering from a magnetic adatom on the surface of a three dimensional topological insulator leads to a magnetic anisotropy whose magnitude and sign may be tuned by adjusting the chemical potential of the helical surface band. As the chemical potential moves from the Dirac point towards the surface band edge, the surface normal is found to change from a magnetic easy to a hard axis. Hexagonal warping is shown to diminish the region with easy axis anisotropy, and to suppress the anisotropy altogether. This indirect contribution can be comparable in magnitude to the intrinsic term arising from crystal field splitting and atomic spin-orbit coupling, and its tunability with the chemical potential makes the two contributions experimentally discernible, and endows this source of anisotropy with potentially interesting magnetic functionality.

Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

Dual nature of magnetic dopants and competing trends in topological insulators

Topological insulators interacting with magnetic impurities have been reported to host several unconventional effects. These phenomena are described within the framework of gapping Dirac quasiparticles due to broken time-reversal symmetry. However, the overwhelming majority of studies demonstrate the presence of a finite density of states near the Dirac point even once topological insulators become magnetic. Here, we map the response of topological states to magnetic impurities at the atomic scale. We demonstrate that magnetic order and gapless states can coexist. We show how this is the result of the delicate balance between two opposite trends, that is, gap opening and emergence of a Dirac node impurity band, both induced by the magnetic dopants. Our results evidence a more intricate and rich scenario with respect to the once generally assumed, showing how different electronic and magnetic states may be generated and controlled in this fascinating class of materials.

Magnetic impurities break time reversal symmetry in topological insulators, but there has been disagreement between theory and experiment. Here, the authors study the response of topological states to magnetic dopants at the atomic level and show that, contrary to what generally believed, magnetic order and gapless states can coexist.

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.

Organic Monolayer Protected Topological Surface State

Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)/Bi2Se3 and Fe/PTCDA/Bi2Se3 heterointerfaces are investigated using scanning tunneling microscopy and spectroscopy. The close-packed self-assembled PTCDA monolayer possesses big molecular band gap and weak molecule-substrate interactions, which leaves the Bi2Se3 topological surface state intact under PTCDA. Formation of Fe-PTCDA hybrids removes interactions between the Fe dopant and the Bi2Se3 surface, such as doping effects and Coulomb scattering. Our findings reveal the functionality of PTCDA to prevent dopant disturbances in the TSS and provide an effective alternative for interface designs of realistic TI devices.

Scanning tunneling microscopy studies of topological insulators

Scanning tunneling microscopy (STM), with surface sensitivity, is an ideal tool to probe the intriguing properties of the surface state of topological insulators (TIs) and topological crystalline insulators (TCIs). We summarize the recent progress on those topological phases revealed by STM studies. STM observations have directly confirmed the existence of the topological surface states and clearly revealed their novel properties. We also discuss STM work on magnetic doped TIs, topological superconductors and crystalline symmetry-protected surface states in TCIs. The studies have greatly promoted our understanding of the exotic properties of the new topological phases, as well as put forward new challenges. STM will continue to play an important role in this rapidly growing field from the point view of both fundamental physics and applications.

Probing the electronic properties of individual MnPc molecules coupled to topological states

Hybrid organic/inorganic interfaces have been widely reported to host emergent properties that go beyond those of their single constituents. Coupling molecules to the recently discovered topological insulators, which possess linearly dispersing and spin-momentum-locked Dirac fermions, may offer a promising platform toward new functionalities. Here, we report a scanning tunneling microscopy and spectroscopy study of the prototypical interface between MnPc molecules and a Bi2Te3 surface. MnPc is found to bind stably to the substrate through its central Mn atom. The adsorption process is only accompanied by a minor charge transfer across the interface, resulting in a moderately n-doped Bi2Te3 surface. More remarkably, topological states remain completely unaffected by the presence of the molecules, as evidenced by the absence of scattering patterns around adsorption sites. Interestingly, we show that, while the HOMO and LUMO orbitals closely resemble those of MnPc in the gas phase, a new hybrid state emerges through interaction with the substrate. Our results pave the way toward hybrid organic-topological insulator heterostructures, which may unveil a broad range of exciting and unknown phenomena.

Scattering of charge carriers by Cr impurities in magnetotransport on a Bi(1 1 1) ultra-thin film

In this investigation we tested the role of Cr impurities on the strongly spin-polarized surface states of ultra-thin epitaxially grown Bi(1 1 1) films by measuring surface magnetoconductance and the Hall effect in conjunction with low-energy electron diffraction at a low temperature (10 K). Compared with Fe and Co, investigated recently, Cr atoms turned out to have scattering cross-sections that are about a factor of three higher than the former atoms. Nevertheless, only a small electron donation (0.03 e/atom) was found for Cr. It also exhibits strong spin-orbit scattering, as judged from quantitative analysis of weak localization effects. As a result, all spin-dependent selection rules are gradually relaxed with increasing Cr concentration, so that the initially observed weak anti-localization shifts towards weak localization. The non-monotonic decrease of conductance as a function of Cr concentration, even at 10 K, indicates high diffusivity and activated adsorption into its final optimal adsorption site.

Ferromagnetism and topological surface states of manganese doped Bi2Te3: insights from density-functional calculations

Based on first-principles calculations, the electronic, magnetic, and topological characters of manganese (Mn) doped topological insulator Bi2Te3 were investigated. The Mn substitutionally doped Bi2Te3, where Mn atoms tend to be uniformly distributed, was shown to be p-type ferromagnetic, arising from hole-mediated Ruderman-Kittel-Kasuya-Yosida interaction. Mn doping leads to an intrinsic band splitting at Γ point, which is substantially different from that of nonmagnetic dopant. The topological surface state of Bi2Te3 is indeed gapped by Mn doping; however, the bulk conductance limits the appearance of an insulating state. Moreover, the n-type doping behavior of Bi2Te3 is derived from Mn entering into the van der Waals gap of Bi2Te3.