Topological insulators are bulk insulators with metallic and fully spin-polarized surface states displaying Dirac-like band dispersion. Due to spin-momentum locking, these topological surface states (TSSs) have a predominant in-plane spin polarization in the bulk fundamental gap. Here, we show by spin-resolved photoemission spectroscopy that the TSS of a topological insulator interfaced with an antimonene bilayer exhibits nearly full out-of-plane spin polarization within the substrate gap. We connect this phenomenon to a symmetry-protected band crossing of the spin-polarized surface states. The nearly full out-of-plane spin polarization of the TSS occurs along a continuous path in the energy-momentum space, and the spin polarization within the gap can be reversibly tuned from nearly full out-of-plane to nearly full in-plane by electron doping. These findings pave the way to advanced spintronics applications that exploit the giant out-of-plane spin polarization of TSSs.

Two-dimensional (2D) van der Waals (vdW) ferromagnetic metals FexGeTe2 with x = 3-5 have raised significant interest in the scientific community. Fe5GeTe2 shows prospects for spintronic applications since the Curie temperature Tc has been reported near or higher than 300 K. In the present work, epitaxial Fe5-dGeTe2 (FGT) heterostructures were grown by Molecular Beam Epitaxy (MBE) on insulating crystalline substrates. The FGT films were combined with Bi2Te3 topological insulator (TI) aiming to investigate the possible beneficial effect of the TI on the magnetic properties of FGT. FGT/Bi2Te3 films were compared to FGT capped only with AlOx to prevent oxidation. SQUID and MOKE measurements revealed that the growth of Bi2Te3 TI on FGT films significantly enhances the saturation magnetization of FGT as well as the Tc well above room temperature (RT) reaching record values of 570 K. First-principles calculations predict a shift of the Fermi level and an associated enhancement of the majority spin ( primarily) as well as the total density of states at the Fermi level suggesting that effective doping of FGT from Bi2Te3 could explain the enhancement of ferromagnetism in FGT. It is also predicted that strain induced stabilization of a high magnetic moment phase in FGT/Bi2Te3 could be an alternative explanation of magnetization and Tc enhancement. Ferromagnetic resonance measurements evidence an enhanced broadening in the FGT/ Bi2Te3 heterostructure when compared to FGT. We obtain a large spin mixing conductance of g"# eff = 4.4 x 1020 m-2, which demonstrates the great potential of FGT/Bi2Te3 systems for spin-charge conversion applications at room temperature.

Topological insulators are considered new states of quantum matter that cannot be systematically related to conventional insulators and semiconductors. Among them, Bi2Se3 has attracted an increasing interest due to a simple surface band structure and due to a strong contribution of the surface to transport. While the dc electric transport properties have been extensively studied, intrinsic fluctuations and their effect on the surface conduction have received less attention. In order to better investigate these aspects, a detailed characterization of the low-frequency noise, also known as noise spectroscopy, has been made in Bi2Se3 thin films. The experimental results have been obtained for different samples thickness and geometry, in a temperature range from 300 down to 8 K, and as a function of dc bias current and gate voltage. While the observed spectral noise shows a typical thermal and shot noise part, an unusual reduction of the 1/f noise component is found, especially in the low-temperature region. A correlation of this behavior with structural and dc electric transport investigations suggests that it could be an indication of the occurrence of the topological regime. Flicker noise measurements, therefore, could be considered as a valid alternative technique to standard topological surface state spectroscopy.

Recently, the topological insulators (TIs) antimony telluride (SbTe) and bismuth telluride (BiTe) are attracting high interest for applications based on spin-charge interconversion mechanisms. Aiming to make a step toward the technology transfer, it is of major importance to achieve and investigate epitaxial quality-TIs on large area Si-based substrates. In view of that, we report here magnetotransport and angle-resolved photoemission spectroscopy (ARPES) studies on SbTe and BiTe thin films grown by metal organic chemical vapor deposition (MOCVD) on top of 4? Si(111) substrates. Clear weak antilocalization (WAL) effects are observed in both TIs, proving the existence of quantum transport mechanism, and the data are successfully interpreted in the framework of the Hikami-Larkin-Nagaoka model. Further, by dedicated magnetotransport experiments, it has been confirmed that the investigated WAL originates from two-dimensional (2D) topological states. ARPES has been performed ex-situ, and in both TIs the gapless Dirac cones have been observed and attributed to the topological surface states. Combining the proofs of the existence of quantum 2D transport as deduced from the analysis of the magnetoconductance curve with the direct observation of the Dirac-like band structure revealed by the ARPES spectra, it is possible to unambiguously confirm the topological nature of our SbTe and BiTe thin films. The results obtained on thin films grown by MOCVD on 4'' Si(111) substrate mark an important step towards the technology transfer of the topological insulators studied in this work.

Large-area antimony telluride (SbTe) thin films are grown by a metal organic chemical vapor deposition technique on 4" Si(111) substrates, and their topological character probed by magnetoconductance measurements. When interfaced with Fe thin films, broadband ferromagnetic resonance spectroscopy (BFMR) shows a clear increase of the damping parameter in Fe/SbTe when compared to a reference Fe layer, which may suggest the occurrence of spin pumping (SP) into SbTe. Simultaneously, X-ray reflectivity and conversion electron Mossbauer spectroscopy evidence the development of a chemically and magnetically pure Fe/SbTe interface. However, by conducting SP-FMR, it is shown that no spin-to-charge conversion (S2C) occurs in Fe/SbTe, while a clear SP signal develops by introducing a 5 nm Au interlayer between Fe and SbTe, with a measured inverse Edelstein effect conversion efficiency of ? = 0.27 nm. The results shed some light on the correlation among the chemical-structural-magnetic properties of the Fe/SbTe interface, the broadening of the magnetic damping parameter as detected by BFMR, and the occurrence of S2C, as probed by SP-FMR.

Topological insulators are materials with time-reversal symmetric states of matter in which an insulating bulk is surrounded by protected Dirac-like edge or surface states. Among topological insulators, Bi Se has attracted special attention due to its simple surface band structure and its relatively large band gap that should enhance the contribution of its surface to transport, which is usually masked by the appearance of defects. In order to avoid this difficulty, several features characteristic of topological insulators in the quantum regime, such as the weak-antilocalization effect, can be explored through magnetotransport experiments carried out on thin films of this material. Here, we review the existing literature on the magnetotransport properties of Bi Se thin films, paying thorough attention to the weak-antilocalization effect, which is omnipresent no matter the film quality. We carefully follow the different situations found in reported experiments, from the most ideal situations, with a strong surface contribution, towards more realistic cases where the bulk contribution dominates. We have compared the transport data found in literature to shed light on the intrinsic properties of Bi Se, finding a clear relationship between the mobility and the phase coherence length of the films that could trigger further experiments on transport in topological systems.

Antimony telluride (SbTe) thin films were prepared by a room temperature Metal-Organic Chemical Vapor Deposition (MOCVD) process using antimony chloride (SbCl) and bis(trimethylsilyl)telluride (Te(SiMe)) as precursors. Pre-growth and post-growth treatments were found to be pivotal in favoring out-of-plane and in-plane alignment of the crystallites composing the films. A comprehensive suite of characterization techniques were used to evaluate their composition, surface roughness, as well as to assess their morphology, crystallinity, and structural features, revealing that a quick post-growth annealing triggers the formation of epitaxial-quality SbTefilms on Si(111).

Since the discovery of the quantum anomalous Hall (QAH) effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)Te and V:(Bi,Sb)Te, the search for the magnetic coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic circular dichroism and density functional theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)Te. State-of-the-art first-principles calculations find pronounced differences in their 3d densities of states, and show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results explain, in an unified picture, the origins of the observed magnetic properties, and establish the essential role of impurity-state-mediated exchange interactions in the magnetism of MTI.

Time- and angle-resolved photoemission spectroscopy (TR-ARPES) provides access to the ultrafast evolution of electrons and many-body interactions in solid-state systems. However, the momentum- and energy-resolved transient photoemission intensity may not be unambiguously described by the intrinsic relaxation dynamics of photoexcited electrons alone. The interpretation of the time-dependent photoemission signal can be affected by the transient evolution of the electronic distribution, and both the one-electron removal spectral function as well as the photoemission matrix elements. Here we investigate the topological insulator Bi1.1Sb0.9Te2S to demonstrate, by means of a detailed probe-polarization dependent study, the transient contribution of matrix elements to TR-ARPES.

When coupled with ferromagnetic layers (FM), topological insulators (TI) are expected to boost the charge-to-spin conversion efficiency across the FM/TI interface. In this context, a thorough control and optimization of the FM/TI interface quality are requested. Here, the evolution of the chemical, structural, and magnetic properties of the Fe/Sb(2)Te(3)heterostructure is presented as a function of a rapid mild thermal annealing conducted on the Sb2Te3-TI (up to 200 degrees C). While the bilayer is not subjected to any thermal treatment upon Fe deposition, the annealing of Sb(2)Te(3)markedly improves its crystalline quality, leading to an increase in the fraction of ferromagnetic Fe atoms at the buried Fe/Sb(2)Te(3)interface and a slight lowering of the magnetic coercivity of the Fe layer. The method is an efficient tool to clean up the Fe/Sb(2)Te(3)interface, which may be extended to different FM/TI heterostructures. Simultaneously to the interface reconstruction, a constant approximate to 20% fraction of FeTe develops at the interface. Since FeTe can display superconductivity, the Fe/Sb(2)Te(3)system could have potentialities for exploiting phenomena at the edge of magnetism, superconductivity and topology.

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.

Interfacing ferromagnetic materials with topological insulators is an intriguing strategy in order to enhance spin-to-charge conversion mechanisms, paving the way toward highly efficient spin-based electronic devices. In particular, the use of large-scale deposition techniques is demanding for a sustainable and cost-effective industrial technology transfer. In this work, we study the magnetic properties of the Co/Sb2Te3 heterostructure, where the ferromagnetic Co layer is deposited by atomic layer deposition on top of the Sb2Te3 topological insulator, which is grown by metal organic chemical vapor deposition. In particular, broadband ferromagnetic resonance is employed to characterize the Co/Sb2Te3 system and the reference Co/Pt heterostructure. For Co/Sb2Te3, we extract an effective magnetic anisotropy constant K = 4.26*10^6 erg / cm^3, which is an order of magnitude higher than in Co/Pt (Keff = 0.43*10^6 erg / cm^3). The large difference in the Keff values observed in Co/Sb2Te3 and Co/Pt is explained in terms of the different Co crystalline structures achieved on top of Sb2Te3 and Pt, respectively. Interestingly, the Co/Sb2Te3 system displays a relatively large Gilbert damping constant (? = 0.095), which we suggest as possibly due to spin pumping from the Co layer into the Sb2Te3 topological insulator.

The recent availability of novel thin films and atom-thick layers, such as graphene, and metal di-chalcogenides and topological insulators, has opened fertile ground for the exploration of fundamental physics in low-dimensional systems. In addition to the continued introduction of new materials, scientists have been searching hard for a new state variable that should be explored for use beyond complementary metal-oxide-semiconductors (CMOS). Such efforts have been remarkably successful in the context of spintronics that use the spin angular momentum to store, transmit and manipulate information, which is more energy-efficient than charge information processing. Spintronics is one of the most active research areas in condensed matter physics and is continuously attracting a large amount of attention from both academia and the semiconductor industry. Efforts have been rewarded with many fundamental discoveries that could lead to important applications such as spin-torque magnetic random access memories and magnetic field sensors. This focus collection highlights recent progress in the development and application of new spintronic materials, such as 2D materials, topological materials, half-metal ferromagnets and low Gilbert damping materials, as well as in novel interfacial phenomena occurring in spintronic materials and heterostructures, such as Spin Hall effect, Rashba effect and proximity effects.

Identifying the two-dimensional (2D) topological insulating (TI) state in new materials and its control are crucial aspects towards the development of voltage-controlled spintronic devices with low-power dissipation. Members of the 2D transition metal dichalcogenides have been recently predicted and experimentally reported as a new class of 2D T1 materials, but in most cases edge conduction seems fragile and limited to the monolayer phase fabricated on specified substrates. Here, we realize the controlled patterning of the 1T' phase embedded into the 2H phase of thin semiconducting molybdenum-disulfide by laser beam irradiation. Integer fractions of the quantum of resistance, the dependence on laser-irradiation conditions, magnetic field, and temperature, as well as the bulk gap observation by scanning tunneling spectroscopy and theoretical calculations indicate the presence of the quantum spin Hall phase in our patterned 1T' phases.

Magnetically doped topological insulators may produce novel states of electronic matter, where for instance the quantum anomalous Hall effect state can be realized. Pivotal to this goal is a microscopic control over the magnetic state, defined by the local electronic structure of the dopants and their interactions. We report on the magnetic coupling among Mn or Co atoms adsorbed on the surface of the topological insulator Bi2Te3. Our findings uncover the mechanisms of the exchange coupling between magnetic atoms coupled to the topological surface state in strong topological insulators. The combination of x-ray magnetic circular dichroism and ab initio calculations reveals that the sign of the magnetic coupling at short adatom-adatom distances is opposite for Mn with respect to Co. For both elements, the magnetic exchange reverses its sign at a critical distance between magnetic adatoms, as a result of the interplay between superexchange, double exchange and Ruderman-Kittel-Kasuya-Yoshida interactions.

A fast control of spins is a major quest in spintronic systems. Ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators have received attention due to the existence, within the insulating band gap of bulk states, of spin-polarized surface states that are protected from backscattering by time-reversal symmetry. We studied sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of BiTe, employing circular-polarized light in time- and angle-resolved photoemission spectroscopy. Noncollinear optical parametric amplification and several nonlinear optical processes result in tunable, ultrashort visible pump pulses with a duration of 30 fs and 1.8 eV energy and ultraviolet probe pulses with about 60 fs duration and about 6 eV energy. The stable optical setup and the high repetition rate of an Yb-laser source gives a high signal-to-noise ratio in our photoemission process. The 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight energy analyzer, provides the opportunity to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism allows access to the spin state of the photoemitted electrons. A signature of femtosecond unpolarized bulk bands dynamics appears in the presence of spin-polarized electrons of the surface states. This helps distinguish the bulk and surface contributions in the spin-electronic current.

By means of angle-resolved photoemission spectroscopy measurements, the electronic band structure of the three-dimensional PbBi4Te7 and PbBi6Te10 topological insulators is compared. The measurements clearly reveal coexisting topological and multiple Rashba-like split states close to the Fermi level for both systems. The observed topological states derive from different surface terminations, as confirmed by scanning tunneling microscopy measurements, and are well-described by the density functional theory simulations. Both the topological and the Rashba-like states reveal a prevalent two-dimensional character barely affected by air exposure. X-ray and valence band photoemission measurements suggest Rashba-like states stem from the van der Waals gap expansion, consistently with density functional theory calculations

Topological insulators are a class of materials which have raised a great interest over the last decade, thanks to their intriguing conduction properties. Indeed, they are insulating in the bulk and metallic at the surface. Moreover, these metallic surface states have linear Dirac dispersion as in the case of graphene [1,2]. Bi2Se3 is among the most promising topological insulators, since its band structure provides only one Dirac cone, while the bulk gap is pretty large (about 300 meV) [3]. It has been demonstrated that by terahertz-infrared spectroscopy it is possible to detect the Dirac surface state by patterning thin films of Bi2Se3 with ribbons of width from 2 to 20 ?m. In this way, a Dirac plasmon is excited and its dispersion recovers very well the theoretical dispersion, calculated by using the parameters of the Dirac carriers [4]. In this scenario, we present here our investigation on the nonlinear regime of patterned films of Bi2Se3 with ribbons of width of 4 and 20 ?m. By exploiting the intense THz electric field of the TeraFERMI beamline at the FEL (Free Electron Laser) Fermi in Trieste [5], we were able to induce a nonlinear behaviour of the Dirac plasmon. Indeed, we observed a redshift of the plasmonic peak as the incoming THz electric field increases (up to MV/cm).

We investigate the band structure and topological phases of silicene embedded on halogenated Si(111) surface using density functional theory calculations. Our results show that the Dirac character of low-energy excitations in silicene is almost preserved in the presence of a silicon substrate passivated by various halogens. Nevertheless, the combined effects of symmetry breaking due to both direct and van der Waals interactions between silicene and the substrate, charge transfer from suspended silicene into the substrate, and, finally, the hybridization which leads to the charge redistribution result in a gap in the spectrum of the embedded silicene. We further take the spin-orbit interaction into account and obtain the resulting modification in the gap. The energy gaps with and without spin-orbit coupling vary significantly when different halogen atoms are used for the passivation of the Si surface, and for the case of iodine, they become on the order of 100 meV. To examine the topological properties, we calculate the projected band structure of silicene from which the Berry curvature and Z 2 invariant based on the evolution of Wannier charge centers are obtained. As a key finding, it is shown that silicene on halogenated Si substrates has a topological insulating state which can survive even at room temperature for the substrates with iodine and bromine at the surface. Therefore, these results suggest that we can have a reliable, stable, and robust silicene-based two-dimensional topological insulator using the considered substrates.

The three-dimensional topological insulator Bi2Se3 presents two cone-like dispersive topological surface states centered at the (Gamma) over bar point. One of them is unoccupied in equilibrium conditions and located 1.8 eV above the other one lying close to the Fermi level. In this work we employ time-and angle-resolved photoemission spectroscopy with circularly polarized pump photons to selectively track the spin dynamics of the empty topological states. We observe that spin-polarized electrons flow along the topological cone and recombine towards the unpolarized bulk states on a timescale of few tens of femtoseconds. This provides direct evidence of the capability to trigger a spin current with circularly polarized light.