Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. Here, we use ab initio simulations and tight-binding models to determine the precise spin texture of electronic states in graphene interfaced with a BiSe topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices.

Dirac material–based heterostructures reveal enhanced spin-orbit coupling with an emerging spin texture.

We demonstrate the growth of twin-free Bi 2 Te 3 and Sb 2 Te 3 topological insulators by molecular beam epitaxy, and a sizable reduction of the twin density in Bi 2 Se 3 , on latticematched BaF 2 (111) substrates. Using x-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se-and Tebased alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches, in which twin-free Bi 2 Se 3 films are achieved by increasing the substrate roughness, the quality of our Bi 2 Te 3 is superior on the flattest BaF 2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab-initio calculations. Bismuth and antimony chalcogenides, Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Se 3 , are prototypical topological insulators (TIs), a new class of bulk insulators with conductive surface states. [1-3] Their surface hosts Dirac fermions protected by time-reversal symmetry and characterized by spinmomentum locking, a property that prevents backscattering and that is highly attractive for spintronics and fault-tolerant quantum computation. [3,4] However, bismuth and antimony chalcogenides are narrow gap semiconductors that are very sensitive to doping, and the surface currents are typically overwhelmed by bulk currents. In order to favour surface transport, it is necessary to suppress the bulk conductivity, which requires the growth of lowdefect thin films, a task that has proven to be very challenging. [5-9] Due to doping by impurities or crystalline defects, the Fermi level of Bi 2 Se 3 and Bi 2 Te 3 is typically shifted to the conduction band. [5,8,10-14] Although counter-doping or electrostatic gating may be used to tune the Fermi energy, structural defects impact negatively the transport properties by coupling surface and bulk channels, by reducing the carrier mobility or by suppressing the surface states in the case of strain localized at grain boundaries. [15] Some common imperfections are Se/Te vacancies, dislocations, mosaïcity, 30° rotational domains and twin domains. [5,7,15-18] In particular, mirror-symmetric twin domains are of special importance owing to the extended planar nature of their boundaries (see Figure 1a). They are known to introduce electron scattering, strain and doping in various semiconductors such as GaAs [19] or HgCdTe. [20] In TIs, it has been demonstrated that twin boundaries hold a spontaneous polarization responsible for self-doping of the surface states, which may reach several hundreds of meV. [21] TIs are usually grown on conventional substrates such as Al 2 O 3 (0001) [7,10,13,22] and Si(111), [8,13,14,23-26] despite l...

Recent progress in the on-surface synthesis and characterization of nanomaterials is facilitating the realization of new carbon allotropes, such as nanoporous graphenes, graphynes, and 2D π-conjugated polymers. One of the latest examples is the biphenylene network (BPN), which was recently fabricated on gold and characterized with atomic precision. This gapless 2D organic material presents uncommon metallic conduction, which could help develop innovative carbon-based electronics. Here, using first principles calculations and quantum transport simulations, we provide new insights into some fundamental properties of BPN, which are key for its further technological exploitation. We predict that BPN hosts an unprecedented spin-polarized multiradical ground state, which has important implications for the chemical reactivity of the 2D material under practical use conditions. The associated electronic band gap is highly sensitive to perturbations, as seen in finite temperature (300 K) molecular dynamics simulations, but the multiradical character remains stable. Furthermore, BPN is found to host in-plane anisotropic (spin-polarized) electrical transport, rooted in its intrinsic structural features, which suggests potential device functionality of interest for both nanoelectronics and spintronics.

We present a first-principles study of electronic properties of ultrathin films of topological insulators and scrutinize the role of disorder on the robustness of topological surface states, which can be analysed through their spin textures. The presence of twin grain boundaries is found to increase the bandgap of the film, while preserving the spin texture of states in first conduction and valence bands. Differently, partial hydrogenation of one surface not only results in some self-doping effect, but also provokes some alteration of the spin texture symmetry of the electronic states. The formation of a new Dirac cone at M -point of the Brillouin zone of the hydrogenated surface, together with a modified spin texture characteristics are consistent with a dominant Dresselhaus spin-orbit interaction type, more usually observed in 3D materials. Our findings indicate that defects can either be detrimental or beneficial for exploring spin transport of surface states in the limit of ultrathin films of topological insulators, which maximizes surface over bulk phenomena.

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