To date, almost all of the discussions on topological insulators (TIs) have focused on two-and three-dimensional systems. One-dimensional (1D) TIs manifested in real materials, in which localized spin states may exist at the end or near the junctions, have largely been unexplored. Previous studies have considered the system of gapped graphene nanoribbons (GNRs) possessing spatial symmetries (e.g. inversion) with only termination patterns commensurate with inversion-or mirror-symmetric unit cells. In this work, we prove that a symmetry-protected Z 2 topological classification exists for any type of termination. In these cases the Berry phase summed up over all occupied bands turns out to be π-quantized in the presence of the chiral symmetry. However, it does not always provide the correct corresponding Z 2 as one would have expected.We show that only the origin-independent part of the Berry phase gives the correct bulk-boundary correspondence by its π-quantized values. The resulting Z 2 invariant depends on the choice of the 1D unit cell (defined by the nanoribbon termination) and is shown to be connected to the symmetry eigenvalues of the wave functions at the center and boundary of the Brillouin zone. Using the cove-edged GNRs as examples, we demonstrate the existence of localized states at the end of some GNR segments and at the junction between two GNRs based on a topological analysis. The current results are expected to shed light on the design of electronic devices based on GNRs as well as the understanding of the topological features in 1D systems.

The integration of quantum materials like topological insulators (TIs) with magnetic insulators (MIs) has important technological implications for spintronics and quantum computing. Here, we report excellent crystallinity of c-axis oriented epitaxial TI films of Bi2Se3 grown on MI films, a rare earth iron garnet (ReIG), such as thulium iron garnet (Tm3Fe5O12, TmIG), by molecular beam epitaxy using Se-buffered low-temperature growth technique. Strained-TmIG films with robust perpendicular magnetic anisotropy were deposited by off-axis sputtering. We demonstrated a streaky reflection high-energy electron diffraction pattern starting from the very first quintuple layer of Bi2Se3, indicating the high-quality interface between TmIG and Bi2Se3, a prerequisite for studying interfacial exchange coupling effects. The strong interfacial exchange interaction was manifested by the observation of an anomalous Hall effect in the Bi2Se3/TmIG bilayer and a shift of the ferromagnetic resonance field of TmIG induced by Bi2Se3. We have reproducibly grown high-quality Bi2Se3/ReIG and interfaces using this TI growth method, which may be applied to grow other types of van der Waals hetero-structures.

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Topological insulating (TI) phases were originally highlighted for their disorder-robust bulk responses, such as the quantized Hall conductivity of 2D Chern insulators. With the discovery of time-reversal-(T -) invariant 2D TIs, and the recognition that their spin Hall conductivity is generically non-quantized, focus has since shifted to boundary states as signatures of 2D and 3D TIs and symmetry-enforced topological crystalline insulators (TCIs). However, in T -invariant (helical) 3D TCIs such as bismuth, BiBr, and MoTe2 -termed higher-order TCIs (HOTIs) -the boundary signatures manifest as 1D hinge states, whose configurations are dependent on sample details. It is hence desirable to elucidate bulk signatures of helical TCIs, and their relationship to sample-independent experimental observables. In this work, we introduce nested spin-resolved Wilson loops and layer constructions as tools to characterize the bulk topological properties of Iand T -protected helical HOTIs. We discover that helical HOTIs realize one of three spin-resolved phases with distinct responses that are quantitatively robust to large deformations of the bulk spin-orbital texture: 3D quantum spin Hall insulators (QSHIs), "spin-Weyl" semimetal states with gapless spin spectra, and T -doubled axion insulator (T-DAXI) states with nontrivial partial axion angles θ ± = π indicative of a 3D spin-magnetoelectric bulk response. We provide experimental signatures of each spin-stable regime of helical HOTIs, including surface Fermi arcs in spin-Weyl semimetals under strong Zeeman fields, and half-quantized 2D TI states on the gapped surfaces of T-DAXIs originating from a partial parity anomaly. Lastly, we use ab-initio calculations to demonstrate that the candidate HOTI β-MoTe2 realizes a spin-Weyl state with 8 total spin-Weyl points.

This study investigates the origin of long-puzzled high frequency dispersion on the accumulation region of capacitance-voltage characteristics in an n-type GaAs-based metal-oxide-semiconductor. Probed adatoms with a high Pauling electronegativity, Ag and Au, unexpectedly donate charge to the contacted As/Ga atoms of as-grown α2 GaAs(001)-2 × 4 surfaces. The GaAs surface atoms behave as charge acceptors, and if not properly passivated, they would trap those electrons accumulated at the oxide and semiconductor interface under a positive bias. The exemplified core-level spectra of the Al2O3/n-GaAs(001)-2 × 4 and the Al2O3/n-GaAs(001)-4 × 6 interfaces exhibit remnant of pristine surface As emission, thereby causing high frequency dispersion in the accumulation region. For the p-type GaAs, electrons under a negatively biased condition are expelled from the interface, thereby avoiding becoming trapped.

Direct deposition of high-dielectric-constant oxides on high-mobility semiconductors with low trap densities is the key to high-performance metal−oxide−semiconductor (MOS) devices. Atomic layer deposition (ALD) has been employed in precise thin oxide depositions with relatively low temperatures in the semiconductor device fabrication industry. Herein, we compare the electronic structures of nanometer-thick ALD-Y 2 O 3 on freshly grown GaAs(001)−4 × 6 without and with ultrahigh vacuum (UHV) annealing to 600 °C. In situ X-ray photoelectron spectroscopy (XPS) and in situ synchrotron radiation photoelectron spectroscopy (SRPES) with the best surface sensitivity were utilized to study the samples, whose pristine conditions were preserved under UHV between the growth and the characterization. In the Y 2 O 3 film, the surface Y−OH bonding and the pure As atoms resulting from the ALD process were completely removed with UHV annealing. Additionally, no O deficiency was detected in the resolution limit of XPS, indicating the intactness of the O−Y bonding. The UHV annealing has reduced traps in the Y 2 O 3 film, leading to smaller frequency dispersions in the measured capacitance−voltage (C−V) curves of the MOS capacitors (MOSCAPs). At the same time, the enriched Ga−O−Y bonds effectively passivated the GaAs surface and strongly stabilized the Y 2 O 3 /GaAs interface, resulting in lower interfacial trap density (D it ) values from (2−3) × 10 12 eV −1 cm −2 to (0.7−2) × 10 12 eV −1 cm −2 and maintaining the high-temperature stability. We have elucidated the effects of UHV annealing on the interfacial chemistry and the thin oxide film of the ALD-Y 2 O 3 /GaAs(001)−4 × 6 in an atomic scale, and have correlated the electronic characteristics of the heterostructure with the electrical properties of the MOSCAPs.