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Register a new userManipulation and Characterization of the Valley Polarized Topological Kink States in Graphene Based Interferometers
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Valley polarized topological kink states, existing broadly in the domain wall of hexagonal lattices systems, are identified in experiments, unfortunately, only very limited physical properties being given. Using an Aharanov-Bohm interferometer composed of domain walls in graphene systems, we study the periodical modulation of pure valley current in a large range by tuning the magnetic field or the Fermi level. For monolayer graphene device, there exists one topological kink state, and the oscillation of transmission coefficients have single period. The $pi$ Berry phase and the linear dispersion relation of kink states can be extracted from the transmission data. For bilayer graphene device, there are two topological kink states with two oscillation periods. Our proposal provides an experimental feasible route to manipulate and characterize the valley polarized topological kink states in classical wave and electronic graphene-type crystalline systems.
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We propose a general and tunable platform to realize high-density arrays of quantum spin-valley Hall kink (QSVHK) states with spin-valley-momentum locking based on a two-dimensional hexagonal topological insulator. Through the analysis of Berry curvature and topological charge, the QSVHK states are found to be topologically protected by the valley-inversion and time-reversal symmetries. Remarkably, the conductance of QSVHK states remains quantized against either nonmagnetic or long-range magnetic disorder, verified by the Green function calculations. Based on first-principles results, we show that QSVHK states, protected with a gap up to 287 meV, can be realized in bismuthene by alloy engineering, surface functionalization, or electric field, supporting non-volatile applications of spin-valley filters, valves, and waveguides even at room temperature.
Bloch states of electrons in honeycomb two-dimensional crystals with multi-valley band structure and broken inversion symmetry have orbital magnetic moments of a topological nature. In crystals with two degenerate valleys, a perpendicular magnetic field lifts the valley degeneracy via a Zeeman effect due to these magnetic moments, leading to magnetoelectric effects which can be leveraged for creating valleytronic devices. In this work, we demonstrate that trilayer graphene with Bernal stacking, (ABA TLG) hosts topological magnetic moments with a large and widely tunable valley g-factor, reaching a value 500 at the extreme of the studied parametric range. The reported experiment consists in sublattice-resolved scanning tunneling spectroscopy under perpendicular electric and magnetic fields that control the TLG bands. The tunneling spectra agree very well with the results of theoretical modelling that includes the full details of the TLG tight-binding model and accounts for a quantum-dot-like potential profile formed electrostatically under the scanning tunneling microscope tip. Our results show that ABA TLG is a compelling quantum material platform.
We theoretically study the correlated insulator states, quantum anomalous Hall (QAH) states, and field-induced topological transitions between different correlated states in twisted multilayer graphene systems. Taking twisted bilayer-monolayer graphene and twisted double-bilayer graphene as examples, we show that both systems stay in spin polarized, $C_{3z}$-broken insulator states with zero Chern number at 1/2 filling of the flat bands under finite displacement fields. In some cases these spin polarized, nematic insulator states are in the quantum valley Hall phase by virtue of the nontrivial band topology of the systems. The spin polarized insulator state is quasi-degenerate with the valley polarized state when only the dominant intra-valley Coulomb interactions are included. Such quasi-degeneracy can be split by atomic on-site interactions such that the spin polarized, nematic state become the unique ground state. Such a scenario applies to various twisted multilayer graphene systems at 1/2 filling, thus can be considered as a universal mechanism. Moreover, under vertical magnetic fields, the giant orbital Zeeman splittings in twisited multilayer graphene systems compete with the atomic Hubbard interactions, which can drive transitions from spin polarized zero-Chern-number states to valley-polarized QAH states with small onset magnetic fields.
Berry phase plays an important role in determining many physical properties of quantum systems. However, a Berry phase altering energy spectrum of a quantum system is comparatively rare. Here, we report an unusual tunable valley polarized energy spectra induced by continuously tunable Berry phase in Bernal-stacked bilayer graphene quantum dots. In our experiment, the Berry phase of electron orbital states is continuously tuned from about pi to 2pi by perpendicular magnetic fields. When the Berry phase equals pi or 2pi, the electron states in the two inequivalent valleys are energetically degenerate. By altering the Berry phase to noninteger multiples of pi, large and continuously tunable valley polarized energy spectra are detected in our experiment. The observed Berry phase-induced valley splitting, on the order of 10 meV at a magnetic field of 1 T, is about 100 times larger than Zeeman splitting for spin, shedding light on graphene-based valleytronics.
We have performed angle-resolved photoemission spectroscopy on (PbSe)5(Bi2Se3)3m, which forms a natural multilayer heterostructure consisting of a topological insulator (TI) and an ordinary insulator. For m = 2, we observed a gapped Dirac-cone state within the bulk-band gap, suggesting that the topological interface states are effectively encapsulated by block layers; furthermore, it was found that the quantum confinement effect of the band dispersions of Bi2Se3 layers enhances the effective bulk-band gap to 0.5 eV, the largest ever observed in TIs. In addition, we found that the system is no longer in the topological phase at m = 1, pointing to a topological phase transition between m = 1 and 2. These results demonstrate that utilization of naturally-occurring heterostructures is a new promising strategy for realizing exotic quantum phenomena and device applications of TIs.
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