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Twisted graphene bilayer around the first magic angle engineered by heterostrain
摘要： Very recently, twisted graphene bilayers (TGBs) around the first magic angle θ ≈ 1.1? have attracted much attention for the realization of exotic quantum states, such as correlated insulator behavior and unconventional superconductivity. Here we elaborately study a series of TGBs around the first magic angle engineered by heterostrain, where each layer is strained independently. Our experiment indicates that a moderate heterostrain enables the structural evolution from the small-angle TGB (θ ～ 1.5°) to the strained magic-angle TGB (θ ～ 1.1°), exhibiting the characteristic low-energy flat bands. The heterostrain can even drive the system into highly strained tiny-angle TGBs (θ (cid:5)1.1°) with large deformed tetragonal superlattices, where a unique network of topological helical edge states emerges. Furthermore, the predicted domain wall modes, which are strongly localized and result in a hexagon-triangle-mixed frustrated lattice derived from the Kagome lattice, are observed in the strained tiny-angle TGBs.
关键词： magic angle,heterostrain,twisted graphene bilayer,topological helical edge states,domain wall modes
更新于2025-09-09 09:28:46
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Photonic topological phases in dispersive metamaterials
摘要： We analyze the photonic topological phases in dispersive metamaterials which satisfy the degenerate condition at a reference frequency. The electromagnetic duality allows for the hybrid modes to be decoupled and described by the spin-orbit Hamiltonians with pseudospin 1, which result in nonzero spin Chern numbers that characterize the topological phases. In particular, the combined Hamiltonian of the hybrid modes complies with a fermionic-like pseudo time-reversal symmetry that ensures the Kramers degeneracy, leading to the topological protection of helical edge states. The transverse spin generated by the evanescent surface waves is perpendicular to the wave vector, which exhibits the spin-momentum locking as in the surface states for three-dimensional topological insulators. The topological properties of the helical edge states are further illustrated with the robust transport of a pair of counterpropagating surface waves with opposite polarization handedness at an irregular boundary of the metamaterial.
关键词： spin-momentum locking,pseudospin 1,Kramers degeneracy,dispersive metamaterials,helical edge states,spin Chern numbers,spin-orbit Hamiltonians,photonic topological phases
更新于2025-09-04 15:30:14
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Dynamic nuclear polarization from topological insulator helical edge states
摘要： Topological insulators are promising for spintronics and related technologies due to their spin-momentum-locked edge states, which are protected by time-reversal symmetry. In addition to the unique fundamental physics that arises in these systems, the potential technological applications of these protected states have also been driving TI research over the past decade. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfine-assisted backscattering. Here, we calculate the scattering probabilities and nuclear polarization for edge channels containing up to 34 nuclear spins using a numerically exact analysis that exploits the symmetries of the problem to drastically reduce the computational complexity. We then show the emergence of universal scaling properties that allow us to extrapolate our findings to vastly larger and experimentally relevant system sizes. We find that significant nuclear polarization can result from relatively weak helical edge currents, suggesting that it may be an important factor affecting spin transport in topological insulator devices.
关键词： Hyperfine coupling,Dynamic nuclear polarization,Helical edge states,Topological insulators,Spintronics
更新于2025-09-04 15:30:14
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Microwave photocurrent from the edge states of InAs/GaInSb bilayers
摘要： We measure microwave photocurrent in devices made from InAs/GaInSb bilayers where both the insulating bulk state and conducting edge state were observed in the inverted-band regime, consistent with the theoretical prediction for a quantum spin Hall (QSH) insulator. It has been theoretically proposed that microwave photocurrent could be a unique probe in studying the properties of QSH edge states. To distinguish a possible photoresponse between a bulk state and helical edge state, we prepare a Hall bar and Corbino disk from the same wafer. Results show that the Corbino-disk samples have a negligible photocurrent in the bulk gap, while clear photocurrent signals from the Hall-bar samples are observed. This finding suggests that the photocurrent may carry information concerning the electronic properties of the edge states.
关键词： quantum spin Hall insulator,microwave photocurrent,helical edge states,InAs/GaInSb bilayers
更新于2025-09-04 15:30:14