Majorana bound states often occur at the end of 1D topological superconductor or at the π Josephson junction mediated by a helical edge state. Validated by a new bulk invariant and an intuitive edge argument, we show the emergence of one Majorana Kramers pair at each corner of a square-shaped 2D topological insulator proximitized by an s±-wave (e.g., Fe-based) superconductor. We obtain a phase diagram that emphasizes the roles of bulk parameters and edge orientations. We propose several experimental realizations in lattice-matched candidate materials. Our scheme offers a high-temperature platform for exploring higher-order non-Abelian quasiparticles.

The emergence of flat bands and correlated behaviors in "magic angle" twisted bilayer graphene (tBLG) has sparked tremendous interest, though many aspects of the system are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93º, which is smaller than the magic angle by 15%. At an electron concentration of ±5 electrons/moiré unit cell, we observe a narrow resistance peak with an activation energy gap ~0.1 meV, indicating the existence of an additional correlated insulating state. This is consistent with theory predicting the presence of a high-energy band with an energetically flat dispersion. At a doping of ±12 electrons/moiré unit cell we observe a resistance peak due to the presence of Dirac points in the spectrum. Our results reveal that the "magic" range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG.

Recently the small-twist-angle (< 2) bilayer graphene has received extraordinary attentions due to its exciting physical properties 1,

Recently twisted bilayer graphene(t-BLG) 1-5 emerges as a new strongly correlated physical platform near a magic twist angle 4 , which hosts many exciting phenomena such as the Mott-like insulating and unconventional superconducting behavior [6][7][8][9][10] . Besides the apparent significance of band flatness 4 , band topology may be another critical element in strongly correlated twistronics yet receives much less attention [11][12][13][14] . Here we report the discovery of nontrivial high dimensional band topology in t-BLG moiré bands through a systematic nonlocal transport study 15,16 , in conjunction with an examination rooted in K-theory 17 . The moiré band topology of t-BLG manifests itself as two pronounced nonlocal responses in the electron and hole superlattice gaps.We further show that the nonlocal responses are robust to the interlayer electric field, twist angle, and edge termination, exhibiting a universal scaling law. While an unusual symmetry of t-BLG trivializes Berry curvature, we elucidate that two high dimensional Z2 invariants characterize the topology of the moiré Dirac bands, validating the topological origin of the observed nonlocal responses. Our findings not only provide a new perspective for understanding the emerging strongly correlated phenomena in twisted van der Waals heterostructures, but also suggest a potential strategy to achieve topologically nontrivial metamaterials from topologically trivial quantum materials based on twist engineering.