We present a two-dimensional (2D) lattice model that exhibits a nontrivial topological phase in the absence of the Berry curvature. Instead, the Berry connection provides the topological nontrivial phase in the model, whose integration over the momentum space, the so-called 2D Zak phase, yields a fractional wave polarization in each direction. These fractional wave polarizations manifest themselves as degenerated edge states with opposite parities in the model.

The quantum spin Hall effect lays the foundation for the topologically protected manipulation of waves, but is restricted to one-dimensional-lower boundaries of systems and hence limits the diversity and integration of topological photonic devices. Recently, the conventional bulk-boundary correspondence of band topology has been extended to higher-order cases that enable explorations of topological states with codimensions larger than one such as hinge and corner states. Here, we demonstrate a higher-order quantum spin Hall effect in a two-dimensional photonic crystal. Owing to the non-trivial higher-order topology and the pseudospin-pseudospin coupling, we observe a directional localization of photons at corners with opposite pseudospin polarizations through pseudospin-momentum-locked edge waves, resembling the quantum spin Hall effect in a higher-order manner. Our work inspires an unprecedented route to transport and trap spinful waves, supporting potential applications in topological photonic devices such as spinful topological lasers and chiral quantum emitters.

The Su-Schrieffer-Heeger (SSH) model on a two-dimensional square lattice exhibits a topological phase transition which is related to the Zak phase determined by bulk band topology. The strong modulation of electron hopping causes nontrivial charge polarization even in the presence of inversion symmetry. The energy band structures and topological edge states have been calculated numerically in previous studies. Here, however, the full energy spectrum and explicit form of wave functions for two-dimensional bulk and one-dimensional ribbon geometries of the SSH model are analytically derived using the wave mechanics approach. Explicit analytic representations of wave functions provide the information of parity for each subband, localization length, and critical point of the topological phase transition in the SSH ribbon. IThe dimensional crossover of the topological transition point for the SSH model from one to two dimensions is also shown.

Typical higher-order topological systems require the fine-tuning of hopping textures and external fields, which considerably hinders their practical realization. Based on a simple picture that corners are “edges” of edges, we determine that in the already-thoroughly-studied monolayer graphene, higher-order topological corner states appear without introducing any additional effects. Unlike quadrupole insulators, owing to degenerate Dirac points in graphene, the emergence of topological corner states depends on the corner angle and edge geometries. We provide a useful expression for the indication of corner states in graphene by the product of Zak phases. We also discuss the methods for experimental detection of the nontrivial higher-order topology in graphene such as the fractional corner anomaly and the disparity of local density of states between trivial and nontrivial corners.

We study topological states of honeycomb photonic crystals in the absence of inversion symmetry using plane wave expansion and finite element methods. The breaking of inversion symmetry in honeycomb lattice leads to contrasting topological valley indices, i.e., the valley-dependent Chern numbers in momentum space. We find that the topological corner states appear for 60^° degree corners, but absent for other corners, which can be understood as the sign flip of valley Chern number at the corner. Our results provide an experimentally feasible platform for exploring valley-dependent higher-order topology in photonic systems.

The Su-Schrieffer-Heeger (SSH) model is fundamental in topological insulators and relevant to understanding higher-order topological phases. This study explores the relationship between the $n$-dimensional SSH model and its $(n-1)$-dimensional counterpart, identifying a hierarchical structure in the Hamiltonian that allows us to solve an arbitrary $n$-dimensional SSH model analytically. By generalizing the bulk-edge correspondence principle to arbitrary dimensions in a higher-order fashion using the vectored Zak phase, we reveal a type of topological insulator called hierarchical topological insulators. In this hierarchical topological insulator, there exist intermediate-order topological interfacial states that are protected by subsymmetry and energy band topology in a partial Brillouin zone. Furthermore, we compare the $n$-dimensional SSH model with the Benalcazar-Bernevig-Hughes (BBH) model, another essential model in higher-order topological phases similar to the two-dimensional SSH model with an extra flux of $\pi$ in each plaque. We find that the BBH model is another example of hierarchical topological insulators.