Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose FeGeTe, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin-orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.

A novel approach to synthesize highly luminescent graphene quantum dots (GQDs) with well-defined sizes was explored based on simple oxidation of herringbone-type carbon nanofibers (HCNFs) and size-selective precipitation.

In previous studies, the topological invariants of 1D non-Hermitian systems have been defined in open boundary condition (OBC) to satisfy the bulk-boundary correspondence. The extreme sensitivity of bulk energy spectra to boundary conditions has been attributed to the breakdown of the conventional bulk-boundary correspondence based on the topological invariants defined under periodic boundary condition (PBC). Here we propose non-Hermitian many-body polarization as a topological invariant for 1D non-Hermitian systems defined in PBC, which satisfies the bulk-boundary correspondence. Employing many-body methodology in the non-Hermitian Su-Schrieffer-Heeger model for fermions, we show the absence of non-Hermitian skin effect due to the Pauli exclusion principle and demonstrate the bulk-boundary correspondence using the invariant defined under PBC. Moreover, we show that the bulk topological invariant is quantized in the presence of chiral or generalized inversion symmetry. Our study suggests the existence of generalized crystalline symmetries in non-Hermitian systems, which give quantized topological invariants that capture the symmetry-protected topology of non-Hermitian systems. PACS numbers:Introduction.-Recent progress in the study of non-Hermitian systems, such as open systems, dissipative systems with gain and loss, and also interacting solid-state systems , has uncovered various intriguing physical phenomena that do not exist in Hermitian systems. For instance, the characteristic complex energy spectra of non-Hermitian systems are theoretically predicted to host exceptional surfaces or bulk Fermi-arcs [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], which are later realized in experiments [54,55]. Nowadays, there are growing research activities to extend the idea of topological Bloch theory developed in Hermitian systems to non-Hermitian Hamiltonians [56][57][58][59][60][61][62][63][64].One central issue in the study of topological phenomena in non-Hermitian systems is to understand the bulk-boundary correspondence (BBC). In Hermitian systems, it is wellestablished that the bulk topological invariants defined by Bloch wave functions in periodic boundary condition (PBC) predict robust boundary states in systems under open boundary condition (OBC) [65][66][67]. Contrary to this, in non-Hermitian systems, the bulk energy spectra exhibit extreme sensitivity to boundary conditions [68,69]. For instance, in recent studies of the non-Hermitian Su-Schrieffer-Heeger (SSH) model, it was shown that the bulk eigenstates, which are extended under PBC, are exponentially localized on one-side of the finite-size system with OBC [57,70]. This phenomenon is named the non-Hermitian skin effect in Ref. 70, which has been extensively discussed recently [70][71][72][73][74][75][76][77]. Since the energy spectra under PBC and OBC differ so drastically, there has been even a common belief that the bulk invariant defined under PBC has intrinsic limitations in explaining BBC of non-Hermitian systems in general. To circumve...

Based on first-principles calculations and tight-binding model analysis, we propose monolayer graphdiyne as a candidate material for a two-dimensional higher-order topological insulator protected by inversion symmetry. Despite the absence of chiral symmetry, the higher-order topology of monolayer graphdiyne is manifested in the filling anomaly and charge accumulation at two corners. Although its low energy band structure can be properly described by the tight-binding Hamiltonian constructed by using only the p z orbital of each atom, the corresponding bulk band topology is trivial. The nontrivial bulk topology can be correctly captured only when the contribution from the core levels derived from p x,y and s orbitals are included, which is further confirmed by the Wilson loop calculations. We also show that the higher-order band topology of a monolayer graphdyine gives rise to the nontrivial band topology of the corresponding three-dimensional material, ABC-stacked graphdiyne, which hosts monopole nodal lines and hinge states.