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We performed density functional theory calculations with self-consistent van der Waals corrected exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves. This observation is relevant for the prediction of the critical temperature Tc. (b) There is a hierarchy of independent parameters that uniquely define a ground state ferroelectric unit cell (and square and rectangular paraelectric unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelectric ground state unit cell, while square and rectangular paraelectric structures are defined by only 3 or 2 independent optimizable variables, respectively. (c) The reduced number of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelectric unit cell when compared to the elastic energy barrier of a square paraelectric structure. This implies that Tc obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temperature on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) The dissociation energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and MoS2. (e) There exists a linear relation among the square paraelectric unit cell lattice parameter and the lattice parameters of the rectangular ferroelectric ground state unit cell. These results highlight the subtle atomistic structure of these novel 2D ferroelectrics.

The ZrSiS family of compounds hosts various exotic quantum phenomena due to the presence of both topological nonsymmorphic Dirac fermions and nodal‐line fermions. In this material family, the LnSbTe (Ln = lanthanide) compounds are particularly interesting owing to the intrinsic magnetism from magnetic Ln which leads to new properties and quantum states. In this work, the authors focus on the previously unexplored compound SmSbTe. The studies reveal a rare combination of a few functional properties in this material, including antiferromagnetism with possible magnetic frustration, electron correlation enhancement, and Dirac nodal‐line fermions. These properties enable SmSbTe as a unique platform to explore exotic quantum phenomena and advanced functionalities arising from the interplay between magnetism, topology, and electronic correlations.

Dirac semimetals (DSMs) have topologically robust three-dimensional Dirac (doubled Weyl) nodes with Fermi-arc states. In heterostructures involving DSMs, charge transfer occurs at the interfaces, which can be used to probe and control their bulk and surface topological properties through surface-bulk connectivity. Here we demonstrate that despite a band gap in DSM films, asymmetric charge transfer at the surface enables one to accurately identify locations of the Dirac-node projections from gapless band crossings and to examine and engineer properties of the topological Fermi-arc surface states connecting the projections, by simulating adatom-adsorbed DSM films using a first-principles method with an effective model. The positions of the Dirac-node projections are insensitive to charge transfer amount or slab thickness except for extremely thin films. By varying the amount of charge transfer, unique spin textures near the projections and a separation between the Fermi-arc states change, which can be observed by gating without adatoms.

We investigate the topological nodal structure of three-dimensional (3D) ZrTe5 driven by Zeeman splitting as a function of the direction of external magnetic (B) field by using a Wannier-functionbased tight-binding (WFTB) model obtained from first-principles calculations. It is known that small external stimuli can drive 3D ZrTe5 into different topological phases including Dirac semimetal. In order to emphasize the effect of Zeeman splitting, we consider 3D ZrTe5 in a strong TI phase with a small band gap. With Zeeman splitting greater than the band gap, the WFTB model suggests that a type-I nodal ring protected by (glide) mirror symmetry is formed when the B field aligns with the crystal a or b axes, and that a pair of type-I Weyl nodes are formed otherwise, when conduction and valence bands touch. We show that a pair of Weyl nodes can disappear through formation of a nodal ring, rather than requiring two Weyl nodes with opposite chirality to come together. Interestingly, a type-II nodal ring appears from crossings of the top two valence bands when the B field is applied along the c axis. This nodal ring gaps out to form type-II Weyl nodes when the B field rotates in the bc plane. Comparing the WFTB and linearized k · p model, we find inadequacy of the latter at some B field directions. Further, using the WFTB model, we numerically compute the intrinsic anomalous Hall conductivity σac induced by Berry curvature as a function of chemical potential and B field direction. We find that σac increases abruptly when the B field is tilted from the a axis within the ab plane. Our WFTB model also shows significant anomalous Hall conductivity induced by avoided level crossings even in the absence of Weyl nodes.

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Weyl semimetals (WSMs) have Weyl nodes where conduction and valence bands meet in the absence of inversion or time-reversal symmetry (TRS), or both. Weyl nodes are topologically protected as long as crystal momentum is conserved, giving rise to Fermi arcs at the surfaces. Interesting phenomena are expected in WSMs such as the chiral magnetic effect, anomalous Hall conductivity or Nernst effect, and unique quantum oscillations. The TRS-broken WSM phase can be driven from a topological Dirac semimetal by magnetic field B or magnetic dopants, considering that Dirac semimetals have degenerate Weyl nodes stabilized by rotational symmetry, i.e. Dirac nodes, near the Fermi level. Here we develop a Wannier-function-based tight-binding (WF-TB) model to investigate the formation of Weyl nodes and nodal rings induced by B field in the topological Dirac semimetal Na3Bi. The field is applied along the rotational axis. So far, studies of B field induced WSMs have been limited to cases with 4×4 effective models, which may not fully capture interesting effects. Remarkably, our study based on the WF-TB model shows that upon B field each Dirac node is split into four separate Weyl nodes along the rotational axis near the Fermi level; two nodes with Chern number ±1 (single Weyl nodes) and two with Chern number ±2 (double Weyl nodes). This result is in contrast to the common belief that each Dirac node consists of only two Weyl nodes with opposite chirality. In the context of the 4 × 4 effective models, the existence of double Weyl nodes ensures nonzero cubic terms in momentum. We further examine the evolution of Fermi arcs at a side surface as a function of chemical potential. This analysis corroborates our finding of the double Weyl nodes. The number of Fermi arcs at a given chemical potential is consistent with the corresponding Fermi surface Chern numbers. Furthermore, our study reveals the existence of nodal rings in the mirror plane below the Fermi level upon B field. These nodal rings persist with spin-orbit coupling, in contrast to many proposed nodal ring/line semimetals. Our WF-TB model can be used to compute interesting features arising from Berry curvature such as anomalous Hall and thermal conductivities, and our findings can be applied to other topological Dirac semimetals like Cd3As2.