The search for topological semimetals is mainly focused on heavy-element compounds as following the footsteps of previous research on topological insulators, with less attention on light-element materials. However, the negligible spin orbit coupling with light elements may turn out to be beneficial for realizing topological band features. Here, using first-principles calculations, we propose a new two-dimensional light-element material-the honeycomb borophene oxide (h-B 2 O), which has nontrivial topological properties. The proposed structure is based on the recently synthesized honeycomb borophene on Al (111) substrate [Sci. Bull. 63, 282 (2018)]. The h-B 2 O monolayer is completely flat, unlike the oxides of graphene or silicene. We systematically investigate the structural properties of h-B 2 O, and find that it has very good stability and exhibits significant mechanical anisotropy. Interestingly, the electronic band structure of h-B 2 O hosts a nodal loop centered around the Y point in the Brillouin zone, protected by the mirror symmetry. Furthermore, under moderate lattice strain, the single nodal loop can be transformed into two loops, each penetrating through the Brillouin zone. The loops before and after the transition are characterized by different ℤ × ℤ topological indices. Our work not only predicts a new two-dimensional material with interesting physical properties, but also offers an alternative approach to search for new topological phases in 2D light-element systems.

Inspirited by the wide range of applications of graphene and the similarity between boron and carbon, 2D boron sheets have gained extensive research interest. In this work, using first-principles combined with a nonequilibrium Green's function method, thermal conductance of fully hydrogenated borophene, named borophane, is studied. Interestingly and in contrast to widely perceived sense, at 300 K, it is found that the thermal conductance of borophane in the armchair direction is remarkably larger than that of graphene. More interesting, a dimensionality crossover is observed in phonon transmission where low-frequency phonons exhibit 2D characteristic, while high-frequency phonons behave like a 1D system, oriented along armchair direction, which results in the ultrahigh thermal conductance. An anomalous increase of thermal conductance with uniaxial tensile strain is observed, which is well explained by the unique puckered structure and chemical bonding in borophane. The excellent in-plane stiffness and flexibility together with the high thermal conductance suggest that borophane is promising for soft thermal channel. Moreover, this unique dimensionality crossover in phonon transmission offers a perfect platform for studying the effect of phonon population in mode space, which is of primary importance for thermal transport in low-dimensional systems.

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