Two-dimensional higher-order topology in monolayer graphdiyne

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.Comment: 19 pages, 4 figures, new titl

Transferable empirical pseudopotenials from machine learning

Machine learning is used to generate empirical pseudopotentials that characterize the local screened interactions in the Kohn-Sham Hamiltonian. Our approach incorporates momentum-range-separated rotation-covariant descriptors to capture crystal symmetries as well as crucial directional information of bonds, thus realizing accurate descriptions of anisotropic solids. Trained empirical potentials are shown to be versatile and transferable such that the calculated energy bands and wave functions without cumbersome self-consistency reproduce conventional ab initio results even for semiconductors with defects, thus fostering faster and faithful data-driven materials researches.Comment: 10 pages, 9 figures, 3 table

Crystalline topological Dirac semimetal phase in rutile structure β′-PtO2

Based on first-principles calculations and symmetry analysis, we propose that a transition-metal rutile oxide, in particular β′-PtO2, can host a three-dimensional topological Dirac semimetal phase. We find that β′-PtO2 possesses an inner nodal chain structure when spin-orbit coupling is neglected. Incorporating spin-orbit coupling gaps the nodal chain while preserving a single pair of three-dimensional Dirac points protected by a screw rotation symmetry. These Dirac points are created by a band inversion of two d bands, which is a realization of a Dirac semimetal phase in a correlated electron system. Moreover, a mirror plane in the momentum space carries a nontrivial mirror Chern number nM=-2, which distinguishes β′-PtO2 from the Dirac semimetals known so far, such as Na3Bi and Cd3As2. If we apply a perturbation that breaks the rotation symmetry and preserves the mirror symmetry, the Dirac points are gapped, and the system becomes a topological crystalline insulator. © 2019 American Physical Societ

Half-metallic ferromagnetism and metal–insulator transition in Sn-doped SrRuO3 perovskite oxides

We investigate the electronic and magnetic properties of SrRu1-xSnxO3 by carrying out density-functional-theory calculations to show that a half-metallic ferromagnetic ground state emerges for the Sn doping of x≳0.5. To examine the effect of on-site Coulomb interactions for the Ru d orbitals, which was suggested to enhance the half-metallicity in SrRuO3, we employed both the local spin-density approximation (LSDA) as well as the LSDA + U method. For all the possible configurations of Sn doping for x=1/8,1/4,1/2,5/8,3/4, and 7/8 within the 2×2×2 unit cell, we monitor the Ru t2g bandwidth as well as the valence band maximum in the majority-spin channel and demonstrate that the Ru d electron hopping is blocked by the Sn-substituted sites so that the Ru t2g bandwidth becomes reduced as the doping x increases. For x0.7, the Ru t2g bandwidth gets so narrow that even a small on-site Coulomb interaction, e.g., Ueff=1.0 eV induces a band-gap, which indeed corresponds to a gap of the Ru impurity bands in the SrSnO3 oxide semiconductor. © 2018 Elsevier B.

Anisotropic suppression of octahedral breathing distortion with the fully strained BaBiO3/BaCeO3 heterointerface

While the physiochemical effects of octahedral tilting and rotating distortions have been studied extensively, octahedral breathing distortion (OBD) at heterointerfaces has rarely been explored. Here, we investigated OBD in fully strained BaBiO3 (BBO) epitaxial films by making a new type of oxide heterointerface with non-breathing BaCeO3 epitaxial films. The integration of first-principles calculations with experimental observations of optical spectroscopy revealed that the oxygen displacement modes in BBO became disordered within six unit cells at the heterointerface and the surface. Controlling OBD in perovskite oxide thin films provides a means to exploit emerging material properties. © 2018 Author(s)

Two-dimensional higher-order topology in monolayer graphdiyne

© 2020, The Author(s).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 pz 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 px,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 state