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Combining first-principles calculations and effective model analysis, we predict that CaTe is a topological node-line semimetal in the absence of the spin-orbit coupling. Using a slab model, we obtain the nearly flat drumhead surface state near the Fermi level. When the spin-orbit coupling is included, three node lines will evolve into a pair of Dirac points along the M − R line. These Dirac points are robust and protected by the C 4 rotation symmetry. Once this crystal symmetry is broken, the Dirac points will be eliminated, and the system becomes a strong topological insulator. Topological physics: a predicted node-line semimetal CaTe Topological insulators are materials with non-trivial topological order that are insulating in their bulk but conductive on their surface. Recent findings extend the topological states to three-dimensional semimetals that host exotic physical phenomena such as Weyl fermion quantum transport and Hall effects. Among the three types of topological semimetals, three-dimensional Dirac semimetals evolve to Weyl analogs upon breaking of time reversal or inversion symmetry. Here, the theoretical work by a team led by Professor Xiangang Wan from Nanjing University in China proposes a new phase that falls into the third category: node-line semimetals. Based on first-principles calculations and effective model analysis, CsCl structured CaTe is predicted to be a node-line semimetals with characteristic drumhead-like surface states if spin-orbit coupling is absent. When spin-orbit coupling is included, CaTe becomes a three-dimensional Dirac semimetal.

The CO and O 2 adsorption as well as CO oxidation on cationic, neutral, and anionic AuM dimers (M = Pd, Ag) are studied by density functional calculations. Our results show that CO and O 2 are adsorbed more stably on AuPd dimers than on AuAg dimers with corresponding charge state. O 2 is favorable to be adsorbed on Pd atom in AuPd + , AuPd and AuPd − dimers. CO is adsorbed on Pd in AuPd and AuPd − , while it is favorable to be adsorbed on Au in AuPd + . For AuAg dimers, O 2 is adsorbed on Ag in AuAg and AuAg − , and it is adsorbed on Au in AuAg + . CO is adsorbed on Ag in AuPd − , while it is adsorbed on Au in AuAg and AuAg + . The CO oxidation reaction is explored along two possible pathways: path-1 involves CO attacking the initial complexes of AuM dimers and O 2 , and path-2 is related to O 2 interacting with the complexes of AuM dimers and CO. The charge state of AuM dimers has a substantial effect on CO oxidation. The reaction on AuPd − prefers path-1, and AuPd + mediated reaction proceeds along path-2, while CO oxidation on AuPd is difficult along both paths. For AuAg, both pathways are viable for AuAg − mediated reactions, while AuAg and AuAg + mediated reactions prefer path-2. Moreover, the energy barriers of CO oxidation on neutral AuAg is comparable with those on AuPd in all charge states while the energy barriers for AuAg − and AuAg + are considerably lower than those for all AuPd dimmers, indicating the impurity atom also plays a significant role in the catalytic activity. Furthermore, AuAg − is proposed to be the most active species due to the lowest barrier involved in the reaction. Figure The simplified diagram of pathways for CO oxidation on cationic, neutral, and anionic AuM dimers (M = Pd and Ag)ᅟ

A systematic density functional theory (DFT) study has been performed to investigate the electronic and magnetic properties of one-dimensional sandwich polymers constructed with benzene (Bz) and the second-row transition metal (TM= Y, Zr, Nb, Mo, and Tc). Within the framework of generalized gradient approximation (GGA), [Tc(Bz)] ∞ is a ferromagnetic half-metal, and [Nb(Bz)] ∞ is a ferromagnetic metal. With the on-site Coulomb interaction for 4d TM atoms being taken into account, [Tc(Bz)] ∞ keeps a robust half-metallic behavior, while [Nb(Bz)] ∞ becomes a spin-selective semiconductor. The stability of the half-metallic [Tc(Bz)] ∞ polymer is discussed based on magnetic anisotropy energy (MAE). Compared with 0.1 meV per metal atom in [Mn(Bz)] ∞, the calculated MAE for [Tc(Bz)] ∞ is 2.3 meV per metal atom. Such a significantly larger MAE suggests that Tc(Bz)] ∞ is practically more promising than its first-row TM equivalent.

Spin-lattice (SL) coupling plays an important role in spintronic applications given its effects on magnetic, ferroelectric, optical, and thermodynamic properties. Experiments and theoretical calculations have revealed a large SL coupling effect in CrGeTe 3 and CrI 3 monolayers. However, the microscopic origin of SL coupling in these systems is still unclear. In this work, we develop a systematic method to explore the atomistic mechanism of SL coupling based on the density functional theory. We find that the first- and second-order SL couplings in ternary system CrGeTe 3 are considerably stronger than those in binary system CrI 3 . For the first-order SL coupling, the Cr ions of the magnetic pair and Ge ions positively contribute to the strain enhancement of ferromagnetism in CrGeTe 3 . However, the Cr ions provide a negative contribution in CrI 3 . Furthermore, our tight-binding analysis suggests that the p-d hopping in CrGeTe 3 gradually decreases with the tensile strain, rapidly enhancing the ferromagnetism under the tensile strain. The large frequency shifts in CrGeTe 3 are caused by the large second-order exchange derivatives (one type of second-order SL coupling) of the Cr ions of the magnetic pair.

A search for new topological quantum systems is challenging due to the requirement of nontrivial band connectivity that leads to protected surface states of electrons. A progress in this field was primarily due to a realization of band inversion mechanism between even and odd parity states that was proven to be very useful in both predicting many of such systems and our understanding their topological properties. Despite many proposed materials assume the band inversion between s and p (or p/d, d/f) electrons, here, we explore a different mechanism where the occupied d states subjected to a tetrahedral crystal field produce an active t2g manifold behaving as a state with an effective orbital momentum equal to negative one, and pushing jeff = 1=2 doublet at a higher energy. Via hybridization with nearest neighbor orbitals realizable, e.g., in a zincblende structural environment, this allows a formation of odd parity state whose subsequent band inversion with an unoccupied s band becomes possible, prompting us to look for the compounds with Cu+1 ionic state. Chemical valence arguments coupled to a search in materials database lead us to systematically investigate electronic structures and topological properties of CuY (Y=F, Cl, Br, I) and CuXO (X=Li, Na, K, Rb) families of compounds. Our theoretical results show that CuF displays a behavior characteristic of an ideal Weyl semimetal with 24 Weyl nodes at the bulk Brillouin Zone. We also find that another compounds CuNaO and CuLiO are the s-d inversion type topological insulators. Results for their electronic structures and corresponding surfaces states are presented and discussed in the context of their topological properties.

Using first--principles density functional calculations, we systematically investigate electronic structures and topological properties of InNbX2 (X=S, Se). In the absence of spin--orbit coupling (SOC), both compounds show nodal lines protected by mirror symmetry. Including SOC, the Dirac rings in InNbS2 split into two Weyl rings. This unique property is distinguished from other dicovered nodal line materials which normally requires the absence of SOC. On the other hand, SOC breaks the nodal lines in InNbSe2 and the compound becomes a type II Weyl semimetal with 12 Weyl points in the Brillouin Zone. Using a supercell slab calculation we study the dispersion of Fermi arcs surface states in InNbSe2, we also utilize a coherent potential approximation to probe their tolernace to the surface disorder effects. The quasi two--dimensionality and the absence of toxic elements makes these two compounds an ideal experimental platform for investigating novel properties of topological semimetals.

The magnetic properties of oxide PbMn(SO 4) 2 consisted of MnO6 octahedra which connected with each other through SO 4 tetrahedra, are well studied in experiments. In this paper, we explored its interesting electronic and magnetic properties with first-principle calculations. Our results show that all Mn ions have high spin states, namely, S = 5/2, and the magnetic couplings between NN and NNN are antiferromagnetic, which agree well with the experimental results. Besides, the surprising results of spin exchange interactions between the NN and NNN are excellently explained with extended Hüuckel tight-binding calculations.

Topological semimetals recently stimulate intense research activities. Combining first-principles calculations and effective model analysis, we predict that CaTe is topological node-line semimetal when spin-orbit coupling (SOC) is ignored. We also obtain the nearly flat surface state which has the drumhead characteristic. When SOC is included, three node lines evolve into a pair of Dirac points along the \\(M-R\\) line. These Dirac points are robust and protected by \\(C_4\\) rotation symmetry. Once this crystal symmetry is broken, the Dirac points will be eliminated, and the system becomes a strong topological insulator.

At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field to induce the half-metallicity decreases with the increase of the ribbon width. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remain half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may leads to some important applications in spinstronics.

We report a first-principles electronic-structure calculation on C and BN hybrid zigzag nanoribbons. We find that half-metallicity can arise in the hybrid nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. This unexpected half-metallicity in the hybrid nanos-tructures stems from a competition between the charge and spin polarizations, as well as from the pi orbital hybridization between C and BN. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of designing metal-free half metals.