First principles study of topological phase in chains of 3d3d transition metals

Recent experiments have shown the signatures of Majorana bound states at the ends of magnetic chains deposited on a superconducting substrate. Here, we employ first principles calculations to directly investigate the topological properties of $3d$ transition metal nanochains (i.e., Mn, Cr, Fe and Co). In contrast to the previous studies [Nadj-Perge et al. Science 346, 602 (2014) and Ruby et al. Nano Lett. 17, 4473 (2017)], we found the exact tight binding models in the Wannier orbital basis for the isolated chains as well as for the surface--deposited wires. Based on these models, we calculate topological invariant of $\mathbb{Z}_2$ phase for all systems. Our results for the isolated chains demonstrate the existence of the topological phase only in the Mn and Co systems. We considered also a non-collinear magnetic order as a source of the non--trivial topological phase and found that this type of magnetic order is not a stable ground state in the Fe and Co isolated chains. Further studies showed that a coupling between the chain and substrate leads to strong modification of the band structure. Moreover, the analysis of the topological invariant indicates a possibility of emergence of the topological phase in all studied nanochains deposited on the Pb surface. Therefore, our results demonstrate an important role of the coupling between deposited atoms and a substrate for topological properties of nanosystems.Comment: 11 pages, 7 figure

Strong Effects of Cation Vacancies on the Electronic and Dynamical Properties of FeO

We report pronounced modifications of electronic and vibrational properties induced in FeO by cation vacancies, obtained within density functional theory incorporating strong local Coulomb interactions at Fe atoms. The insulating gap of FeO is reduced by about 50% due to unoccupied electronic bands introduced by trivalent Fe ions stabilized by cation vacancies. The changes in the electronic structure along with atomic displacements induced by cation vacancies affect strongly phonon dispersions via modified force constants, including those at atoms beyond nearest neighbors of defects. We demonstrate that theoretical phonon dispersions and their densities of states reproduce the results of inelastic neutron and nuclear resonant x-ray scattering experiments \emph{only} when Fe vacancies and Coulomb interaction $U$ are both included explicitly in \emph{ab initio} simulations, which also suggests that the electron-phonon coupling in FeO is strong.Comment: 5 pages, 4 figure

Influence of ff electrons on the electronic band structure of rare-earth nickelates

Recently, superconductivity was discovered in the infinite layer of hole-doped nickelates NdNiO$_{2}$. Contrary to this, superconductivity in LaNiO$_{2}$ is still under debate. This indicates the crucial role played by the $f$ electrons on the electronic structure and the pairing mechanism of infinite-layer nickelates. Here, we discuss the role of the electron correlations in the $f$ electron states and their influence on the electronic structure. We show that the lattice parameters are in good agreement with the experimental values, independent of the chosen parameters within the DFT+U approach. Increasing Coulomb interaction U tends to shift the $f$ states away from the Fermi level. Surprisingly, independently of the position of $f$ states with respect to the Fermi energy, these states play an important role in the electronic band structure, which can be reflected in the modification of the NdNiO$_{2}$ effective models

Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors

In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic band structure of iron-based superconductors and thus the obtained results can explain the experimental observations in FeSe and Co-doped BaFe2As2 compounds

Electronic band structure and surface states in Dirac semimetal LaAgSb2LaAgSb_{2}

LaAgSb$_{2}$ is a Dirac semimetal showing charge density wave (CDW) order. Previous angle-resolved photoemission spectroscopy (ARPES) results suggest the existence of the Dirac-cone-like structure in the vicinity of the Fermi level along the $\Gamma$–M direction. This paper is devoted to a complex analysis of the electronic band structure of LaAgSb$_{2}$ by means of ARPES and theoretical studies within the ab initio method as well as tight binding model formulation. To investigate the possible surface states, we performed the direct DFT slab calculation and the surface Green function calculation for the (001) surface. The appearance of the surface states, which depends strongly on the surface, points to the conclusion that LaSb termination is realized in the cleaved crystals. Moreover, the surface states predicted by our calculations at the $\Gamma$ and X points are found by ARPES. Nodal lines, which exist along the X–R and M–A paths due to crystal symmetry, are also observed experimentally. The calculations reveal other nodal lines, which originate from the vanishing of spin–orbit splitting and are located at the X–M–A–R plane at the Brillouin zone boundary. In addition, we analyze the band structure along the $\Gamma$–M path to verify whether Dirac surface states can be expected. Their appearance in this region is not confirmed

Density functional theory study of Au-fcc/Ge and Au-hcp/Ge interfaces

In recent years, nanostructures with hexagonal polytypes of gold have been synthesised, opening new possibilities in nanoscience and nanotechnology. As bulk gold crystallizes in the fcc phase, surface effects can play an important role in stabilizing hexagonal gold nanostructures. Here, we investigate several heterostructures with Ge substrates, including the fcc and hcp phases of gold that have been observed experimentally. We determine and discuss their interfacial energies and optimized atomic arrangements, comparing the theory results with available experimental data. Our DFT calculations for the Au-fcc(011)/Ge(001) junction show how the presence of defects in the interface layer can help to stabilize the atomic pattern, consistent with microscopic images. Although the Au-hcp/Ge interface is characterized by a similar interface energy, it reveals large atomic displacements due to significant mismatch. Finally, analyzing the electronic properties, we demonstrate that Au/Ge systems have metallic character, but covalent-like bonding states between interfacial Ge and Au atoms are also present

Anharmonicity and structural phase transition in the Mott insulator Cu2_2P2_2O7_7

Ab initio investigations of structural, electronic, and dynamical properties of the high-temperature $\beta$ phase of copper pyrophosphate were performed using density functional theory. The electronic band structure shows the Mott insulating state due to electron correlations in copper ions. By calculating phonon dispersion relations, the soft mode at the A point of the Brillouin zone was revealed, showing the dynamical instability of the $\beta$ phase at low temperatures. The double-well potential connected with the soft mode is derived and the mechanism of the structural phase transition to the $\alpha$ phase is discussed. The self-consistent phonon calculations based on the temperature-dependent effective potential show the stabilization of the $\beta$ phase at high temperatures, due to the anharmonic effects. The pronounced temperature dependence and the large line width of the soft mode indicate an essential role of anharmonicity in the structural phase transition