First-principles investigation of the magnetoelectric properties of Ba7Mn4O15.

Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of Ba7Mn4O15which has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics. When the constraints on the magnetic moments are removed, the spins self-consistently relax into a canted antiferromagnetic ground state configuration where two magnetic modes transforming as distinct irreducible representations coexist. While the dominant magnetic mode matches well with the previous experimental observations, the second mode is found to possess a different character resulting in a non-polar ground state. Interestingly, the non-polar magnetic ground state exhibits a significantly strong linear magnetoelectric (ME) coupling comparable to the well-known multiferroic BiFeO3, suggesting strategies to design new linear MEs.

Dynamic chiral magnetic effect and anisotropic natural optical activity of tilted Weyl semimetals.

We study the dynamic chiral magnetic conductivity (DCMC) and natural optical activity in an inversion-broken tilted Weyl semimetal (WSM). Starting from the Kubo formula, we derive the analytical expressions for the DCMC for two different directions of the incident electromagnetic wave. We show that the angle of rotation of the plane of polarization of the transmitted wave exhibits remarkable anisotropy and is larger along the tilt direction. This striking anisotropy of DCMC results in anisotropic optical activity and rotary power, which can be experimentally observed as a topological magneto-electric effect of inversion-broken tilted WSMs. Finally, using the low energy Hamiltonian, we show that the DCMC follows the universal [Formula: see text] decay in the high frequency regime. In the low frequency regime, however, the DCMC shows sharp peaks at the tilt dependent effective chemical potentials of the left-handed and right-handed Weyl points. This can serve as a signature to distinguish between the type-I and type-II Weyl semimetals.

Bulk band inversion and surface Dirac cones in LaSb and LaBi: Prediction of a new topological heterostructure.

We perform ab initio investigations of the bulk and surface band structures of LaSb and LaBi and resolve the existing disagreements about the topological property of LaSb, considering LaBi as a reference. We examine the bulk band structure for band inversion, along with the stability of surface Dirac cones (if any) to time-reversal-preserving perturbations, as a strong diagnostic test for determining the topological character of LaSb, LaBi and LaSb-LaBi multilayer. A detailed ab initio investigation of a multilayer consisting of alternating unit cells of LaSb and LaBi shows the presence of band inversion in the bulk and a massless Dirac cone on the (001) surface, which remains stable under the influence of time-reversal-preserving perturbations, thus confirming the topologically non-trivial nature of the multilayer in which the electronic properties can be tailored as per requirement. A detailed [Formula: see text] invariant calculation is performed to arrive at a holistic conclusion.

Antisite-disorder engineering in La-based oxide heterostructures via oxygen vacancy control.

It has been realized lately that disorder, primarily in the form of oxygen vacancies, cation stoichiometry, atomic inter-diffusion and antisite defects, has a major effect on the electronic and transport properties of a 2D electron liquid at oxide hetero-interfaces - the first and the last being the two key players. In order to delineate the roles of these two key factors, we have investigated the effect of oxygen vacancies on the antisite disorder at a large number of interfaces separating two La-based transition metal oxides, using density functional theory. The oxygen vacancy is found to suppress antisite disorder in some heterostructures thereby stabilizing the ordered structure, while in some others, it tends to favor disorder, opening up the possibility of using it to control the order. Our calculations show that the oxygen vacancy offers an opportunity to generate new magnetic states by manipulating the inter-site coupling. Moreover, it can be used to control the electrical transport. The oxygen vacancy and antisite disorder are intrinsic to oxide heterostructures and it is therefore incumbent to engineer the latter and tune the magnetic and transport properties by controlling the oxygen partial pressure during growth.

Comparative study of the compensated semi-metals LaBi and LuBi: a first-principles approach.

We have investigated the electronic structures of LaBi and LuBi, employing the full-potential all electron method as implemented in Wien2k. Using this, we have studied in detail both the bulk and the surface states of these materials. From our band structure calculations we find that LuBi, like LaBi, is a compensated semi-metal with almost equal and sizable electron and hole pockets. In analogy with experimental evidence in LaBi, we thus predict that LuBi will also be a candidate for extremely large magneto-resistance (XMR), which should be of immense technological interest. Our calculations reveal that LaBi, despite being gapless in the bulk spectrum, displays the characteristic features of a [Formula: see text] topological semi-metal, resulting in gapless Dirac cones on the surface, whereas LuBi only shows avoided band inversion in the bulk and is thus a conventional compensated semi-metal with extremely large magneto-resistance.