Author

# Birabar Nanda

Bio: Birabar Nanda is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topic(s): Band gap & Ferromagnetism. The author has an hindex of 17, co-authored 73 publication(s) receiving 1046 citation(s). Previous affiliations of Birabar Nanda include Indian Institutes of Technology & University of Missouri.

##### Papers
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TL;DR: In this paper, the authors applied the full-potential linearized muffin tin orbital method and the tight-binding linearized MTL orbital method to investigate the electronic structure and magnetism of a series of half-Heusler compounds XMZ with X = Fe,Co,Ni, M = Ti,V,Nb,Zr,Cr,Mo,Mn and Z = Sb,Sn.
Abstract: In this paper we have applied the full-potential linearized muffin tin orbital method and the tight-binding linearized muffin tin orbital method to investigate in detail the electronic structure and magnetism of a series of half-Heusler compounds XMZ with X = Fe,Co,Ni, M = Ti,V,Nb,Zr,Cr,Mo,Mn and Z = Sb,Sn. Our detailed analysis of the electronic structure using various indicators of chemical bonding suggests that covalent hybridization of the higher-valent transition element X with the lower-valent transition element M is the key interaction responsible for the formation of the d–d gap in these systems. However, the presence of the sp-valent element is crucial to provide stability to these systems. The influence of the relative ordering of the atoms in the unit cell on the d–d gap is also investigated. We have also studied in detail some of these systems with more than 18 valence electrons which exhibit novel magnetic properties, namely half-metallic ferro- and ferrimagnetism. We show that the d–d gap in the paramagnetic state, the relatively large X–Sb hybridization and the large exchange splitting of the M atoms are responsible for the half-metallic property of some of these systems.

163 citations

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TL;DR: In this paper, the electronic structure of graphene with a single substitutional vacancy was studied using a combination of the density-functional, tight-binding and impurity Green's function approaches. And the results showed that the long-range nature of the V? wave function is a unique feature of the graphene vacancy and that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.
Abstract: We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding and impurity Green's function approaches. Density-functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three sp2? dangling bonds adjacent to the vacancy introduce localized states (V?) in the mid-gap region, which split due to the crystal field and a Jahn?Teller distortion, while the pz? states introduce a sharp resonance state (V?) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the ? and the ? states, so that these bands are clearly identifiable in the calculated band structure. As to the magnetic moment of the vacancy, the Hund's rule coupling aligns the spins of the four localized V?1??, V?2? and V?? electrons, resulting in an S?=?1 state, with a magnetic moment of 2?B, which is reduced by about 0.3?B due to the anti-ferromagnetic spin polarization of the ? band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of 1.7?B. Using the Lippmann?Schwinger equation, we reproduce the well-known ?1/r decay of the localized V? wave function with distance, and in addition, find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the V? wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.

85 citations

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TL;DR: In this paper, the electronic structure of the bilayer graphene (BLG) is changed by electric field and strain from ab initio density-functional calculations using the linear muffin-tin orbital and the linear augmented plane wave methods.
Abstract: We study how the electronic structure of the bilayer graphene (BLG) is changed by electric field and strain from ab initio density-functional calculations using the linear muffin-tin orbital and the linear augmented plane wave methods. Both hexagonal and Bernal stacked structures are considered. We only consider interplanar strain where only the interlayer spacing is changed. The BLG is a zero-gap semiconductor like the isolated layer of graphene. We find that while strain alone does not produce a gap in the BLG, an electric field does so in the Bernal structure but not in the hexagonal structure. The topology of the bands leads to Dirac circles with linear dispersion in the case of the hexagonally stacked BLG due to the interpenetration of the Dirac cones, while for the Bernal stacking, the dispersion is quadratic. The size of the Dirac circle increases with the applied electric field, leading to an interesting way of controlling the Fermi surface. The external electric field is screened due to polarization charges between the layers, leading to a reduced size of the band gap and the Dirac circle. The screening is substantial in both cases and diverges for the Bernal structure for small fields as has been noted by earlier authors. As a biproduct of this work, we present the tight-binding parameters for the free-standing single layer graphene as obtained by fitting to the density-functional bands, both with and without the slope constraint for the Dirac cone and keeping the hopping integral up to four near neighbors.

64 citations

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TL;DR: In this paper, the authors study how strain affects orbital ordering and magnetism at the interface between a three-site Mn-O-Mn model and interpret the basic results in terms of a three site model.
Abstract: We study how strain affects orbital ordering and magnetism at the interface between ${\text{SrMnO}}_{3}$ and ${\text{LaMnO}}_{3}$ from density-functional calculations and interpret the basic results in terms of a three-site Mn-O-Mn model. Magnetic interaction between the Mn atoms is governed by a competition between the antiferromagnetic superexchange of the $\text{Mn}\text{ }{t}_{2g}$ core spins and the ferromagnetic double exchange of the itinerant ${e}_{g}$ electrons. While the core electrons are relatively unaffected by the strain, the orbital character of the itinerant electron is strongly affected, which in turn causes a large change in the strength of the ferromagnetic double exchange. The epitaxial strain produces the tetragonal distortion of the ${\text{MnO}}_{6}$ octahedron, splitting the $\text{Mn}\text{ }{e}_{g}$ states into ${x}^{2}\ensuremath{-}{y}^{2}$ and $3{z}^{2}\ensuremath{-}1$ states, with the former being lower in energy, if the strain is tensile in the plane and opposite if the strain is compressive. For the case of the tensile strain, the resulting higher occupancy of the ${x}^{2}\ensuremath{-}{y}^{2}$ orbital enhances the in-plane ferromagnetic double exchange owing to the larger electron hopping in the plane, causing at the same time a reduction in the out-of-plane double exchange. This reduction is large enough to be overcome by antiferromagnetic superexchange, which wins to produce a net antiferromagnetic interaction between the out-of-plane Mn atoms. For the case of the in-plane compressive strain, the reverse happens, viz., that the higher occupancy of the $3{z}^{2}\ensuremath{-}1$ orbital results in the out-of-plane ferromagnetic interaction, while the in-plane magnetic interaction remains antiferromagnetic. Concrete density-functional results are presented for the ${({\text{LaMnO}}_{3})}_{1}/{({\text{SrMnO}}_{3})}_{1}$ and ${({\text{LaMnO}}_{3})}_{1}/{({\text{SrMnO}}_{3})}_{3}$ superlattices for various strain conditions.

54 citations

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TL;DR: In this paper, the magnetic properties of superlattices were studied from density functional calculations, and it was shown that the magnetism changes with the layer thickness of the lattice, and the reason for the different magnetic structures is the varying potential barrier across the interface.
Abstract: We study the magnetic structure of the ${({\text{LaMnO}}_{3})}_{2n}/{({\text{SrMnO}}_{3})}_{n}$ superlattices from density-functional calculations. In agreement with the experiments, we find that the magnetism changes with the layer thickness $n$. The reason for the different magnetic structures is shown to be the varying potential barrier across the interface, which controls the leakage of the $\text{Mn-}{e}_{g}$ electrons from the ${\text{LaMnO}}_{3}$ side to the ${\text{SrMnO}}_{3}$ side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For the $n=1$ superlattice, the $\text{Mn-}{e}_{g}$ electrons are more or less spread over the entire lattice so that the magnetic behavior is similar to the equivalent alloy compound ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}$. For larger $n$, the ${e}_{g}$ electron transfer occurs mostly between the two layers adjacent to the interface, thus leaving the magnetism unchanged and bulklike away from the interface region.

50 citations

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01 Jan 1955

2,246 citations

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TL;DR: The experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene are reviewed.
Abstract: The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials

1,089 citations

Journal ArticleDOI
TL;DR: The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and the also discusses orbital magnetism, phonons and the influence of strain on electronic properties.
Abstract: We review the electronic properties of bilayer graphene, beginning with a description of the tight-binding model of bilayer graphene and the derivation of the effective Hamiltonian describing massive chiral quasiparticles in two parabolic bands at low energies. We take into account five tight-binding parameters of the Slonczewski–Weiss–McClure model of bulk graphite plus intra- and interlayer asymmetry between atomic sites which induce band gaps in the low-energy spectrum. The Hartree model of screening and band-gap opening due to interlayer asymmetry in the presence of external gates is presented. The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and we also discuss orbital magnetism, phonons and the influence of strain on electronic properties. We conclude with an overview of electronic interaction effects.

729 citations

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TL;DR: A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented in this article, where the effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered.
Abstract: A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

656 citations

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TL;DR: This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces, identifying the most exciting new scientific results and pointing to promising future research directions.
Abstract: This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

581 citations