IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

We investigate the adhesion at the interface of face-centered tungsten-carbide (001) and diamond (001) from density-functional calculations. Four high-symmetry model interfaces, representing different lattice orientations for either side of the interface, are constructed to incorporate different degrees of strain arising due to lattice mismatch. The adhesion, estimated from the ideal work of separation, is found to be in the range of 4 - 7 J m -2 and is comparable to that of metal-carbide interfaces. Maximum adhesion occurs when WC and diamond slabs have the same orientation, even though such a growth induces large epitaxial strain at the interface. From electronic structure calculations, we attribute the adhesion to covalent interaction between carbon p-orbitals as well as partial ionic interaction between the tungsten d- and carbon p-orbitals across the interface. © 2015 AIP Publishing LLC.

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Department Of Physics

AIP Conference Proceedings

Adhesion at WC/diamond interfaces - A theoretical study

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New Journal of Physics

Erratum: Electronic structure of the substitutional vacancy in graphene: Density-functional and Green's function studies(New Journal of Physics (2012) 14 (083004))

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 sp 2σ dangling bonds adjacent to the vacancy introduce localized states (Vσ) in the mid-gap region, which split due to the crystal field and a Jahn-Teller distortion, while the p zπ states introduce a sharp resonance state (Vπ) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the σ and the π 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 Vσ 1 Vσ 2 and Vπ electrons, resulting in an S = 1 state, with a magnetic moment of 2μB, which is reduced by about 0.3μB due to the anti-ferromagnetic spin polarization of the π band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of 1.7μB. Using the Lippmann-Schwinger equation, we reproduce the well-known ̃1/r decay of the localized Vπ wave function with distance, and in addition, find aninterference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the Vπ 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. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Electronic structure of the substitutional vacancy in graphene: Density-functional and green's function studiesauth

We show that the substitutional vacancy in graphene forms a dynamical Jahn-Teller center. The adiabatic potential surface resulting from the electron-lattice coupling was computed using density-functional methods, and subsequently the Schrödinger equation was solved for the nuclear motion. Our calculations show a large tunneling splitting 3Γ estimated to be about 65 cm -1. The effect results in a large delocalization of the carbon nuclear wave functions around the vacancy, leading to a significant broadening of the Jahn-Teller active sp2σ electron states. The tunneling splitting should be observable in electron paramagnetic resonance and two-photon resonance scattering experiments. © 2012 American Physical Society.

Physical Review B - Condensed Matter and Materials Physics

Nuclear tunneling and dynamical Jahn-Teller effect in graphene with vacancy

From a mean-field solution of the Hubbard-Holstein model, we show that a rich variety of different electronic phases can result at the interface between two polar materials such as LaAlO3/SrTiO3. Depending on the strengths of the various competing interactions, viz., the electronic kinetic energy, electron-phonon interaction, Coulomb energy, and electronic screening strength, the electrons could (i) either be strongly confined to the interface forming a 2D metallic or an insulating phase, (ii) spread deeper into the bulk making a 3D phase, or (iii) become localized at individual sites forming a Jahn-Teller polaronic phase. In the polaronic phase, the Coulomb interaction could lead to unpaired electrons resulting in magnetic Kondo centers. Under appropriate conditions, electronic phase separation may also occur resulting in the coexistence of metallic and insulating regions at the interface. © 2011 American Physical Society.

Electronic phases and phase separation in the Hubbard-Holstein model of a polar interface

From calculations using ab initio density-functional methods, the authors show that analogous to the much-studied polar LaAlO3/SrTiO 3 interface, there also forms a two-dimensional electron gas at the LaMnO3/SrMnO3 (100) interface due to the polar catastrophe. The electron gas originates from the accumulation of half an electron per cell to quench the polar catastrophe and from the leakage of Mn (eg) electrons from the LaMnO3 side to the SrMnO 3 side. These electrons mediate a ferromagnetic double-exchange interaction between the Mn moments at the interface, causing the interface region to be ferromagnetic and in turn the electrons becoming spin-polarized owing to the Zeeman field of the Mn moments. © 2010 American Vacuum Society.

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Journal of Vacuum Science and Technology B:Nanotechnology and Microelectronics

Polar catastrophe and the spin-polarized electron gas at the LaMnO 3/SrMnO3 interface

Electronic reconstruction at the polar interface LaMnO3 / SrMnO3 (LMO/SMO) (100) resulting from the polar catastrophe is studied from a model Hamiltonian that includes the double and superexchange interactions, the Madelung potential, and the Jahn-Teller coupling terms relevant for the manganites. We show that the polar catastrophe, originating from the alternately charged LMO layers and neutral SMO layers, is quenched by the accumulation of an extra half electron per cell in the interface region as in the case of the LaAlO3 / SrTiO3 interface. In addition, the Mn eg electrons leak out from the LMO side to the SMO side, the extent of the leakage being controlled by the interfacial potential barrier and the substrate induced epitaxial strain. The leaked electrons mediate a Zener double exchange, making the layers adjacent to the interface ferromagnetic, while the two bulk materials away from the interface retain their original type A or G antiferromagnetic structures. A half-metallic conduction band results at the interface, sandwiched by the two insulating bulks. We have also studied how the electron leakage and consequently the magnetic ordering are affected by the substrate-induced epitaxial strain. Comparisons are made with the results of the density-functional calculations for the (LMO) 6 / (SMO) 4 superlattice. © 2010 The American Physical Society.

Polar catastrophe, electron leakage, and magnetic ordering at the LaMnO3 / SrMnO3 interface

We study the electronic and magnetic properties of tetragonal LaMnO 3 (LMO) under uniaxial strain, appropriate for epitaxially grown LMO heterostructures, from density functional calculations. The optimized tetragonal structure without strain has volume, magnetic order, and JahnTeller distortions similar to the bulk LMO, which forms in the orthorhombic structure. Strain affects the relative magnitudes of these distortions and changes the magnetic and conduction properties. While unstrained LMO is a type A antiferromagnet and insulating, we find that it changes to a ferromagnetic metal under tensile strain condition (c-axis stretched). The latter is the result of a diminishing magnitude of the JahnTeller distortion with strain, which in turn reduces the splitting of the Mn-eg orbitals, eventually leading to a metallic state, and finally to a ferromagnet due to the double exchange interaction. The calculated Poisson's ratio from geometry optimization agrees with the experimental values for the bulk. © 2010 Elsevier B.V. All rights reserved.

Journal of Magnetism and Magnetic Materials

Density functional studies of LaMnO3 under uniaxial strain

The effect of uniaxial strain on electronic structure and magnetism in LaMnO3 is studied from a model Hamiltonian that illustrates the competition between the Jahn-Teller, superexchange, and double-exchange interactions. We retain in our model the three main octahedral distortions (Q1, Q2, and Q3), which couple to the Mn (eg) electrons. Our results show the ground state to be a type A antiferromagnetic (AFM) insulating state for the unstrained case, consistent with experiments. With tensile strain (stretching along the c axis), the ground state changes into a ferromagnetic and eventually into a type G′ AFM structure, while with compressive strain, we find the type A switching into a type G structure. The orbital ordering, which displays the well-known checkerboard x2-1/ y2-1 structure for the unstrained case, retains more or less the same character for compressive strain, while changing into the z2-1 character for tensile strains. While Q1and Q 3are fixed by the strain components εxx and εzz in our model, the magnitude of the in-plane distortion mode (Q2), which varies to minimize the total energy, slowly diminishes with tensile strain, completely disappearing as the FM state is entered. Within our model, the FM state is metallic, while the three AFM states are insulating. © 2010 The American Physical Society.

Magnetic and orbital order in LaMnO3 under uniaxial strain: A model study

We study the magnetic structure of the (LaMnO3) 2n / (SrMnO3) 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 Mn- eg electrons from the LaMnO3 side to the SrMnO3 side. This in turn affects the interfacial magnetism via the carrier-mediated Zener double exchange. For the n=1 superlattice, the Mn- eg electrons are more or less spread over the entire lattice so that the magnetic behavior is similar to the equivalent alloy compound La2/3 Sr1/3 MnO3. For larger n, the eg electron transfer occurs mostly between the two layers adjacent to the interface, thus leaving the magnetism unchanged and bulklike away from the interface region. © 2009 The American Physical Society.

Electronic and magnetic structure of the (LaMnO3) 2n / (SrMnO3) n superlattices

We study how strain affects orbital ordering and magnetism at the interface between SrMnO3 and LaMnO3 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 Mn t2g core spins and the ferromagnetic double exchange of the itinerant eg 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 MnO6 octahedron, splitting the Mn eg states into x2 - y2 and 3 z2 -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 x2 - y2 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 z2 -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 (LaMnO3) 1/(SrMnO3) 1 and (LaMnO3) 1/(SrMnO3) 3 superlattices for various strain conditions. © 2008 The American Physical Society.

Effects of strain on orbital ordering and magnetism at perovskite oxide interfaces: LaMnO3/SrMnO3

The possibility of formation of a fully spin-polarized 2D electron gas at the SrMnO3/(LaMnO3)1/SrMnO3 heterostructure is predicted from density-functional calculations. The La(d) electrons become confined in the direction normal to the interface in the electrostatic potential well of the positively charged layer of La atoms, acting as electron donors. These electrons mediate a ferromagnetic alignment of the Mn t2g spins near the interface via Zener double exchange and become, in turn, spin-polarized due to the internal magnetic fields of the Mn moments. © 2008 The American Physical Society.

Physical Review Letters

Spin-polarized two-dimensional electron gas at oxide interfaces

Density-functional electronic structure studies of a prototype interface between a paramagnetic metal and an antiferromagnetic (AFM) insulator (CaRuO3/CaMnO3) reveal the exponential leakage of the metallic electrons into the insulator side. The leaked electrons in turn control the magnetism at the interface via the ferromagnetic (FM) Anderson-Hasegawa double exchange, which competes with the AFM superexchange of the bulk CaMnO3. The competition produces a FM interfacial CaMnO3 layer (possibly canted); but beyond this layer, the electron penetration is insufficient to alter the bulk magnetism. © 2007 The American Physical Society.

Electron leakage and double-exchange ferromagnetism at the interface between a metal and an antiferromagnetic insulator: CaRuO3/CaMnO3

We have analyzed the electronic structure of half-metallic magnets based on first principles electronic structure calculations of a series of semi-Heusler alloys. The characteristic feature of the electronic structure of semi-Heusler systems is a d-d gap in the density of states lying at/close to the Fermi level depending on the number of valence electrons. We have employed various indicators of chemical bonding to understand the origin of the gap in these systems, which is crucial for their half-metallic property. The density of states of other half-metallic magnets also supports a gap and it is a generic feature of these systems. We have discussed in some details the origin of magnetism, in particular, how the presence of the gap is crucial to stabilize half-metallic ferro- and ferri-magnetism in these systems. Finally, we have studied the role of magnetic impurities in semiconducting semi-Heusler systems. We show with the aid of model supercell calculations that these systems are not only ferromagnetic but also half-metallic with possibly high Curie temperature. © 2005 Elsevier B.V. All rights reserved.

Computational Materials Science

Electronic structure of half-metallic magnets

We have studied in detail the electronic structure and magnetism in M (Mn and Cr)-doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn (XYxM1-xZ) in a wide concentration range using the local-spin density functional method in the framework of the tight-binding linearized muffin tin orbital method (TB-LMTO) and supercell approach. Our calculations indicate that some of these compounds are not only ferromagnetic but also half-metallic and may be useful for spintronics applications. The electronic structure of the doped systems is analysed with the aid of a simple model where we have considered the interaction between the dopant transition metal (M) and the valence band X-Z hybrid. We have shown that the strong X-d-M-d interaction places the M-d states close to the Fermi level with the M-t 2g states lying higher in energy in comparison to the M-eg states. Depending on the number of available d electrons, ferromagnetism is realized provided that the d manifold is partially occupied. The tendencies toward ferromagnetic (FM) or antiferromagnetic (AFM) behaviour are discussed within Anderson-Hasegawa models of super-exchange and double-exchange. In our calculations for Mn-doped NiTiSn, the strong preference for FM over AFM ordering suggests a possible high Curie temperature for these systems. © 2005 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Electronic structure and magnetism in doped semiconducting half-Heusler compounds

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.