Jawaharlal Nehru Centre for Advanced Scientific Research
Jakkur, Bangalore-560 064, India

Materials Theory Group


Areas of Research


 

Ferroelectrics


  • Prediction of 2-dimensional ferroelectricity (which is contraindicative) emerging at a metal-semiconductor phase transition in MoS2.
  • Demonstrating how anomalously large dielectric response of ferroelectric oxides arises from the inhomogeneously ordered states, for example, states with domain structure.
  • Development of a technique to derive Ginzburg-Landau theory of fluctuation-driven 1st order ferroelectric phase transition from first-principles, and pointing out corrections to the conventional Landau theory.
  • Determination of epitaxial strain-temperature phase diagrams of nano-thin films of ferroelectric BaTiO3 and PbTiO3, which guide experimentalists in design of devices based on ultra-thin ferroelectric films.
  • Prediction of lead-free ferroelectrics, BiAlO3 and BiGaO3, which was later verified experimentally in Japan.
  • Demonstrating how short-range chemical ordering gives rise to nano-polar regions that are responsible for diffuse phase transition and giant dielectric response of relaxor ferroelectrics.

 

Electronic Topological Materials


  • Demonstration of broken adiabaticity at the Dirac semi-metallic state at a transition from normal to topological insulating states, and developing a model to explain the observed Raman anomalies serving as bulk signature of the transition.
  • Prediction of strain-induced topological insulating state of As2Te3, and proposing its application as a charge pump.
  • Demonstration of surface multiferroic behavior in elemental Se as a consequence of its electronic topology and chirality.

 

Nano-scale Materials


  • Proposed symmetry dependent renormalization of phonons to explain the observed Raman spectra of MoS2 as a function of doping, useful in characterization of field effect transistors based on MoS2.
  • Determination of the structure of point and line defects in graphene and 2-dimensional BN, and providing phenomenological explanation of how they lead to buckling (line defects seen later in experiments).
  • Calculation of dynamical effects of e-phonon coupling to explain the observed Raman spectra of graphene as a function of doping, useful in characterization of graphene-based field effect transistors.
  • Provided theoretical basis to substitutional, molecular and electrochemical doping in graphene identifying their spectroscopic signatures.
  • Explained experiments on how few-layers or bilayers of graphene (rather than graphene) are effective in storage of hydrogen as a green fuel.

 

Mechanical Deformation of Materials


  • Introduced the idea of multi-scale hyperelasticity as a mechanism for size-dependent plasticity in nano-scale hcp metals. This has relevance to a number of light-weight (Mg, Ti) alloys used in a automobile and aerospace technologies.
  • Demonstrating how soft phonon modes are responsible for the observed mechanical failure in SiC to be used in power electronic devices.
  • Analyzed interfacial strength of nano-precipitates in Ni-Al superalloys used in making of blades of jet engines.

 

Multiferroics and Dilute Magnetic Semiconductors


  • Identified specific phonons that couple strongly with spins in multiferroic perovskite oxides.
  • Contributed to theoretical explanation of anomalous disparity in ferroelectric polarization observed in the bulk form and thin films of BiFeO3.
  • Showed how cationic disorder and spin-phonon coupling are responsible for magnetoelectric effects, interpreting the observed Raman spectra of AlFeO3.
  • Theory of magnetoelectric coupling observed in nano-particles of BaTiO3 in terms of coupling between magnetic ordering due to O vacancies at the surface and ferroelectricity at the core.
  • Provided evidence for a strong spin-phonon coupling in a double perovskite La2NiMnO6, establishing it to be the mechanism of its observed magnetocapacitance.
  • First-principles prediction of room temperature ferromagnetism in Co-Li co-doped ZnO, which was verified experimentally.

 

Materials for Energy and Environment


  • Uncovering mechanisms of high performance thermoelectrics based on metal-semiconductor superlattices and chalcogenides.
  • Identified a number of oxides and 2-dimensional materials suitable as photo-catalysts for splitting of water using solar energy.
  • Uncovered the mechanism of charge transfer in cathodes of Li ion batteries during charging and discharging operation.
  • Identified the rate limiting step in use of metal hydrides to store Hydrogen.
  • Determined the structural origin of activation of oxygen and enhanced catalytic activity of CeO2 with cation substitution, with applications to oxidation of CO and hydrocarbons.

 

Development of Formalism and Methods


  • Showed how Wannier functions of electrons in 1-dimensional crystal are eigenfunctions of non-Abelian Berry phase matrices.
  • Developed the formalism of phonon Wannier functions for construction of model Hamiltonian of structural phase transitions in crystals.
  • Developed a “polarization-stat” which can be used within molecular dynamics and thermodynamic integration to obtain free energies and Landau theory of ferroelectrics.
  • Contributed to generalization of the technique to model site occupancy disorder in crystals to grand-canonical ensembles.
  • Developed mixed-space molecular dynamics for efficient simulations of model hamiltonians of ferroelectrics (contributed to release of free software FERAM).

 

Pnictide Superconductors


    • Established the relevance of magnetic frustration and spin-phonon coupling to superconducting transition in Iron pnictides using theory and the observed Raman spectra.