Overview

­­My research has concentrated on theoretical and computational studies of the physics of phonons and electrons in crystalline solids, surfaces and nano-structures. I have collaborated with various physicists, both experimentalists and theorists, of international reputation. This has led to over 495 publicat­ions, including three postgraduate books entitled The Physics of Phonons (1990), Theoretical Modelling of Semiconductor Surfaces (1999) and Length-Scale Dependent Phonon Interactions (2014).

Phonon Engineering of Nanocomposite Thermoelectric Materials

Thermoelectricity (TE) is the process of generating either electricity from heat engines or heating devices from electricity. Examples of modern TE applications include portable refrigerators, beverage coolers, electronic component coolers, infrared sensing, etc. Possible future applications of TE devices include efficient conversion of waste heat (e.g. from waste and during powering of vehicles, etc) into usable energy, in improving the efficiency of photovoltaic cells, etc.

It is being realised that material choice with reduced dimensionality is a prudent strategy for increasing the TE figure-of-merit (ZT) relative to bulk values. It is further believed that significant enhancement in ZT can be achieved by producing huge reduction in the lattice (phonon) thermal conductivity by fabricating 'nanocomposites' (in the form of superlattices, nanowired embedded matrices of larger dimensions, or nanodots embedded in matrices of larger dimensions). We are making a systematic theoretical investigations of this aspect by considering Si-based, Bi2Te3-based and PbTe-based nanocomposites.

Nanophononic Solids

Our aim is to establish trends and criteria for the development of polarisation gaps as well as total band gaps in the phonon spectrum of 1D, 2D and 3D nanophononic semiconductor systems. These studies are made by employing a combination of the adiabatic bond charge model and the DFT-based first-principles pseudopotential method. We are developing a theory of phonon-defect and phonon-phonon interactions, and of phonon conductivity in such systems. These investigations are based on a combination of anharmonic elastic continuum theory, time-dependent perturbation theory, and a model relaxation time approach for the solution of the phonon Boltzmann equation. Project support: Leverhulme Trust.

Nanomaterials

We are working on studies of the structural (equilibrium atomic positions), electronic states, phonon modes, phonon interactions, and thermal conductivity of semiconductor nanomaterials, such as wires and superlattices. Theoretical methods employed include the adiabatic bond charge model, the DFT-based first-principles pseudopotential method, a model relaxation time theory for thermal conductivity.

Graphene Systems

We investigate equilibrium atomic geometry, stable structural shapes, electronic states, magnetic properties, phonon modes, and phonon lifetimes in graphene systes including monolayer graphene, bilayer graphene, multilayer graphene, graphite, and graphene nanoribbons. Theoretical methods include a combination of the DFT-based first-principles pseudopotential method, and an anharmonic elastic continuum theory. Collaborator: Dr. R. H. Miwa (University of Uberlandia, Brazil).

Solid Surfaces

We make first-principles studies of adsorption and reactions on solids surfaces. (a) Atomic geometry, electronic states and phonon dispersion relations of clean semiconductor and metal surfaces. Collaborator: Professor H. M. Tutuncu (Sakarya University, Turkey). (b) Adsorption of atoms on III-V(001) surfaces. (c) Molecular adsorption on Si(111). Collaborators: Professor A. B. McLean (Queen's University, Canada) and Dr. R. H. Miwa (University of Uberlandia, Brazil). (d) Molecular adsorption on Si(100). Collaborator: Dr. M. Shimomura (Shizuoka University, Japan). (e) Dilute magnetic systems - atomic adsorption on III-V(110). Collaborator: Dr. M. Migliorato (Manchester University).

Bulk crystals

First-principles studies of ground-state properties, electronic structure, lattice dynamics, electron-phonon interactions in the context of BCS theory of superconductivity. Collaborator: Professor H. M. Tutuncu (University of Sakarya, Turkey).

Previous Research

1970s:

Anharmonic phonon interactions; Theory of lattice thermal conductivity; Empirical pseudopotential method for electronic band structure of semiconductors; Pseudopotential theory of deep impurities in semiconductors.

1980s:

First-principles studies of atomic geometry, reconstrction and electronic structure of solids and semiconductor surfaces using the planewave pseudopotential method and the Density Function theory; Development of the pseudopotential-DFT and a simple GW theory and computer codes; Formulation and use of Broyden's Jacobian update method for accelerated self-consistent calculations.

1990s:

Development of theory and computer codes for lattice dynamical studies (i.e. phonon dispersion relations and phonon eigen displacements) of semiconductors and their surfaces using the adiabatic bond charge model and the first-principles pseudopotential method.

2000s:

First-principles studies of surface adsorption and reactions. First-principles studies of lattice dynamics using the pseudopotential method and the density functional scheme, to carry out full lattice dynamics of bulk, surfaces, and nanostructures. Application of our previously developed theory of anharmonic interactions and lattice thermal conductivity to nanostructures.

A brief description of my past research projects is available below.

• Physics of phonons
• Physics of electrons

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Further details of my research projects can be found at the page.

Education and Employment

• 1967 BSc (Physics, Mathematics, Chemistry) University of Gorakhpur, India
• 1969 MSc (Physics) Banaras Hindu Iniversity, Varanasi, India
• 1972 PhD (Physics) Banaras Hindu Iniversity, Varanasi, India
• 1980 DSc (Physics) Banaras Hindu Iniversity, Varanasi, India
• 1974 - 1976 University of Wales Research Fellow, UWIST, Cardiff
• 1976 - 1977 Research Associate, University of Newcastle
• 1977 - 1987 Lecturer in Physics, (New) University of Ulster, Coleraine
• 1987 - 1988 Senior Lecturer in Physics, University of Ulster, Coleraine
• 1988 - 1990 Senior Lecturer in Physics, University of Exeter
• 1990 - 2000 Reader in Theoretical Semiconductor Physics, University of Exeter
• 2000 - present Professor of Theoretical Condensed Matter Physics, University of Exeter
• Dec 1982 - Sept 1983 Sabbatical at Solar Energy Research Institute, Golden, Colorado, USA
• May 2004 - Sept 2004 Sabbatical at Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan

Professional Qualifications

• Fellow of the Institute of Physics; Chartered Physicist; Fellow of the Institute of Mathematics and its Applications; Chartered Mathematician

Funded Research Projects (since 2005)

"Tuning thermal transport in nanocomposites with size, shape and interface control", Leverhulme Trust, Sept 2016 - Feb 2020, £228,338

• "Phonon Engineering of Nanocomposite Thermoelectric Materials", EPSRC, Nov 2009 - Dec 2013, £326,408
• "International Conference on Transport and Optical Properties of Nanomaterials", Roy Soc, Jan 2009, £1,900
• "Vibrational and thermal properties of phononic crystals", Leverhulme Trust, Jan 2006 - Dec 2009, £92,634
• "Ab initio study of electrons and phonons in multiferroic BiFeO3", EPSRC, Oct 2006 - Sept 2007, £15,978
• "Ab initio investigations of phonon modes and electron-phonon interactions", EPSRC, Sept 2005 - Aug 2006, £23,689

Consultancy

• Wellcome Trust (2012): Phononic Devices Tech Review

Invited Presentations

Invited presentations since Jan 2008:

  "Thermal transport in nanocomposites" (Plenary talk), International conference CTMSE2019, Kolkata, 18 July 2019.

"Non-equilibrium phonon dynamics" (Invited talk), Hot carriers workshop, Kavli Royal Society International Centre, Chicheley Hall, Buckinghamshire, 1 Oct 2018.

"Theoretical studies of solid surfaces and interfaces", (Invited talk), International congress on the world of technology and advanced materials, Kirsehir, Turkey, 21 Sept 2018

"Theoretical ingredients for tuning thermoelectric properties of semiconducting materials" (Invited talk), EPSRC-UK TE workshop, University of manchester, 15 Feb 2017.

• "Tuning phonon properties for enhancement in thermoelectric figure of merit" (Invited talk), International Conference on Nanomaterials, Western University, London, Ontario, Canada, 14 Aug 2013
• "Phonons in Nanostructures" (Invited talk), Insulating Films on Semiconductors, Krakow, 25 June, 2013
• "Tuning phonon properties of nanostructures for future technological applications" (Invited talk), Computational materials Design and Engineering Workshop, Jodhpur, India, 8-10 Feb 2013
• "Theory of hot phonon effect in phononic-gap materials" (Invited talk),High Effeciency Materials for Photovoltaics Conference, Imperial College, London, 17-18 Sept, 2012.
• "Theory of phonon transport in nanocomposite materials" (Invited talk), Phonon School, University of Wisconsin, Madison, USA, May 21-22, 2012
• "Phonons: characeristics, and role in thermal and thermoelectric phenomena" (Invited talk), Phonons day, Warwick University, October 2011.
• "Theoretical analysis of semiconductor surface passivation by adsorption of alkaline-earth metals and chalcogens" (Invited talk), International Seminar on Semiconductor Surface Passivation, Sept 2011, Krakow, Poland
• "Theory of phonon transport in superlattices and nanowires" (Invited talk), CECAM workshop on Thermal and Electronic Transport at the Nanoscale, Lugano, Switzerland, 20-23 June, 2011
• "Theory of phonon transport in nanostructured semiconductors" (Invited talk), International conference on nanostructures and condensed matter theory, University of Western Ontario, London, Canada, 9-11 March 2011
• "Phonon Transport in Nanophononic Semiconductor" (Invited talk), Phonon Engineering Workshop, ICREA, Barcelona, Spain, 24-27 May 2010
• "Theory of Thermal Conductivity of Micro- and Nano-structured Materials" (Invited talk), MRS Sprint 2009 meeting, San Francisco, USA, April 2009
• "Atomic Theory of Phononic Gaps in Nano-patterned Semiconductors" (Invited talk), Int. Conf. on Transport and Optical Props. of Nanomaterials, Allahabad, India, Jan 2009
• "Theoretical modelling of surface phonons" (Invited talk), 6th Int. Conf. on Solid State Surface and Interfaces, Bratislava, Slovakia, Oct. 2008

Contributions to the Community

• Co-organiser, Symposium W 'Phonons in Nanomaterials - Theory, Experiments, and Applications', MRS Fall 2011 Meeting, Boston, USA
• Co-organiser, CECAM workshop 'Empirical methods in semiconductor nano-structures design and modelling', Manchester, UK, 2010
• Co-organiser, Symposium CC 'Phonon engineering for advanced materials solution - Theory and applications, MRS Fall 2009 Meeting, Boston, USA
• Scientific Advisory Committee member, VII International Workshop on Semiconductor Surface Passivation, Krakow, Poland, 2011
• Scientific Advisory Committee member, VI International Workshop on Semiconductor Surface Passivation, Poland, 2009

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Publications

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