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Papers by Rmaheshkaran Mahesh

Research paper thumbnail of Large-scale computing with Quantum-Espresso

Nuovo Cimento C, Jan 1, 2009

This paper gives a short introduction to Quantum ESPRESSO: a distribution of software for atomist... more This paper gives a short introduction to Quantum ESPRESSO: a distribution of software for atomistic simulations in condensed-matter physics, chemical physics, materials science, and to its usage in large-scale parallel computing.

Research paper thumbnail of Interface structure and mechanics between graphene and metal substrates: a first-principles study

Journal of Physics: Condensed Matter, Jan 1, 2010

Graphene is a fascinating material not only for technological applications, but also as a test be... more Graphene is a fascinating material not only for technological applications, but also as a test bed for fundamental insights into condensed matter physics due to its unique two-dimensional structure. One of the most intriguing issues is the understanding of the properties of graphene and various substrate materials. In particular, the interfaces between graphene and metal substrates are of critical importance in applications of graphene in integrated electronics, as thermal materials, and in electromechanical devices. Here we investigate the structure and mechanical interactions at a graphene-metal interface through density functional theory (DFT)-based calculations. We focus on copper (111) and nickel (111) surfaces adhered to a monolayer of graphene, and find that their cohesive energy, strength and electronic structure correlate directly with their atomic geometry. Due to the strong coupling between open d-orbitals, the nickel-graphene interface has a much stronger cohesive energy with graphene than copper. We also find that the interface cohesive energy profile features a well-and-shoulder shape that cannot be captured by simple pair-wise models such as the Lennard-Jones potential. Our results provide a detailed understanding of the interfacial properties of graphene-metal systems, and help to predict the performance of graphene-based nanoelectronics and nanocomposites. The availability of structural and energetic data of graphene-metal interfaces could also be useful for the development of empirical force fields for molecular dynamics simulations.

Research paper thumbnail of The rutile TiO (110) surface: Obtaining converged structural properties from first-principles calculations

… of chemical physics, Jan 1, 2006

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate t... more We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first ...

Research paper thumbnail of Band gaps and dielectric constants of amorphous hafnium silicates: A first-principles investigation

Applied physics letters, Jan 1, 2007

[Applied Physics Letters 90 ... Molecular dynamics calculations and other numerical simulations (... more [Applied Physics Letters 90 ... Molecular dynamics calculations and other numerical simulations (condensed matter electronic structure) including Car–Parinello. ... we carry out ab initio molecular dynamics using the implementation provided in the QUANTUM-ESPRESSO package. ...

Research paper thumbnail of Influence of magnetism on phonons in CaFe_ {2} As_ {2} as seen via inelastic x-ray scattering

Physical Review B, Jan 1, 2009

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position... more In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe 2 As 2 using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.

Research paper thumbnail of Predicted High-Temperature Superconducting State in the Hydrogen-Dense Transition-Metal Hydride YH_ {3} at 40 K and 17.7 GPa

Physical review letters, Jan 1, 2009

Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivi... more Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivity at pressures which still lie beyond the reach of contemporary experimental techniques. Hydrogen-dense materials offer an opportunity to study related phenomena at experimentally achievable pressures. Here we report the prediction of high-temperature superconductivity in yttrium hydride (YH 3 ), with a T c of 40 K at 17.7 GPa, the lowest reported pressure for hydrogen-dense materials to date. Specifically, we find that the face-centered cubic structure of YH 3 exhibits superconductivity of different origins in two pressure regions. The evolution of T c with pressure follows the corresponding change of s-d hybridization between H and Y, due to an enhancement of the electron-phonon coupling by a matching of the energy level from Y-H vibrations with the peak of the s electrons from the octahedrally coordinated hydrogen atoms.

Research paper thumbnail of Unconventional Pairing Originating from the Disconnected Fermi Surfaces of Superconducting LaFeAsO_ {1-x} F_ {x}

Physical review letters, Jan 1, 2008

For a newly discovered iron-based high Tc superconductor LaFeAsO1−xFx, we have constructed a mini... more For a newly discovered iron-based high Tc superconductor LaFeAsO1−xFx, we have constructed a minimal model, where inclusion of all the five Fe d bands is found to be necessary. Random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.

Research paper thumbnail of First-principles codes for computational crystallography in the Quantum-ESPRESSO package

Zeitschrift für …, Jan 1, 2005

The Quantum-ESPRESSO package is a multipurpose and multi-platform software for ab-initio calculat... more The Quantum-ESPRESSO package is a multipurpose and multi-platform software for ab-initio calculations of condensed matter (periodic and disordered) systems. Codes in the package are based on density functional theory and on a plane wave/pseudopotential description of the electronic ground state and are ideally suited for structural optimizations (both at zero and at finite temperature), linear response calculations (phonons, elastic constants, dielectric and Raman tensors, etc.) and high-temperature molecular dynamics. Examples of applications of the codes included in the package are briefly discussed.

Research paper thumbnail of QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Journal of Physics: …, Jan 1, 2009

QUANTUM ESPRESSO is an integrated suite of computer codes for electronicstructure calculations an... more QUANTUM ESPRESSO is an integrated suite of computer codes for electronicstructure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). QUANTUM ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newlyrestructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Research paper thumbnail of Large-scale computing with Quantum-Espresso

Nuovo Cimento C, Jan 1, 2009

This paper gives a short introduction to Quantum ESPRESSO: a distribution of software for atomist... more This paper gives a short introduction to Quantum ESPRESSO: a distribution of software for atomistic simulations in condensed-matter physics, chemical physics, materials science, and to its usage in large-scale parallel computing.

Research paper thumbnail of Interface structure and mechanics between graphene and metal substrates: a first-principles study

Journal of Physics: Condensed Matter, Jan 1, 2010

Graphene is a fascinating material not only for technological applications, but also as a test be... more Graphene is a fascinating material not only for technological applications, but also as a test bed for fundamental insights into condensed matter physics due to its unique two-dimensional structure. One of the most intriguing issues is the understanding of the properties of graphene and various substrate materials. In particular, the interfaces between graphene and metal substrates are of critical importance in applications of graphene in integrated electronics, as thermal materials, and in electromechanical devices. Here we investigate the structure and mechanical interactions at a graphene-metal interface through density functional theory (DFT)-based calculations. We focus on copper (111) and nickel (111) surfaces adhered to a monolayer of graphene, and find that their cohesive energy, strength and electronic structure correlate directly with their atomic geometry. Due to the strong coupling between open d-orbitals, the nickel-graphene interface has a much stronger cohesive energy with graphene than copper. We also find that the interface cohesive energy profile features a well-and-shoulder shape that cannot be captured by simple pair-wise models such as the Lennard-Jones potential. Our results provide a detailed understanding of the interfacial properties of graphene-metal systems, and help to predict the performance of graphene-based nanoelectronics and nanocomposites. The availability of structural and energetic data of graphene-metal interfaces could also be useful for the development of empirical force fields for molecular dynamics simulations.

Research paper thumbnail of The rutile TiO (110) surface: Obtaining converged structural properties from first-principles calculations

… of chemical physics, Jan 1, 2006

We investigate the effects of constraining the motion of atoms in finite slabs used to simulate t... more We investigate the effects of constraining the motion of atoms in finite slabs used to simulate the rutile (110) surface in first-principles calculations. We show that an appropriate choice of fixing atoms in a slab eliminates spurious effects due to the finite size of the slabs, leading to a considerable improvement in the simulation of the (110) surface. The method thus allows for a systematic improvement in convergence in calculating both geometrical and electronic properties. The advantages of this approach are illustrated by presenting the first ...

Research paper thumbnail of Band gaps and dielectric constants of amorphous hafnium silicates: A first-principles investigation

Applied physics letters, Jan 1, 2007

[Applied Physics Letters 90 ... Molecular dynamics calculations and other numerical simulations (... more [Applied Physics Letters 90 ... Molecular dynamics calculations and other numerical simulations (condensed matter electronic structure) including Car–Parinello. ... we carry out ab initio molecular dynamics using the implementation provided in the QUANTUM-ESPRESSO package. ...

Research paper thumbnail of Influence of magnetism on phonons in CaFe_ {2} As_ {2} as seen via inelastic x-ray scattering

Physical Review B, Jan 1, 2009

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position... more In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe 2 As 2 using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin-polarized calculations imposing the antiferromagnetic order present in the low-temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.

Research paper thumbnail of Predicted High-Temperature Superconducting State in the Hydrogen-Dense Transition-Metal Hydride YH_ {3} at 40 K and 17.7 GPa

Physical review letters, Jan 1, 2009

Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivi... more Metallization in pure hydrogen has been proposed to give rise to high-temperature superconductivity at pressures which still lie beyond the reach of contemporary experimental techniques. Hydrogen-dense materials offer an opportunity to study related phenomena at experimentally achievable pressures. Here we report the prediction of high-temperature superconductivity in yttrium hydride (YH 3 ), with a T c of 40 K at 17.7 GPa, the lowest reported pressure for hydrogen-dense materials to date. Specifically, we find that the face-centered cubic structure of YH 3 exhibits superconductivity of different origins in two pressure regions. The evolution of T c with pressure follows the corresponding change of s-d hybridization between H and Y, due to an enhancement of the electron-phonon coupling by a matching of the energy level from Y-H vibrations with the peak of the s electrons from the octahedrally coordinated hydrogen atoms.

Research paper thumbnail of Unconventional Pairing Originating from the Disconnected Fermi Surfaces of Superconducting LaFeAsO_ {1-x} F_ {x}

Physical review letters, Jan 1, 2008

For a newly discovered iron-based high Tc superconductor LaFeAsO1−xFx, we have constructed a mini... more For a newly discovered iron-based high Tc superconductor LaFeAsO1−xFx, we have constructed a minimal model, where inclusion of all the five Fe d bands is found to be necessary. Random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.

Research paper thumbnail of First-principles codes for computational crystallography in the Quantum-ESPRESSO package

Zeitschrift für …, Jan 1, 2005

The Quantum-ESPRESSO package is a multipurpose and multi-platform software for ab-initio calculat... more The Quantum-ESPRESSO package is a multipurpose and multi-platform software for ab-initio calculations of condensed matter (periodic and disordered) systems. Codes in the package are based on density functional theory and on a plane wave/pseudopotential description of the electronic ground state and are ideally suited for structural optimizations (both at zero and at finite temperature), linear response calculations (phonons, elastic constants, dielectric and Raman tensors, etc.) and high-temperature molecular dynamics. Examples of applications of the codes included in the package are briefly discussed.

Research paper thumbnail of QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Journal of Physics: …, Jan 1, 2009

QUANTUM ESPRESSO is an integrated suite of computer codes for electronicstructure calculations an... more QUANTUM ESPRESSO is an integrated suite of computer codes for electronicstructure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). QUANTUM ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newlyrestructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively-parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and inter-operable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.