Roxana Margine - Academia.edu (original) (raw)
Papers by Roxana Margine
Bulletin of the American Physical Society, Mar 23, 2011
Physical Review Research, 2021
Journal of Physics: Condensed Matter, 2021
Designing materials with advanced functionalities is the main focus of contemporary solid-state p... more Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture...
Computer Physics Communications, 2016
The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses... more The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses densityfunctional perturbation theory and maximally localized Wannier functions for computing electron-phonon couplings and related properties in solids accurately and efficiently. The EPW v4 program can be used to compute electron and phonon self-energies, linewidths, electron-phonon scattering rates, electron-phonon coupling strengths, transport spectral functions, electronic velocities, resistivity, anisotropic superconducting gaps and spectral functions within the Migdal-Eliashberg theory. The code now supports spin-orbit coupling, time-reversal symmetry in noncentrosymmetric crystals, polar materials, and k and q-point parallelization. Considerable effort was dedicated to optimization and parallelization, achieving almost a ten times speedup with respect to previous releases. A computer test farm was implemented to ensure stability and portability of the code on the most popular compilers and architectures. Since April 2016, version 4 of the EPW code is fully integrated in and distributed with the Quantum ESPRESSO package, and can be downloaded through QE-forge at http://qe-forge.org/gf/project/q-e.
Scientific Reports, 2016
Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate ... more Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets.
Raman scattering is used to probe the charge transfer distribution in Bromine-doped double-walled... more Raman scattering is used to probe the charge transfer distribution in Bromine-doped double-walled carbon nanotubes (DWNT). Using 1064 nm and 514.5 nm laser excitation we are able to study the charge-transfer sensitive phonons in the inner ( (5,5)) and outer ( (10,10)) tubes of the double-walled pair. The experimental results are compared to our tight binding band structure calculations that
Science, 2012
Moving Dislocations The mechanical properties of crystalline materials are limited by the presenc... more Moving Dislocations The mechanical properties of crystalline materials are limited by the presence and motion of defects caused by extra or missing atoms in the crystal lattice. Plastic deformation of a material causes these defects, known as dislocations, to move and multiply. Much is known about the motion of dislocations in three dimensions but less so in two. Warner et al. (p. 209 ; see the Perspective by Bonilla and Carpio ) used graphene as a model material to track dislocation dynamics in real time. The strain fields in the graphene sheet were mapped, which suggests that the dislocation motion is connected to the stretching, rotating, and breaking of individual carbon bonds.
The doping dependence of the high-frequency Raman-active modes in single-walled semiconducting ca... more The doping dependence of the high-frequency Raman-active modes in single-walled semiconducting carbon nanotubes is studied by density functional theory. We find that the A1g longitudinal mode in (3*n+1,0) zigzag tubes shows a small anomalous upshift, followed by a large downshift under electron doping. This doping-induced stiffening of the A1g mode is related to the large anharmonicity of the mode. Connections are made to recent experiments in the group of P. C. Eklund.
Physical Review B, 2013
We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation base... more We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation based on maximally-localized Wannier functions, in order to perform reliable and highly accurate calculations of the anisotropic temperature-dependent superconducting gap and critical temperature of conventional superconductors. Compared with the widely used McMillan approximation, our methodology yields a more comprehensive and detailed description of superconducting properties, and is especially relevant for the study of layered or low-dimensional systems as well as systems with complex Fermi surfaces. In order to validate our method we perform calculations on two prototypical superconductors, Pb and MgB2, and obtain good agreement with previous studies.
Bulletin of the American Physical Society, Mar 22, 2011
We have identified a synthesizable candidate FeB4 material with a potential for conventional supe... more We have identified a synthesizable candidate FeB4 material with a potential for conventional superconductivity at 15-20 K [1, 2]. The strong electron-phonon coupling in the proposed material is unexpected as the recently discovered iron-based superconductors are considered to display an unconventional pairing mechanism. The new nonmagnetic ground state crystal structure has been predicted with an ab initio evolutionary search [3] and shown to be marginally stable at ambient pressures.\\[4pt][1] AN Kolmogorov, S. Shah, ER ...
Bulletin of the American Physical Society, Mar 23, 2011
APS Bulletin of the American Physical Society. APS March Meeting 2011 Volume 56, Number 1. Monday... more APS Bulletin of the American Physical Society. APS March Meeting 2011 Volume 56, Number 1. Monday–Friday, March 21–25, 2011; Dallas, Texas. ...
Recent experiments show that alkali atoms can induce a graphitization of nanoporous carbon. We st... more Recent experiments show that alkali atoms can induce a graphitization of nanoporous carbon. We study the effect theoretically in a representative model of a fully sp2 connected defect (a ``wormhole'') in a carbon network. We investigate possible mechanisms of the wormhole annihilation in the presence of impurities. Variations in charge transfer and partial covalency between different alkali atoms leads to
Bulletin of the American Physical Society, Mar 23, 2011
Physical Review Research, 2021
Journal of Physics: Condensed Matter, 2021
Designing materials with advanced functionalities is the main focus of contemporary solid-state p... more Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture...
Computer Physics Communications, 2016
The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses... more The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that uses densityfunctional perturbation theory and maximally localized Wannier functions for computing electron-phonon couplings and related properties in solids accurately and efficiently. The EPW v4 program can be used to compute electron and phonon self-energies, linewidths, electron-phonon scattering rates, electron-phonon coupling strengths, transport spectral functions, electronic velocities, resistivity, anisotropic superconducting gaps and spectral functions within the Migdal-Eliashberg theory. The code now supports spin-orbit coupling, time-reversal symmetry in noncentrosymmetric crystals, polar materials, and k and q-point parallelization. Considerable effort was dedicated to optimization and parallelization, achieving almost a ten times speedup with respect to previous releases. A computer test farm was implemented to ensure stability and portability of the code on the most popular compilers and architectures. Since April 2016, version 4 of the EPW code is fully integrated in and distributed with the Quantum ESPRESSO package, and can be downloaded through QE-forge at http://qe-forge.org/gf/project/q-e.
Scientific Reports, 2016
Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate ... more Using the ab initio anisotropic Eliashberg theory including Coulomb interactions, we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported superconducting Ca-intercalated bilayer graphene. We find that C6CaC6 can support phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data. Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing and that it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi surface pockets.
Raman scattering is used to probe the charge transfer distribution in Bromine-doped double-walled... more Raman scattering is used to probe the charge transfer distribution in Bromine-doped double-walled carbon nanotubes (DWNT). Using 1064 nm and 514.5 nm laser excitation we are able to study the charge-transfer sensitive phonons in the inner ( (5,5)) and outer ( (10,10)) tubes of the double-walled pair. The experimental results are compared to our tight binding band structure calculations that
Science, 2012
Moving Dislocations The mechanical properties of crystalline materials are limited by the presenc... more Moving Dislocations The mechanical properties of crystalline materials are limited by the presence and motion of defects caused by extra or missing atoms in the crystal lattice. Plastic deformation of a material causes these defects, known as dislocations, to move and multiply. Much is known about the motion of dislocations in three dimensions but less so in two. Warner et al. (p. 209 ; see the Perspective by Bonilla and Carpio ) used graphene as a model material to track dislocation dynamics in real time. The strain fields in the graphene sheet were mapped, which suggests that the dislocation motion is connected to the stretching, rotating, and breaking of individual carbon bonds.
The doping dependence of the high-frequency Raman-active modes in single-walled semiconducting ca... more The doping dependence of the high-frequency Raman-active modes in single-walled semiconducting carbon nanotubes is studied by density functional theory. We find that the A1g longitudinal mode in (3*n+1,0) zigzag tubes shows a small anomalous upshift, followed by a large downshift under electron doping. This doping-induced stiffening of the A1g mode is related to the large anharmonicity of the mode. Connections are made to recent experiments in the group of P. C. Eklund.
Physical Review B, 2013
We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation base... more We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation based on maximally-localized Wannier functions, in order to perform reliable and highly accurate calculations of the anisotropic temperature-dependent superconducting gap and critical temperature of conventional superconductors. Compared with the widely used McMillan approximation, our methodology yields a more comprehensive and detailed description of superconducting properties, and is especially relevant for the study of layered or low-dimensional systems as well as systems with complex Fermi surfaces. In order to validate our method we perform calculations on two prototypical superconductors, Pb and MgB2, and obtain good agreement with previous studies.
Bulletin of the American Physical Society, Mar 22, 2011
We have identified a synthesizable candidate FeB4 material with a potential for conventional supe... more We have identified a synthesizable candidate FeB4 material with a potential for conventional superconductivity at 15-20 K [1, 2]. The strong electron-phonon coupling in the proposed material is unexpected as the recently discovered iron-based superconductors are considered to display an unconventional pairing mechanism. The new nonmagnetic ground state crystal structure has been predicted with an ab initio evolutionary search [3] and shown to be marginally stable at ambient pressures.\\[4pt][1] AN Kolmogorov, S. Shah, ER ...
Bulletin of the American Physical Society, Mar 23, 2011
APS Bulletin of the American Physical Society. APS March Meeting 2011 Volume 56, Number 1. Monday... more APS Bulletin of the American Physical Society. APS March Meeting 2011 Volume 56, Number 1. Monday–Friday, March 21–25, 2011; Dallas, Texas. ...
Recent experiments show that alkali atoms can induce a graphitization of nanoporous carbon. We st... more Recent experiments show that alkali atoms can induce a graphitization of nanoporous carbon. We study the effect theoretically in a representative model of a fully sp2 connected defect (a ``wormhole'') in a carbon network. We investigate possible mechanisms of the wormhole annihilation in the presence of impurities. Variations in charge transfer and partial covalency between different alkali atoms leads to