Excitons
Many-body effects on electronic and optical properties
My Ph. D. research was focused on the optical properties of low-dimensional systems using an ab initio approach. The method employed was taken from solid-state physics and was based on density functional theory plus many-body Green's function theory. In particular, we used the, so-called, GW approximation and the Bethe-Salpeter equation. I worked directly on the implementation of these methods and then used them to study the properties of conjugated polymers and carbon nanotubes. I also started the development of a package for GW and Bethe-Salpeter calculation which was later completed by Layla Martin-Samos and released with an open source licence.
Excitonic effects in conjugated polymers: the role of interchain interaction
A. Ruini, M. J. Caldas, G. Bussi, and E. Molinari,
Solid state effects on exciton states and optical properties of PPV,
Phys. Rev. Lett. 88, 206403 (2002)
Preprint: arXiv:cond-mat/0203471
G. Bussi, A. Ruini, E. Molinari, M. J. Caldas, P. Puschnig, and C. Ambrosch-Draxl,
Interchain interaction and Davydov splitting in polythiophene crystals: an ab initio approach,
Appl. Phys. Lett. 80, 4118 (2002)
G. Bussi, A. Ferretti, A. Ruini, M. J. Caldas, and E. Molinari,
Optics and transport in conjugated polymer crystals: interchain interaction effects,
Adv. Solid State Phys. 43, 313 (2003)
In these works we investigate the role of excitons in the optical properties of conjugated polymers, with a special focus on the effects of interchain coupling.
Excitonic effects in carbon nanotubes: exploiting the symmetry
E. Chang, G. Bussi, A. Ruini, and E. Molinari, Excitons in carbon nanotubes: an ab initio symmetry-based approach, Phys. Rev. Lett. 92, 196401 (2004) Preprint: arXiv:cond-mat/0405185E. Chang, G. Bussi, A. Ruini, and E. Molinari, First-principles approach for the calculation of optical properties of one-dimensional systems with helical symmetry: the case of carbon nanotubes, Phys. Rev. B 72, 195423 (2005)In these works we develop and use a new method which allows to completely exploit the roto-translational symmetry of nanotubes. In this way, any nanotube can be described using a unit cell with only two atoms. Moreover, selection rules arise which simplify the calculation of optical properties and the interpretation of the results. With this method we study a chiral tube (4,2) and we find that the excitonic effects are crucial.
Quantum interference in the Raman spectrum of carbon nanotubes
G. Bussi, J. Menéndez, J. Ren, M. Canonico, and E. Molinari, Quantum interferences in the Raman cross section for the radial breathing mode in metallic carbon nanotubes, Phys. Rev. B 71, 041404R (2005) Preprint: arXiv:cond-mat/0408583 Here we investigate, in collaboration with an experimental group, the coupling of electronic structure with the radial breathing mode. We discover that the sign of the derivative of the electronic transitions with respect to the breathing mode can have a different sign in different transitions, with implications on the analysis of resonant Raman experiments.
Lecture notes on the Bethe-Salpeter equation
G. Bussi,
Effects of the electron-hole interaction on the optical properties of materials: the Bethe-Salpeter equation,
These are notes that I wrote when I was invited to give a lecture at the EXC!TING summer school in Riksgränsen, Sweden.
An open source package for self-energy and excitons
L. Martin-Samos and G. Bussi,
SaX: An open source package for electronic-structure and optical-properties calculations in the GW approximation,
Comp. Phys. Comm. 180, 1416 (2009)
Sax is a package to perform GW and Bethe-Salpeter calculations at different degrees of accuracy. It uses as starting point the DFT wavefunctions calculated using a plane-wave code. Presently, it is interfaced with the Quantum-ESPRESSO package.
Complex band structure of organic polymers
A. Ferretti, G. Mallia, L. Martin-Samos, G. Bussi, A. Ruini, B. Montanari, and N. M. Harrison,
Ab initio complex band structure of conjugated polymers: Effect of hybrid DFT and GW schemes,
Phys. Rev. B 85, 235105 (2012)
Preprint: arXiv:1205.3282
In this paper, we present the calculation of the complex band structure for organic polymers using a variety of computational schemes, including standard local, semilocal, and hybrid-exchange density functionals, and many-body perturbation theory within the GW approximation.
