An atomistic description of excitonic effects in the optical response of two-dimensional materials

• Autores: Juan José Esteve Paredes
• Directores de la Tesis: Juan José Palacios Burgos (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2025
• Idioma: inglés
• Número de páginas: 130
• Tribunal Calificador de la Tesis: Alejandro Molina-Sanchez (presid.), Francesca Maria Marchetti (secret.), Bartomeu Monserrat Sánchez (voc.), Julen Ibañez Azpiroz (voc.), María Reyes Calvo Urbina (voc.)
• Programa de doctorado: Programa de Doctorado en Física de la Materia Condensada, Nanociencia y Biofísica por la Universidad Autónoma de Madrid; la Universidad de Murcia y la Universidad de Oviedo
• Materias:
  • Física
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • The discovery of experimentally accessible two-dimensional (2D) materials has become one of the most groundbreaking accomplishments in materials science. These new materials include semiconductors with bandgaps that lie within the visible spectrum, some of which are ideal candidates for future optoelectronic device implementations. In this thesis, we mainly focus on studying the interaction of a system with an electromagnetic wave through two types of optical response functions, which are: (i) the absorptive properties, related to the linear conductivity, and (ii) the so-called bulk photovoltaic effect (BPVE), a nonlinear phenomenon in which incident light is directly converted into a flow of current within the material through a second-order quantum process.

    This dissertation is presented in two separate parts. In the first part, we start by reviewing state-of-the-art methods to calculate the ground state and excited-state properties of a system composed of electrons and static ions. We introduce, from first principles, the concepts of mean-field eigenstates, quasi-particle excitations, and electron-hole correlated states, namely excitons. With these foundations, the interaction of the system with a classical electric field is considered in the time domain to later extract the frequency-dependent conductivities at any order in a perturbative manner, also including the electron-hole interaction aspect. Additionally, we provide an analysis of fundamental operators, such as position, momentum, and velocity, and their treatment in a basis of Bloch states in an extended system.

    In the second part, we focus on the computational implementation of equations required to describe linear and second-order optical responses. We develop a computational approach to evaluate the frequency-dependent linear conductivity and the BPVE shift conductivity (and associated shift current), which accounts for the BPVE response to linearly polarized incident light. Within the independent-particle approximation (IPA), we develop a workflow to extract optical conductivities after obtaining the self-consistent Kohn-Sham solutions, in the case of using a local orbital expansion for Bloch states. Expanding in complexity, we study the case of including electron-hole interactions, namely excitons, on top of a single-particle solution. In this case, we start with a first-principles tight-binding description of the band structure, obtained by Wannier interpolation. Next, we solve the Bethe-Salpeter equation together with an effective electron-hole interaction kernel suitable for 2D samples. By exploiting the tight-binding form of Bloch states, the four-body Coulomb interaction is written in a computationally efficient way. The previous theoretical description is applied to the study of several 2D crystals. First, we study the non-interacting optical responses in graphene and hBN, which serve as benchmark studies for more sophisticated materials. Next, we explain our results for the optical conductivity and BPVE responses at the independent-particle level. Simulations of the interaction with light in the time domain are also addressed, with the inclusion of exciton effects. Finally, we study the excitonic optical responses of MoS2 and GeS monolayers. We compute the shift current response as a function of the frequency of incident light, as it would be measured in a photocurrent spectroscopy experiment. We show that excitonic effects red-shift the IPA response function in energy, generally increasing the shift current by nearly an order of magnitude. This occurs due to the appearance of a finite and sometimes dominant contribution at energies below the bandgap. Interestingly, we show that 2p-like excitons, dark in linear response, are bright in the second-order optical response and can therefore contribute to the BPVE.