Affiliated project

PhD student: Riccardo Reho

Project Leader: Zeila Zanolli

First-principles methods for superconducting and optical properties of quantum materials

I specialize in formulating and developing theoretical frameworks to elucidate quantum phenomena in materials, including superconductivity (Pillar 3) and light-matter interaction (Pillar 4). My contribution in spectroscopy focuses on examining the optical properties of two-dimensional (2D) multilayer structures formed by Transition Metal Dichalcogenides (TMDs) and topological insulators like Bi2Se3.

These multilayer 2D structures enhance the Coulomb interaction between charge carriers leading to strongly bound electron-hole (exciton) pairs. The physics of these excitons are both of fundamental interest and of crucial importance for engineering and exploiting the properties of these materials in potential applications, for example, in photovoltaic devices where the silicon era has reached its efficiency limit. My research revealed a pronounced excitonic shift due to twisting and strain for MoS2/WS2 and MoSe2/WSe2 heterostructures. .For the analysis of the optical spectra of these systems I implemented a method for computing the Photoluminescence (PL) spectra using the YAMBO code .[1,2].
Moreover, in collaboration with the group of D. Vanmaekelbergh, we have discovered that the higher-energy surface excitations in a 6 Quintuple Layer of Bi2Se3 are distinctive, preserving the polarization of incident light, which results in the creation of chiral excitons .[3].

In the realm of superconductivity, I investigate both conventional superconductors, which are described by the Bardeen-Cooper-Schrieffer (BCS) theory .[4] and function at extremely low temperatures, and unconventional superconductors, which exhibit high-temperature superconductivity from non-electron-phonon mechanisms. In the Quantum Materials by Design group we combined Bogoliubov-de-Gennes and Density Functional Theory (DFT) in a unified approach, Superconducting Density Functional Theory (SCDFT) .[5,6], and extended it to unconventional superconductors. We implemented and optimized a novel approach in the SIESTA code .[7,8].
I’ve applied this method to study complex heterostructures like PbTe/Pb, contributing evidence for induced superconductivity through proximity effects.

[1] R. Reho et al., in preparation (2024)
[2] D. Sangalli et al., J. Phys. Condens. Matter 31 325902 (2019).
[3] J. Vliem et al., in preparation (2024)
[4] J. Bardeen et al., Phys. Rev. 108, 1175(1957)
[5] M. Suvasini et al., Phys. Rev B 48, 1202 (1993)
[6] L. N. Oliveira et al., Phys. Rev. Letters 60 2430 (1988)
[7] García et al., J. Chem. Phys. 152 204108 (2020).
[8] R. Reho et al., in preparation (2023)

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