Banca de DEFESA: FUJUN LIU

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : FUJUN LIU
DATE: 03/03/2023
TIME: 09:00
LOCAL: IF
TITLE:

"Study on the Topological, Electrical and Optical Properties and Carrier Dynamics of 2D Materials"

KEY WORDS:

Topological properties, Excitons, Exciton-phonon coupling, 2D materials, Heterojunctions.

PAGES: 122
BIG AREA: Ciências Exatas e da Terra
AREA: Física
SUMMARY:

First, we propose an simple and general method to create robust topological nodal line semi-metal, by introducing periodic vacancies into the lattice. Normally the nodal lines are suppressed in the presence of spin orbit coupling, unless they are protected by some topological symetries. In this section, we turn a 2D material into a lattice with nonsymmorphic symmetry, by periotically removing selected atoms. As a case study, we investigate borophene (a honeycomb-like boron sheet). Pristine borophene is employed as the reference. While the Dirac cones of pristine borophene are gapped out by spin orbit coupling and magnetic exchange, robust nodal lines emerge in the energy spectrum of borophene with nonsymmorphic symmetry. We propose an effective 2D model and a symmetry analysis to demonstrate that these nodal lines are topological and protected by a nonsymmorphic glide plane. Our findings offer a paradigm shift to the design of nodal line semi-metals: instead of searching for nonsymmorphic materials, robust nodal line semi-metals may be realized simply by removing atoms from ordinary symmorphic crystals.

 Next, we study the the excitonic spectrum of 2D material and the heterostructure derivatives. Our approach takes into account the anomalous screening in two dimensions and the presence of a magnetic substrate. The Bethe-Salpeter equation is solved for a DFT-based tight-binding description of the single particle spectrum. We obtain the main features of the optical spectrum analyzing the localization in light zone of the excitonic peaks. We study the effect of different stacking forms on the main optical features of heterostructures, namely, the contributions of interlayer interaction, and the validity of valley polorizations. We compare the absolute magnitude of the linear optical conductivity.

 Finally, we investigate the inter-/intra- valley scattering of carriers in 2D transition metal dichalcogenide monolayers. A detailed analysis of the electron-phonon and exciton-phonon inter-/intra- valley scattering is described. The scattering of electrons at conduction bands are studied by several teams, but the scattering of holes at valence bands is never reported yet, due to the big energy difference induced by spin-orbit coupling. To solve this issue, we study the scattering possibility by exciton exchange effect between different valleys.

BANKING MEMBERS:
Externo à Instituição - DIEGO RABELO DA COSTA - UFC
Externa à Instituição - SABRINA MENDONÇA RUFO
Presidente - 3272139 - ALEXANDRE CAVALHEIRO DIAS
Interno - 407633 - DAVID LIMA AZEVEDO
Externa ao Programa - 1767433 - MARIANA MALARD SALES ANDRADE