Ph Thesis Defense | Javier Sivianes

Charge and spin photocurrents in topological systems, surfaces and unconventional magnets

This thesis investigates the bulk photovoltaic effect (BPVE) and related charge and spin optical responses as sensitive probes of electronic structure, band topology, and symmetry. By combining model Hamiltonians with first-principles calculations, three platforms are explored. First, an analysis of the Haldane model reveals a robust sign reversal of the shift photoconductivity at topological phase transitions, establishing an optical fingerprint of band inversion. Second, spin-split surface states bypass the inversion-symmetry constraint of the BPVE: an extended Rashba model quantitatively captures the response of Au(111), while Tl/Si(111) exhibits a shift photocurrent comparable to ferroelectrics alongside a tunable nonlinear spin photocurrent, opening routes for all-optical spintronics. Third, spin symmetries constrain the optical response of unconventional magnets, yielding experimentally verifiable predictions for altermagnets and angular fingerprints in Mn5Si3 that distinguish competing non-collinear spin configurations. Collectively, photocurrents encode rich information about quantum geometry, spin texture, and symmetry, positioning the BPVE as a versatile spectroscopic tool for optoelectronics and spintronics.