Photoemission Studies of Ferroelectric Rashba Semiconductors—From Static to Transient Properties

Ferroelectric Rashba semiconductor are bulk materials with noncentrosymmetric atomic structures in their ferroelectric phase combined with a significant spin-orbit interaction, leading to a strong coupling between spins, electrons and the underlying lattice. These properties are directly imprinted on their electronic structure through the occurrence of a giant Rashba splitting in their valence band. This gives the possibility of studying their paraelectric-to-ferroelectric phase transition in the momentum space with angle-resolved photoemission spectroscopy (ARPES). α-GeTe and α-SnTe are two prime examples of such a class of materials. Moreover, it has been recently demonstrated that it is possible to reversibly manipulate the spin polarization in GeTe by an external electric field, a promising behavior for spintronics applications.<br/>A stimulating direction of research is now to investigate how it is possible to modify the ferroelectric properties of these materials with temperature or light pulses.<br/><br/>In this talk, I will start with the case of SnTe and will show high-energy-resolution ARPES data acquired as a function temperature across its ferroelectric phase transition. Our excellent experimental resolution and unprecedented thin film quality allow us to follow the evolution of the Rashba splitting induced by the ferroelectric distortion in the bulk bands and to reveal drastic deviations from a mean-field-like transition. I will discuss how the persistence of a splitting at room temperature supports an order-disorder type of transition and will elaborate on the implications for the topological nature of surface states in SnTe.<br/><br/>In a second part, I will move to the case of GeTe. Upon photoexcitation, we have driven GeTe out-of-equilibrium and probed its transient low-energy electronic structure with time-resolved ARPES. As a consequence, the Rashba splitting of its bulk states is enhanced after 200 fs, a very surprising result, especially given that bulk-sensitive time-resolved experiments have only led to a suppression of ferroelectric upon photoexcitation. In addition, our data show a coherent modulation of the Rashba splitting with a frequency that corresponds to the amplitude mode of the ferroelectric distortion. I will explain what is the physical mechanism behind this unexpected behavior.