Dynamics of lattice disorder in perovskite materials, polarization nanoclusters and ferroelectric domain wall structures

The nexus between classic ferroelectricity and the structure of perovskite materials hinges on the concept of lattice disorder. Although the ordered perovskites display short-range displacements of the central cations around their equilibrium points, the lattice disorder dynamically unfolds to generate a myriad of distorted rhombohedral lattices characterized by the hopping of the central cations across directions. It is discovered that the lattice disorder correlates with the emergence of minimum configuration energy pathways for the central cations, resulting in spatially modulated ultrafast polarization nanocluster arrangements that are stabilized by the electric charge defects in the material. Through high-resolution phonon dispersion analyses encompassing molecular dynamics (MD) and density functional theory (DFT) simulations, we provide unequivocal evidence linking the hopping of central cations to the development of diffuse soft phonon modes observed throughout the phase transitions of the perovskite. Through massive MD simulations, we unveil the impact of lattice disorder on the structures of domain walls at finite-temperature vis-à-vis collective activation and deactivation of pathways. Furthermore, our simulations demonstrate the development of hierarchical morphotropic phase boundary (MPB) nanostructures under the combined influence of externally applied pressure and stress relaxation, characterized by sudden emergence of zig-zagged monoclinic arrangements that involve dual shifts of the central cations. These findings have implications for tailoring MPBs in thin-film structures and for the light-induced mobilization of DWs. Avenues are finally uncovered to the exploration of lattice disorder through gradual shear strain application.Peer ReviewedPostprint (published version

Nature of the metallic and in-gap states in Ni-doped SrTiO3_3

Epitaxial thin films of SrTiO$_3$(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy (ARPES) at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band (VB) of pristine SrTiO$_3$ towards the Fermi level (p-doping) and reduces its band gap. This is accompanied by an upward energy shift of the Ti t2g-derived mobile electron system (MES). Thereby, the in-plane dxy-derived bands reduce the embedded electron density, as evidenced by progressive reduction of their Fermi momentum with the Ni concentration, and the out-of-plane dxz/yz-derived bands depopulate, making the MES purely two-dimensional. Furthermore, the Ti and Ni L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum, and decipher the full spectrum of the VO-induced in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES and in-gap states in Ni-doped SrTiO$_3$ may help development of this material towards its device applications associated with the reduced optical band gap

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

The ferroelectric semiconductor $\alpha$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $\alpha$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature.Comment: 26 pages, 8 figure

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe2_2

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides