Unprecedentedly high-quality ARPES experiments have been performed at the URANOS beamline and revealed the band and spin behavior in a semiconductor undergoing a ferroelectric transition. The temperature dependent measurements unraveled the impact of the ferroelectric phase transition on the electronic spin texture.

Ferroelectric Rashba semiconductor are a sought-after new class of materials due to their remarkable properties. Their spin texture is controllable by an external electric field, which makes these multifunctional materials highly attractive for spintronics technology like spin-FET transistors or bipolar memories. However, their experimental demonstrations are still rare and elusive.
A ferroelectric phase results in a permanent electric polarization intrinsic to the material, which lifts the spin degeneracy of the electronic bands. This spin splitting is called the Rashba effect and has been directly observed using the ARPES setup of the URANOS beamline (see Fig. 1). Temperature dependent measurements (10 to 300 K) have revealed the emergent Rashba splitting when temperature is decreased below the critical temperature of the phase transition. In this way, the ferroelectric phase of the Pb_1-xGe_xTe compounds have been fully probed in a wide range of composition (x=0 to 10 %). Remarkably, the ferroelectric behavior is seen to persist down to very thin film thickness (8 nm).
The high-quality ARPES images have allowed for an accurate quantitative evaluation of the Rashba spin-texture in the ferroelectric phase and its evolution with temperature. The order parameter of the phase transition is defined in relation to the spin splitting, and is determined when ARPES is compared to a k.p model developed for ferroelectric quantum wells. 
The Rashba splitting, which defined the spin degeneracy lifting or the degree of spin polarization in the structure, has been determined as high as 2 eV.Å in these compounds, which is comparable to state-of-the-art ferroelectric Rashba semiconductors like GeTe or SnTe. Moreover, the Pb_1-xGe_xTe system is better suited for the perspectives of device applications, because it shows a room-temperature critical temperature, a low carrier concentration that prevents from carrier leakage when subject to an external electric field, and a direct optical gap. This work highlights the outstanding electronic and spin properties of ferroelectric Rashba semiconductor by direct observations, and that ultra-high definition ARPES is a powerful technique to discover and study new ferroelectric Rashba materials.

Figure 1. Temperature dependent ARPES spectra measured on a Pb_0.93Ge_0.07Te 8 nm quantum well. The band degeneracy lifting is clearly observed above a critical temperature Tc, which indicates the ferroelectric phase transition. The Rashba effect is seen to increase as temperature is decreased.

Author: Gauthier Krizman

Link to the publication : G. Krizman, T. Zakusylo, L. Sajeev, M. Hajlaoui, T. Takashiro, M. Rosmus, N. Olszowska, J. Kołodziej, G. Bauer, O. Caha, G. Springholz, A Novel Ferroelectric Rashba Semiconductor, Advanced Materials, 2310278(2023), https://doi.org/10.1002/adma.202310278