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Register a new userRashba spin-splitting in ferroelectric oxides: from rationalizing to engineering
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Ferroelectric Rashba semiconductors (FERSC), in which Rashba spin-splitting can be controlled and reversed by an electric field, have recently emerged as a new class of functional materials useful for spintronic applications. The development of concrete devices based on such materials is, however, still hampered by the lack of robust FERSC compounds. Here, we show that the coexistence of large spontaneous polarisation and sizeable spin-orbit coupling is not sufficient to have strong Rashba effects and clarify why simple ferroelectric oxide perovskites with transition metal at the B-site are typically not suitable FERSC candidates. By rationalizing how this limitation can be by-passed through band engineering of the electronic structure in layered perovskites, we identify the Bi$_2$WO$_6$ Aurivillius crystal as the first robust ferroelectric with large and reversible Rashba spin-splitting, that can even be substantially doped without losing its ferroelectric properties. Importantly, we highlight that a unidirectional spin-orbit field arises in layered Bi$_2$WO$_6$, resulting in a protection against spin-decoherence.We highlight moreover that a unidirectional spin-orbit field arises in Bi$_2$WO$_6$, in which the spin-texture is so protected against spin-decoherence.
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In transition metal perovskites (ABO3) most physical properties are tunable by structural parameters such as the rotation of the BO6 octahedra. Examples include the Neel temperature of orthoferrites, the conductivity of mixed-valence manganites, or the band gap of rare-earth scandates. Since oxides often host large internal electric dipoles and can accommodate heavy elements, they also emerge as prime candidates to display Rashba spin-orbit coupling, through which charge and spin currents may be efficiently interconverted. However, despite a few experimental reports in SrTiO3-based interface systems, the Rashba interaction has been little studied in these materials, and its interplay with structural distortions remain unknown. In this Letter, we identify a bismuth-based perovskite with a giant, electrically-switchable Rashba interaction whose amplitude can be controlled by both the ferroelectric polarization and the breathing mode of oxygen octahedra. This particular structural parameter arises from the strongly covalent nature of the Bi-O bonds, reminiscent of the situation in perovskite nickelates. Our results not only provide novel strategies to craft agile spin-charge converters but also highlight the relevance of covalence as a powerful handle to design emerging properties in complex oxides.
Ferroelectric Rashba semiconductors (FERSC) are a novel class of multifunctional materials showing a giant Rashba spin splitting which can be reversed by switching the electric polarization. Although they are excellent candidates as channels in spin field effect transistors, the experimental research has been limited so far to semiconducting GeTe, in which ferroelectric switching is often prevented by heavy doping and/or large leakage currents. Here, we report that CsBiNb2O7, a layered perovskite of Dion-Jacobson type, is a robust ferroelectric with sufficiently strong spin-orbit coupling and spin texture reversible by electric field. Moreover, we reveal that its topmost valence bands spin texture is quasi-independent from the momentum, as a result of the low symmetry of its ferroelectric phase. The peculiar spin polarization pattern in the momentum space may yield the so-called persistent spin helix, a specific spin-wave mode which protects the spin from decoherence in diffusive transport regime, potentially ensuring a very long spin lifetime in this material.
In systems with broken inversion symmetry spin-orbit coupling (SOC) yields a Rashba-type spin splitting of electronic states, manifested in a k-dependent splitting of the bands. While most research had previously focused on 2D electron systems, recently a three-dimensional (3D) form of such Rashba-effect was found in a series of bismuth tellurohalides. Whereas these materials exhibit a very large spin-splitting, they lack an important property concerning functionalization, namely the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba-splitting coupled to ferroelectricity: the ferroelectric Rashba semiconductors (FERS). Using spin- and angle-resolved photoemission spectroscopy (SARPES) we show that GeTe(111) forms a prime member of this class, displaying a complex spin-texture for the Rashba-split surface and bulk bands arising from the intrinsic inversion symmetry breaking caused by the ferroelectric polarization of the bulk (FE). Apart from pure surface and bulk states we find surface-bulk resonant states (SBR) whose wavefunctions entangle the spinors from the bulk and surface contributions. At the Fermi level their hybridization results in unconventional spin topologies with cochiral helicities and concomitant gap opening. The GeTe(111) surface and SBR states make the semiconductor surface conducting. At the same time our SARPES data confirm that GeTe is a narrow-gap semiconductor, suggesting that GeTe(111) electronic states are endowed with spin properties that are theoretically challenging to anticipate. As the helicity of the spins in Rashba bands is connected to the direction of the FE polarization, this work paves the way to all-electric non-volatile control of spin-transport properties in semiconductors.
We report the observation of Shubnikov-de Haas (SdH) oscillations in single crystals of the Rashba spin-splitting compound BiTeI, from both longitudinal ($R_{xx}(B)$) and Hall ($R_{xy}(B)$) magnetoresistance. Under magnetic field up to 65 T, we resolved unambiguously only one frequency $F = 284.3pm 1.3$ T, corresponding to a Fermi momentum $k_{F} = 0.093pm 0.002$AA$^{-1}$.The amplitude of oscillations is strongly suppressed by tilting magnetic field, suggesting a highly two-dimensional Fermi surface. Combining with optical spectroscopy, we show that quantum oscillations may be consistent with a bulk conduction band having a Rashba splitting momentum $k_{R}=0.046pm$AA$^{-1}$.
The realization of multifunctional two-dimensional (2D) materials is fundamentally intriguing, such as combination of piezoelectricity with topological insulating phase or ferromagnetism. In this work, a Janus monolayer $mathrm{SrAlGaSe_4}$ is built from 2D $mathrm{MA_2Z_4}$ family with dynamic, mechanical and thermal stabilities, which is piezoelectric due to lacking inversion symmetry. The unstrained $mathrm{SrAlGaSe_4}$ monolayer is a narrow gap normal insulator (NI) with spin orbital coupling (SOC). However, the NI to topological insulator (TI) phase transition can be induced by the biaxial strain, and a piezoelectric quantum spin Hall insulator (PQSHI) can be achieved. More excitingly, the phase transformation point is only about 1.01 tensile strain, and nontrivial band topology can hold until considered 1.16 tensile strain. Moreover, a Rashba spin splitting in the conduction bands can exit in PQSHI due to the absence of a horizontal mirror symmetry and the presence of SOC. For monolayer $mathrm{SrAlGaSe_4}$, both in-plane and much weak out-of-plane piezoelectric polarizations can be induced with a uniaxial strain applied. The calculated piezoelectric strain coefficients $d_{11}$ and $d_{31}$ of monolayer $mathrm{SrAlGaSe_4}$ are -1.865 pm/V and -0.068 pm/V at 1.06 tensile strain as a representative TI. In fact, many PQSHIs can be realized from 2D $mathrm{MA_2Z_4}$ family. To confirm that, similar to $mathrm{SrAlGaSe_4}$, the coexistence of piezoelectricity and topological orders can be realized by strain (about 1.04 tensile strain) in the $mathrm{CaAlGaSe_4}$ monolayer. Our works suggest that Janus monolayer $mathrm{SrAlGaSe_4}$ is a pure 2D system for PQSHI, enabling future studies exploring the interplay between piezoelectricity and topological orders, which can lead to novel applications in electronics and spintronics.
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