Giant Rashba‐Type Spin Splitting in Ferroelectric GeTe(111)

Abstract

A comprehensive mapping of the spin polarization of the electronic bands in ferroelectric α-GeTe(111) films has been performed using a time-of-flight momentum microscope equipped with an imaging spin filter that enables a simultaneous measurement of more than 10.000 data points (voxels). A Rashba type splitting of both surface and bulk bands with opposite spin helicity of the inner and outer Rashba bands is found revealing a complex spin texture at the Fermi energy. The switchable inner electric field of GeTe implies new functionalities for spintronic devices. The strong coupling of electron momentum and spin in low-dimensional structures allows an electrically controlled spin manipulation in spintronic devices [1-4], e.g. via the Rashba effect [5]. The Rashba effect has first been experimentally demonstrated in semiconductor heterostructures, where an electrical field perpendicular to the layered structure, i.e. perpendicular to the electron momentum, determines the electron spin orientation relative to its momentum [6-8]. An asymmetric interface structure causes the necessary inversion symmetry breaking and accounts for the special spin-splitting of electron states, the Rashba effect [5], the size of which can be tuned by the strength of the electrical field. For most semiconducting materials the Rashba effect causes only a quite small splitting of the order of 10 −2 ˚ A −1 and thus requires experiments at very low temperatures [9-11] and also implies large lateral dimensions for potential spintronic applications. A considerably larger splitting has been predicted theoretically [12] and was recently found experimentally for the surface states of GeTe(111) [13, 14]. GeTe is a ferroelectric semiconductor with a Curie temperature of 700 K. Thus, besides the interface induced Rashba splitting, the ferroelectric properties also imply a broken inversion symmetry within the bulk and thus would allow for the electrical tuning of the bulk Rashba splitting via switching the ferroelectric polarization [12, 15, 16]. This effect is of great interest for non-volatile spin orbitronics [10]. For GeTe a bulk Rashba splitting of 0.19Å19Å −1 has been predicted theoretically [12]. Experimentally, bulk-Rashba bands are rare and have only been found in the layered polar semiconductors BiTeCl and BiTeI [17-20] that, however, are not switchable. A characterization of the ferroelectric properties and a measurement of the spin polarization of the surface states of GeTe(111) at selected k-points has been performed previously by force microscopy [21, 22] and spin-resolved angular resolved photoemission spectroscopy, respectively [13]. A recent experimental and theoretical study revealed that at the Fermi level the hybridization of surface and bulk states causes surface-bulk resonant states resulting in unconventional spin topologies with chiral symmetry [14]. Here, we demonstrate the spin structure of surface and bulk bands of the GeTe(111) surface using the novel pho-toemission technique of spin-resolved time-of-flight momentum microsco...

Electronic origin of the high thermoelectric performance of GeTe among the p-type group IV monotellurides

“…The calculated band structures for GeTe are consistent with those in the available literature. [71][72][73][74][75][76][77][78][79] It is seen Figure 4 Temperature-dependent Seebeck coefficient and resistivity for Bi x Ge 1 − x Te with x = 0.075 (n H = 1.37 × 10 20 cm − 3 ) showing a transition from conduction by the Σ band in α-GeTe at To~550 K to that by both the L and Σ bands in β-GeTe at T4~625 K due to the phase transition. The insets show that the Fermi surface evolution is related to the transition.…”

Disentangling bulk and surface Rashba effects in ferroelectricα-GeTe

“…We show that the α-GeTe(111) surface exhibits a multitude of the surface and resonantly enhanced bulk states at the Fermi level that induces conductivity in the otherwise semiconducting α-GeTe thin films. In particular, we reveal that the previously reported bulklike Rashba state [16] is actually a surface resonance and thus cannot contribute to spin-charge conversion in realistic devices [18]. Quenching of these states is demonstrated by amorphous tellurium capping of the GeTe surface rendering data dominated by bulk effects.…”

“…This represents a major challenge for surface sensitive techniques such as angle-resolved photoemission (ARPES). For this reason up to now ARPES measurements on α-GeTe surfaces performed in the surface-sensitive UV regime have been dominated by surface effects [16,17] and clear information of the three-dimensional bulk electronic structure and its spin texture has not been obtained. …”

“…Although 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 so-called ferroelectric Rashba semiconductors (FERS) [13,14].Recent photoemission experiments on α-GeTe-the stable rhombohedral room temperature configuration of the GeTe phase change material [15]-indicate that this system is a hallmark candidate for entanglement of the ferroelectric and spin-orbit order [16,17]. Due to the giant Rashba splitting spin injection from magnetic systems into GeTe appears viable in order to achieve spin-to-charge conversion [18].…”

“…Recent photoemission experiments on α-GeTe-the stable rhombohedral room temperature configuration of the GeTe phase change material [15]-indicate that this system is a hallmark candidate for entanglement of the ferroelectric and spin-orbit order [16,17]. Due to the giant Rashba splitting spin injection from magnetic systems into GeTe appears viable in order to achieve spin-to-charge conversion [18].…”