Interplay of spin, charge, orbital and lattice degrees of freedom in oxide heterostructures results in a plethora of fascinating properties, which can be exploited in new generations of electronic devices with enhanced functionalities. The paradigm example is the interface between the two band insulators LaAlO3 and SrTiO3 that hosts a two-dimensional electron system. Apart from the mobile charge carriers, this system exhibits a range of intriguing properties such as field effect, superconductivity and ferromagnetism, whose fundamental origins are still debated. Here we use soft-X-ray angle-resolved photoelectron spectroscopy to penetrate through the LaAlO3 overlayer and access charge carriers at the buried interface. The experimental spectral function directly identifies the interface charge carriers as large polarons, emerging from coupling of charge and lattice degrees of freedom, and involving two phonons of different energy and thermal activity. This phenomenon fundamentally limits the carrier mobility and explains its puzzling drop at high temperatures.

Macroscopic ferroelectric order in α-GeTe with its noncentrosymmetric lattice structure leads to a giant Rashba spin splitting in the bulk bands due to strong spin-orbit interaction. Direct measurements of the bulk band structure using soft x-ray angle-resolved photoemission (ARPES) reveals the three-dimensional electronic structure with spindle torus shape. By combining high-resolution and spin-resolved ARPES as well as photoemission calculations, the bulk electronic structure is disentangled from the two-dimensional surface electronic structure by means of surface capping, which quenches the complex surface electronic structure. This unravels the bulk Rashba-split states in the ferroelectric Rashba α-GeTe(111) semiconductor exhibiting a giant spin splitting with Rashba parameter α R around 4.2 eVÅ, the highest of so-far known materials. DOI: 10.1103/PhysRevB.94.205111 In spintronics an important goal is to be able to control the spin of the electron in solids without applying magnetic fields [1,2]. The most promising mechanism is based on the Rashba effect [3] and the subsequent spin precession induced in such systems [4]. While most research has previously focused on 2D electron systems [5,6], recently a threedimensional (3D) form of such Rashba effect was found in a series of bismuth tellurohalides BiTeX (X = I, Br, or Cl) [7][8][9][10][11][12]. 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]. Therefore, ferroelectric [13] or multiferroic [19] Rashba semiconductors bring new multifunctional assets for spintronic devices. A crucial issue for the understanding of FERS is to disentangle the Rashba effect in the bulk caused by the bulk ferroelectric lattice distortion and surface effects arising from particular surface terminations and/or possible band bendings. 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.

Spin-and angle-resolved photoemission spectroscopy is used to reveal that a large spin polarization is observable in the bulk centrosymmetric transition metal dichalcogenide MoS 2 . It is found that the measured spin polarization can be reversed by changing the handedness of incident circularly polarized light. Calculations based on a three-step model of photoemission show that the valley and layer-locked spinpolarized electronic states can be selectively addressed by circularly polarized light, therefore providing a novel route to probe these hidden spin-polarized states in inversion-symmetric systems as predicted by Zhang et al. [Nat. Phys. 10, 387 (2014).]. DOI: 10.1103/PhysRevLett.118.086402 Transition metal dichalcogenide (TMDC) monolayers have been heavily investigated due to the locking of the spin with valley pseudospins and the presence of a direct gap, which makes them ideal candidates for valleytronic devices [1]. Thanks to the lack of inversion symmetry and the non-negligible spin-orbit coupling, TMDC monolayers also feature well-defined spin-polarized ground states [2], which can, in principle, be investigated by spin-and angleresolved photoemission spectroscopy (spin-ARPES). However, while few ARPES studies clearly observed the indirect to direct band gap transition going from the bulk crystal to the monolayer [3-6], spin-ARPES investigation of TMDC monolayers is more challenging, given the low cross section of photoemission from single layers.ARPES measurements on the bulk system are instead less demanding, but early studies detected spin-resolved signals only from TMDCs with broken inversion symmetry [7]. Interestingly, a recent theoretical study [8] suggested that the spin texture of the TMDC could be probed by photoemission, even in the inversion symmetric bulk TMDC crystals, as a result of the localization of two spin-degenerated valence band maxima on different layers of the unit cell and of the finite penetration depth of the photoemission process probing preferentially the uppermost layer. Experimentally this effect has been observed for WSe 2 , where the spin orbit (SO) coupling is the strongest one among the TMDCs [9].Here we show that a large out-of-plane spin-polarization is observable in the bulk dichalcogenide MoS 2 , and more importantly, that its sign depends on the handedness of the incident circularly polarized light. Our calculations, based on a three step model of the photoemission process demonstrate that the observed spin reversal is an initial state effect. Using left-(C L ) and right-handed (C R ) circularly polarized light results in selecting different initial states that present a positive or negative out-of-plane spin polarization depending on their localization on the S-Mo-S layers of the 2H-stacked MoS 2 unit cell. Our findings not only highlight the locking of the spin with layer and valley pseudospins in MoS 2 but also provide a novel and improved route, other than taking profit of the inelastic mean free path (IMFP) [8], for selectively probing hidden spin-polari...

The spinel/perovskite heterointerface γ-Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.The search for high-mobility two-dimensional electron systems (2DES) at atomically engineered transition metal oxide heterointerfaces is an ongoing endeavor, since the strong electronic correlations in partially occupied d-orbitals promise an even richer physical behavior than found in conventional semiconductor heterostructures [1]. However, the charge carrier mobilities in the most prominent complex oxide 2DES, found at the perovskite-perovskite heterointerface between the band insulators LaAlO 3 and SrTiO 3 , still fall short of those in semiconductors by several orders of magnitude [2]. The hitherto-highest mobility in SrTiO 3 -based structures (140,000 cm 2 /Vs at 2 K) is found at the spinel/perovskite heterointerface between γ-Al 2 O 3 thin films and SrTiO 3 [3, 4], thus making it a promising candidate for applications in oxide electronics or fundamental studies of quantum transport. The mechanism of 2DES formation in SrTiO 3 -based heterostructures has been under debate for many years.