Absence of giant spin splitting in the two-dimensional electron liquid at the surface ofSrTiO3(001)

Abstract

We reinvestigate the putative giant spin splitting at the surface of SrTiO3 reported by SantanderSyro et al. [Nature Mat. 13, 1085]. Our spin-and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is fully consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.Two-dimensional electron liquids (2DELs) formed at the interfaces between insulating transition metal oxides are important for the rapidly growing field of oxide electronics. Their potential utility lies in their exotic responses to external fields which, for the prototypical case of the LaAlO 3 /SrTiO 3 (LAO/STO) interface, includes gate-tunable superconductivity 1,2 possibly coexisting with magnetism 3 , and gate-tunable Rashba interaction [4][5][6] . It has been shown that STO can support such a two-dimensional electron liquid in many other configurations, for example when interfaced with amorphous LAO 7 , by electrolyte gating 8 or by reduction of the bare surface by UV radiation 9,10 or Al capping 11 . Irrespective of their origin, all these systems show a similar electronic structure with multiple subbands and a characteristic orbital polarization, commonly understood as a consequence of quantum confinement of the Ti t 2g states in a potential well induced by band bending 9,10,12-17 .Ab initio density functional theory (DFT) of both interface and surface geometries predicts an unconventional Rashba-like spin splitting of these quantum confined subbands due to broken inversion symmetry and the interplay of orbital and spin degrees of freedom. The lifting of spin degeneracy is found to be of the order of ∼ 1 meV at the Fermi surface except in the vicinity of avoided crossings of subbands with different orbital character where it can be enhanced by almost an order of magnitude 15,[18][19][20][21][22][23] . The resulting k-space spin texture is complex and has not yet been observed experimentally. However, the magnitude and carrier density dependence of the Rashba splitting inferred from transport and quantum oscillation experiments 4,5,24,25 is in good agreement with these calculations.Recently, a completely different interpretation of the basic electronic structure of the 2DEL at the (001) Fig. 1) arise from a single band with a giant Rashba splitting of approximately 100 meV at the chemical potential. In order to reconcile this claim with the large subband splitting at the Γ point that is well established from high-resolution angle-resolved photoemission spectroscopy (HR-ARPES) 9-11,13,15 , Santander-Syro et al. propose the existence of strong ferromagnetism with significant out-of-plane moments. To date, a Rashba splitting of this magnitude has not been reproduced experimentally or explained theoretically 22,23,27 . More...

Dichotomy of the photo-induced 2-dimensional electron gas on SrTiO 3 surface terminations

“…Recent experiments suggest the scenario with itinerant charge coming solely from the topmost SrO layer is at least plausible 12,15 . A higher oxygen vacancy formation rate has been observed in fractured STO crystals compared to annealed STO wafers, which suggests exposure of the SrO termination can increase the vacancy formation rate 15 . Additionally, modeling x-ray photoelectron spectroscopy data of the mixed Ti 3+ /Ti 4+ state fits a scenario with all the itinerant charge isolated to the topmost termination layer, but the authors of this study ultimately dismiss this model as nonphysical 12 .…”

“…The 2DEG on the surface of STO has been heavily studied using angle resolved photoemission spectroscopy (ARPES) 9-12, 14, 15 . Measurements have been carried out on the (001), (110) 16 and (111) 17 More generally, perovskite oxides are typically divided into sub-unit cell layers that play different roles, like an active layer that drives the physical phenomena of interest sandwiched between passive doping layers.…”

A spin–orbit playground: surfaces and interfaces of transition metal oxides

“…Based on the analysis of the data it was concluded that the Rashba spin-orbit energy was on the order of 10 meV. There was some debate about the linear versus cubic nature of the effect, but it was suggested by Baumberger et al [54,55] that the size and momentum dependence of the surface spin-orbit term can be tied to the proximity of E F to avoided crossing points of the heavy and light bands. Surface rumpling and atomic displacements imply that the d xz /d yz orbitals now have a finite, oxygen mediated, overlap with the d xy orbitals [46,47].…”

“…Their analysis of magnetotransport led them to advocate a cubic Rashba term as the cause of the observed weak antilocalization behavior with a strength of about 0.5 meV, one order of magnitude less than reported in the above measurements. The low value of the carrier concentration (3 -8×10 12 cm −2 ) could explain both the cubic-like term and the discrepancy in the determination of its size [55]. The tunability of the Rashba contribution as well as its magnitude allows one to envisage spintronic applications, exploiting for example the so-called inverse Edelstein effect [44].…”

High‐temperature 2D Fermi surface of SrTiO3 studied by energy‐filtered PEEM

“…The electronic structure has been revealed by angle‐resolved photoelectron spectroscopy (ARPES). The highest spectroscopic resolution is obtained at low temperature, typically 10 to 20 K . This allows to resolve 2D and 3D band character, calculate the Fermi surface (FS) volume, and estimate the electron effective mass.…”