Quasiparticle dynamics and spin–orbital texture of the SrTiO3 two-dimensional electron gas

Abstract

Two-dimensional electron gases (2DEGs) in SrTiO 3 have become model systems for engineering emergent behaviour in complex transition metal oxides. Understanding the collective interactions that enable this, however, has thus far proved elusive. Here we demonstrate that angle-resolved photoemission can directly image the quasiparticle dynamics of the d-electron subband ladder of this complex-oxide 2DEG. Combined with realistic tight-binding supercell calculations, we uncover how quantum confinement and inversion symmetry breaking collectively tune the delicate interplay of charge, spin, orbital and lattice degrees of freedom in this system. We reveal how they lead to pronounced orbital ordering, mediate an orbitally enhanced Rashba splitting with complex subband-dependent spin-orbital textures and markedly change the character of electron-phonon coupling, co-operatively shaping the low-energy electronic structure of the 2DEG. Our results allow for a unified understanding of spectroscopic and transport measurements across different classes of SrTiO 3 -based 2DEGs, and yield new microscopic insights on their functional properties.

Highly efficient and tunable spin-to-charge conversion through Rashba coupling at oxide interfaces

“…As pointed out in several studies 31,33 , at low carrier density the electrons occupy a single low-lying band with dxy character, whereas raising the Fermi level EF through the application of a gate voltage promotes the population of dxz,yz bands. The amplitude of R and the associated spin textures are strongly dependent on the relative energy and the orbital symmetry of these bands 29,31,34 .…”

Nature of orbital and spin Rashba coupling in the surface bands ofSrTiO3andKTaO3

“…Transport measurements do reveal the Rashba term, of cubic order in momentum, through analysis of the orientation-dependent magneto-resistance data on STO surface [8,9]. Existing theories treat Rashba effects of t 2g -derived bands phenomenologically [6, 10] and cannot, for instance, explain the complex band-specific spin and orbital angular momentum structures observed in the electronic structure calculation [7].…”

“…Transport measurements do reveal the Rashba term, of cubic order in momentum, through analysis of the orientation-dependent magneto-resistance data on STO surface [8,9]. Existing theories treat Rashba effects of t 2g -derived bands phenomenologically [6, 10] and cannot, for instance, explain the complex band-specific spin and orbital angular momentum structures observed in the electronic structure calculation [7].It has recently been argued that multi-orbital bands, subject to the surface ISB electric field, must give rise to an entity called the chiral orbital angular momentum (OAM) in momentum space [11,12]. The argument remains valid as long as the crystal field splitting does not quench the multi-orbital degrees of freedom in a given band structure.…”

Quasiparticle mass enhancement and Fermi surface shape modification in oxide two-dimensional electron gases

“…Before concluding this section we again mention that angle-resolved photoemission spectra of SrTiO 3 twodimensional electron gases 25,30 reveal that several xy subbands are often occupied, and tight-binding/selfconsistent Hartree calculations 21,22 indicate that their contribution to the total density is small. We thus expect that their influence on FSSM of the anisotropic xz and yz bands will be weak.…”

“…We take the xz and yz bands to be isotropic with mass m DOS = √ m H m L as in Eq. (30). The influence of anisotropy on the quasiparticle mass is isolated and studied in Section V B, but we neglect it here.…”