Subband structure of a two-dimensional electron gas formed at the polar surface of the strong spin-orbit perovskite KTaO$_3$

TL;DR

This study reveals the formation of a two-dimensional electron gas at the polar surface of KTaO$_3$, showing quantum confinement effects, subband structure, and minimal Rashba spin splitting despite strong spin-orbit coupling.

Contribution

It provides the first direct observation of a 2DEG at the polar surface of KTaO$_3$ and characterizes its subband structure and spin-orbit effects.

Findings

Quantum confinement lifts orbital degeneracy.

2DEG consists of light and heavy carrier subbands.

Rashba spin splitting is minimal at the Fermi level.

Abstract

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the $(100)$ surface of the $5d$ transition-metal oxide KTaO$_3$. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for potential Rashba spin splitting of only $\Delta{k}_\parallel\sim\0.02$ \AA$^{-1}$ at the Fermi level. The polar nature of the KTaO$_3(100)$ surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO$_3(100)$.