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Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction

Chun Sum Brian Pang, Bruce A. Davidson, Fengmiao Li, Mohamed Oudah, Peter C. Moen, Steef Smit, Cissy T. Suen, Simon Godin, Sergey A. Gorovikov, Marta Zonno, Sergey Zhdanovich, Giorgio Levy, Matteo Michiardi, Alannah M. Hallas, George A. Sawatzky, Robert J. Green, Andrea Damascelli, Ke Zou

Abstract

Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy (MBE). Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (less than 1-2 monolayers) MgO layer that is transparent to soft x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with an approximately 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.6 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111), and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces.

Direct Fabrication of a Superconducting Two-Dimensional Electron Gas on KTaO3(111) via Mg-Induced Surface Reduction

Abstract

Two-dimensional electron gases (2DEGs) at the surfaces of KTaO3 have become an exciting platform for exploring strong spin-orbit coupling, Rashba physics, and low-carrier-density superconductivity. Yet, a large fraction of reported KTaO3-based 2DEGs has been realized through chemically complex overlayers that both generate carriers and can obscure the native electronic structure, making spectroscopic access to the underlying 2DEG challenging. Here, we demonstrate a simple and direct method to generate a superconducting 2DEG on KTaO3(111) using Mg-induced surface reduction in molecular-beam epitaxy (MBE). Mg has an extremely low sticking coefficient at elevated temperatures, enabling the formation of an ultrathin (less than 1-2 monolayers) MgO layer that is transparent to soft x-ray photoemission spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). This allows direct measurement of the surface chemistry and low-energy electronic structure of the pristine reduced surface without the need for a several-nanometer-thick capping layer. XPS shows clear reduction of Ta5+ to lower oxidation states, while ARPES reveals a parabolic Ta 5d conduction band with an approximately 150 meV bandwidth and additional subband features arising from quantum confinement. Transport measurements confirm a superconducting transition below 0.6 K. Together, these results demonstrate a chemically straightforward and controllable pathway for fabricating spectroscopically accessible superconducting 2DEGs on KTaO3(111), and provide a powerful new platform for investigating the mechanisms underlying orientation-dependent superconductivity in KTaO3-based oxide interfaces.

Paper Structure

This paper contains 5 sections, 4 figures.

Table of Contents

  1. Author Declarations
  2. Data Availability
  3. References

Figures (4)

  • Figure 1: Fabrication of the MgO/KTO(111) sample. Schematic diagrams of (a) Stage 1, substrate degassing; (b) Stage 2, high-temperature surface reduction using Mg; and (c) Stage 3, room-temperature capping. The corresponding RHEED patterns along the $[\bar{1}\bar{1}2]$ direction are shown in (d) to (f).
  • Figure 2: XPS spectra of an MgO/KTO(111) sample around the 4f core level of Ta. The spectra are taken right after the degassing of KTO(111) substrate (top), and after the surface reduction with Mg (bottom).
  • Figure 3: Low-temperature transport measurements. Resistance as a function of temperature between 60 mK and 1.6 K for out-of-plane magnetic fields ranging from 0 to 3 T. The critical temperature at half the normal-state resistance ($T_{\mathrm{c,1/2}}$) is fitted and shown.
  • Figure 4: ARPES data of the Mg-reduced KTO(111) surface. (a) Fermi surface in the first surface Brillouin zone with high-symmetry points labeled. (b) Band dispersion along the M--$\Gamma$--M direction, with the momentum distribution curve (MDC, top) at $E_F$. All data were acquired with p-polarized 105.5 eV photons at 15 K. White dotted lines are included in both the Fermi surface and band-dispersion panels as guides to the eye.