High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces

Weiwei Luo,Alexey B Kuzmenko,Jean-Marie Poumirol,Stefano Gariglio,Jean-Marc Triscone,Dirk Van Der Marel,Margherita Boselli,Ivan Ardizzone,Jérémie Teyssier

doi:10.1038/s41467-019-10672-5

Weiwei Luo, Alexey B Kuzmenko + Show 7 more

Open Access

https://doi.org/10.1038/s41467-019-10672-5

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Journal: Nature Communications	Publication Date: Jun 24, 2019
Citations: 73	License type: open-access

Affiliation: University of Geneva

Abstract

Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature down to 6 K. We show that the near-field optical signal, in particular its phase component, is highly sensitive to the transport properties of the electron system present at the interface. Our modeling reveals that such sensitivity originates from the interaction of the AFM tip with coupled plasmon–phonon modes with a small penetration depth. The model allows us to quantitatively correlate changes in the optical signal with the variation of the 2DES transport properties induced by cooling and by electrostatic gating. To probe the spatial resolution of the technique, we image conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope.

Highlights

Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range
The rapidly developing technique of scattering-type scanning near-field optical microscopy (s-SNOM)[16,17] overcomes such limitation and allows the investigation of the amplitude and phase of the optical response with the resolution of 10–20 nm. This approach rests on the near-field interaction of the sample surface with a metal-coated atomic force microscope (AFM) tip illuminated by an infrared laser: the back-scattered light, recorded as a function of the tip position, brings information about the surface conductivity
We demonstrate that 160–190 nm wide conducting channels electrically created by an AFM tip can be detected in an insulating background, illustrating the high spatial resolution and high sensitivity of this technique applied to oxide interfaces

Summary

Introduction

Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. It was predicted that, in the presence of a Rashba-type spin–orbit interaction, whose relevance for the LAO/STO interface has been revealed in magnetotransport[9,10] and spin-charge conversion experiments[11,12], the 2D correlated electron system undergoes an electronic phase separation with an intrinsically inhomogeneous charge distribution[13] Visualizing such effects remains challenging: it requires a local probe with nanometer-scale resolution capable of imaging the dynamic electric response of the 2DES. We employ a cryogenic s-SNOM system (cryo-SNOM, neaspec GmbH) to study the near-field response of LAO/STO heterostructures in the range of wavelengths from 9.3 to 10.7 μm (CO2 laser) from room temperature to 6 K We demonstrate that both the amplitude and the phase of the backscattered optical signal are sensitive to the presence of the 2DES at the LAO/STO interface. We demonstrate that 160–190 nm wide conducting channels electrically created by an AFM tip can be detected in an insulating background, illustrating the high spatial resolution and high sensitivity of this technique applied to oxide interfaces

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