Bridging atomic and mesoscopic length scales with Replica Scanning Tunneling Microscopy: Visualizing the intra-unit cell pair density modulation of superconducting FeSe at micron length scale

TL;DR

This paper introduces Replica STM (R-STM), a novel method that enables visualization of atomic-scale electronic density modulations over micron-scale areas, revealing persistent pair density modulations in FeSe at large length scales.

Contribution

The paper presents R-STM, a new technique that allows tracking atomic-scale phenomena over large areas, bridging the gap between atomic and mesoscopic length scales in STM.

Findings

Discovered large-wavelength signals as replicas of atomic-scale modulations.

Showed pair density modulation in FeSe persists up to hundreds of nanometers.

Demonstrated R-STM's ability to efficiently analyze large-area STM data.

Abstract

Scanning Tunneling Microscopy (STM) is a cornerstone technique for visualizing the electronic density of states with atomic resolution (typically below 0.1 nm). While the field of view of most STM setups extends up to a few microns, obtaining atomic resolution over these large areas is often impractical and excessively time-consuming. This is due to the need to acquire maps with a point number reaching 107 or more with a full current or conductance vs voltage curve at each point. The standard procedure is to make large scale maps and then select small regions to zoom-in for high-resolution atomic scale analysis. However, this approach fails to address a question which is often critical: Does a specific atomic-scale modulation of the electronic density of states persist over much larger, mesoscopic length scales? Here we present a new method: Replica STM (R-STM), that overcomes this limitation, allowing the study of atomic-scale phenomena up to micron length scales. We obtained new large-area STM tunneling conductance maps in UTe2 and FeSe, spanning areas over 200 nm in size. In these large scale maps we discovered signals with wavelengths significantly exceeding interatomic distances. We show that these large-wavelength signals are replicas of the underlying atomic-scale density of states modulations. R-STM leverages these replica signals to efficiently track atomic-scale features over large areas. Using this novel technique, we show that the pair density modulation discovered recently in FeSe persists with the same characteristic wavelength up to hundreds of nm length scales. R-STM provides a powerful and practical new capability for STM to compare atomic scale with micrometer scale phenomena. The proof of principle of R-STM can be extended to any other scanning probe microscopy experiment where a periodic signal is traced as a function of position.