Ultrahigh-resolution scanning microwave impedance microscopy of moiré lattices and superstructures

Abstract

Two-dimensional heterostructures composed of layers with slightly different lattice vectors exhibit new periodic structure known as moiré lattices, which, in turn, can support novel correlated and topological phenomena. Moreover, moiré superstructures can emerge from multiple misaligned moiré lattices or inhomogeneous strain distributions, offering additional degrees of freedom in tailoring electronic structure. High-resolution imaging of the moiré lattices and superstructures is critical for understanding the emerging physics. Here, we report the imaging of moiré lattices and superstructures in graphene-based samples under ambient conditions using an ultrahigh-resolution implementation of scanning microwave impedance microscopy. Although the probe tip has a gross radius of ~100 nm, spatial resolution better than 5 nm is achieved, which allows direct visualization of the structural details in moiré lattices and the composite super-moiré. We also demonstrate artificial synthesis of novel superstructures, including the Kagome moiré arising from the interplay between different layers.

The limits of near field immersion microwave microscopy evaluated by imaging bilayer graphene moiré patterns

“…The scanning microwave impedance microscope (sMIM) is one of the latest additions to the family of scanning probe microscopes. Commercially available 8 tools can now be used to retrofit existing equipment, and exciting results in multiple applications have been published 9 – 12 describing exquisite spatial detail and vector analysis of the microwave reflected signal at each pixel. A 3 GHz microwave signal is coupled to an Atomic Force Microscope (AFM) probe tip that works as a waveguide and performs as an apertureless near-field microscope 9 .…”

“…The possibility offered by van der Wall heterostructures such as graphene to explore the potential modulation parameters in a more detailed fashion presented surprising opportunities that went beyond metal-insulator transition and Wigner crystallization 17 when observation of additional electron-correlation physics such as superconductivity was reported in TBG with a magical angle of ≈1.1 ∘ 19 . Tools that can expeditiously analyze and provide answers on the electronic structure, preferably at ambient conditions, are currently being pursued 12 , 20 .…”

Optical Responses of Chiral Majorana Edge States in Two-Dimensional Topological Superconductors

“…Recently, a microwave microscopy experiment with an ultrahigh spatial resolution of 5nm has been reported. [28] With this size used for d, [σ xx ] achieves 4000e 2 /h.…”

“…In this case, the chemical potential µ should be inside the surface magnetization gap (∆ m ∼ 50meV [31,32] ). Assuming µ = 50meV , the SC gap ∆ sc = 0.1meV and the detection spot size d = 5nm [28], we estimate that the optical conductivity has a maximum value of [σ xx (ω 0 )] ≈ 10e 2 /h at the peak position ω 0 ≈ 0.003meV , or ω 0 ≈ 4.5 GHz.…”

Local Raman spectroscopy of chiral Majorana edge modes in Kitaev spin liquids and topological superconductors

“…But the optical response of the edge states remains an open question. Particularly, optical microscopy and spectroscopy methods [26][27][28] have been successful in detecting local optical responses with high spatial resolution (up to a few nanometers [29,30]), which is ideal for the investigation of topological edge states [31,32].…”