Imaging quantum interference in a monolayer Kitaev quantum spin liquid candidate

TL;DR

This study visualizes quantum interference patterns in a monolayer Kitaev quantum spin liquid candidate, revealing exotic oscillations linked to Majorana fermions, and introduces a new method to probe such quantum states locally.

Contribution

First atomic-scale imaging of quantum interference in a Kitaev quantum spin liquid, demonstrating oscillations related to Majorana fermions and proposing a novel local probing technique.

Findings

Observed incommensurate, decaying density oscillations around defects.

Oscillations are unique to $ ext{RuCl}_3$ and not found in other Mott insulators.

Numerical models link oscillations to scattering of Majorana fermions.

Abstract

Single atomic defects are prominent windows to look into host quantum states because collective responses from the host states emerge as localized states around the defects. Friedel oscillations and Kondo clouds in Fermi liquids are quintessential examples. However, the situation is quite different for quantum spin liquid (QSL), an exotic state of matter with fractionalized quasiparticles and topological order arising from a profound impact of quantum entanglement. Elucidating the underlying local electronic property has been challenging due to the charge neutrality of fractionalized quasiparticles and the insulating nature of QSLs. Here, using spectroscopic-imaging scanning tunneling microscopy, we report atomically resolved images of monolayer $\alpha-RuCl_3$, the most promising Kitaev QSL candidate, on metallic substrates. We find quantum interference in the insulator manifesting as incommensurate and decaying spatial oscillations of the local density of states around defects with a characteristic bias dependence. The oscillation differs from any known spatial structures in its nature and does not exist in other Mott insulators, implying it is an exotic oscillation involved with excitations unique to $\alpha-RuCl_3$. Numerical simulations suggest that the observed oscillation can be reproduced by assuming that itinerant Majorana fermions of Kitaev QSL are scattered across the Majorana Fermi surface. The oscillation provides a new approach to exploring Kitaev QSLs through the local response against defects like Friedel oscillations in metals.