Low-lying magnon frequency comb in skyrmion crystals

A stable, low-power, and tunable magnon frequency comb (MFC) is crucial for magnon-based precision measurements, quantum information processing, and chip integration. The original method for creating a MFC utilizes the nonlinear interactions between propagating spin waves and localized oscillations of an isolated magnetic texture, e.g., skyrmion. It requires a driving frequency well above the ferromagnetic resonance (FMR) and the spectrum frequency of a MFC will quickly approach the detection limit of a conventional microwave technique after only tens of comb teeth. In addition, the detection and manipulation of a single skyrmion is challenging in experiments due to its high degree of locality. These issues hinder the applications of a MFC. In this work, we report the low-lying MFC with comb frequencies below the FMR in a skyrmion crystal (SkX). We show that the MFC originates from the three-wave mixing between the collective skyrmion gyration and breathing in the SkX. Our findings significantly improve the efficiency of the nonlinear frequency conversion from a single-frequency microwave input and establish a synergistic relationship between the SkX and MFC, which paves the way to coherent information processing and ultrasensitive metrology based on the MFC.