Inducing Kondo screening of vacancy magnetic moments in graphene with gating and local curvature

Yuhang Jiang,Guohong Li,Eva Y Andrei,Guang-Yu Guo,Po-Wei Lo,Kenji Watanabe,Daniel May,Frithjof B Anders,Takashi Taniguchi,Jinhai Mao

doi:10.1038/s41467-018-04812-6

Abstract

In normal metals the magnetic moment of impurity-spins disappears below a characteristic Kondo temperature which marks the formation of a cloud of conduction-band electrons that screen the local-moment. In contrast, moments embedded in insulators remain unscreened at all temperatures. What then is the fate of magnetic-moments in intermediate, pseudogap systems, such as graphene? Theory predicts that coupling to the conduction-band electrons will drive a quantum phase transition between a local-moment phase and a Kondo-screened phase. However, attempts to experimentally confirm this prediction and its intriguing consequences, such as electrostatically tunable magnetic-moments, have been elusive. Here we report the observation of Kondo-screening and the quantum phase-transition between screened and unscreened phases of vacancy magnetic moments in graphene. Using scanning tunneling spectroscopy and numerical renormalization-group calculations we show that this transition enables to control the screening of local moments by tuning the gate voltage and the local curvature of the graphene membrane.

Highlights

In normal metals the magnetic moment of impurity-spins disappears below a characteristic Kondo temperature which marks the formation of a cloud of conduction-band electrons that screen the local-moment
We employed scanning tunneling spectroscopy (STS)[23,24] to identify Kondo screening of the vacancy magnetic moment by the distinctive zero-bias resonance it produces in the dI/dV curves (I is the tunneling current and V the junction bias), hereafter called Kondo peak
In the asymmetric Anderson impurity model (AIM), which is relevant to screening of vacancy magnetic moments in graphene, the particle-hole symmetry is broken by next-nearest neighbor hopping and by U≠2jεdj

Summary

Introduction

In normal metals the magnetic moment of impurity-spins disappears below a characteristic Kondo temperature which marks the formation of a cloud of conduction-band electrons that screen the local-moment. Theory predicts that coupling to the conduction-band electrons will drive a quantum phase transition between a local-moment phase and a Kondo-screened phase. We report the observation of Kondo-screening and the quantum phase-transition between screened and unscreened phases of vacancy magnetic moments in graphene. The removal of a carbon atom from the honeycomb lattice induces a magnetic moment stemming from the unpaired electrons at the vacancy site. In flat graphene the magnetic moment from the dangling σ-bond is unscreened because the σ-orbital is orthogonal to the π-band conduction electrons[16,17]. We employ the spectroscopic signature of the Kondo effect to demonstrate that screening of vacancy magnetic moments in graphene is enabled by corrugated substrates. As we will show, the quantum critical transition between Kondo screened and local moment phases in this system, can only be observed through a local measurement

Methods

Results

Conclusion