Evidence for spinarons in Co adatoms

The collective screening of an impurity spin by conduction electrons in the Kondo effect represents a fundamental aspect of many-body physics. The experimental identification of the Kondo effect in individual atoms is associated with the emergence of a local electronic state at the Fermi level, exhibiting characteristic temperature and field-dependent behaviors. This phenomenon is marked by a zero-bias anomaly (ZBA) in the differential conductance signal, first observed in scanning tunneling spectroscopy (STS) experiments on Co atoms on the Au (111) surface. The ZBA results from a Fano resonance caused by the interference of tunneling paths into the Kondo state and atomic orbitals. Recent first-principle calculations suggest an alternative interpretation of the ZBA, involving the formation of a novel collective excitation, the spinaron, in contrast to the prevailing Kondo-based explanation.

In this study, state-of-the-art spin-averaged and spin-polarized scanning tunneling spectroscopy measurements were conducted on Co atoms on the Cu(111) surface in magnetic fields up to 12 T. The results revealed a field-induced energy shift and experimentally determined spin characteristics, challenging the Kondo-based interpretation of the ZBA in favor of the spinaron, marking the first experimental detection of this novel many-body excitation. These findings disrupt the long-standing model system for the single-atom Kondo effect for Co atoms on coinage (111) metal surfaces, opening exciting possibilities for further exploration of spinaron physics.