’Kondo state ’ and Kondo resonance in a two-dimensional electron gas1

The delicate balance of spin-screening and spin-aligning interactions determines many of the pecu-liar properties of dilute magnetic systems. We study a surface-supported all-organic multi-impurity Kondo spin system at the atomic scale by low-temperature scanning tunnelling microscopy and-spectroscopy. The model system consists of spin-1/2 radicals that are aligned in one-dimensional chains and interact via a ferromagnetic RKKY interaction mediated by the 2DEG of the supporting substrate. Due to the RKKY-induced enhanced depopulation of one spin-subband in the 2DEG, we finally succeeded to detect the so far unobserved ’Kondo state ’ as opposed to the well-established Kondo resonance. Its cloud of screening electrons, that are virtually bound to the radicals below the Kondo temperature, represents the extended exchange hole of the ferromagnetically polarized spin chain imaged here in real space. The interaction of localized electron spins of magnetic4 impurities with delocalized conduction electrons provides5 a key handle for controlling the spin polarization in di-6 luted magnetic systems essential for future spintronics1,27 and molecular electronics3 applications. Many of their8 unique properties are determined by the competition be-