Spinaronic excitations in magnetic nanostructures on noble metals

• Applicants: Professor Dr. Samir Lounis; Dr. Artem Odobesko
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550485402

Understanding the interaction between an atomic spin and the conduction electrons of the substrate is a fundamental issue in condensed-matter physics. According to the seminal Kondo model, the spins of the electrons effectively screen the impurity spin, resulting in a correlated many-electron state. For over two decades, this Kondo model has been the prevailing explanation for spectroscopic observations involving individual magnetic adatoms at the surfaces of coinage metals. However, a recent theoretical proposal challenges this traditional viewpoint by providing an alternative explanation for the zero-bias resonance. In contrast to the Kondo model, the magnetic anisotropy of the impurity, that is the tendency to orient the magnetic moment in space, plays an important role. A Co atom on Cu(111) surface, exhibits a notable out-of-plane magnetic anisotropy, resulting in spin excitations that interact with the conduction electrons. The resulting many-electron state has been termed a "spinaron," exhibiting a spectroscopic signature in Scanning Tunneling Spectroscopy (STS) nearly indistinguishable from the Kondo resonance. Now, in a groundbreaking study published in Nature Physics, we have employed a spin-polarised-STS to challenge the validity of a long-standing model system for the Kondo effect. Our experimental work provides the first empirical evidence for the existence of spinarons in Co atoms on Cu surface. In light of these intriguing findings, numerous questions emerge: Can spinarons be observed in other sample systems by combining 3d transition metal adatoms such such as Fe, Cr, or Mn, with (111)-, (110)-, or (100)-oriented noble metal surfaces? If so, what factors influence the formation of spinarons, including the magnitude and orientation of the magnetic anisotropy energy (MAE)? How does the strength of hybridization with the substrate affect spinaron formation? What role does the magnetic adatom's spin-orbit coupling (SOC) play? Can spinarons interact with each other, potentially forming a collective and delocalized spinaron state? Lastly, could spinarons contribute to an increase in resistivity? The proposed project and research plan aims at addressing these questions by a concerted effort of the experimental and theoretical expertise of the research groups participating in this project. By conducting a systematic investigation, we aim to shed further light on the properties and behaviors of spinarons and their relevance in the field of condensed matter physics.

• DFG Programme: Research Grants