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Magnetism of molecules is in the focus of intense research, because it could lead to new spin-based electronic devices. In this context, two molecules adsorbed on a metal surface and carrying magnetic moments constitute an important model system which helps to understand the fundamental properties of molecular magnetism – in particular the interaction between their magnetic moments. Typically the interaction of spins on a metal surface is dominated and tuned by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction – an indirect exchange interaction mediated by the conduction electrons of the metal. Here we show that the non-magnetic, chemical interaction between the molecules can become the decisive effect if the spin-moment-carrying molecular orbitals extend in space and therefore lead to the formation of bonding and antibonding orbitals due to wave function overlap. We demonstrate that this splitting has a crucial influence on the magnetic properties exemplified by two neighbouring metal-molecule (Au-PTCDA) complexes, i.e. dimers, on the Au(111) surface. Interestingly, in this particular case, competition between the binding energy gain, due to the chemical interaction between the moment-carrying orbitals, and the gain of additional hybridization energy, due to the strong entanglement between the local moment and the conduction band of the substrate, is the driving force of a quantum phase transition (QPT) in the dimer. Small changes in the wave function overlap, involving slightly different dimer configurations, drive the system through a quantum critical point (QCP) from a partially Kondo-screened triplet to a singlet ground state, with one dimer configuration being located extremely close to a quantum critical point.

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Taner Esat, Peter-Gruenberg-Institut Juelich
Seminar room Institute of physics 2
Contact: T. Michely