Christophe Stroh

Temperature and magnetic field dependence of a Kondo system in the weak coupling regime

The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host. Studies of individual magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics; however, a quantitative comparison with theoretical predictions remained challenging. Here we show that the zero-bias anomaly detected on an organic radical weakly coupled to a Au (111) surface can be described with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago. Our results demonstrate that Kondo physics can only be fully conceived by studying both temperature and magnetic field dependence of the resonance. The identification of a spin 1/2 Kondo system is of relevance not only as a benchmark for predictions for Kondo physics but also for correlated electron materials in general.

Synthesis, structure and properties of benzoic acids bearing para- or meta-imino nitroxides or ortho-nitronyl nitroxide radical centres

2-(4-Carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl ( p-IMBAH), 2-(3-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl (m-IMBAH) and 2-(2-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-3-oxido-1H-imidazol-3-ium-1-yloxyl (o-NITBAH) have been prepared. The m-IMBAH and p-IMBAH radicals crystallised in monoclinic space group P2 1 /n while the o-NITBAH radical occupied the monoclinic space group Cc. In p-IMBAH, a 12-membered ring is formed via strong hydrogen bonds between two water molecules and two radicals, this unit being propagated along the b axis via additional hydrogen bonds. In m-IMBAH, the radicals are arranged in dimers held together by strong hydrogen bonds between the acid and the imine functions, each dimer being connected with its neighbours via hydrogen bonds forming alternating chains along the c axis. No cyclic arrangements are evident in o-NITBAH, the radicals being organized into infinite chains. In each case, the product of paramagnetic susceptibility with T lies in the 0.362 to 0.374 emu K mol –1 range at room temperature while the reciprocal susceptibility follows a Curie-Weiss law above 20 K with θ=+1.38, +5.24 and –0.81 K, respectively, for p-IMBAH, m-IMBAH and o-NITBAH. The isotropic 14 N hyperfine coupling constants are found from solution experiments to be 9.20 (a N1 ) and 4.28 (a N2 ) G for the imino radicals and 7.59 G for the nitronyl nitroxide radical. The principal values of the g-tensor measured on single crystals afforded an average of 2.0066/7 while in solution g iso =2.0060 for p/m-IMBAH and 2.0065 for o-NITBAH.

Intramolecular exchange interaction in twofold spin-labelled platinum complexes.

The synthesis and the crystal structures of two platinum bis-ethynyl-phenyl-nitronyl-nitroxide complexes are reported and solution EPR investigations evidenced the intramolecular exchange interaction through a diamagnetic transition metal based spacer.

A Tripodal Molecule on a Gold Surface: Orientation-Dependent Coupling and Electronic Properties of the Molecular Legs

The realization of molecular electronics demands a detailed knowledge of the correlation between chemical groups and electronic function. It has become obvious during the last years that the conformation of a molecule and its coupling to the connecting electrodes plays a crucial role in its conductance behavior and its electronic function, e.g., as a switch. Knowledge about these relationships is therefore essential for future design of molecular electronic building blocks. We present a new three-dimensional molecule, consisting of three identical molecular wires connected to a headgroup. Due to the well-defined spatial arrangement of the molecule in a nonplanar geometry, it is possible to investigate the conductance behavior of these wires with respect to their position and coupling to the surface electrode with the submolecular resolution of a scanning tunneling microscope. The experimental findings are supported by calculations of the electronic structure and conformation of the molecule on the surface by density functional theory with dispersion corrections.

Strong intramolecular exchange interactions between nitronyl nitroxide radicals bridged by olefinic spacers

Nitronyl nitroxide radicals bridged by olefinic spacers display strong intramolecular and antiferromagnetic exchange interactions, the magnitude of which depends on the length of the spacer and on steric crowding around the radicaloid subunits.