We investigated the conductance of C 60 molecules using mechanically controllable break-junction electrodes at 10 K in UHV. The molecules are evaporated in situ. With this method, we obtain clean and low-resistance contacts. From the analysis of conductance histograms and differential-conductance ͑dI / dV͒ traces, we deduce the preferred conductance value of a single C 60 molecule between gold electrodes to be close to 0.1 G 0 . The presence of C 60 is evidenced by features in the derivative of the dI / dV at energies close to the molecular vibration energies. The characteristics of the molecules disappear upon cold working of the junctions. DOI: 10.1103/PhysRevB.76.125432 Over the last years, much progress has been achieved in the understanding of the electronic transport through single molecules. Since the seminal experiment by Joachim et al. 1 in which a C 60 molecule was contacted by a scanningtunneling microscope tip, the fullerenes have become benchmark systems for the development of new measurement schemes for molecular electronics applications. Much theoretical work has been performed on different metal-C 60 systems including the noble metals 2 and aluminum. 3 It has been measured that C 60 between gold electrodes would form ionic contacts, giving rise to high conductances. 4,5 Due to its high symmetry, only a few different contact geometries have to be explored although the molecule bears manifold possible functionality including the possibility of endodoping by magnetic 6,7 or other ions. 8 Most of the single-molecule transport experiments performed so far can be categorized into two groups: The first set of experiments is characterized by rather high resistance contacts allowing for the investigation of Coulomb blockade ͑CB͒ and molecular level spectroscopy by inelastic electron tunneling spectroscopy. These devices can be fabricated by different techniques including scanningtunneling microscopy ͑STM͒ and spectroscopy, 1 electromigrated electrode structures, 9,10 and mechanically controllable break junctions ͑MCBJs͒.11 Among those, STM provides the largest variability in the combination of metals and molecules, to tune the contact after its formation, to reestablish a new contact after it has been broken, and to image the configuration of the environment. With electromigrated electrodes, very stable devices are achieved but without any possibility to tune the coupling after the contact has been established and with usually very low yield. The MCBJ technique interpolates between these two extremes. It combines tunability of the contacts with high mechanical stability, enabling measurements over several hours on the same contact configuration at low temperatures and the application of external fields.For the study of CB, oxide or other contamination layers between electrodes and molecules might be helpful for ensuring the weak coupling which is necessary for considering the molecular systems as independent from the metal electrodes. However, for the application of single-molecule contacts in ele...

We discuss the use of mechanically controllable break-junctions (MCBs) as electrodes for contacting individual objects such as clusters or molecules. A big advantage of MCB electrodes is the possibility of varying the contact geometry and, accordingly, the contact resistances and the quantum coherent transport properties between the object under study and the electrodes. We compare the suitability of single-atom contacts of different elements as electrode material. Finally, we present preliminary results on electronic transport through individual (or a few) fullerene molecules contacted by gold MCB electrodes. The typical resistances are in the order of the quantum of conductance. The circuits sustain voltages of the order of 1 V and are mechanically more stable than single-atom contacts between the gold electrodes alone.

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Electrical contacts of the width of only one atom can be realized by the break‐junction technique. The conductance decreases stepwise due to structural reconfigurations when tearing a nano‐bridge in the few‐atom range. Transport is described by an ensemble of channels with possibly quite high transmission probabilities. For a single break‐junction the last one‐atom contact consists of a material‐specific channel ensemble, determined by the chemical valance as verified for quite a number of metals. d‐electrons in half‐metals and spin‐effects in magnetic materials will complicate this simple model. Break‐junctions also provide ideal contacts to investigate transport through freely suspended clusters or molecules like DNA.

We report the observation of strong resonances at zero bias in the differential conductance through Al-C 60 -Al junctions with tunable electrode distance, measured above T = 10 K. The conductance value at resonance ranges from a few percent up to eighty percent of the quantum of conductance. The resonances may disappear or reoccur completely and discontinuously upon very small changes of the electrode distance. However, once they are formed they are very robust with respect to changes of the electrode distance. We discuss similarities and differences to the common theories of the Kondo screening of a spontaneous spin polarization of the C 60 molecule. We deduce Kondo temperatures in the range from 35 to 160 K and demonstrate that the temperature dependence is in agreement with the scaling behavior of the Kondo effect in the temperature range of our experiment.