The idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than its zero-resistance critical temperature (T-c) has been explored for unconventional, interfacial,... Show moreThe idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than its zero-resistance critical temperature (T-c) has been explored for unconventional, interfacial, and disordered superconductors, but direct experimental evidence is lacking. We used scanning tunneling noise spectroscopy to show that preformed Cooper pairs exist up to temperatures much higher than T-c in the disordered superconductor titanium nitride by observing an enhancement in the shot noise that is equivalent to a change of the effective charge from one to two electron charges. We further show that the spectroscopic gap fills up rather than closes with increasing temperature. Our results demonstrate the existence of a state above T-c that, much like an ordinary metal, has no (pseudo)gap but carries charge through paired electrons. Show less
For the technological integration of molecular switches in electronic devices, self-assembling nanomaterials of such switches are highly sought after. The syntheses of a new tetrapyridyl ligand... Show moreFor the technological integration of molecular switches in electronic devices, self-assembling nanomaterials of such switches are highly sought after. The syntheses of a new tetrapyridyl ligand bearing a C12 alkyl chain and two N-H bridges (compound 1) and of its iron(II) complex [Fe(1)(NCS)(2)] (compound 2), are described. Magnetic susceptibility data for bulk samples of 2 confirmed their gradual spin-crossover properties. The self-assembly of 1 and 2 on highly ordered pyrolytic graphite surfaces (HOPG) was investigated by Scanning Tunneling Microscopy (STM). Both compounds 1 and 2 formed ordered monolayers after deposition by drop casting. The patterns of the two compounds are very different, which is attributed to the fundamentally different hydrogen bonding networks before and after coordination of Fe(NCS)(2) to the tetradentate chelate. Two possible models for the self-assembly of 1 and 2 are provided. This work suggests that it is possible to design molecular switches that self-assemble on surfaces in highly ordered monolayer films. This is a significant step in the development of spin-switching materials, which may streamline the integration of molecular switches in for example memory and sensing devices. Show less
Rodríguez-Fernandez, C.; Akius, K.A.C.; Morais de Lima, M.; Cantarero, A.; Ruitenbeek, J.M. van; Sabater, C. 2021
Electronic transport properties of single-molecule junctions have been widely measured by several techniques, including mechanically controllable break junctions, electromigration break junctions,... Show moreElectronic transport properties of single-molecule junctions have been widely measured by several techniques, including mechanically controllable break junctions, electromigration break junctions, and by means of scanning tunneling microscopes. In parallel, many theoretical tools have been developed and refined for describing such transport properties and for obtaining numerical predictions. Most prominent among these theoretical tools are those based upon density functional theory. In this review, theory and experiment are critically compared, and this confrontation leads to several important conclusions. The theoretically predicted trends nowadays reproduce the experimental findings well for series of molecules with a single well-defined control parameter, such as the length of the molecules. The quantitative agreement between theory and experiment usually is less convincing, however. Two main sources for the quantitative discrepancies can be identified. Experimentally, the atomic structure of the junction typically realized in the measurement is not well known, so simulations rely on plausible scenarios. In theory, correlation effects can be included only in approximations that are difficult to control for experimentally relevant situations. Therefore, one typically expects qualitative agreement with present modeling tools; in exceptional cases a quantitative agreement has already been achieved. For further progress, benchmark systems are required that are sufficiently well defined by experiment to allow quantitative testing of the approximation schemes underlying the theoretical modeling. Several key experiments can be identified suggesting that the present description may even be qualitatively incomplete in some cases. Such key experimental observations and their current models are also discussed here, leading to several suggestions for extensions of the models toward including dynamic image charges, electron correlations, and polaron formation. Show less
Evers, F.; Korytár, R.; Tewari, S.; Ruitenbeek, J.M. van 2020
A new way to control individual molecules and monoatomic chains is devised by preparing a human–machine augmented system in which the operator and the machine are connected by a real-time... Show moreA new way to control individual molecules and monoatomic chains is devised by preparing a human–machine augmented system in which the operator and the machine are connected by a real-time simulation. Here, a 3D motion control system is integrated with an ultra-high vacuum (UHV) low-temperature scanning tunnelling microscope (STM). Moreover, we coupled a real-time molecular dynamics (MD) simulation to the motion control system that provides a continuous visual feedback to the operator during atomic manipulation. This allows the operator to become a part of the experiment and to make any adaptable tip trajectory that could be useful for atomic manipulation in three dimensions. The strength of this system is demonstrated by preparing and lifting a monoatomic chain of gold atoms from a Au(111) surface in a well-controlled manner. We have demonstrated the existence of Fabry–Pérot-type electronic oscillations in such a monoatomic chain of gold atoms and determined its phase, which was difficult to ascertain previously. We also show here a new geometric procedure to infer the adatom positions and therefore information about the substrate atoms, which are not easily visible on clean metallic surfaces such as gold. This method enables a new controlled atom manipulation technique, which we will refer to as point contact pushing (PCP) technique. Show less
We demonstrate that the notched-wire mechanically controllable break junction technique can be exploited for the study of single molecule junctions. We have developed a protocol for deposition of... Show moreWe demonstrate that the notched-wire mechanically controllable break junction technique can be exploited for the study of single molecule junctions. We have developed a protocol for deposition of thiol-coupled molecules onto Au electrodes from solution. We find surprisingly sharp conductance historgrams at low temperatures, which suggest that the commonly observed large width of the peaks in conductance historgams is the result of inhomogeneous broadening. Show less
Atesci, H.; Coneri, F.; Leeuwenhoek, M.; Hilgenkamp, H.; Ruitenbeek, J.M. van 2017
