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A combined experimental and theoretical study is presented revealing the influence of metal-molecule coupling on electronic transport through single-molecule junctions. Transport experiments through tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current-voltage characteristics to a single-level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal-molecule coupling. The values found for these parameters are rationalized with the help of density-functional-theory-based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.

More than a decade after the first report of single-molecule conductance, it remains a challenging goal to prove the exact nature of the transport through single molecules, including the number of transport channels and the origin of these channels from a molecular orbital point of view. We demonstrate for the archetypical organic molecule, benzenedithiol (BDT), incorporated into a mechanically controllable break junction at low temperature, how this information can be deduced from studies of the elastic and inelastic current contributions. We are able to tune the molecular conformation and thus the transport properties by displacing the nanogap electrodes. We observe stable contacts with low conductance in the order of 10(-3) conductance quanta as well as with high conductance values above ∼0.5 quanta. Our observations show unambiguously that the conductance of BDT is carried by a single transport channel provided by the same molecular level, which is coupled to the metallic electrodes, through the whole conductance range. This makes BDT particularly interesting for applications as a broad range coherent molecular conductor with tunable conductance.

We report on the experimental analysis of the charge transport through single-molecule junctions of the open and closed isomers of photoswitching molecules. Sulfur-free diarylethene molecules are developed and studied via electrical and optical measurements as well as density functional theory calculations. The single-molecule conductance and the current-voltage characteristics are measured in a mechanically controlled break-junction system at low temperatures. Comparing the results with the single-level transport model, we find an unexpected behavior of the current-dominating molecular orbital upon isomerization. We show that both the side chains and end groups of the molecules are crucial to understand the charge transport mechanism of photoswitching molecular junctions.

The use of spin waves as information carriers in spintronic devices can substantially reduce energy losses by eliminating the ohmic heating associated with electron transport. Yet, the excitation of short-wavelength spin waves in nanoscale magnetic systems remains a significant challenge. Here, we propose a method for their coherent generation in a heterostructure composed of antiferromagnetically coupled magnetic layers. The driven dynamics of naturally formed nanosized stacked pairs of magnetic vortex cores is used to achieve this aim. The resulting spin-wave propagation is directly imaged by time-resolved scanning transmission X-ray microscopy. We show that the dipole-exchange spin waves excited in this system have a linear, non-reciprocal dispersion and that their wavelength can be tuned by changing the driving frequency.

We observe transport of electrons through a metallic island on the tip of a nanomechanical pendulum. The resulting tunneling current shows distinct features corresponding to the discrete mechanical eigenfrequencies of the pendulum. We report on measurements covering the temperature range from 300 K down to 4.2 K. We explain the I-V curve, which unexpectedly differs from previous theoretical predictions, with model calculations based on a Master equation approach. PACS. 68.60.Bs,87.80.Mj One of the traditional experiments in the electrodynamics class is set up by two large capacitor plates and a metallized ball suspended in between the plates. Applying a constant voltage of several 100 V across the plates leads to the onset of periodic charge transfer by the ball bouncing back and forth, similar to a classical bell [1]. The number of electrons transferred by the metallized ball in each revolution naturally depends on the volume of the metal, but can be estimated to be of the order of 10 10 . At an oscillation frequency of some 10 Hz up into the audible kHz-range this gives a typical current of 1 − 10 µA.The question arising is whether such an experiment can be performed on the microscopic level in order to obtain a transfer not of a multitude but of only one electron per cycle of operation at frequencies of some 100 MHz. Indeed this can be achieved by simply scaling down the setup and applying a nanomechanical resonator. In recent experiments [2] the importance of the excitation of mechanical modes for electronic transport through single fullerenes was discussed.Here we want to present our results on shrinking the mechanical electron shuttle to submicron dimensions by integration of an electron island into a nanomechanical resonator functioning as an electromechanical transistor (EMT). The clear advantages are the increased speed of operation and the reduction of the transfer rate, allowing to count electrons one by one. A similar combination was proposed theoretically by Gorelik et al. [3] for metallic particles, which are connected to the reservoirs by elastically deformable organic molecular links. The main difference to common single electron transistor devices is the fact that only one tunneling barrier is open at a certain time. This leads to an exponential suppression of cotunneling effects and thus increases the accuracy of current transport. Detailed calculations for theoretical limits of the accuracy, which were presented elsewhere [4], show that these devices will allow measuring quantum fluctuations. In the present work we report on measurements of transport of electrons through a nanomechanical electron shuttle. The I-V characteristics which unexpectedly differs from theoretical predictions [4] can be explained by taking into account the driving voltage.An important feature of the device is the possibility to effectively modulate the tunneling rate onto and off the electron island given by the large speed of operation (f ∼ 100 MHz). This basically enables to mechanically filter and select...

We introduce a new concept for the solution-based fabrication of conductive gold nanowires using DNA templates. To this end, we employ DNA nanomolds, inside which electroless gold deposition is initiated by site-specific attached seeds. Using programmable interfaces, individual molds self-assemble into micrometer-long mold superstructures. During subsequent internal gold deposition, the mold walls constrain the metal growth, such that highly homogeneous nanowires with 20-30 nm diameters are obtained. Wire contacting using electron-beam lithography and electrical conductance characterization at temperatures between 4.2 K and room temperature demonstrate that metallic conducting wires were produced, although for part of the wires, the conductance is limited by boundaries between gold grains. Using different mold designs, our synthesis scheme will, in the future, allow the fabrication of complex metal structures with programmable shapes.

The transport behavior of a system of gravitationally driven superparamagnetic colloidal particles is investigated. The motion of the particles through a narrow channel is governed by magnetic dipole interactions, and a layered structure forms parallel to the walls. The arrangement of the particles is perturbed by diffusion and the motion induced by gravity leading to a density gradient along the channel. Our main result is the reduction of the number of layers. Experiments and Brownian dynamics simulations show that this occurs due to the density gradient along the channel. DOI: 10.1103/PhysRevLett.97.208302 PACS numbers: 82.70.Dd, 61.72.ÿy, 64.60.Cn, 75.40.Mg Lattice defects can be introduced in perfectly ordered crystals by deformations due to external forces. Despite their importance, the dynamical behavior of single dislocations is still rather poorly understood, because they are difficult to generate and observe on the atomic scale. Therefore, the observation of this behavior in a model system will lead to insights that can be transferred to a broad range of systems. Studies of isolated lattice defects have recently become possible in static systems of colloidal crystals [1]. Experiments on the behavior of such defects in a nonequilibrium system can lead to a better understanding of the transport behavior of a wide range of systems.In biological systems, the transport of interacting particles through narrow constrictions is of high importance for many processes, for example, for the size selectivity of transport in ion channels [2]. The complexity of such systems allows one to only make hypotheses on the underlying physics governing such phenomena. Model systems that can be easily accessed experimentally can reveal many of the underlying processes [3]. This requires studies of particle transport through channels with variously shaped walls. In order to perform these studies, transport through channels with straight walls has to be well understood in experiments shown in this Letter.In this work, we report on studies of the transport behavior of colloids in a quasi-two-dimensional (2D) setup. The colloids are superparamagnetic; therefore, the interaction energy can be continuously tuned by the application of an external magnetic field [4]. The particles are driven by gravity through a narrow constriction (channel). Such driven diffusive systems serve as model systems for theoretical studies of nonequilibrium behavior [5]. In addition, such a system resembles the classical case of a quantum point contact in mesoscopic electronics [6,7] or in metallic single atom contacts [8,9]. These contacts exhibit transport in electronic channels due to quantization effects. These quantum channels can be seen as analogous to the layers in the macroscopic transport, since both occur due to the interaction of the particles with the confining potential. A classical version of a similar scenario can be built on a liquid helium surface, which is loaded with charges. For such a system, the formation of layers has been repor...