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Register a new userA Device Structure for Electronic Transport Through Individual Molecules with Strong Coupling to Metallic Electrodes
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We present a simple and reliable method for making electrical contacts to small organic molecules with thiol endgroups. Nanometer-scale gaps between metallic electrodes have been fabricated by passing a large current through a lithographically-patterned Au-line with appropriate thickness. Under appropriate conditions, the passage of current breaks the Au-line, creating two opposite facing electrodes separated by a gap comparable to the length of small organic molecules. Current-voltage characteristics have been measured both before and after deposition of short organic molecules. The resistance of single 1,4-benzenedithiol and 1,4-bezenedimethanedithiol molecules were found to be 9M$Omega$ and 26M$Omega$, respectively. The experimental results indicate strong electronic copuling to the contacts and are discussed using a relatively simple model of mesoscopic transport. The use of electrodes formed on an insulating surface by lithography and electromigration provides a stable structure suitable for integrated circuit applications.
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Within the hierarchical quantum master equation (HQME) framework, an approach is presented, which allows a numerically exact description of nonequilibrium charge transport in nanosystems with strong electronic-vibrational coupling. The method is applied to a generic model of vibrationally coupled transport considering a broad spectrum of parameters ranging from the nonadiabatic to the adiabatic regime and including both resonant and off-resonant transport. We show that nonequilibrium effects are important in all these regimes. In particular in the off-resonant transport regime, the inelastic co-tunneling signal is analyzed for a vibrational mode in full nonequilibrium, revealing a complex interplay of different transport processes and deviations from the commonly used $G_0/2$-thumb-rule. In addition, the HQME-approach is used to benchmark approximate master equation and nonequilibrium Greens function methods.
We present a combined scanning force and tunneling microscope working in a dilution refrigerator that is optimized for the study of individual electronic nano-devices. This apparatus is equipped with commercial piezo-electric positioners enabling the displacement of a sample below the probe over several hundred microns at very low temperature, without excessive heating. Atomic force microscopy based on a tuning fork resonator probe is used for cryogenic precise alignment of the tip with an individual device. We demonstrate the local tunneling spectroscopy of a hybrid Josephson junction as a function of its current bias.
Using a first principles approach, we study the electron transport properties of a new class of molecular wires containing fluorenone units, whose features open up new possibilities for controlling transport through a single molecule. We show that the presence of side groups attached to these units leads to Fano resonances close to the Fermi energy. As a consequence electron transport through the molecule can be controlled either by chemically modifying the side group, or by changing the conformation of the side group. This sensitivity, opens up new possibilities for novel single-molecule sensors. We also show that transport can be controlled by tilting a molecule with respect to the electrode surfaces. Our results compare favorably with recent experiments.
The exchange scattering at magnetic adsorbates on superconductors gives rise to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the exchange coupling, the magnetic moment perturbs the Cooper pair condensate only weakly, resulting in a free-spin ground state, or binds a quasiparticle in its vicinity, leading to a (partially) screened spin state. Here, we use the flexibility of Fe-porphin molecules adsorbed on a Pb(111) surface to reversibly and continuously tune between these distinct ground states. We find that the FeP moment is screened in the pristine adsorption state. Approaching the tip of a scanning tunneling microscope, we exert a sufficiently strong attractive force to tune the molecule through the quantum phase transition into the free-spin state. We ascertain and characterize the transition by investigating the transport processes as function of tip-molecule distance, exciting the YSR states by single-electron tunneling as well as (multiple) Andreev reflections.
An easy to implement and powerful method for the solution of 3D scattering problems that can be well described by Helmholtz equation is presented. The matrix algebra used provides excellent stability versus the number of junctions as well as great computational speed. The matrix truncation method yields an easy single-parameter convergence procedure. Subsequently, some aspects of the electronic transport through metal nanowires are studied by the use of Landauers scattering approach to the conductance. We predict the existence of current vortex-rings patterns due to sharp enough narrow-wide connections in atomic size point contacts. Longitudinal resonances between scattering centers provide a simple physical picture for the understanding of negative differential resistance in ideal monoatomic contacts. Relatively long nanowires with high geometrical perfection -like those recently observed by Transmission Electron Microscopy- are modelled exhibiting resonant tunnelling and total reflection at given incident energy intervals.
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