Andrey Danilov

Single-electron transistor of a single organic molecule with access to several redox states.

A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots, metallic and semiconducting nanoparticles, carbon nanotubes, and single molecules. Recently, transport has been shown to be also influenced by spin—through the Kondo effect—for both nanotubes and single molecules, as well as by vibrational fine structure. Here we describe a single-electron transistor where the electronic levels of a single π-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.

Express Optical Analysis of Epitaxial Graphene on SiC: Impact of Morphology on Quantum Transport

We show that inspection with an optical microscope allows surprisingly simple and accurate identification of single and multilayer graphene domains in epitaxial graphene on silicon carbide (SiC/G) and is informative about nanoscopic details of the SiC topography, making it ideal for rapid and noninvasive quality control of as-grown SiC/G. As an illustration of the power of the method, we apply it to demonstrate the correlations between graphene morphology and its electronic properties by quantum magneto-transport.

Magnetic field resilient superconducting fractal resonators for coupling to free spins

We demonstrate a planar superconducting microwave resonator intended for use in applications requiring strong magnetic fields and high quality factors. In perpendicular magnetic fields of 20 mT, the niobium resonators maintain a quality factor above 25 000 over a wide range of applied powers, down to single photon population. In parallel field, the same quality factor is observed above 160 mT, the field required for coupling to free spins at a typical operating frequency of 5 GHz. We attribute the increased performance to the current branching in the fractal design. We demonstrate that our device can be used for spectroscopy by measuring the dissipation from a pico-mole of molecular spins.

Tunneling through a multigrain system: Deducing sample topology from nonlinear conductance

We study a current transport through a system of a few grains connected with tunneling links. The exact solution is given for an arbitrarily connected double-grain system with a shared gate in the framework of the orthodox model. The result is generalized for multigrain systems with strongly different tunneling resistances. We analyze the large-scale nonlinear conductance and demonstrate how the sample topology can be unambiguously deduced from the spectroscopy pattern (differential conductance versus gate-bias plot). We present experimental data for a multigrain sample and reconstruct the sample topology. A simple selection rule is formulated to distinguish samples with spectral patterns free from spurious disturbance caused by recharging of some grains nearby. As an example, we demonstrate experimental data with additional peaks in the spectroscopy pattern, which cannot be attributed to coupling to additional grains. The approach can be used to judge the sample topology when it is not guaranteed by fabrication and direct imaging is not possible.

Coulomb blockade effects at room temperature in thin-film nanoconstrictions fabricated by a novel technique

A technique was developed to fabricate and probe nanosize tunneling structures in thin metallic films. Using oblique evaporation through conventional undercut electron-beam lithographic masks, as the sample resistance was measured in situ, we defined constrictions with widths and lengths of about 10 nm in thin granular palladium films. The tunneling conductivity through a network of metallic grains was studied. Single electron tunneling transistor effects were registered. An electrostatic gate voltage at room temperature could clearly modulate Coulomb blockade offsets of the order of 0.1 V in the current–voltage curves.

Mixed valence radical cations and intermolecular complexes derived from indenofluorene-extended tetrathiafulvalenes

Engineering of mixed-valence (MV) radical cations and intermolecular complexes based on π-extended tetrathiafulvalenes (TTFs) is central for the development of organic conductors. On another front, redox-controlled dimerization of radical cations has recently been recognized as an important tool in supramolecular chemistry. Here we show that π-extended TTFs based on the indenofluorene core, prepared by Horner–Wadsworth–Emmons reactions, undergo reversible and stepwise one-electron oxidations and that the detectable, intermediate radical cation forms remarkably strong intermolecular MV ([neutral·cation]) and π-dimer ([cation·cation]) complexes with near-infrared radical cation absorptions. The radical cation itself seems to be a so-called Class III MV species in the Robin–Day classification. The formation of MV dimers was corroborated by ESR spectroelectrochemical studies, revealing two slightly different ESR signals upon oxidation, one assigned to the MV dimer and the other to the cation monomer. Crystals of the radical cation with different anions (PF6−, BF4−, and TaF6−) were grown by electrocrystallization. Conductance studies revealed that the salts behave as semiconductors with the hexafluorotantalate salt exhibiting the highest conductance. Using a custom-built ESR spectrometer with sub-femtomole sensitivity, the magnetic properties of one crystal were investigated. While the spin-to-spin interaction between radical cations was negligible, a high cooperativity coupling to the microwave field was observed – as a result of an exceptionally narrow spin line width and high spin density. This could have great potential for applications in quantum computation where crystalline spin ensembles are exploited for their long coherence times.

Nanoelectromechanical switch operating by tunneling of an entire C60 molecule.

We present a solid state single molecule electronic device where switching between two states with different conductance happens predominantly by tunneling of an entire C60 molecule. This conclusion is based on a novel statistical analysis of approximately 10(5) switching events. The analysis yields (i) the relative contribution of tunneling, current induced heating and thermal fluctuations to the switching mechanism, (ii) the voltage dependent energy barrier (approximately 100-200 meV) separating the two states of the switch and (iii) the switching attempt frequency, omega0, corresponding to a 2.8 meV mode, which is most likely rotational.

A near-field scanning microwave microscope based on a superconducting resonator for low power measurements

We report on the design and performance of a cryogenic (300 mK) near-field scanning microwave microscope. It uses a microwave resonator as the near-field sensor, operating at a frequency of 6 GHz and microwave probing amplitudes down to 100 μV, approaching low enough photon population (N ∼ 1000) of the resonator such that coherent quantum manipulation becomes feasible. The resonator is made out of a miniaturized distributed fractal superconducting circuit that is integrated with the probing tip, micromachined to be compact enough such that it can be mounted directly on a quartz tuning-fork, and used for parallel operation as an atomic force microscope (AFM). The resonator is magnetically coupled to a transmission line for readout, and to achieve enhanced sensitivity we employ a Pound-Drever-Hall measurement scheme to lock to the resonance frequency. We achieve a well localized near-field around the tip such that the microwave resolution is comparable to the AFM resolution, and a capacitive sensitivity down to 6.4 × 10(-20) F/Hz, limited by mechanical noise. We believe that the results presented here are a significant step towards probing quantum systems at the nanoscale using near-field scanning microwave microscopy.

Tunneling through a single quench-condensed cluster

Quench-condensed bismuth films of 3–5 nm thickness have beenused as a cluster source to prepare Single ElectronTransistors (SET) based on a single cluster with high chargingenergy. We used electron-beam defined shadow evaporation masksto pattern 10 nm wide constrictions in these films. Byincremental depositions through these masks controlled by insitu sample conductance measurements, we obtained a SETgeometry for clusters with charging energies up to 90 meV. Ourexperiment showed that the SET geometry can be achieved inevery sample preparation run, despite the apparent randomnature of cluster formation in granular films. The resultingcharging energy of the transistor varied from experiment toexperiment. Its value, however, was always higher than 10 meV.

Galvanically split superconducting microwave resonators for introducing internal voltage bias

We present the design and performance of high-Q superconducting niobium nitride microwave resonators intended for use in hybrid quantum systems, coupling spin degrees of freedom to the cavity mode, both magnetically and electrically. We demonstrate a solution that allows to introduce static electric fields in the resonator without compromising the microwave performance. Quality factors above 105 remain unchanged in strong applied static electric fields above 10 MV/m and magnetic fields up to ∼400 mT. By design, the configuration of the dc field matches that of the microwave field, especially advantageous for experiments on electrostatically controlled spin systems.