Jonas Sköldberg

Critical roles of metal-molecule contacts in electron transport through molecular-wire junctions

We study the variation of electron transmission through Au-S-benzene-S-Au junctions and related systems as a function of the structure of the Au:S contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly coordinated in-hollow and bridge positions are connected with broad transmission peaks around the Fermi level, due to a broad range of transmission angles from transverse motion, resulting in high conductivity and weak dependence on geometrical variations. In contrast, for (unstable) S adsorption on-top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission peaks narrow up due to suppression of large transmission angles. Such more one-dimensional situations may describe more common types of contacts and junctions, resulting in large variations in conductivity and sensitivity to bonding sites, tilting, and gating. In particular, if S is adsorbed in an Au vacancy, sharp spectral features appear near the Fermi level due to essential changes of the level structure and hybridization in the contacts, admitting order-of-magnitude variations of the conductivity. Possibly such a system, can it be fabricated, will show extremely strong nonlinear effects and might work as uni- or bi-directional voltage-controlled two-terminal switches and nonlinear mixing elements. Finally, density-functional theory based transport calculations seem relevant, being capable of describing a wide range of transmission peak structures and conductivities. Prediction and interpretation of experimental results probably require more precise modeling of realistic experimental situations.

Nanocell Devices and Architecture for Configurable Computing With Molecular Electronics

We develop a method to configure a 3-D nonlinear nanoparticle-molecule network to performing ten out of twelve possible combinations of two 2-bit logic gates with shared inputs. The logic gates are based on a simple circuit with adjustable linear and fixed negative differential resistance (NDR) elements. A bistable latch for signal restoration is an integral part of this target circuit. The simulations show that conductive patterns can be formed by applying voltages on the input-output pins of the nanocell. They also show that one-link gaps (short highly resistive links) can be created within the conductive channels. Furthermore, we discuss methods for introducing NDR molecules in these gaps, a crucial element of the target circuit. The structures resulting from the simulations are put in an architectural context, in which complex functions can be realized from the individual nanocell logic gates.

Spectrum of Andreev Bound States in a Molecule Embedded Inside a Microwave-Excited Superconducting Junction

Nondissipative Josephson current through nanoscale superconducting constrictions is carried by spectroscopically sharp energy states, the so-called Andreev states. Although theoretically predicted almost 40 years ago, no direct spectroscopic evidence of these Andreev bound states exists to date. We propose a novel type of spectroscopy based on embedding a superconducting constriction, formed by a single-level molecule junction, in a microwave QED cavity environment. In the electron-dressed cavity spectrum we find a polariton excitation at twice the Andreev bound state energy, and a superconducting-phase-dependent ac Stark shift of the cavity frequency. Dispersive measurement of this frequency shift can be used for Andreev bound state spectroscopy.

Reconfigurable logic in nanoelectronic switching networks

We demonstrate how to configure and reconfigure a nanoelectronic nonlinear network to a universal set of logic gates by applying sequences of voltage pulses to the edges of the network. The nanoelectronic device is designed to consist of a self-assembled network of nanoparticles connected by two-terminal linker elements with hysteretic behaviour, allowing voltage-controlled switching between a linear and nonlinear current–voltage characteristic (IVC), making reconfigurable logic possible.

Robustness of logic gates and reconfigurability of neuromorphic switching networks

Nanoparticle networks with functional molecular links that show current-voltage characteristics (IVC) with negative differential resistance (NDR) can be trained to perform XOR-AND logic gates (Husband et al. [1]; Skoldberg and Wendin [2]). In this work we investigate the robustness of the Nanocell network by removing links until desired logic gates no longer can be configured or operated within our simulation of the network. We present results for the robustness of XOR-AND configured (halfadder) Nanocells, as well as the effects of varying the IVC and NDR characteristics of the linker molecules.