We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting π-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level.

We develop a π-electron effective field theory (π-EFT) wherein the two-body Hamiltonian for a π-electron system is expressed in terms of three effective parameters: the π-orbital quadrupole moment, the on-site repulsion, and a dielectric constant. As a first application of this π-EFT, we develop a model of screening in molecular junctions based on image multipole moments, and use this to investigate the reduction of the HOMO-LUMO gap of benzene. Beyond this, we also use π-EFT to calculate the differential conductance spectrum of the prototypical benzenedithiol-Au single-molecule junction and the π-electron contribution to the van der Waals interaction between benzene and a metallic electrode.

Summary Background: The transport through a quantum-scale device may be uniquely characterized by its transmission eigenvalues τn. Recently, highly conductive single-molecule junctions (SMJ) with multiple transport channels (i.e., several τn > 0) have been formed from benzene molecules between Pt electrodes. Transport through these multichannel SMJs is a probe of both the bonding properties at the lead–molecule interface and of the molecular symmetry. Results: We use a many-body theory that properly describes the complementary wave–particle nature of the electron to investigate transport in an ensemble of Pt–benzene–Pt junctions. We utilize an effective-field theory of interacting π-electrons to accurately model the electrostatic influence of the leads, and we develop an ab initio tunneling model to describe the details of the lead–molecule bonding over an ensemble of junction geometries. We also develop a simple decomposition of transmission eigenchannels into molecular resonances based on the isolated resonance approximation, which helps to illustrate the workings of our many-body theory, and facilitates unambiguous interpretation of transmission spectra. Conclusion: We confirm that Pt–benzene–Pt junctions have two dominant transmission channels, with only a small contribution from a third channel with τn << 1. In addition, we demonstrate that the isolated resonance approximation is extremely accurate and determine that transport occurs predominantly via the HOMO orbital in Pt–benzene–Pt junctions. Finally, we show that the transport occurs in a lead–molecule coupling regime where the charge carriers are both particle-like and wave-like simultaneously, requiring a many-body description.

In many organizations, actively engaging in user-centered design (UCD) techniques is standard practice when delivering products into the commercial marketplace or to external customers. But in these same organizations -or in organizations not delivering products to an external customer -the creation of systems for use by employees is a conversation between IT and the business unit. UCD professionals are either not participating, or they have very limited influence. This SIG creates a forum for people with real-world experience and challenges to discuss how -and whether it is even possible -to bring UCD into the IT organization.

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