Electronic/spintronic transport, spectroscopy, and dynamics

Abstract

Modulation of the molecular orbitals in molecular systems is useful to tune the performance of electron/spin transport [1]. Such transport phenomena in molecular electronic/spintronic devices can be understood based on density functional theory (DFT) coupled to non-equilibrium Green function theory (NEGF) with which we have developed the Postrans program package [2]. The intriguing transport phenomena are found particularly from graphene [3] and graphene analogs [4]. Graphene nanoribbon (GNR) spin valves show the super magnetoresistance behavior as a spin filter [5]. Electronics spectroscopy of a GNR placed across a fluidic nanochannel could lead to a DNA sequencing method. In this device, while a single-stranded DNA passes beneath the GNR, a single base interacts with the GNR, giving a sharp conductance change due to Fano resonance [6]. These unique resonance profiles reflect the characteristic features and conformations of physisorbed molecules. The differential conductance with respect to bias and gate voltages not only distinguishes different types of nucleobases for DNA sequencing but also recognizes cancerous methylated nucleobases. This two-dimensional molecular electronics spectroscopy (2D MES) [7] could recognize single molecule signatures at atomic resolution. The advantages of the 2D MES over the one-dimensional (1D) current analysis can be comparable to those of 2D NMR over 1D NMR analysis. Finally, the development of attosecond spectroscopy to detect electronic motions in attosecond timescale is addressed. We have developed a code for real-time electron dynamics based on the real-time time-dependent density functional theory (TDDFT) [8] to investigate optical band gap, excitation energy, and corresponding electronic motions. We are also working on the development of a new coupled electron-nuclear dynamics for nonadiabatic situations. Electronic states with the electron-nuclear correlation show continuous and analytic behavior without any additional phase. The molecular Berry phase can be viewed as an artefact of the Born-Oppenheimer approximation [9].