Michael Galperin of the University of California San Diego is supported by an award from the Theory, Models and Computational Methods program in the Chemistry Division to develop non-equilibrium many-body methods for calculating electron transport in molecular junctions. Both resonant and non-resonant inelastic electron scattering are of concern in molecular-scale electronics where a single molecule may form a current-carrying bridge between different parts of a nanoscale device. At the core of the work is the incorporation of molecular quantum states into transport formalisms based on Hubbard non-equilibrium Green functions and quantum master equations. This allows simultaneous consideration of coupling between a single molecule and electrodes, optical and plasmonic excitation and scattering, electronic transitions and charge transfer, and vibrations and heat transfer. A need for development of these capabilities comes from emerging experiments such as the simultaneous measurement of conductance and Raman scattering for molecules bridging metallic break junctions under an applied potential bias. Addition of a mixed classical-quantum approach allows accommodation of nonadiabatic molecular motion and switching.
The drive toward ever-smaller electronic devices has as a limit the case of current traversing a single molecule that is attached to leads on either side. Usually current-carrying devices have so many atoms that they are treated statistically and not individually. If a single molecule forms a bridge this is no longer possible, and the electronic and vibrational motions of this molecule strongly affect the measured conductance and other properties. The work under this award develops a consistent theoretical picture joining the statistical treatment of the leads and the rigorous quantum treatment of the attached molecule. This project requires merging elements from several different disciplines, including quantum chemistry, molecular spectroscopy, transport theory and statistical physics. Molecular electronics courses for graduates and undergraduates will be developed that provide these interconnections in ways that traditional disciplinary courses cannot.
Our research is focused on developing theoretical approahes to description and understanding of dynamics in open non-equilibirum molecular systems. Such studies are facilitated by huge experimental advancements in laser technologies and fabrication at nanoscale, with the promise to revolutionize technology by miniaturization of devices, and developing qualitatively new quantum capabilities of functioning. Theoretical understanding of dynamics in such systems is crucial for develpment of crucial in developing memory and logic molecular devices, molecule based sensors, photovoltaics for solar conversion, molecular optoelectronics and spintronics. Intellectual merit of our research consiists in developing new theories and suggestions for realistic devices in the fields of quantum transport, nanoplasmonics, optical response of molecular devices, and spin transistors. In particular, 1. In nanoplasmonics we developed theoretical approaches for self-consistent description of electronic dynamics in open systems under driving by surface plasmons excited by external laser field. In additon ot developing a mixed quantum-classical theory (quantum for electron transport and classical for light propagation), we showed that self-consistent treatment of local field formation (due to both surface plasmon-polariton excitations in the contacts and the molecular response) is crucial for the proper description of the device characteristics. This fact was not accounted for in many of the previous works on the subject. 2. Description of optical response of open molecular systems is qualitativey different form the standard (isolated molecule) molecular spectroscopy. We are proud to be the first to develop a theoretical methodology capable of extending teh standard description ot teh case of current carrying junctions. Such formulation allowed us to successfully describe experimentally observed dynamical features (temporal correlations in conductance and optical signal, Raman staircase) in junctions, and to establish a solid ground for development of ab inito methodoogies for realistic simulations (projects currently underway in the group). 3. Molecular spintronics is a new field of research where spin (rather than charge) current is the detected signal. Recent experimental data on spin-flip inelastic electron tunneling spectroscopy and spin filtering capabilities in DNA junctions suggest possibility of building miniature molecular spintronic devices. Possibility of electric (rather than magnetic) field control is one of desired properties in such devices. We developed theories which allowed us to propose stable biradicals (particularly bis(nitronyl nitroxide) based biradicals) as potential molecular spin-pumps which should be persistent at ambient conditions. Also we demonstrated feasibility of DNA junctions to perform as spin filters. Within a physically relevant range of parameters we showed the possibility of generating pure spin currents in DNA by time-dependent electric fields. Broader impact is based on both clear impact of the development of electronics as a whole in our society, and inter-disciplinary nature of our research. In particular, 1. Our projects interconnect between several research communities (such as mesoscopic physics, quanutm chemistry, general biochemistry, statistical mechanics, quantum field theory, moleuclar spectrscopy and electrodynamics). Combination of the methods and ideas from different fields creates a unique research experience with graduate students and postdocs being exposed to several important branches of theoretical chemistry and physics. 2. Results of our research were disseminated through scientific journal publications and conferences. On one hand students attending those conferences were exposed. On the otehr participation of graduate students and postdocs in such meetings contributed to their own prepeartion for indepenedent research careers of their own. 3. We employed undergraduate students in research related to the project (Mr. Garrett Williams, summer quarter of 2011; Mrs. Roh Song, spring quarter of 2013), which helped them to get a glimpse of scinetific research and helped in developing their careers. 4. Several results of our research were used in teaching undergraduate (CHEM 132 - Chemical Physics (Statistical Mechanics); CHEM 133 - Chemical Physics (Quantum Mechanics)), and graduate (CHEM 230A - Advanced Quantum Chemistry) courses.
