Session Z3: Invited Session: Frontiers of Non-Equilibrium Transport Theories

DFT treatment of transport through Anderson junction: exact results and approximations

Since the pioneering break-junction experiments of Reed and Tour measuring the conductance of dithiolated benzene between gold leads, many researchers in physics and chemistry have been calculating conductance for such systems using density functional theory (DFT). Off resonance, the predicted current is often 10-100 times larger than that measured. This error is often ascribed to the application of ground-state DFT to a non-equilibrium problem. I will argue that, in fact, this is largely due to errors in the density functional approximations in popular use, rather than necessarily errors in the methodology. A stark illustration of this principle is the ability of DFT to reproduce the exact transmission through an Anderson junction at zero-temperature and weak bias, including the Kondo plateau, but only if the exact ground-state density functional is used. In fact, this case can be used to reverse-engineer the exact functional for this problem. Popular approximations can also be tested, including both smooth and discontinuous functionals of the density, as well as symmetry-broken approaches. \\[4pt] [1] Kondo effect given exactly by density functional theory, J. P. Bergfield, Z. Liu, K. Burke, and C. A. Stafford, arXiv:1106.3104; \\[0pt] [2] Broadening of the Derivative Discontinuity in Density Functional Theory, F. Evers, and P. Schmitteckert, arXiv:1106.3658; \\[0pt] [3] DFT-based transport calculations, Friedel's sum rule and the Kondo effect, P. Tr\"{o}ster, P. Schmitteckert, and F. Evers, arXiv:1106.3669; \\[0pt] [4] Towards a description of the Kondo effect using time-dependent density functional theory, G. Stefanucci, and S. Kurth, arXiv:1106.3728.   

Inelastic single-spin transport theory

Spin-flip inelastic spectroscopy is a powerful tool for investigating the magnetic excitations of nano-scale magnets deposited on a metallic surface. In this talk I will present a perturbative approach to the calculation of the inelastic spin-flip spectra of magnetic adatoms, small magnetic clusters and magnetic molecules. The theory is based on the non-equilibrium Green's function formalism combined with a model spin Hamiltonian, where the conduction electrons are exchanged coupled to a system of quantum spins. By expanding the self-energy describing the electron-spin interaction to the third order we are able to capture both inelastic spin-flip events and the signature of Kondo resonances. Furthermore, when our approach is combined with a Master equation describing spin-relaxation, effects related to the spin-pumping at the single spin level can be described. In the talk I will demonstrate that the method offers an extremely good quantitative agreement with published experimental data. Importantly the formalism is amenable to be implemented together with highly accurate electronic structure methods.   

Electron correlation effects on the diode properties and the local heating

Single molecular bridge junctions and atomic wires provide one of the best test fields for non-equilibrium transport theories whose progress gives benefits over wide range of physics. Experimental progresses in inelastic tunneling spectroscopy (IETS) and break junction techniques have played very important roles to make this possible. Inelastic scatterings between electrons and phonons give ``local heating'' of the junctions. The effective temperature due to the local heating was discussed successfully in terms of a fully self-consistent theory treating energy dissipation processes as well as inelastic heat generation on equal footing [1]. Recently, we found two cases where electron correlation gives distinct changes. The first case was found in the local heating problem in the resonant systems, where phonon damping due to its coupling with electron-hole excitation is suppressed by the correlation. The suppression enhances heat release to electrodes leading to the effective temperature suppression [2]. Another example is the single molecular rectifier. First principle NEGF-GGA calculation fails to explain the large rectification ratio (RR) at high bias voltage. Separate GW calculation based on Keldysh Green's function gives clear enhancement of RR over the mean field NEGF results suggesting that RR could be enhanced by the electron correlation effect [3]. Thus latest non-equilibrium transport theories enable us to treat the important physical processes accompanying electric conduction allowing us to make more direct comparisons with experimental phenomena at nano-scale. \\[4pt] [1] Y. Asai, Phys. Rev. B78, 045434 (2008).\\[0pt] [2] Y. Asai, Phys. Rev. B84, 085436 (2011).\\[0pt] [3] Y. Asai, H. Nakamura, J. Hihath, C. Bruot, and N.J Tao, Phys. Rev. B 84, 115436 (2011).   

Electron-ion correlations in electromigration: Coulomb's law and Landauer transport at the nanoscale

Electromigration has gained increased prominence in recent years, as the rise of nanoelectronics has given way to higher current and power densities in computing interconnects and devices. In this context we address the fundamental materials question: what drives electromigration at the nanoscale? Our understanding of the forces that drive electromigration has remained at an uneasy juncture between the mesoscopic semi-classical and atomistic quantum mechanical regimes. At the nanoscale an atomistic understanding of materials is required. Through first-principles quantum transport calculations we show that a self-consistent Laundaer transport framework provides much needed atomistic insight into the fundamental Coulombic forces which drive both current flow and electromigration at the nanoscale. These insights provide a general timely overview of the importance of electromigration in modern nanoelectronic devices and materials, not only from an operational perspective but also from a novel materials design perspective.   

Probing DNA in nanopores via tunneling: from sequencing to ``quantum'' analogies

Fast and low-cost DNA sequencing methods would revolutionize medicine: a person could have his/her full genome sequenced so that drugs could be tailored to his/her specific illnesses; doctors could know in advance patients' likelihood to develop a given ailment; cures to major diseases could be found faster [1]. However, this goal of ``personalized medicine'' is hampered today by the high cost and slow speed of DNA sequencing methods. In this talk, I will discuss the sequencing protocol we suggest which requires the measurement of the distributions of transverse currents during the translocation of single-stranded DNA into nanopores [2-5]. I will support our conclusions with a combination of molecular dynamics simulations coupled to quantum mechanical calculations of electrical current in experimentally realizable systems [2-5]. I will also discuss recent experiments that support these theoretical predictions. In addition, I will show how this relatively unexplored area of research at the interface between solids, liquids, and biomolecules at the nanometer length scale is a fertile ground to study quantum phenomena that have a classical counterpart, such as ionic quasi-particles, ionic ``quantized'' conductance [6,7] and Coulomb blockade [8]. Work supported in part by NIH. \\[4pt] [1] M. Zwolak, M. Di Ventra, Physical Approaches to DNA Sequencing and Detection, Rev. Mod. Phys. 80, 141 (2008).\\[0pt] [2] M. Zwolak and M. Di Ventra, Electronic signature of DNA nucleotides via transverse transport, Nano Lett. 5, 421 (2005).\\[0pt] [3] J. Lagerqvist, M. Zwolak, and M. Di Ventra, Fast DNA sequencing via transverse electronic transport, Nano Lett. 6, 779 (2006).\\[0pt] [4] J. Lagerqvist, M. Zwolak, and M. Di Ventra, Influence of the environment and probes on rapid DNA sequencing via transverse electronic transport, Biophys. J. 93, 2384 (2007).\\[0pt] [5] M. Krems, M. Zwolak, Y.V. Pershin, and M. Di Ventra, Effect of noise on DNA sequencing via transverse electronic transport, Biophys. J. 97, 1990, (2009).\\[0pt] [6] M. Zwolak, J. Lagerqvist, and M. Di Ventra, Ionic conductance quantization in nanopores, Phys. Rev.Lett. 103, 128102 (2009).\\[0pt] [7] M. Zwolak, J. Wilson, and M. Di Ventra, Dehydration and ionic conductance quantization in nanopores, J. Phys. Cond. Matt. 22 454126 (2011). \\[0pt] [8] M. Krems and M. Di Ventra, Ionic Coulomb blockade in nanopores arXiv:1103.2749.