The theory of charge and heat transport in nano-scale devices has evolved rapidly over the past decade as advances in experimental techniques have made it possible to probe transport properties down to the single molecule scale. The development of accurate theoretical methods for the description of quantum transport at the molecular level is essential for continued progress in a number of areas including molecular- and organic electronics, molecular spintronics, and molecular thermoelectrics. Furthermore, the atomic and electronic structure of metal-molecule interfaces and charge transfer processes across such interfaces, play a key role for photo- and electrochemical reactions, surface enhanced Raman spectroscopy, organic and dye-sensitized photovoltaics, etc..
The theory of charge and heat transport in nano-scale devices has evolved rapidly over the past decade as advances in experimental techniques have made it possible to probe transport properties down to the single molecule scale. The development of accurate theoretical methods for the description of quantum transport at the molecular level is essential for continued progress in a number of areas including molecular- and organic electronics, molecular spintronics, and molecular thermoelectrics. Furthermore, the atomic and electronic structure of metal-molecule interfaces and charge transfer processes across such interfaces, play a key role for photo- and electrochemical reactions, surface enhanced Raman spectroscopy, organic and dye-sensitized photovoltaics, etc.. The field of molecular transport involves central aspects of computational nanoscience, mesoscopic transport theory, organic- and electro-chemistry, and even biology. The interdisciplinary character of the subject makes the direct interaction between scientists with different fields of expertise particularly important. By bringing together theoretical and experimental researchers covering the different aspects of molecular transport, this workshop will provide the optimal opportunity to identify, discuss and even resolve the main challenges presently faced in the field.
The workshop is intended as a 'kick-off' activity of the newly established Psi-k working group "Quantum transport in nanostructures" which is coordinated by the organisers of the present workshop proposal. Following the two succesful CECAM sponsored workshops on the topic in 2010 (one in Zuerich organized by Evers et al. and one in Bremen organized by Frauenheim et al. ) there was no CECAM workshop on quantum transport in 2011. We feel this makes the need for a workshop in 2012 even more important and timely.
The role of the molecule-metal interface and its importance for the transport properties of molecular junctions cannot be emphasized enough. The most commonly used thiol anchoring group has been found to introduce significant variation of the molecular conductance. Recent studies indicate that the gold-sulphur interface can be much more complex than previously assumed with important consequences for the transport properties. As alternative to the strong sulphur-gold bond, weaker amine based linkers have been shown to lead to more well defined conductance characteristics , and very recently fullerene-, dithiocarbamate- and nitrile-based anchoring groups have also been shown to yield promising results in terms of reproducibility, robustness and lowering of the charge injection barriers [3-5]. The identification of the most stable atomic structures of metal-molecule interfaces is an essential first step for successful modeling of the junction transport properties. This poses a challenge since metal-molecule bonding often involves a significant portion of dispersive interactions which are not well described by the standard exchange-correlation functionals.
One of the longstanding problems of molecular charge transport is the establishment of a theoretical framework which allows for quantitatively accurate predictions of conductance from first principles. The need for methods going beyond the standard approach based on density functional theory (DFT) has been clear for a number of years, but only recently more advanced methods, based e.g. the many-body GW approximation, have appeared [7,8]. Such calculations are (almost) free from self-interaction errors and incorporate dynamical screening effects, i.e. image charge effects, which are very important for a proper description of the energy level alignment at the junction . In parallel, semi empirical corrections to the Kohn-Sham energy levels have been found to improve the agreement with experiments significantly [10,11]. In some situations electronic correlations on the molecule are large compared to the metal-molecule hopping rate leading to Coulomb blockade and Kondo physics at low temperatures. This transport regime calls for more advanced treatments of the electron-electron interactions based on e.g. dynamical mean field theory or numerical renormalization group theory[12,13]. The development of a method capable of describing both the coherent tunneling and strong correlation regimes for realistic systems remains an open challenge.
Inelastic tunneling spectroscopy constitutes an important basis for spectroscopy of molecular junctions yielding insight into the vibrational modes and ultimately the atomic structure of the junction. Recently, surface enhanced Raman spectroscopy on molecular junctions has been introduced as a complementary tool yielding equivalent information but under less restrictive conditions. Apart from its use in spectroscopy, the electron-phonon interactions give rise to heating and eventually break down of molecular junctions in the presence of an applied bias voltage. Steps towards a microscopic understanding of the break down mechanisms in molecular transport junctions have just been taken[16,17], but experimental support of these predictions are still lacking. The idea of using metal-molecule interfaces as basis for thermoelectric devices has been explored recently and it has been shown theoretically that very high thermoelectric figures of merits can be achieved through chemical tuning of the electronic states [18-21]. Most of these calculations treat neglect the phonon contribution to the heat transport, and it should be important to investigate how the phonons and the electron-phonon interactions affect the thermoelectric properties.
It is often stated that one of the main advantages of molecule-based electronics is the rich flexibility in design and functionality offered by these systems. Indeed, it has been shown that a molecule's conductance can be controlled by varying its conformation or by attaching different functional groups[22,23]. These effects can often be explained in terms of the induced changes in the width and position of the frontier molecular orbitals relative to the Fermi level. Quantum interference effects can also induce very pronounced changes in the electron transmission function. Several groups have recently addressed this phenomenon theoretically[24-26], and simple rules for its relation to the molecular structure have been developed. Very recently, experimental evidence for quantum interference in molecular junctions was obtained. In general, the effect of the interplay between molecular topology (including effects of interference) and the chemical nature of functional groups (accepting/donating character) on the transport properties can be very complex. A deeper understanding of this relation is a crucial prerequisite for designing molecules with specific electronic properties.
The access to sub-mK temperatures and large magnetic fields in STM experiments has recently made it possible to use inelastic electron tunneling spectroscopy to probe elementary spin excitations[29, 30]. This can provide unique insights into magnetism at the nanoscale and can be a powerful tool for understanding the magnetic interaction of atoms and molecules on surfaces. The conventional approach to model this problem consists in using the master equation scheme together with model Hamiltonian fitted from experiments . More recently however perturbative approaches have been proposed . These are amenable to ab initio implementations and can open a completely new area of research for first principles techniques applied to the electron transport problem.
Time-dependent density functional theory (TDDFT) provides a theoretical framework to address time-dependent transport phenomena such as transients, AC bias, interaction with light, fast switching processes etc. One of the specific technical difficulties of the transport problem is the proper treatment of the open boundary conditions. This problem has been tackled by including a sufficiently large but finite portion of the leads coupled to the device, by the use of absorbing potentials, or using embedding self energies[35,36]. While it is probably fair to say that the methodology for applying TDDFT to transport is still under development , applications to atomistic molecular models with adiabatic local density functionals are beginning to appear[38,39]. At the same time, applications to simple model systems have recently shown that memory effects and derivative discontinuities in the exchange-correlation potential can have dramatic effects on the system dynamics  which indicates that the choice of an appropriate approximation for the exchange-correlation potential may be crucial to describe certain transport properties. An alternative framework to time-dependent transport is given by many-body perturbation theory based on the Kadanoff-Baym equations where correlations are taken into account by using conserving approximations for the many-body self energy [41,42].