Organisers

• Thomas Frederiksen (DIPC - Donostia International Physics Center, Spain)
• Ferdinand Evers (Karlsruhe Institute of Technology, Germany)
• Kyungwha Park (Virginia Tech, USA)

Supports

   CECAM

Donostia International Physics Center (DIPC)

Karlsruhe Institute of Technology (KIT)

Description

Recently, substantial progress has been made in various subfields of molecular nanostructures. We list a few examples. First-principles calculations based on density-functional theory (DFT) have been successfully used for qualitative understanding of coherent transport through molecules in a strong coupling regime. However, due to inherent limitations in DFT, this approach does not provide qualitatively correct transport features for molecules where electron correlations are strong or for a weak coupling regime. Thus, in those cases, it becomes increasingly important to combine DFT with other computational and analytical tools, such as the Anderson impurity model [1], the dynamic mean field theory [2], the density matrix and the numerical renormalization group [4], and the GW approximation [5]. Additionally, there has been some progress in understanding how the interactions between internal degrees of freedom of molecules and local environments affect transport characteristics. A few examples are the interplays between transport properties and vibrational [6-8] and magnetic degrees of freedom [1-3,23], spin-flip processes, adsorbate motions, dissipation, and noises. To push further developments, it is crucial to have the crosstalk among researchers pursuing these very different approaches as well as a continuous feedback from experiments. 

Recent experimental progress has been highly promising. It surely motivates theorists to incorporate additional degrees of freedom into their studies and to look into new hybrid structures [21]. Simultaneous measurements of conductance and Raman spectra for molecular junctions open a new avenue in our search for the effects of bonding geometry, chemical environments, and vibrational degrees of freedom on the transport [22]. Experiments on break junctions reveal intriguing mechanical and collective effects [12-14]. Binding and transport properties of new hybrids and new anchor groups have been tested, including C60, with encouraging results [16-18, 21]. Low-temperature STM experiments show a possibility of controlling the Kondo effect atom by atom [1] and the intriguing Kondo effect for single adatoms with magnetic anisotropy [23]. STM measurements also provide access to force constants for single adatoms [20] and allow to map out vibrational states of adsorbates [19]. 

Many attempts to cure the problem caused by the approximations in the DFT functionals, at least partially, are hampered by computational issues or by the necessity to keep too many degrees of freedom. A growing number of examples show how to overcome some of the difficulties by a close collaboration between experimental and computational groups. In these cases, a lack of quantitative predictability is compensated by a good understanding of qualitative trends. This positive development is partly due to (i) a better understanding of the effects of functional approximations, which is what the mathematical and analytical colleagues can take credit for, and (ii) an enhanced cooperation between the different sub-disciplines. 

Scientific Objectives

The objective of the workshop is to provide a platform where computational scientists interact with mathematically and experimentally oriented experts in the different subfields such as Mesoscopic Physics, Surface Science, and Material Science. 

More specifically, we intend to provide a forum

(i)where the DFT-based community interacts with experts from strongly correlated systems in order to (a) identify and remove modeling artifacts and (b) develop methodology, 

(ii)where mathematically oriented people present qualitative features based on model systems: important in order to properly interpret experiments and first-principles calculations,

(iii) where experimentalists share findings to suggest new test systems for modeling and simulations 

(iv) where computational experts report the features of specific molecular nanostructures, which is crucial feedback for experimentalists and model building

References

First-principles methods for Kondo effect, electron correlations, and spin-filtering in nanoscale contacts:
[1] N. Néel, J. Kröger, R. Berndt, T. O. Wehling, A. I. Lichtenstein, and M. I. Katsnelson: “Controlling the Kondo Effect in CoCun Clusters Atom by Atom”, Phys. Rev. Lett. 101, 266803 (2008).
[2] D. Jacob, K. Haule, and G. Kotliar: ”Kondo Effect and Conductance of Nanocontacts with Magnetic Impurities”, Phys. Rev. Lett. 103, 016803 (2009).
[3] S. Barraza-Lopez, K. Park, V. García-Suárez, and J. Ferrer: “First-principles Study of Electron Transport through the Single-Molecule Magnet Mn12”, Phys. Rev. Lett. 102, 246801 (2009).

Beyond DFT:
[4] P. Schmitteckert and F. Evers: “Exact Ground State Density-Functional Theory for Impurity Models Coupled to External Reservoirs and Transport Calculations”, Phys. Rev. Lett. 100, 086401 (2008); P. Lucignano, R. Mazzarello, A. Smogunov, M. Fabrizio and E. Tosatti, “Kondo Conductance in an Atomic Nanocontact from First Principles”, Nature Materials, 8, 563 (2009).
[5] C. D. Spataru, M. S. Hybertsen, S. G. Louie, and A. J. Millis: “GW approach to Anderson model out of equilibrium: Coulomb blockade and false hysteresis in the I-V characteristics”, Phys. Rev. B 79, 155110 (2009); P. Myohanen, A. Stan, G. Stefanucci, and R. van Leeuwen, “Kadanoff-Baym approach to quantum transport through interacting nanoscale systems: From the transient to the steady-state regime”, arXiv:0906.2136.

Inelastic effects in electron transport (nuclear vibrations, spin-flip, adsorbate motion, etc.):
[6] S. G. Tikhodeev and H. Ueba: “How Vibrationally Assisted Tunneling with STM Affects the Motions and Reactions of Single Adsorbates”, Phys. Rev. Lett. 102, 246101 (2009).
[7] M. Paulsson, C. Krag, T. Frederiksen, and M. Brandbyge: “Conductance of alkanedithiol single-molecule junctions: a molecular dynamics study”, Nano Lett. 9, 117 (2009).
[8] M. Paulsson, T. Frederiksen, H. Ueba, N. Lorente, and M. Brandbyge: “Unified description of Inelastic Propensity Rules for Electron Transport through Nanoscale Junctions”, Phys. Rev. Lett. 100, 226604 (2008).
[9] M. C. Lüffe, J. Koch, and F. von Oppen: “Theory of vibrational absorption sidebands in the Coulomb-blockade regime of single-molecule transistors”, Phys. Rev. B 77, 125306 (2008).
[10] O. Entin-Wohlmann, Y. Imry, and A. Aharony, “Voltage-induced singularities in transport through
single molecular junctions”, arXiv:0904.4385; also R. Egger and A.O. Gogolin, Phys. Rev. B 77, 113405 (2008).
[11] R. Haertle, C. Benesch, and M. Thoss, “Vibrational Nonequilibrium Effects in the Conductance of Single Molecules with Multiple Electronic States”, Phys. Rev. Lett. 102, 146801 (2009).

Experimental works:
[12] M. L. Trouwborst, E. H. Huisman, F. L. Bakker, S. J. van der Molen, and B. J. van Wees: “Single Atom Adhesion in Optimized Gold Nanojunctions”, Phys. Rev. Lett. 100, 175502 (2008).
[13] E. H. Huisman, M. L. Trouwborst, F. L. Bakker, B. de Boer, B. J. van Wees, and S. J. van der Molen., “Stablezing single atom contacts by molecular bridge formation”, Nano Lett. 8, 3381 (2008).
[14] M.L. Trouwborst, E. H. Huisman, S. J. van der Molen, and B.J. Van Wees, “Bistable hysteresis and resistance switching in hydrogen gold junctions”, arXiv:0906.5006.
[15] A.K. Huettel, B. Witkamp, M. Leijnse, M.R. Wegewijs, and H.S. J. van der Zant,
“Pumping of vibrational excitations in a Coulomb blockaded suspended carbon nanotube”,
Phys. Rev. Let. 102, 225501 (2009).
[16] C. A. Martin, D. Ding, H. S. J. van der Zant, and J. M. van Ruitenbeek, “Lithographic mechanical break junctions for single-molecule vaccum: possibilities and limitations”, New. J. Phys. 10, 065008 (2008).
[17] C.A. Martin, D. Ding, J. K. Sorensen, Th. Bjoernholm, J. M. van Ruitenbeek, and H. S. J. van der Zant, “Fullerene-Based Anchoring Groups for Molecular Electronics”, J. Am. Chem. Soc. 130, 13198 (2008).
[18] J. A. Larsson et al., “Orientation of individual C60 molecules adsorbed on Cu(111)”, Phys. Rev. B 77, 115434 (2008).
[19] A. F. Takacs, F. Witt, S. Schmauss et al. “Electron transport through single phthalocyanine molecules studied using STM”, Phys. Rev. B 78, 233404 (2008) and unpublished (2009).
[20] M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, and A. J. Heinrich: “The Force Needed to Move an Atom on a Surface”, Science 319, 1066 (2008).
[21] L. Bogani, C. Danieli, E. Biavardi, N. Bendiab, A.-L. Barra, E. Dalcanale, W. Wernsdorfer, and A. Cornia, “Single-Molecule-Magnet Carbon-Nanotube Hybrids,” Angew. Chem. Int. Ed. 48, 746 (2009).
[22] D.R. Ward, N.J. Halas, J.W. Ciszek, J.M. Tour, Y. Wu, P. Nordlander, and D. Natelson, “Simultaneous measurements of electronic conduction and Raman response in molecular junctions,” Nano Lett. 8, 919 (2008).
[23] A.F. Otte, M. Ternes, K. von Bergmann, S. Loth, H. Brune, C.P. Lutz, C.F. Hirjibehedin, and A. J. Heinrich, “The role of magnetic anisotropy in the Kondo effect,” Nature Physics 4, 847 (2008).