Suchergebnisse

We present a computational method to quantitatively describe the linear-response conductance of nanoscale devices in the Kondo regime. This method relies on a projection scheme to extract an Anderson impurity model from the results of density functional theory and nonequilibrium Green's functions calculations. The Anderson impurity model is then solved by continuous-time quantum Monte Carlo. The developed formalism allows us to separate the different contributions to the transport, including coherent or noncoherent transport channels, and also the quantum interference between impurity and background transmission. We apply the method to a scanning tunneling microscope setup for the 1,3,5-triphenyl-6-oxoverdazyl (TOV) stable radical molecule adsorbed on gold. The TOV molecule has one unpaired electron, which when brought in contact with metal electrodes behaves like a prototypical single Anderson impurity. We evaluate the Kondo temperature, the finite-temperature spectral function, and transport properties, finding good agreement with published experimental results. ; The work was sponsored by the European Union through the Project No. 618082 ACMOL. A.D. received additional support from the "Ministerio de Economia y Competitividad" (Mineco) of Spain Grant No. FPDI-2013-16641. ; Peer Reviewed

The energy gap of a molecular crystal is one of the most important properties since it determines the crystal charge transport when the material is utilized in electronic devices. This is, however, a quantity difficult to calculate and standard theoretical approaches based on density functional theory (DFT) have proven unable to provide accurate estimates. In fact, besides the well-known band-gap problem, DFT completely fails in capturing the fundamental gap reduction occurring when molecules are packed in a crystal structures. The failure has to be associated with the inability of describing the electronic polarization and the real space localization of the charged states. Here we describe a scheme based on constrained DFT, which can improve upon the shortcomings of standard DFT. The method is applied to the benzene crystal, where we show that accurate results can be achieved for both the band gap and also the energy level alignment. ; A.D. and I.R. were sponsored by the European Union through Project No. 618082 ACMOL. A.D. received additional support from the "Ministerio de Economia y Competitividad" (Mineco) of Spain (Grant No. FPDI-2013-16641). S.S. acknowledges the Quest project funded by European Research Council. Computational resources were provided by the Trinity Centre for High Performance Computing (TCHPC) and the Irish Centre for High-End Computing (ICHEC). ; Peer reviewed

Spin filtering at organic-metal interfaces is often determined by the details of the interaction between the organic molecules and the inorganic magnets used as electrodes. Here we demonstrate a spin-filtering mechanism based on the dynamical spin relaxation of the long-living interface states formed by the magnet and weakly physisorbed molecules. We investigate the case of Alq3 on Co and, by combining two-photon photoemission experiments with electronic structure theory, show that the observed long-time spin-dependent electron dynamics is driven by molecules in the second organic layer. The interface states formed by physisorbed molecules are not spin-split, but acquire a spin-dependent lifetime, that is the result of dynamical spin-relaxation driven by the interaction with the Co substrate. Such spin-filtering mechanism has an important role in the injection of spin-polarized carriers across the interface and their successive hopping diffusion into successive molecular layers of molecular spintronics devices. ; The experimental work carried out at the University of Kaiserslautern was partly funded by the SFB/TRR 173 Spin+X: spin in its collective environment (Project B05) from the DFG. A.D. and I.R. were sponsored by the European Union through the FP7 project 618082 ACMOL. S.S. acknowledges the European Research Council, Quest project, for financial support. P.T. and B.S. thankfully acknowledge financial support from the Graduate School of Excellence MAINZ (Excellence Initiative DFG/GSC 266). ; Peer reviewed