Steering and ro-vibrational effects in the dissociative adsorption and associative desorption of H_2/Pd(100)

The interaction of hydrogen with many transition metal surfaces is characterized by a coexistence of activated with non-activated paths to adsorption with a broad distribution of barrier heights. By performing six-dimensional quantum dynamical calculations using a potential energy surface derived from ab initio calculations for the system H_2/Pd(100) we show that these features of the potential energy surface lead to strong steering effects in the dissociative adsorption and associative desorption dynamics. In particular, we focus on the coupling of the translational, rotational and vibrational degrees of freedom of the hydrogen molecule in the reaction dynamics.Comment: 8 pages, 5 figures, subm. to the Proceedings of ISSP-18, June 1996, Poland, to appear in Prog. Surf. Sc

Pinning of Fermionic Occupation Numbers

The Pauli exclusion principle is a constraint on the natural occupation numbers of fermionic states. It has been suspected since at least the 1970's, and only proved very recently, that there is a multitude of further constraints on these numbers, generalizing the Pauli principle. Here, we provide the first analytic analysis of the physical relevance of these constraints. We compute the natural occupation numbers for the ground states of a family of interacting fermions in a harmonic potential. Intriguingly, we find that the occupation numbers are almost, but not exactly, pinned to the boundary of the allowed region (quasi-pinned). The result suggests that the physics behind the phenomenon is richer than previously appreciated. In particular, it shows that for some models, the generalized Pauli constraints play a role for the ground state, even though they do not limit the ground-state energy. Our findings suggest a generalization of the Hartree-Fock approximation

Spin transport and spin dephasing in zinc oxide

The wide bandgap semiconductor ZnO is interesting for spintronic applications because of its small spin-orbit coupling implying a large spin coherence length. Utilizing vertical spin valve devices with ferromagnetic electrodes (TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in all-electrical experiments. The measured magnetoresistance agrees well with the prediction of a two spin channel model with spin-dependent interface resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K), 10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns (2K), 2.0ns (10K), and 31ps (200K).Comment: 7 pages, 5 figures; supplemental material adde