Formation of hydrogen impurity states in silicon and insulators at low implantation energies

The formation of hydrogen-like muonium (Mu) has been studied as a function of implantation energy in intrinsic Si, thin films of condensed van der Waals gases (N2, Ne, Ar, Xe), fused and crystalline quartz and sapphire. By varying the initial energy of positive muons (mu+) between 1 and 30 keV the number of electron-hole pairs generated in the ionization track of the mu+ can be tuned between a few and several thousand. The results show the strong suppression of the formation of those Mu states that depend on the availability of excess electrons. This indicates, that the role of H-impurity states in determining electric properties of semiconductors and insulators depends on the way how atomic H is introduced into the material.Comment: 4 pages, 4 enscapulated postscript figures, uses revtex4 twocolumn style to be published in Physical Review Letter

An ab initio and dynamics study of the photodissociation of nitric acid HNO3

We investigated the photodissociation of HNO3 within the first (300 nm) and the third (200 nm) absorption band. The relevant S1 and S3 potential energy surfaces were calculated by taking into account the N-O single bond and N=O “double” bond distances. The striking feature of the dynamical analysis is a bifurcation of the wave packet on the S3 surface which explains the branching into the two reaction pathways with the products OH+NO2 and O+HONO found in experiments. Dissociation on the S1 surface is predicted to proceed along a single channel leading to OH+NO2, both in their electronic ground states. Corresponding author

Quantum information processing using strongly-dipolar coupled nuclear spins

Dipolar coupled homonuclear spins present challenging, yet useful systems for quantum information processing. In such systems, eigenbasis of the system Hamiltonian is the appropriate computational basis and coherent control can be achieved by specially designed strongly modulating pulses. In this letter we describe the first experimental implementation of the quantum algorithm for numerical gradient estimation on the eigenbasis of a four spin system.Comment: 5 pages, 5 figures, Accepted in PR

Cosmogenic rare gases and 10-Be in a cross section of Knyahinya

The concentrations of cosmogenic nuclides were studied as a function of shielding on samples from a cross section of the 293 kg main fragment of the L5 chondrite Knyahinya. The stone broke into two nearly symmetrical parts upon its fall in 1866. The planar cross section has diameters between 40 and 55 cm. He, Ne, and Ar were measured on about 20 samples by mass spectrometry and the 10-Be activities on aliquots of 10 selected samples were determined by AMS. The 10-Be data are presented and the abundances of spallogenic nuclides are compared with the model calculations reported by Reedy for spherical L chondrites. The 10-Be production rates in Knyahinya are shown versus the shielding parameter 22-Ne/21-Ne

Charge degree of freedom and single-spin fluid model in YBa_2Cu_4O_8

We present a 17O nuclear magnetic resonance study in the stoichiometric superconductor YBa_2Cu_4O_8. A double irradiation method enables us to show that, below around 180 K, the spin-lattice relaxation rate of plane oxygen is not only driven by magnetic, but also significantly by quadrupolar fluctuations, i.e. low-frequency charge fluctuations. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. These findings show that, with the opening of the pseudo spin gap, a charge degree of freedom of mainly oxygen character is present in the electronic low-energy excitation spectrum.Comment: 4 pages, 3 figures, REVTE

Depth dependent spin dynamics of canonical spin glass films: A low-energy muon spin rotation study

We have performed depth dependent muon spin rotation/relaxation studies of the dynamics of single layer films of {\it Au}Fe and {\it Cu}Mn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5-70 nm). A significant reduction in the muon spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5-10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to e.g. 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction

First-Principles Calculations of Hyperfine Interactions in La_2CuO_4

We present the results of first-principles cluster calculations of the electronic structure of La_2CuO_4. Several clusters containing up to nine copper atoms embedded in a background potential were investigated. Spin-polarized calculations were performed both at the Hartree-Fock level and with density functional methods with generalized gradient corrections to the local density approximation. The distinct results for the electronic structure obtained with these two methods are discussed. The dependence of the electric-field gradients at the Cu and the O sites on the cluster size is studied and the results are compared to experiments. The magnetic hyperfine coupling parameters are carefully examined. Special attention is given to a quantitative determination of on-site and transferred hyperfine fields. We provide a detailed analysis that compares the hyperfine fields obtained for various cluster sizes with results from additional calculations of spin states with different multiplicities. From this we conclude that hyperfine couplings are mainly transferred from nearest neighbor Cu^{2+} ions and that contributions from further distant neighbors are marginal. The mechanisms giving rise to transfer of spin density are worked out. Assuming conventional values for the spin-orbit coupling, the total calculated hyperfine interaction parameters are compared to informations from experiments.Comment: 23 pages, 9 figure