Proposed parameter-free model for interpreting the measured positron annihilation spectra of materials using a generalized gradient approximation

Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements opening new avenues for highly precise and advanced positron characterization of materials.Comment: 5 pages, 2 figures, 2 table

Improved generalized gradient approximation for positron states in solids

Several first-principles calculations of positron-annihilation characteristics in solids have added gradient corrections to the local-density approximation within the theory by Arponen and Pajanne [Ann. Phys. (N.Y.) 121, 343 (1979)] since this theory systematically overestimates the annihilation rates. As a further remedy we propose to use gradient corrections for other local density approximation schemes based on perturbed hypernetted-chain and on Quantum Monte Carlo results. Our calculations for various metals and semiconductors show that the proposed schemes generally improve the positron lifetimes when they are confronted with experiment. We also compare the resulting positron affinities in solids with data from slow-positron measurements.Comment: 16 pages, 5 figures, 3 table

Hydrogen bonding and coordination in normal and supercritical water from X-ray inelastic scattering

A direct measure of hydrogen bonding in water under conditions ranging from the normal state to the supercritical regime is derived from the Compton scattering of inelastically-scattered X-rays. First, we show that a measure of the number of electrons $n_e$ involved in hydrogen bonding at varying thermodynamic conditions can be directly obtained from Compton profile differences. Then, we use first-principles simulations to provide a connection between $n_e$ and the number of hydrogen bonds $n_{HB}$. Our study shows that over the broad range studied the relationship between $n_e$ and $n_{HB}$ is linear, allowing for a direct experimental measure of bonding and coordination in water. In particular, the transition to supercritical state is characterized by a sharp increase in the number of water monomers, but also displays a significant number of residual dimers and trimers.Comment: 14 pages, 5 figures, 1 tabl

Positron surface state as a spectroscopic probe for characterizing surfaces of topological insulator materials

Topological insulators are attracting considerable interest due to their potential for technological applications and as platforms for exploring wide-ranging fundamental science questions. In order to exploit, fine-tune, control, and manipulate the topological surface states, spectroscopic tools which can effectively probe their properties are of key importance. Here, we demonstrate that positrons provide a sensitive probe for topological states and that the associated annihilation spectrum provides a technique for characterizing these states. Firm experimental evidence for the existence of a positron surface state near Bi2Te2Se with a binding energy of Eb=2.7±0.2eV is presented and is confirmed by first-principles calculations. Additionally, the simulations predict a significant signal originating from annihilation with the topological surface states and show the feasibility to detect their spin texture through the use of spin-polarized positron beams.Academy of Finland (Projects No. 285809)Academy of Finland (Projects No. 293932)United States. Department of Energy. Office of Basic Energy Sciences (Grant No. DE-FG02- 07ER4635)United States. Department of Energy (Grant No. DE-AC02-05CH11231)National Science Foundation (U.S.) (Grants No. DMR-MRI-1338130)National Science Foundation (U.S.) (Grants No. DMR-1508719)National Science Foundation (U.S.) (Grants No. DMR-1231319)National Science Foundation (U.S.) (DMR-1207469)United States. Office of Naval Research (Grant No. N00014-13-1-0301)Laboratoire de physique théorique. École Normale Supérieule. International Centre for Fundamental Physics (Grant No. ANR-10-LABX-0010/ANR- 10-IDEX-0001-02 PSL