Highly charged ions: optical clocks and applications in fundamental physics

Recent developments in frequency metrology and optical clocks have been based on electronic transitions in atoms and singly charged ions as references. These systems have enabled relative frequency uncertainties at a level of a few parts in $10^{-18}$. This accomplishment not only allows for extremely accurate time and frequency measurements, but also to probe our understanding of fundamental physics, such as variation of fundamental constants, violation of the local Lorentz invariance, and forces beyond the Standard Model of Physics. In addition, novel clocks are driving the development of sophisticated technical applications. Crucial for applications of clocks in fundamental physics are a high sensitivity to effects beyond the Standard Model and Einstein's Theory of Relativity and a small frequency uncertainty of the clock. Highly charged ions offer both. They have been proposed as highly accurate clocks, since they possess optical transitions which can be extremely narrow and less sensitive to external perturbations compared to current atomic clock species. The selection of highly charged ions in different charge states offers narrow transitions that are among the most sensitive ones for a change in the fine-structure constant and the electron-to-proton mass ratio, as well as other new physics effects. Recent advances in trapping and sympathetic cooling of highly charged ions will in the future enable high accuracy optical spectroscopy. Progress in calculating the properties of selected highly charged ions has allowed the evaluation of systematic shifts and the prediction of the sensitivity to the "new physics" effects. This article reviews the current status of theory and experiment in the field.Comment: 53 pages, 16 figures, submitted to RM

Thermally activated delayed fluorescence in neutral and cationic copper(I) complexes with the 2-(4-Thiazolyl)benzimidazole ligand

Cationic [Cu(P^P)(Htbz)]PF6 [P^P = xantphos, dpephos; Htbz = 2-(4-thiazolyl)benzimidazole] and the corresponding neutral complexes [Cu(P^P)(tbz)], obtained through deprotonation of the diimine ligand, have been synthesized with the aim of analyzing the role of the diphosphane and Htbz deprotonation in the emissive properties of these complexes. For the study of the diphosphane effect, the luminescence properties of these compounds have been compared with those of the reported analogous derivatives with Htbz and carborane diphosphanes. Complexes [Cu(P^P)(Htbz)]PF6 (P^P = xantphos, dpephos) and [Cu(dpephos)(tbz)] display thermally activated delayed fluorescence, which has been studied, revealing a ΔE(S1–T1) between 658 and 455 cm–1. Theoretical calculations indicate different origins for the absorptions, leading to the observed emissions

Band gap control via tuning of inversion degree in CdIn2_2S4_4 spinel

Based on theoretical arguments we propose a possible route for controlling the band-gap in the promising photovoltaic material CdIn$_2$S$_4$. Our \textit{ab initio} calculations show that the experimental degree of inversion in this spinel (fraction of tetrahedral sites occupied by In) corresponds approximately to the equilibrium value given by the minimum of the theoretical inversion free energy at a typical synthesis temperature. Modification of this temperature, or of the cooling rate after synthesis, is then expected to change the inversion degree, which in turn sensitively tunes the electronic band-gap of the solid, as shown here by accurate screened hybrid functional calculations.Comment: In press in Applied Physics Letters (2012); 4 pages, 2 figures, 1 tabl

Decay rate measurement of the first vibrationally excited state of MgH+^+ in a cryogenic Paul trap

We present a method to measure the decay rate of the first excited vibrational state of simple polar molecular ions being part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the $|\nu$=$1,J$=$1 \rangle_X$ towards the $|\nu$=$0,J$=$0 \rangle_X$ level in MgH$^+$ by saturated laser excitation of the $|\nu$=$0,J$=$2 \rangle_X$-$|\nu$=$1,J$=$1 \rangle_X$ transition followed by state selective resonance enhanced two-photon dissociation out of the $|\nu$=$0,J$=$2 \rangle_X$ level. The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.Comment: 5 pages, 4 figure

Effect of Progesterone, Cortisol and Dhea on the ITR of maedivisna virus transcripcional activity

Estudios previos sugieren que, al igual que en otras infecciones por retrovirus, las hormonas esteroideas serían capaces de dirigir la expresión del virus de Maedi-Visna (MVV) mediante la interacción con los Elementos de Respuesta a Hormona (HRE) de la región promotora/reguladora LTR (Repeticiones Largas Terminales) del genoma del provirus. El objetivo de este trabajo fue la evaluación del efecto del cortisol, progesterona y dehidroepiandrosterona (DHEA) sobre la capacidad transcripcional de la región LTR de MVV mediante ensayos de transfección en fibroblastos ovinos con plásmidos pAcGFP (que contiene el gen para la GFP, proteína verde fluorescente) en los que se había clonado la región U3-cap del LTR de distintas cepas de MVV. La actividad transcripcional del LTR se evaluó a través de la cuantificación de la expresión de la GFP por citometría de flujo con las distintas concentraciones de cada hormona tras 48 horas de incubación. En la mayoría de los ensayos se observó un claro efecto inhibitorio de la transcripción del LTR a elevadas concentraciones hormonales, disminuyendo el efecto a medida que se diluía la hormona, llegando incluso en el caso de cortisol y de DHEA a producirse un incremento de la expresión a partir de 10-7M. En general no se pudo asociar una diferente respuesta con el origen de la cepa estudiada lo que sugiere que no está relacionado con los distintos orígenes/tropismos de los virus. Estos datos sugieren la presencia de un sitio HRE capaz de responder a estimulación hormonal en el LTR de MVV.Previous studies suggest that steroid hormones may direct the expression of Maedi-Visna virus (MVV), as has been observed in other retroviral infections. This would be achieved through the promoter/regulator region of the LTR (long terminal repeats) of the proviral genome, which would contain hormone responsive elements (HRE). The aim of this study was to evaluate the effect of cortisol, progesterone and dehydroepiandrosterone (DHEA) on the transcriptional ability of the MVV LTR region. For this, sheep fibroblasts were transfected with pAcGFP plasmids (containing the gene for green fluorescent protein, GFP) in which the U3-cap region of the LTR of different strains of MVV had been cloned. Different concentrations of each hormone were added to transfected cells and the transcriptional activity of the LTR was evaluated after 48 hours of incubation by quantifying the expression of GFP by flow cytometry. A clear inhibitory effect of the transcriptional ability of the LTR was observed in most of the assays at high hormonal concentrations. This effect decreased with the increasing dilutions of the hormones, to the point that GFP expression was above baseline in cells transfected with several of the plasmids and treated with dilutions above 10-7M of cortisol and DHEA. In general terms, a different response could not be associated to the origin of the strain under study, suggesting that the effect of steroids is not related to the different origins/tropisms of the virus. These data suggest the presence of a hormone responsive element (HRE) in the MVV LTR able to respond to hormonal stimulation