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

Sliding Nanofriction in Low Dimensional Model-Systems

Thanks to novel experimental techniques, physicists are now able to characterize the dynamics of interacting surfaces down to molecular length scales. This possibility has brought fresh interest in the field of friction and has opened a new branch of research, nanotribology, whose aim is the study of tribological properties of sliding systems in terms of fundamental atomistic dissipative mechanisms. Far from being a mere academic problem, understanding and controlling friction at the nano-scales can have a great impact on many technological applications, from energy conversion and saving, to transportation and micro-machining. To reach this goal it is essential to develop theoretical tools able to tackle this problem. In absence of a general theory of friction, molecular dynamics (MD) simulations represent, at the moment, one of the most powerful approaches able to explain and predict the behaviour of nanoscale interfaces. Within the context of nanotribology, this thesis deals with some fundamental aspects of friction between dry crystalline surfaces. Anticipating experiments, it reports results obtained by means of realistic MD simulations of artificial model-systems currently under experimental investigation for application in nanotribology. Qualitatively, the sliding properties of crystalline interfaces can be interpreted based on the mutual interaction between the potential energy landscapes generated by the two touching lattices. These may be described as periodic, two dimensional sequences of wells and hills, corresponding to repulsive and attractive regions of the surfaces. Commensurate geometries correspond to atomically locked configurations, where the atoms of each interacting plane adapt into the wells of the potential landscape generated by the other one. When driven out of equilibrium by an applied shear force, this kind of atomic locking, or interdigitation, generally determines mechanical instabilities, which in turn lead to violent depinning events and high dissipation. Atomic locking may still occur, but may also be avoided in incommensurate interfaces, in which case friction shows more smooth and gentle sliding regimes. The ordered atomic arrangement of crystalline surfaces therefore offers a peculiar way to reduce friction, that is by controlling the geometry of the interface, e.g., by rotating relative to each other two originally aligned and commensurate surfaces. The above picture applies to clean, chemically inert, and atomically flat crystal surfaces. To describe in a quantitative way real systems, one has to take into account many other effects whose interplay determine the overall tribological response. They include elasticity of the surfaces, plastic deformations, and the presence of steps, defects and impurities, not counting chemical interactions and other processes involving the direct excitations of electronic degrees of freedom. From the experimental point of view, the fine features of the interface are hardly accessible because buried. Usually only macroscopic average values of some arising physical quantities are measured, which hinders the possibility to keep track of each distinct mechanism at play. From the theoretical point of view, developing a general theory different from brute-force with quantitative predictive power is also difficult, and phenomenological models are usually adopted which apply only to a reduced number of cases. However, there exist a class of real systems where these complications are absent or mitigated, allowing for detailed experimental and theoretical investigations. On one hand, one (1D) and two dimensional (2D) artificial crystals of charged particles trapped inside optical lattices offer the possibility to study the dynamics of ideal crystalline interfaces with all interface parameters under control, including commensurability and substrate interaction strength. On the other hand, surface science and ultra high vacuum techniques supply clean and atomically flat substrates, suitable for the study of the sliding properties of two dimensional monolayers of adsorbate atoms. Both these systems are very well characterized, and allow for accurate realistic MD simulations where geometrical effects, and interface elastic and plastic deformations effects are investigated in great details. In view of future experiments, this thesis reports results of MD investigations of some fundamental tribological aspects in low dimensional incommensurate interfaces formed by: (i) 1D cold-ion chains trapped in optical lattices, (ii) 2D charged colloids monolayers interacting with laser-induced periodic potentials, (iii) 2D islands of rare gas atoms physisorbed on clean metallic substrates. These simulations show that incommensurate linear chains of trapped cold ions display a rich dynamics when forced to slide over a periodic corrugated potential. That suggests that they can be adopted to investigate in detail the external-load dependent transition between the intermittent stick-slip motion and the smooth sliding regime, as well as the precursor dynamics preceding the onset of motion. Both of them are shown to display paradigmatic behaviours observed in sliding contacts at any length scales. Incommensurate two dimensional interfaces realized by colloidal crystals in periodic fields have been simulated to study the pinning transition from the locked (finite static friction) state, to the ``superlubric'' -- zero static friction -- free-sliding state, which is predicted to occur as a function of decreasing substrate potential strength. In a range of parameters compatible with recent experiments, that is a first order (structural) phase transition of the colloid monolayer, showing analogies with the superlubric to pinned Aubry transition" extensively studied in the one dimensional discrete Frenkel-Kontorova model of dry friction. Moreover, realistic simulations show that relative misfit rotations between the colloidal slider and substrate may significantly affect the dissipation under steady sliding even in genuinely incommensurate geometries, by changing the degree to which the soft deformable slider interdigitates within the hard, non deformable optical substrate. This is a result of more general validity, since a similar mechanism must be at play in any incommensurate 2D crystalline interface. Finally, in order to understand the persistent static friction force observed in quartz crystal microbalance experiments on highly clean surfaces, extensive numerical simulations of substrate-incommensurate model rare-gas islands have been performed, which, in absence of any other sources of pinning, describe how the island edges alone may play the ultimate role in determining the overall barrier preventing the onset of global sliding

Loading an Equidistant Ion Chain in a Ring Shaped Surface Trap and Anomalous Heating Studies with a High Optical Access Trap

Microfabricated segmented surface ion traps are one viable avenue to scalable quantum information processing. At Sandia National Laboratories we design, fabricate, and characterize such traps. Our unique fabrication capabilities allow us to design traps that facilitate tasks beyond quantum information processing. The design and performance of a trap with a target capability of storing hundreds of equally spaced ions on a ring is described. Such a device could aid experimental studies of phenom- ena as diverse as Hawking radiation, quantum phase transitions, and the Aharonov - Bohm effect. The fabricated device is demonstrated to hold a 3c 400 ion circular crystal, with 9 μm average spacing between ions. The task is accomplished by first characterizing undesired electric fields in the trapping volume and then designing and applying an electric field that substantially reduces the undesired fields. In addition, experimental efforts are described to reduce the motional heating rates in a surface trap by low energy in situ argon plasma treatment that reduces the amount of surface contaminants. The experiment explores the premise that carbonaceous compounds present on the surface contribute to the anomalous heating of secular motion modes in surface traps. This is a research area of fundamental interest to the ion trapping community, as heating adversely affects coherence and thus gate fidelity. The de- vice used provides high optical laser access, substantially reducing scatter from the surface, and thus charging that may lead to excess micromotion. Heating rates for different axial mode frequencies are compared before and after plasma treatment. The presence of a carbon source near the plasma prevents making a conclusion on the observed absence of change in heating rates

ICP Enhanced HIPIMS System Design and Characterisation

A custom inductively coupled plasma assisted high-power impulse magnetron sputtering system was designed and assembled for the study of E x B plasma spoke instabilities. The magnetron plasma was characterized using time-resolved Langmuir probe, floating probe, optical emission spectroscopy and high-speed camera methods. Fluctuations in the floating potential of the plasma were observed, this was attributed to localized rotating spokes. Localized spoke structures were observed in the camera footage, the shape and size of these spokes were found to be influenced by argon gas rarefaction

Mass spectrometry combined with strategic enzymatic digestion, selective derivatization and ultraviolet photodissociation for the identification and characterization of Immunoglobulin G antibodies

Immunoglobulin G (IgG) antibodies represent important analytical targets both for their therapeutic properties and for their critical role in the adaptive immune response. While much of the primary structure is conserved across the IgG class, subtle changes in amino acid sequence and the presence or absence of post-translational modifications can have a profound effect on the function and therapeutic potential of a given antibody. As such, there remains a high demand for versatile analytical tools capable of both identification and complete structural characterization of IgGs. The work presented in this dissertation largely focuses on the development of mass spectrometry-based methods for the improved analysis of antibodies. This was accomplished using strategic enzymatic Brodbeltselectivity for regions of particular diagnostic value or to facilitate comprehensive structural characterization. A method based on chromophore-mediated 351 nm UVPD was developed as a means to streamline the identification of antibodies in mixtures by enhancing selectively for the third complementarity determining region of the IgG heavy chain (CDR-H3). The hypervariable sequences within this region serve as the primary determinant of antigen binding specificity and thus provide a molecular signature by which to differentiate unique antibodies. To accomplish this, a highly conserved cysteine residue located in the framework preceding the CDR-H3 region was exploited for selective tagging with an Alexa Fluor 350 (AF350) thiol-selective maleimide. This site-specific tagging combined with strategic enzymatic digestion and 351 nm UVPD allowed selective dissociation of only AF350-labeled peptides for facile discrimination of CDR-H3 sequences within a high-throughput liquid chromatography-tandem mass spectrometry (LC-MS/MS) based workflow. Two variations of middle-down mass spectrometry based on either restricted Lys-C proteolysis or hinge-selective IdeS digestion combined with 193 nm UVPD were used for the characterization of monoclonal antibodies. Both strategies yielded considerably greater diagnostic sequence information when benchmarked against conventional collision- and electron-based activation methods. The Lys-C proteolysis method was found to have considerable implications for the analysis of serological antibody repertoires owing to its facile implementation into high-throughput proteomic workflows and ability to unambiguously differentiate unique CDR-H3 sequences. The development and implementation of a front-end dual spray reactor for high-throughput ion/ion-mediated bioconjugation is demonstrated for the enhanced structural characterization of unmodified and post-translationally modified peptide cations by 193 nm UVPD and CID. The ability to generate ion/ion complexes in real-time followed by efficient covalent conversion allowed integration of the dual spray reactor into a high-throughput LC-MS [superscript n] workflow for rapid derivatization of peptide mixtures.Chemistr