Missing Shapiro steps and the 8π8\pi-periodic Josephson effect in interacting helical electron systems

Two-particle backscattering in time-reversal invariant interacting helical electron systems can lead to the formation of quasiparticles with charge $e/2$. We propose a way to detect such states by means of the Josephson effect in the presence of proximity-induced superconductivity. In this case, the existence of $e/2$ charges leads to an $8 \pi$-periodic component of the Josephson current which can be identified through measurement of Shapiro steps in Josephson junctions. In particular, we show that even when there is weak explicit time-reversal symmetry breaking, which causes the two-particle backscattering to be a sub-leading effect at low energies, its presence can still be detected in driven, current-biased Shapiro step measurements. The disappearance of some of these steps as a function of the drive frequency is directly related to the existence of non-Abelian zero-energy states. We suggest that this effect can be measured in current state-of-the-art Rashba wires.Comment: 9 pages, 5 figures. A new submission extending and expanding our analysis in arXiv:1507.08881. (v2) References adde

Emotion regulation capacity in older adults: Effects on facial expression and memory

This thesis primarily investigated the extent to which the capacity for emotion regulation is preserved in older adults. In doing this, the current research explored possible contributors that might help explain how older adults regulate their emotions as well as young adults, and the limitations to emotion regulation ability in ageing. Subtle changes in muscle activity associated with positive and negative facial expressions were measured with zygomaticus and corrugator facial electromyography (EMG) as a novel technique to determine age differences in specific emotion regulation use, as well as age differences in emotional reactivity

Electronic spin-triplet nematic with a twist

We analyze a model of itinerant electrons interacting through a quadrupole density-density repulsion in three dimensions. At the mean field level, the interaction drives a continuous Pomeranchuk instability towards $d$-wave, spin-triplet nematic order, which simultaneously breaks the SU(2) spin-rotation and spatial rotational symmetries. This order results in spin antisymmetric, elliptical deformations of the Fermi surfaces of up and down spins. We show that the effects of quantum fluctuations are similar to those in metallic ferromagnets, rendering the nematic transition first-order at low temperatures. Using the fermionic quantum order-by-disorder approach to self-consistently calculate fluctuations around possible modulated states, we show that the first-order transition is pre-empted by the formation of a nematic state that is intertwined with a helical modulation in spin space. Such a state is closely related to $d$-wave bond density wave order in square-lattice systems. Moreover, we show that it may coexist with a modulated, $p$-wave superconducting state.Comment: 15 pages, 9 figure

Dynamic response functions and helical gaps in interacting Rashba nanowires with and without magnetic fields

A partially gapped spectrum due to the application of a magnetic field is one of the main probes of Rashba spin-orbit coupling in nanowires. Such a "helical gap" manifests itself in the linear conductance, as well as in dynamic response functions such as the spectral function, the structure factor, or the tunnelling density of states. In this paper, we investigate theoretically the signature of the helical gap in these observables with a particular focus on the interplay between Rashba spin-orbit coupling and electron-electron interactions. We show that in a quasi-one-dimensional wire, interactions can open a helical gap even without magnetic field. We calculate the dynamic response functions using bosonization, a renormalization group analysis, and the exact form factors of the emerging sine-Gordon model. For special interaction strengths, we verify our results by refermionization. We show how the two types of helical gaps, caused by magnetic fields or interactions, can be distinguished in experiments.Comment: 15 pages, 7 figures, v2 refs adde

The Glow of Significance: Narrating stories using natural history specimens

The subject of this project is natural history specimens and the exploration of their qualities in visual artwork. The first part is a 533cm watercolour painting composed of an image of at least one specimen (or part thereof) to represent each flowering plant family, of which there are 505. The ‘Herbarium Specimen Painting’ was created using dried plant specimens from the herbarium collection at the Royal Botanic Gardens, Kew. The plant families are painted in systematic order following one of the recently developed DNA classification systems. The painting was produced with scientific rigor and under the constant supervision of Kew botanists. It aims not only to illustrate the chosen classification system but to explore the aesthetic beauty of herbarium specimens and celebrate many of the incredible and varied narratives contained within the Kew collection. The second element of this thesis constructs a context for the above artwork among similar projects. Natural history institutions worldwide were contacted for information about artists using natural history collections to produce art with a strong narrative element that ‘discussed’ the notion of the specimen. These artists were then contacted and many interviewed. In parallel, the literature review concentrated on theories developed in the field of material culture where the human relationships between groups of objects are analysed. These theories proved fundamental and on occasion inspirational in uncovering deeper meanings and narrative possibilities. The concluding section of this research discusses whether the findings of this project, which uses and develops material culture theory can contribute to that field of research. It analyses the possibility that specimen-based artwork can benefit science and/or help revitalise museum collections, and comments on whether institutions can improve the public communicability of the objects in their care by treating them as a potential source for new art