IST Austria Thesis

Asymmetries have long been known about in the central nervous system. From gross anatomical differences, such as the presence of the parapineal organ in only one hemisphere of the developing zebrafish, to more subtle differences in activity between both hemispheres, as seen in freely roaming animals or human participants under PET and fMRI imaging analysis. The presence of asymmetries has been demonstrated to have huge behavioural implications, with their disruption often leading to the generation of neurological disorders, memory problems, changes in personality, and in an organism's health and well-being. For my Ph.D. work I aimed to tackle two important avenues of research. The first being the process of input-side dependency in the hippocampus, with the goal of finding a key gene responsible for its development (Gene X). The second project was to do with experience-induced laterality formation in the hippocampus. Specifically, how laterality in the synapse density of the CA1 stratum radiatum (s.r.) could be induced purely through environmental enrichment. Through unilateral tracer injections into the CA3, I was able to selectively measure the properties of synapses within the CA1 and investigate how they differed based upon which hemisphere the presynaptic neurone originated. Having found the existence of a previously unreported reversed (left-isomerism) i.v. mutant, through morpholocal examination of labelled terminals in the CA1 s.r., I aimed to elucidate a key gene responsible for the process of left or right determination of inputs to the CA1 s.r.. This work relates to the previous finding of input-side dependent asymmetry in the wild-type rodent, where the origin of the projecting neurone to the CA1 will determine the morphology of a synapse, to a greater degree than the hemisphere in which the projection terminates. Using left- and right-isomerism i.v. mice, in combination with whole genome sequence analysis, I highlight Ena/VASP-like (Evl) as a potential target for Gene X. In relation to this topic, I also highlight my work in the recently published paper of how knockout of PirB can lead to a lack of input-side dependency in the murine hippocampus. For the second question, I show that the environmental enrichment paradigm will lead to an asymmetry in the synapse densities in the hippocampus of mice. I also highlight that the nature of the enrichment is of less consequence than the process of enrichment itself. I demonstrate that the CA3 region will dramatically alter its projection targets, in relation to environmental stimulation, with the asymmetry in synaptic density, caused by enrichment, relying heavily on commissural fibres. I also highlight the vital importance of input-side dependent asymmetry, as a necessary component of experience-dependent laterality formation in the CA1 s.r.. However, my results suggest that it isn't the only cause, as there appears to be a CA1 dependent mechanism also at play. Upon further investigation, I highlight the significant, and highly important, finding that the changes seen in the CA1 s.r. were predominantly caused through projections from the left-CA3, with the right-CA3 having less involvement in this mechanism

Finite temperature superfluid density in very underdoped cuprates

The combination of a large superconducting gap, low transition temperature, and quasi two-dimensionality in strongly underdoped high temperature superconductors severely constrains the behavior of the ab-plane superfluid density \rho with temperature T. In particular, we argue that the contribution of nodal quasiparticles to \rho(T) is essential to account both for the amplitude of, and the recently observed deviations from, the Uemura scaling. A relation between T_c and \rho(0) which combines the effects of quasiparticle excitations at low temperatures and of vortex fluctuations near the critical temperature is proposed and discussed in light of recent experiments.Comment: 5 RevTex pages, 4 figures (one new); more discussion and comparison with experiment; version to appear in Phys. Rev.

Renormalization group approach to 2D Coulomb interacting Dirac fermions with random gauge potential

We argue that massless Dirac particles in two spatial dimensions with $1/r$ Coulomb repulsion and quenched random gauge field are described by a manifold of fixed points which can be accessed perturbatively in disorder and interaction strength, thereby confirming and extending the results of arXiv:0707.4171. At small interaction and small randomness, there is an infra-red stable fixed curve which merges with the strongly interacting infra-red unstable line at a critical endpoint, along which the dynamical critical exponent $z=1$.Comment: 4 pages, 4 figure