Unique and redundant functions of ATM and DNA-PKcs during V(D)J recombination

Lymphocyte antigen receptor genes are assembled through the process of V(D)J recombination, during which pairwise DNA cleavage of gene segments results in the formation of four DNA ends that are resolved into a coding joint and a signal joint. The joining of these DNA ends occurs in G(1)-phase lymphocytes and is mediated by the non-homologous end-joining (NHEJ) pathway of DNA double-strand break (DSB) repair. The ataxia telangiectasia mutated (ATM) and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), two related kinases, both function in the repair of DNA breaks generated during antigen receptor gene assembly. Although these proteins have unique functions during coding joint formation, their activities in signal joint formation, if any, have been less clear. However, two recent studies demonstrated that ATM and DNA-PKcs have overlapping activities important for signal joint formation. Here, we discuss the unique and shared activities of the ATM and DNA-PKcs kinases during V(D)J recombination, a process that is essential for lymphocyte development and the diversification of antigen receptors

Empire circumscribed: silence, disconnection, public secrets, and the absent-presence of the British Empire in Bristol

This thesis explores the ways in which the British Empire is understood and represented in historical discourse and heritage practice in the city of Bristol. It attempts to develop a wider literature on metropolitan post-colonial memory, looking at the ways in which European Empires are understood and talked about in their former metropoles. While commentators including journalists and other scholars have suggested that Britain has an amnesiac relationship with its history of Empire, this thesis uses a more nuanced framework, largely based off of Ann Stoler’s concept of colonial aphasia. As with Stoler, I suggest that this is not so much a matter of memory or forgetting as much as it is about processes of silence, displacement, and disconnection. A central assumption concerning histories of Empire is that they happened a long time ago, somewhere over there and thus, have limited relevance in present-day Britain as artefacts of the past. This thesis looks at both anthropological theory and its own ethnographic data to critically explore what work this central temporal and spatial assumption does, arguing that it is a way in which Britain can effectively displace or write around this fundamentally constitutive and uncomfortably ambivalent aspect of its own history and construction, despite its ongoing material presence. The work ultimately seeks to further destabilise this central assumption, noting the ways in which a port city like Bristol was both fundamental to and fundamentally built by the imperial encounter, both historically and in the present, not least of all through ongoing debates about Bristol’s contentious history as an epicentre of the transatlantic slave trade. Based off of 12 months of ethnographic fieldwork with an emphasis on history workers (particularly heritage volunteers and alternative [non-council] historical actors), the thesis explores various practices taken by these history workers to interpret, narrate, and represent the both the city’s material, urban landscape and its history. Looking at materiality through walking tours, the thesis explores the ways in which the present-absence of Empire is manifested in the cityscape, even if it is not explicitly addressed in the Council’s narrative of the city’s history. Furthermore, Council museum representations effectively circumscribe the wider history of Empire from present day Bristol in accordance with the central assumption above with the exception of discussions of the slave trade which is contained and compartmentalised from the rest of the city’s history. However, while these circumscribed histories of Empire are not effectively addressed by Council actors, they are confronted through alternative, non-Council heritage actors and sometimes framed as a conspiracy by the city’s elite (with ties and roots to Bristol’s mercantile trade) to conceal Bristol’s problematic histories. In this light, taking into account theorisations of materiality and memory, silence, and public secrets, I ultimately argue that Bristol’s history debates over its past are debates about the very nature of British and English identities, as well as the time and place of Empire in the politics of the present

Martensitic transformation in V_3Si single crystal: ^51V NMR evidence for coexistence of cubic and tetragonal phases

The Martensitic transformation (MT) in A15 binary-alloy superconductor V_3Si is a second-order, displacive structural transition from cubic to tetragonal symmetry, at temperature T_m a few K above the superconducting transition temperature T_c = 17 K. Though studied extensively, the MT has not yet been conclusively linked with a transition to superconductivity, and remains relevant, e.g. due to renewed interest in soft phonon modes, while V_3Si continues to be of interest, e.g. due to similarities with Fe-As superconductors. Previous NMR studies on the MT in V_3Si have mainly been on powder samples, and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting and Knight shift of NMR spectra for ^51V allowed us to distinguish between spectra from transverse chains of V as a function of temperature. Our data revealed that the MT is not abrupt, but rather there is a microscopic coexistence of untransformed cubic phase and transformed tetragonal phase over a few K below and above T_m, and that the Martensitic lengthening of one axis occurs predominantly in a plane perpendicular to the crystal growth axis, as twinned domains

On narrowing coated conductor film: emergence of granularity-induced field hysteresis of transport critical current

Critical current density Jc in polycrystalline or granular superconducting material is known to be hysteretic with applied field H due to the focusing of field within the boundary between adjacent grains. This is of concern in the so-called coated conductors wherein superconducting film is grown on a granular, but textured surface of a metal substrate. While previous work has mainly been on Jc determined using induced or magnetization currents, the present work utilizes transport current via an applied potential in strip geometry. It is observed that the effect is not as pronounced using transport current, probably due to a large difference in criterion voltage between the two types of measurements. However, when the films are narrowed by patterning into 200-, 100-, or 80-micron, the hysteresis is clearly seen, because of the forcing of percolation across higher-angle grain boundaries. This effect is compared for films grown on ion-beam-assisted-deposited (IBAD) YSZ substrate and those grown on rolling-assisted-biaxially-textures substrates (RABiTS) which have grains that are about ten times larger. The hysteresis is more pronounced for the latter, which is more likely to have a weak grain boundary spanning the width of the microbridge. This is also of concern to applications in which coated conductors will be striated in order to reduce of AC losses.Comment: text-only: 10 pages, plus 5 figures on 5 page

Theoretical Description of Nearly Discontinuous Transition in Superconductors with Paramagnetic Depairing

Based on a theoretical argument and Monte Carlo simulations of a Ginzburg-Landau model derived microscopically, it is argued that, in type-II superconductors where {\it both} the paramagnetic {\it and} orbital depairings are important, a strong first-order transition (FOT) at $H_{c2}$ expected in the mean field (MF) approximation never occurs in real systems and changes due to the fluctuation into a crossover. The present result explains why a {\it nearly} discontinuous crossover at $H_{c2}$ with {\it no} intrinsic hysteresis is observed only in a clean superconducting material with a singlet pairing and a high condensation energy such as CeCoIn$_5$.Comment: Publication version. See cond-mat/0306060 regarding a corresponding long pape

Through-thickness superconducting and normal-state transport properties revealed by thinning of thick film ex situ YBa2Cu3O7-x coated conductors

A rapid decrease in the critical current density (Jc) of YBa2Cu3O7-x (YBCO) films with increasing film thickness has been observed for multiple YBCO growth processes. While such behavior is predicted from 2D collective pinning models under certain assumptions, empirical observations of the thickness dependence of Jc are believed to be largely processing dependent at present. To investigate this behavior in ex situ YBCO films, 2.0 and 2.9 um thick YBCO films on ion beam assisted deposition (IBAD) - yttria stabilized zirconia (YSZ) substrates were thinned and repeatedly measured for rho(T) and Jc(H). The 2.9 um film exhibited a constant Jc(77K,SF) through thickness of ~1 MA/cm2 while the 2.0 um film exhibited an increase in Jc(77K,SF) as it was thinned. Neither film offered evidence of significant dead layers, suggesting that further increases in critical current can be obtained by growing thicker YBCO layers.Comment: To appear in Applied Physics Letter