344 research outputs found
Phase--coherence Effects in Antidot Lattices: A Semiclassical Approach to Bulk Conductivity
We derive semiclassical expressions for the Kubo conductivity tensor. Within
our approach the oscillatory parts of the diagonal and Hall conductivity are
given as sums over contributions from classical periodic orbits in close
relation to Gutzwiller's trace formula for the density of states. Taking into
account the effects of weak disorder and temperature we reproduce recently
observed anomalous phase coherence oscillations in the conductivity of large
antidot arrays.Comment: 11 pages, 2 figures available under request, RevTe
Weak-Localization and Integrability in Ballistic Cavities
We demonstrate the existence of an interference contribution to the average
magnetoconductance, G(B), of ballistic cavities and use it to test the
semiclassical theory of quantum billiards. G(B) is qualitatively different for
chaotic and regular cavities, an effect explained semiclassically by the
differing classical distribution of areas. The magnitude of G(B) is poorly
explained by the semiclassical theory of coherent backscattering (elastic
enhancement factor)-- correlations beyond time-reversed pairs of trajectories
must be included-- but is in agreement with random matrix theory.Comment: 12 pages + 3 figures, revtex, hub-92-w
Mesoscopic Transport Through Ballistic Cavities: A Random S-Matrix Theory Approach
We deduce the effects of quantum interference on the conductance of chaotic
cavities by using a statistical ansatz for the S matrix. Assuming that the
circular ensembles describe the S matrix of a chaotic cavity, we find that the
conductance fluctuation and weak-localization magnitudes are universal: they
are independent of the size and shape of the cavity if the number of incoming
modes, N, is large. The limit of small N is more relevant experimentally; here
we calculate the full distribution of the conductance and find striking
differences as N changes or a magnetic field is applied.Comment: 4 pages revtex 3.0 (2-column) plus 2 postscript figures (appended),
hub.pam.94.
The genetic contribution of single male immigrants to small, inbred populations: A laboratory study using drosophila melanogaster
This study examined the genetic contribution of single male immigrants to small, inbred laboratory populations of Drosophila melanogaster. Genetic contribution was assessed by measuring the relative frequency of immigrant marker alleles in the first and second generations after immigration, while controlling for any selection effects at the marker locus, and for the experience of male immigrants. When immigrants were outbred, the mean frequency of the immigrant allele was significantly higher than its initial frequency, in both the first and second generations after immigration. There was no significant change in allele frequency for populations receiving inbred immigrants. The increase in allele frequency for outbred immigrants was attributed to an initial outbred vigour fitness advantage of immigrant males over resident males experiencing inbreeding depression. Hybrid vigour of immigrant progeny and the rare-male effect did not have a statistically significant role in the fitness advantage of the immigrant allele. The results suggest that inbreeding may have a considerable impact on the contribution of immigrants to the genetic diversity of populations
The Big Society and the Conjunction of Crises: Justifying Welfare Reform and Undermining Social Housing
The idea of the âBig Societyâ can be seen as culmination of a long-standing debate about the regulation of welfare. Situating the concept within governance theory, the article considers how the UK coalition government has justified a radical restructuring of welfare provision, and considers its implications for housing provision. Although drawing on earlier modernization processes, the article contends that the genesis for welfare reform was based on an analysis that the government was forced to respond to a unique conjunction of crises: in morality, the state, ideology and economics. The government has therefore embarked upon a programme, which has served to undermine the legitimacy of the social housing sector (most notably in England), with detrimental consequences for residents and raising significant dilemmas for those working in the housing sector
Effects of the field modulation on the Hofstadter's spectrum
We study the effect of spatially modulated magnetic fields on the energy
spectrum of a two-dimensional (2D) Bloch electron. Taking into account four
kinds of modulated fields and using the method of direct diagonalization of the
Hamiltonian matrix, we calculate energy spectra with varying system parameters
(i.e., the kind of the modulation, the relative strength of the modulated field
to the uniform background field, and the period of the modulation) to elucidate
that the energy band structure sensitively depends on such parameters:
Inclusion of spatially modulated fields into a uniform field leads occurrence
of gap opening, gap closing, band crossing, and band broadening, resulting
distinctive energy band structure from the Hofstadter's spectrum. We also
discuss the effect of the field modulation on the symmetries appeared in the
Hofstadter's spectrum in detail.Comment: 7 pages (in two-column), 10 figures (including 2 tables
Reassessing Britainâs âpost-war consensusâ: the politics of reason 1945â1979
Since the late-1970s, scholars have been engaged in a vibrant debate about the nature of post-war British politics. While some writers have suggested that the three decades that succeeded the Second World War witnessed a bi-partisan consensus on key policy questions, others have argued that it was conflict, not agreement, that marked the period. This article offers a novel contribution to this controversy by drawing attention to the epistemological beliefs of the Labour and Conservative parties. It argues that once these beliefs are considered, it becomes possible to reconcile some of the competing claims made by proponents and critics of the âpost-war consensusâ thesis. Labour and Conservative leaders may have been wedded to different beliefs, but they also shared a common enthusiasm for empiricist reasoning and were both reluctant to identify fixed political âendsâ that they sought to realise. Consequently, they were both committed to evolutionary forms of change, and they eschewed the notion that any social or political arrangement was of universal value
Reinventing grounded theory: some questions about theory, ground and discovery
Grounded theoryâs popularity persists after three decades of broad-ranging critique. In this article three problematic notions are discussedââtheory,â âgroundâ and âdiscoveryââwhich linger in the continuing use and development of grounded theory procedures. It is argued that far from providing the epistemic security promised by grounded theory, these notionsâembodied in continuing reinventions of grounded theoryâconstrain and distort qualitative inquiry, and that what is contrived is not in fact theory in any meaningful sense, that âgroundâ is a misnomer when talking about interpretation and that what ultimately materializes following grounded theory procedures is less like discovery and more akin to invention. The procedures admittedly provide signposts for qualitative inquirers, but educational researchers should be wary, for the significance of interpretation, narrative and reflection can be undermined in the procedures of grounded theory
National identity - A multiculturalist's approach
âMulticulturalistsâ in Britain have advocated the importance of national identity since the 1970s. Yet many claim that multiculturalists do not do this and few study how they do it. We thus do not know why and how multiculturalists in Britain advocate the importance of national identity. In this article I will examine how one of Britainâs most prominent multiculturalists, Bhikhu Parekh, has done so since the 1970s. I will show that Parekhâs way of thinking about national identity is distinct from the ways in which other prominent thinkers discuss such an identity, and valuable, as Parekh has insights into what such an identity is and why we should value it. This article will identify a previously unnoticed, distinct and valuable way of thinking about national identity, which comes from a multiculturalist in Britain
In The Shadows: Conservative Epistemology and Ideological Value
This article intervenes in the debate about the nature of conservatism. Some contributors to this debate have claimed that this ideology can be defined as an adjectival disposition. They claim, that is, that a conservative possesses an attitude towards shared values rather than a distinct set of substantive values. The following discussion interrogates this account of conservatism and concludes that it can only be coherent if we ignore the epistemological limits of conservative thinking
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