Klein paradox for a pn junction in multilayer graphene

Charge carriers in single and multilayered graphene systems behave as chiral particles due to the particular lattice symmetry of the crystal. We show that the interplay between the meta-material properties of graphene multilayers and the pseudospinorial properties of the charge carriers result in the occurrence of Klein and anti-Klein tunneling for rhombohedral stacked multilayers. We derive an algebraic formula predicting the angles at which these phenomena occur and support this with numerical calculations for systems up to four layers. We present a decomposition of an arbitrarily stacked multilayer into pseudospin doublets that have the same properties as rhombohedral systems with a lower number of layers.Comment: 5 pages, 4 figure

Four band tunneling in bilayer graphene

The conductance, the transmission and the reflection probabilities through rectangular potential barriers and pn-junctions are obtained for bilayer graphene taking into account the four bands of the energy spectrum. We have evaluated the importance of the skew hopping parameters {\gamma}3 and {\gamma}4 to these properties and show that for energies E>{\gamma}1/100 their effect is negligible. For high energies two modes of propagation exist and we investigate scattering between these modes. For perpendicular incidence both propagation modes are decoupled and scattering between them is forbidden. This extends the concept of pseudospin as defined within the two band approximation to a four band model and corresponds to the (anti)symmetry of the wavefunctions under in-plane mirroring. New transmission resonances are found that appear as sharp peaks in the conductance which are absent in the two band approximation. The application of an interlayer bias to the system: 1) breaks the pseudospin structure, 2) opens a bandgap that results in a distinct feature of suppressed transmission in the conductance, and 3) breaks the angular symmetry with respect to normal incidence in the transmission and reflection

Comment on "Creating in-plane pseudomagnetic fields in excess of 1000 T by misoriented stacking in a graphene bilayer"

In a recent paper [Phys. Rev. B 89, 125418 (2014)], the authors argue that it is possible to map the electronic properties of twisted bilayer graphene to those of bilayer graphene in an in-plane magnetic field. However, their description of the low-energy dynamics of twisted bilayer graphene is restricted to the extended zone scheme and therefore neglects the effects of the superperiodic structure. If the energy spectrum is studied in the supercell Brillouin zone, we find that the comparison with an in-plane magnetic field fails because (i) the energy spectra of the two situations exhibit different symmetries and (ii) the low-energy spectra are very different.Comment: 3 pages, 2 figure

Shifting Grounds: How Urban Gardening Practices Enact the Relations between Play and Work

This thesis discusses how urban gardening practices enact the relations between play and work, based upon a comparative ethnographic study of allotment, community and guerrilla gardening in London. It deploys participant observation and garden go-alongs, and makes use of photography to engage with the corporealities, textures, and creativities of these practices. Rather than binary conceptions of play and work, this thesis understands urban gardening practices as creating entangled contingencies of play and work in social life. It sees urban gardens as paradoxical spaces of play and work; and thereby develops cultural geography’s understanding of gardens as sites imbued with multiple and contradictory meanings. Gardening is enjoyed for its visceral experience confirming ideas of play as being fun, embodied and absorbing. Yet, it also demands work, because this seemingly voluntary activity implicates social, material and legal obligations. Furthermore, the research demonstrates how objects travel across spaces of play and work, as permeable garden boundaries are made and unmade constantly. By showing these relations between inside and outside, the thesis challenges ideas of the ‘garden’ and the ‘playground’ as fixed, enclosed time-spaces set apart from everyday life. Moreover, this ambiguity is further exemplified by how gardeners have varying perceptions of play, which overlap but also contest each other. This research also enhances debates on public spaces in cities, and more-than-human geographies, by showing how gardening breathes life into the urban through on-going encounters between people, plants and animals. It identifies four types of encounters, namely festive, chance, care-taking and contestation. The range of others encountered is multiple and diverse, and gardeners’ openness towards the contingencies of inhabiting these urban spaces alludes to a playful mode of engaging with the world. The thesis argues that urban gardening practices feed into, complement and offer an alternative to neoliberal conceptions of play and work in post-fordist economies

Plasmons and their interaction with electrons in trilayer graphene

The interaction between electrons and plasmons in trilayer graphene is investigated within the Overhauser approach resulting in the 'plasmaron' quasi-particle. This interaction is cast into a field theoretical problem, nd its effect on the energy spectrum is calculated using improved Wigner-Brillouin perturbation theory. The plasmaron spectrum is shifted with respect to the bare electron spectrum by $\Delta E(\mathbf{k})\sim 50\div200\,{\rm meV}$ for ABC stacked trilayer graphene and for ABA trilayer graphene by $\Delta E(\mathbf{k})\sim 30\div150\,{\rm meV}$ ($\Delta E(\mathbf{k})\sim 1\div5\,{\rm meV}$) for the hyperbolic linear) part of the spectrum. The shift in general increases with the electron concentration $n_{e}$ and electron momentum. The dispersion of plasmarons is more pronounced in \textit{ABC} stacked than in ABA tacked trilayer graphene, because of the different energy band structure and their different plasmon dispersion.Comment: arXiv admin note: substantial text overlap with arXiv:1310.623

Seeing Patterns on the Ground: Reflections on Field-based Photography

This paper reflects on field-based photography practices that are informed by the ‘shooting script’ approach and its potential for social science and design researchers to analyse urban spaces. By discussing an ethnographic study of allotment, community and guerrilla gardeners in London, it examines the shooting script in conjunction with grounded theory as a way of structuring the use of photography in fieldwork and analysis. The paper critiques the methodological underpinnings of the shooting script and reframes it as a performed embodied practice of documentation, interpretation and translation. Following on, it suggests finding ways to include self-reflections in publications. Dispersed throughout the paper, images and captions provide an insight into the research process and they evidence the potential of this visual methodology – when triangulated with participant observation and interviews – for analysing the distinctive patterning on the ground produced by gardeners and drawing out the ambiguities involved in their spatial boundary-making practices. Furthermore, the paper discusses the implications of moving from analogue to digital photography in fieldwork, and how the navigations between virtual and material technologies consulted during analysis co-constitute research outcomes. It continues by arguing that the notion of a ‘script’ might be too rigidly interpreted and proposes instead to nurture openness towards the accidental and contingent in fieldwork and analysis

Multiband tunneling in trilayer graphene

The electronic tunneling properties of the two stable forms of trilayer graphene (TLG), rhombohedral ABC and Bernal ABA, are examined for pn and pnp junctions as realized by using a single gate (SG) or a double gate (DG). For the rhombohedral form, due to the chirality of the electrons, the Klein paradox is found at normal incidence for SG devices while at high energy interband scattering between additional propagation modes can occur. The electrons in Bernal ABA TLG can have a monolayer- or bilayer-like character when incident on a SG device. Using a DG however both propagation modes will couple by breaking the mirror symmetry of the system which induces intermode scattering and resonances that depend on the width of the DG pnp junction. For ABC TLG the DG opens up a band gap which suppresses Klein tunneling. The DG induces also an unexpected asymmetry in the tunneling angle for single valley electrons

Empirical description of beta-delayed fission partial half-lives

Background: The process of beta-delayed fission (bDF) provides a versatile tool to study low-energy fission in nuclei far away from the beta-stability line, especially for nuclei which do not fission spontaneously. Purpose: The aim of this paper is to investigate systematic trends in bDF partial half-lives. Method: A semi-phenomenological framework was developed to systematically account for the behavior of bDF partial half-lives. Results: The bDF partial half-life appears to exponentially depend on the difference between the Q value for beta decay of the parent nucleus and the fission-barrier energy of the daughter (after beta decay) product. Such dependence was found to arise naturally from some simple theoretical considerations. Conclusions: This systematic trend was confirmed for experimental bDF partial half-lives spanning over 7 orders of magnitudes when using fission barriers calculated from either the Thomas-Fermi or the liquid-drop fission model. The same dependence was also observed, although less pronounced, when comparing to fission barriers from the finite-range liquid-drop model or the Thomas-Fermi plus Strutinsky Integral method.Comment: accepted for publication in Phys. Rev.