Efficient simulations with electronic open boundaries

We present a reformulation of the hairy-probe method for introducing electronic open boundaries that is appropriate for steady-state calculations involving nonorthogonal atomic basis sets. As a check on the correctness of the method we investigate a perfect atomic wire of Cu atoms and a perfect nonorthogonal chain of H atoms. For both atom chains we find that the conductance has a value of exactly one quantum unit and that this is rather insensitive to the strength of coupling of the probes to the system, provided values of the coupling are of the same order as the mean interlevel spacing of the system without probes. For the Cu atom chain we find in addition that away from the regions with probes attached, the potential in the wire is uniform, while within them it follows a predicted exponential variation with position. We then apply the method to an initial investigation of the suitability of graphene as a contact material for molecular electronics. We perform calculations on a carbon nanoribbon to determine the correct coupling strength of the probes to the graphene and obtain a conductance of about two quantum units corresponding to two bands crossing the Fermi surface. We then compute the current through a benzene molecule attached to two graphene contacts and find only a very weak current because of the disruption of the π conjugation by the covalent bond between the benzene and the graphene. In all cases we find that very strong or weak probe couplings suppress the current.

Shadow policing: the boundaries of community-based ‘policing’ in Northern Ireland

The intention of this article is to provide a structural and operational analysis of policing beyond the police in Northern Ireland. While the polity enjoys low levels of ‘officially’ recorded crime as part of its post-conflict status, little empirical analysis exists as to the epistemological roots of security production outside that of the Police Service of Northern Ireland. The empirical evidence presented seeks to establish that beyond more prominent analyses related to paramilitary ‘policing’, the country is in fact replete with a substantial reservoir of legitimate civil society policing – the collective mass of which contributes to policing, community safety and quality of life issues. While such non-state policing at the level of locale was recognised by the Independent Commission for Policing, structured understandings have rarely permeated governmental or academic discourse beyond anecdotal contentions. Thus, the present argument provides an empirical assessment of the complex, non-state policing landscape beyond the formal state apparatus; examines definitions and structures of such community-based policing activities; and explores issues related to co-opting this non-state security ‘otherness’ into more formal relations with the state.