Mapping, sensing and visualising the digital co-presence in the public arena

This paper reports on work carried out within the Cityware project using mobile technologies to map, visualise and project the digital co-presence in the city. This paper focuses on two pilot studies exploring the Bluetooth landscape in the city of Bath. Here we apply adapted and ‘digitally augmented’ methods for spatial observation and analysis based on established methods used extensively in the space syntax approach to urban design. We map the physical and digital flows at a macro level and observe static space use at the micro level. In addition we look at social and mobile behaviour from an individual’s point of view. We apply a method based on intervention through ‘Sensing and projecting’ Bluetooth names and digital identity in the public arena. We present early findings in terms of patterns of Bluetooth flow and presence, and outline initial observations about how people’s reaction towards the projection of their Bluetooth names practices in public. In particular we note the importance of constructing socially meaningful relations between people mediated by these technologies. We discuss initial results and outline issues raised in detail before finally describing ongoing work

Holography and D3-branes in Melvin Universes

Recently,in a paper hep-th/0511197, it was found that non-commutative super Yang-Mills (NCSYM) theory with space-dependent noncommutativity can be formulated as a decoupling limit of open strings ending on D3-branes wrapping a Melvin universe supported by a flux of the NSNS B-field. Under S-duality, we show that this theory turns into a noncommutative open string (NCOS) theory with space-dependent space-time noncommutativity and effective space-dependent string scale. It is an NCOS theory with both space-dependent space-space and space-time noncommutativities under more general $SL(2,\mathbb{Z})$ transformation. These space-dependent noncommutative theories (NCSYM and NCOS) have completely the same thermodynamics as that of ordinary super YM theory, NCSYM and NCOS theories with constant noncommutativity in the dual supergravity description. Starting from black D3-brane solution in the Melvin universe and making a Lorentz boost along one of spatial directions on the worldvolume of D3-branes, we show that the decoupled theory is a light-like NCSYM theory with space-dependent noncommutativity in a static frame or in an infinite-momentum frame depending on whether there is a gravitational pp-wave on the worldvolume of the D3-branes.Comment: Latex, 13 pages, v2: typos fixed, the version in PR

Thermodynamics of Dual CFTs for Kerr-AdS Black Holes

Recently Gibbons {\it et al.} in hep-th/0408217 defined a set of conserved quantities for Kerr-AdS black holes with the maximal number of rotation parameters in arbitrary dimension. This set of conserved quantities is defined with respect to a frame which is non-rotating at infinity. On the other hand, there is another set of conserved quantities for Kerr-AdS black holes, defined by Hawking {\it et al.} in hep-th/9811056, which is measured relative to a frame rotating at infinity. Gibbons {\it et al.} explicitly showed that the quantities defined by them satisfy the first law of black hole thermodynamics, while those quantities defined by Hawking {\it et al.} do not obey the first law. In this paper we discuss thermodynamics of dual CFTs to the Kerr-AdS black holes by mapping the bulk thermodynamic quantities to the boundary of the AdS space. We find that thermodynamic quantities of dual CFTs satisfy the first law of thermodynamics and Cardy-Verlinde formula only when these thermodynamic quantities result from the set of bulk quantities given by Hawking {\it et al.}. We discuss the implication of our results.Comment: Revtex4, twocolumn, 6 pages, v2: 7 pages, more references and comments added, an err correcte

Current-induced domain wall motion in Rashba spin-orbit system

Current-induced magnetic domain wall motion, induced by transfer of spin transfer effect due to exchange interaction, is expected to be useful for next generation high-density storages. We here show that efficient domain wall manipulation can be achieved by introduction of Rashba spin-orbit interaction, which induces spin precession of conduction electron and acts as an effective magnetic field. Its effect on domain wall motion depends on the wall configuration. We found that the effect is significant for Bloch wall with the hard axis along the current, since the effective field works as $\beta$ or field-like term and removes the threshold current if in extrinsic pinning is absent. For N\'eel wall and Bloch wall with easy axis perpendicular to Rashba plane, the effective field induces a step motion of wall corresponding to a rotation of wall plane by the angle of approximately $\pi$ at current lower than intrinsic threshold. Rashba interaction would therefore be useful to assist efficient motion of domain walls at low current

Diffusive versus local spin currents in dynamic spin pumping systems

Using microscopic theory, we investigate the properties of a spin current driven by magnetization dynamics. In the limit of smooth magnetization texture, the dominant spin current induced by the spin pumping effect is shown to be the diffusive spin current, i.e., the one arising from only a diffusion associated with spin accumulation. That is to say, there is no effective field that locally drives the spin current. We also investigate the conversion mechanism of the pumped spin current into a charge current by spin-orbit interactions, specifically the inverse spin Hall effect. We show that the spin-charge conversion does not always occur and that it depends strongly on the type of spin-orbit interaction. In a Rashba spin-orbit system, the local part of the charge current is proportional to the spin relaxation torque, and the local spin current, which does not arise from the spin accumulation, does not play any role in the conversion. In contrast, the diffusive spin current contributes to the diffusive charge current. Alternatively, for spin-orbit interactions arising from random impurities, the local charge current is proportional to the local spin current that constitutes only a small fraction of the total spin current. Clearly, the dominant spin current (diffusive spin current) is not converted into a charge current. Therefore, the nature of the spin current is fundamentally different depending on its origin and thus the spin transport and the spin-charge conversion behavior need to be discussed together along with spin current generation

Critical Dimension for Stable Self-gravitating Stars in AdS

We study the self-gravitating stars with a linear equation of state, $P=a \rho$, in AdS space, where $a$ is a constant parameter. There exists a critical dimension, beyond which the stars are always stable with any central energy density; below which there exists a maximal mass configuration for a certain central energy density and when the central energy density continues to increase, the configuration becomes unstable. We find that the critical dimension depends on the parameter $a$, it runs from $d=11.1429$ to 10.1291 as $a$ varies from $a=0$ to 1. The lowest integer dimension for a dynamically stable self-gravitating configuration should be $d=12$ for any $a \in [0,1]$ rather than $d=11$, the latter is the case of self-gravitating radiation configurations in AdS space.Comment: Revtex, 11 pages with 7 eps figure

Holographic Superconductors with Ho\v{r}ava-Lifshitz Black Holes

We discuss the phase transition of planar black holes in Ho\v{r}ava-Lifshitz gravity by introducing a Maxwell field and a complex scalar field. We calculate the condensates of the charged operators in the dual CFTs when the mass square of the complex scalar filed is $m^2=-2/L^2$ and $m^2=0$, respectively. We compute the electrical conductivity of the \hl superconductor in the probe approximation. In particular, it is found that there exists a spike in the conductivity for the case of the operator with scaling dimension one. These results are quite similar to those in the case of Schwarzschild-AdS black holes, which demonstrates that the holographic superconductivity is a robust phenomenon associated with asymptotic AdS black holes.Comment: 12 pages, 7 figures,refs adde

Relation between the Dynamics of the Reduced Purity and Correlations

A general property of the relation between the dynamics of the reduced purity and correlations is investigated in quantum mechanical systems. We show that a non-zero time-derivative of the reduced purity of a system implies the existence of non-zero correlations with its environment under any unbounded Hamiltonians with finite variance. This shows the role of local dynamical information on the correlations, as well as the role of correlations in the mechanism of purity change.Comment: 7 page

Ricci Flat Black Holes and Hawking-Page Phase Transition in Gauss-Bonnet Gravity and Dilaton Gravity

It is well-known that there exists a Hawking-Page phase transition between a spherical AdS black hole and a thermal AdS space. The phase transition does not happen between a Ricci flat AdS black hole whose horizon is a Ricci flat space and a thermal AdS space in the Poincare coordinates. However, the Hawking-Page phase transition occurs between a Ricci flat AdS black hole and an AdS soliton if at least one of horizon coordinates for the Ricci flat black hole is compact. We show a similar phase transition betwen the Ricci flat black holes and deformed AdS solitons in the Gauss-Bonnet gravity and the dilaton gravity with a Liouville-type potential including the gauged supergravity coming from the spherical reduction of Dp-branes in type II supergravity. In contrast to Einstein gravity, we find that the high temperature phase can be dominated either by black holes or deformed AdS solitons depending on parameters.Comment: Latex, 17 pages without figure