Spending time with money: from shared values to social connectivity

This article has been made available through the Brunel Open Access Publishing Fund.There is a rapidly growing momentum driving the development of mobile payment systems for co-present interactions, using near-field communication on smartphones and contactless payment systems. The design (and marketing) imperative for this is to enable faster, simpler, effortless and secure transactions, yet our evidence shows that this focus on reducing transactional friction may ignore other important features around making payments. We draw from empirical data to consider user interactions around financial exchanges made on mobile phones. Our findings examine how the practices around making payments support people in making connections, to other people, to their communities, to the places they move through, to their environment, and to what they consume. While these social and community bonds shape the kinds of interactions that become possible, they also shape how users feel about, and act on, the values that they hold with their co-users. We draw implications for future payment systems that make use of community connections, build trust, leverage transactional latency, and generate opportunities for rich social interactions

Superfluid-Insulator and Roughening Transitions in Domain Walls

We have performed quantum Monte Carlo simulations to investigate the superfluid behavior of one- and two-dimensional interfaces separating checkerboard solid domains. The system is described by the hard-core Bose-Hubbard Hamiltonian with nearest-neighbor interaction. In accordance with Ref.1, we find that (i) the interface remains superfluid in a wide range of interaction strength before it undergoes a superfluid-insulator transition; (ii) in one dimension, the transition is of the Kosterlitz-Thouless type and is accompanied by the roughening transition, driven by proliferation of charge 1/2 quasiparticles; (iii) in two dimensions, the transition belongs to the 3D U(1) universality class and the interface remains smooth. Similar phenomena are expected for domain walls in quantum antiferromagnets.Comment: 6 pages, 7 figures; references added, typo corrected in fig

Level crossings in a cavity QED model

In this paper I study the dynamics of a two-level atom interacting with a standing wave field. When the atom is subjected to a weak linear force, the problem can be turned into a time dependent one, and the evolution is understood from the band structure of the spectrum. The presence of level crossings in the spectrum gives rise to Bloch oscillations of the atomic motion. Here I investigate the effects of the atom-field detuning parameter. A variety of different level crossings are obtained by changing the magnitude of the detuning, and the behaviour of the atomic motion is strongly affected due to this. I also consider the situation in which the detuning is oscillating in time and its impact on the atomic motion. Wave packet simulations of the full problem are treated numerically and the results are compared with analytical solutions given by the standard Landau-Zener and the three-level Landau-Zener models.Comment: 12 pages, 10 figure

Symmetry conserving non-perturbative s-wave renormalization of the pion in hot and baryon dense medium

A non-perturbative s-wave renormalization of the pion in a hot and baryon rich medium is presented. This approach proceeds via a mapping of the canonical pion into the axial Noether's charge. The mapping was made dynamical in the Hartree-Fock-Bogoliubov random phase approximation (HFB-RPA). It is shown that this approach, while order mixing, is still symmetry conserving both in the baryon free and baryon rich sectors, at zero as well as finite temperature. The systematic character of this approach is emphasized and it is particularly argued that it may constitute an interesting alternative for the non-perturbative assessment of the nuclear matter saturation properties.Comment: Latex, 22 pages, 3 figure

Phonon Bloch oscillations in acoustic-cavity structures

We describe a semiconductor multilayer structure based in acoustic phonon cavities and achievable with MBE technology, designed to display acoustic phonon Bloch oscillations. We show that forward and backscattering Raman spectra give a direct measure of the created phononic Wannier-Stark ladder. We also discuss the use of femtosecond laser impulsions for the generation and direct probe of the induced phonon Bloch oscillations. We propose a gedanken experiment based in an integrated phonon source-structure-detector device, and we present calculations of pump and probe time dependent optical reflectivity that evidence temporal beatings in agreement with the Wannier-Stark ladder energy splitting.Comment: PDF file including 4 figure

Propagators in Coulomb gauge from SU(2) lattice gauge theory

A thorough study of 4-dimensional SU(2) Yang-Mills theory in Coulomb gauge is performed using large scale lattice simulations. The (equal-time) transverse gluon propagator, the ghost form factor d(p) and the Coulomb potential V_{coul} (p) ~ d^2(p) f(p)/p^2 are calculated. For large momenta p, the gluon propagator decreases like 1/p^{1+\eta} with \eta =0.5(1). At low momentum, the propagator is weakly momentum dependent. The small momentum behavior of the Coulomb potential is consistent with linear confinement. We find that the inequality \sigma_{coul} \ge \sigma comes close to be saturated. Finally, we provide evidence that the ghost form factor d(p) and f(p) acquire IR singularities, i.e., d(p) \propto 1/\sqrt{p} and f(p) \propto 1/p, respectively. It turns out that the combination g_0^2 d_0(p) of the bare gauge coupling g_0 and the bare ghost form factor d_0(p) is finite and therefore renormalization group invariant.Comment: 10 pages, 7 figure

Spin-orbit coupled Bose-Einstein condensate in a tilted optical lattice

Bloch oscillations appear for a particle in a weakly tilted periodic potential. The intrinsic spin Hall effect is an outcome of a spin-orbit coupling. We demonstrate that both these phenomena can be realized simultaneously in a gas of weakly interacting ultracold atoms exposed to a tilted optical lattice and to a set of spatially dependent light fields inducing an effective spin-orbit coupling. It is found that both the spin Hall as well as the Bloch oscillation effects may coexist, showing, however, a strong correlation between the two. These correlations are manifested as a transverse spin current oscillating in-phase with the Bloch oscillations.Comment: 12 pages, 7 figure

Measuring the temporal coherence of an atom laser beam

We report on the measurement of the temporal coherence of an atom laser beam extracted from a $^{87}$Rb Bose-Einstein condensate. Reflecting the beam from a potential barrier creates a standing matter wave structure. From the contrast of this interference pattern, observed by magnetic resonance imaging, we have deduced an energy width of the atom laser beam which is Fourier limited by the duration of output coupling. This gives an upper limit for temporal phase fluctuations in the Bose-Einstein condensate.Comment: 4 pages, 3 figure