Frustration effects in magnetic molecules

By means of exact diagonalization we study the ground-state and the low-temperature physics of the Heisenberg antiferromagnet on the cuboctahedron and the icosidodecahedron. Both are frustrated magnetic polytopes and correspond to the arrangement of magnetic atoms in the magnetic molecules Cu12La8 and Mo72Fe30. The interplay of strong quantum fluctuations and frustration influences the ground state spin correlations drastically and leads to an interesting magnetization process at low temperatures. Furthermore the frustration yields low-lying non-magnetic excitations resulting in an extra low-temperature peak in the specific heat.Comment: 4 pages, 7 figure

Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma

We derive the space-time distribution of energy and momentum deposited by a fast parton traversing a weakly coupled quark-gluon plasma by treating the fast part on as the source of an external color field perturbing the medium. We then use our result as a source term for the linearized hydrodynamical equations of the medium. We show that the solution contains a sonic Mach cone and a dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio

Contact tracing and epidemics control in social networks

A generalization of the standard susceptible-infectious-removed (SIR) stochastic model for epidemics in sparse random networks is introduced which incorporates contact tracing in addition to random screening. We propose a deterministic mean-field description which yields quantitative agreement with stochastic simulations on random graphs. We also analyze the role of contact tracing in epidemics control in small-world networks and show that its effectiveness grows as the rewiring probability is reduced.Comment: 4 pages, 4 figures, submitted to PR

Trip-Based Public Transit Routing

We study the problem of computing all Pareto-optimal journeys in a public transit network regarding the two criteria of arrival time and number of transfers taken. We take a novel approach, focusing on trips and transfers between them, allowing fine-grained modeling. Our experiments on the metropolitan network of London show that the algorithm computes full 24-hour profiles in 70 ms after a preprocessing phase of 30 s, allowing fast queries in dynamic scenarios.Comment: Minor corrections, no substantial changes. To be presented at ESA 201

Solitary-wave description of condensate micro-motion in a time-averaged orbiting potential trap

We present a detailed theoretical analysis of micro-motion in a time-averaged orbiting potential trap. Our treatment is based on the Gross-Pitaevskii equation, with the full time dependent behaviour of the trap systematically approximated to reduce the trapping potential to its dominant terms. We show that within some well specified approximations, the dynamic trap has solitary-wave solutions, and we identify a moving frame of reference which provides the most natural description of the system. In that frame eigenstates of the time-averaged orbiting potential trap can be found, all of which must be solitary-wave solutions with identical, circular centre of mass motion in the lab frame. The validity regime for our treatment is carefully defined, and is shown to be satisfied by existing experimental systems.Comment: 12 pages, 2 figure

Programmable trap geometries with superconducting atom chips

We employ the hysteretic behavior of a superconducting thin film in the remanent state to generate different traps and flexible magnetic potentials for ultra-cold atoms. The trap geometry can be programmed by externally applied fields. This new approach for atom-optics is demonstrated by three different trap types realized on a single micro-structure: a Z-type trap, a double trap and a bias field free trap. Our studies show that superconductors in the remanent state provide a new versatile platform for atom-optics and applications in ultra-cold quantum gases