Seeing the light : experimental signatures of emergent electromagnetism in a quantum spin ice

The "spin ice" state found in the rare earth pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 offers a beautiful realisation of classical magnetostatics, complete with magnetic monopole excitations. It has been suggested that in "quantum spin ice" materials, quantum-mechanical tunnelling between different ice configurations could convert the magnetostatics of spin ice into a quantum spin liquid which realises a fully dynamical, lattice-analogue of quantum electromagnetism. Here we explore how such a state might manifest itself in experiment, within the minimal microscopic model of a such a quantum spin ice. We develop a lattice field theory for this model, and use this to make explicit predictions for the dynamical structure factor which would be observed in neutron scattering experiments on a quantum spin ice. We find that "pinch points", seen in quasi-elastic scattering, which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its zero-temperature ground state. We also make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the "photons" of this emergent electromagnetism. The predictions of this field theory are shown to be in quantitative agreement with Quantum Monte Carlo simulations at zero temperature.Comment: 26 pages, 18 figures, minor revision

Strategies in crowd and crowd structure

In an emergency situation, imitation of strategies of neighbours can lead to an order-disorder phase transition, where spatial clusters of pedestrians adopt the same strategy. We assume that there are two strategies, cooperating and competitive, which correspond to a smaller or larger desired velocity. The results of our simulations within the Social Force Model indicate that the ordered phase can be detected as an increase of spatial order of positions of the pedestrians in the crowd.Comment: 5 pages, 7 figure

Extraction of the electron mass from gg factor measurements on light hydrogenlike ions

The determination of the electron mass from Penning-trap measurements with $^{12}$C$^{5+}$ ions and from theoretical results for the bound-electron $g$ factor is described in detail. Some recently calculated contributions slightly shift the extracted mass value. Prospects of a further improvement of the electron mass are discussed both from the experimental and from the theoretical point of view. Measurements with $^4$He$^+$ ions will enable a consistency check of the electron mass value, and in future an improvement of the $^4$He nuclear mass and a determination of the fine-structure constant

Baryon Loading of AGN Jets Mediated by Neutrons

Plasmas of geometrically thick, black hole (BH) accretion flows in active galactic nuclei (AGNs) are generally collisionless for protons, and involve magnetic field turbulence. Under such conditions a fraction of protons can be accelerated stochastically and create relativistic neutrons via nuclear collisions. These neutrons can freely escape from the accretion flow and decay into protons in dilute polar region above the rotating BH to form relativistic jets. We calculate geometric efficiencies of the neutron energy and mass injections into the polar region, and show that this process can deposit luminosity as high as L_j ~ 2e-3 dot{M} c^2 and mass loading dot{M}_j ~ 6e-4 dot{M} for the case of the BH mass M ~ 1e8 M_sun, where dot{M} is mass accretion rate. The terminal Lorentz factors of the jets are Gamma ~ 3, and they may explain the AGN jets having low luminosities. For higher luminosity jets, which can be produced by additional energy inputs such as Poynting flux, the neutron decay still can be a dominant mass loading process, leading to e.g., Gamma ~ 50 for L_{j,tot} ~ 3e-2 dot{M}c^2.Comment: 7 pages, 6 figures; accepted for publication in Ap

Polarization of synchrotron emission from relativistic reconfinement shocks with ordered magnetic fields

We calculate the polarization of synchrotron radiation produced at the relativistic reconfinement shocks, taking into account globally ordered magnetic field components, in particular toroidal and helical fields. In these shocks, toroidal fields produce high parallel polarization (electric vectors parallel to the projected jet axis), while chaotic fields generate moderate perpendicular polarization. Helical fields result in a non-axisymmetric distribution of the total and polarized brightness. For a diverging downstream velocity field, the Stokes parameter U does not vanish and the average polarization is neither strictly parallel nor perpendicular. A distance at which the downstream flow is changing from diverging to converging can be easily identified on polarization maps as the turning point, at which polarization vectors switch, e.g., from clockwise to counterclockwise.Comment: 10 pages, 6 figures, accepted for publication in A&

Measurement of Electron Trapping in the CESR Storage Ring

The buildup of low-energy electrons has been shown to affect the performance of a wide variety of particle accelerators. Of particular concern is the persistence of the cloud between beam bunch passages, which can impose limitations on the stability of operation at high beam current. We have obtained measurements of long-lived electron clouds trapped in the field of a quadrupole magnet in a positron storage ring, with lifetimes much longer than the revolution period. Based on modeling, we estimate that about 7% of the electrons in the cloud generated by a 20-bunch train of 5.3 GeV positrons with 16-ns spacing and $1.3x10^{11}$ population survive longer than 2.3 $\mu$s in a quadrupole field of gradient 7.4 T/m. We have observed a non-monotonic dependence of the trapping effect on the bunch spacing. The effect of a witness bunch on the measured signal provides direct evidence for the existence of trapped electrons. The witness bunch is also observed to clear the cloud, demonstrating its effectiveness as a mitigation technique.Comment: 6 pages, 9 figures, 28 citation