3,201 research outputs found
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
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&
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