Jeudi 14 mars 2013
Magnetic monopoles were first proposed to exist by Dirac many decades ago as the
natural counterparts of electrically charged particles such as the electron. Despite
much searching, no elementary monopoles have ever been observed, even though
many theories of high-energy physics suggest that they should be present. Here,
we present an alternative route for the observation of monopoles, as a low- rather
than a high-energy phenomenon. It involves a process known as fractionalisation,
which is a striking emergent phenomenon, in which an ’elementary’ particle breaks
up into two independent entities. A celebrated example of this is spin-charge
separation, in which an electron’s magnetic (spin) and electric (charge) properties
appear to become independent degrees of freedom. The spin ice materials
Dy2Ti2O7 and Ho2Ti2O7 provide a rare instance of fractionalisation in three
dimensions : their atomic magnetic dipole moments fractionalise, resulting in
elementary excitations which can be thought of as magnetic monopoles.
This colloquium presents a self-contained introduction to theoretical concepts and
experimental phenomena in the physics of spin ice. It focuses on the unique
signatures of the peculiar nature of its ground state and its excitations. These
include unusual neutron scattering structure factors, rich non-equilibrium physics,
as well as a response to external magnetic fields that promotes spin ice as a
magnetic Coulomb liquid, a magnetic analogue of an electrolyte. Finally, this talk
addresses open questions and future perspectives for detecting individual
monopoles, among them a (thought-)experiment inspired by high energy physics.
RECRUTEMENT ACCES DIRECT
Le département de physique
Fête de la Science 2018 