Posted: Jul 20, 2016 
When magnetism meets topology
(Nanowerk News) The search for exotic particles is one of the most fascinating pursuits of modern physics. While this area of research is traditionally associated with elementary particles created in highenergy physics, simple ‘quasiparticles’, with sometimes highly unconventional properties, can also emerge in the complex, manybody world of materials. One such particle is the Weyl fermion. First postulated by Hermann Weyl in 1929, the Weyl fermion is a solution to the Dirac equation of fermion fields that describes a massless fermion with definite handedness. However, no elementary particles that behave as Weyl fermions have yet been observed.


Energy dispersion of magnetic excitations of the allin allout magnetic structure (shown in the inset) of Sm2Ir2O7 as a function of momentum transfer, measured by resonant Xray scattering.

Inside materials, the occurrence of Weyl fermions as quasiparticles hinges on stringent symmetry constraints. Mathematically, the Weyl fermion is described by a twocomponent spinor, compared to the fourcomponent spinor of Dirac fermions (describing e. g. conventional electrons). Due to the twocomponent spinor, only two states must meet at the Fermi level of the material. This requires that either timereversal or inversion symmetry must be broken, to ensure that the touching conduction and valence bands are nondegenerate.

Materials hosting Weyl fermions are known as Weyl semimetals. The nontrivial topology of the Weyl points manifests in striking properties, such as Fermi arc surface states, which provide a ‘smoking gun’ experimental signature. In a series of breakthrough experiments in 2015, Weyl semimetals were discovered in a class of materials with broken inversion symmetry.

So far, all Weyl fermions have been found in ‘uncorrelated’ materials, where the electrons only interact weakly with each other. In general, electronic interactions can enrich topological phases. For Weyl semimetals, electronic correlations can stabilise magnetic order, which offers the possibility of a timereversal symmetry breaking Weyl semimetal. This correlated Weyl semimetal state is predicted to exhibited yet undiscovered exotic properties, such as the anomalous Hall effect.

The most promising candidate materials for this state are iridiumbased pyrochlore oxides, known as pyrochlore iridates. While the frustrated pyrochlore lattice hosts many intriguing magnetic states, such as the spin liquids, spin glasses and spin ices, the presence of iridium atoms with strong spinorbit coupling can induce nontrivial band topology. In pyrochlore iridates, to stabilise a correlated Weyl semimetal state, the microscopic magnetic order has to preserve inversion symmetry.

Theoretical studies have identified that the ‘allin allout’ magnetic order would fulfil the required symmetries. In this allin allout magnetic structure, the magnetic moments either point all towards or away from the centre of the tetrahedra formed by the iridium ions

A team of researchers from the London Centre for Nanotechnology and the University of Oxford, working with collaborators at central facilities, have now discovered that the magnetic moments in pyrochlore iridates indeed order in the allin allout structure. This confirms theoretical predictions and allows the elusive correlated Weyl semimetal state to be realised in these materials. Using resonant Xray scattering techniques, both the magnetic order and excitations were comprehensively characterised in the prototypical pyrochlore iridate Sm2Ir2O7. By analysing the excitation spectrum it was possible to deduce the effective Hamiltonian describing this class of material for the first time (Physical Review Letters, "Allin–allOut Magnetic Order and Propagating Spin Waves in Sm2Ir2O7").

The experiments were performed at the ESRF (Grenoble, France) and PETRA III (Hamburg, Germany) synchrotron radiation facilities where the use of stateoftheart instrumentation was critical; enabling magnetic excitations with a dispersion bandwidth of only 15 meV to be discerned. This presents a remarkable achievement for resonant Xray scattering.
