Magnetism orders exotic fermions

2 August 2016

A team from Oxford and UCL has shown that an iridium oxide compound satisfies a necessary requirement for a correlated Weyl semi-metal. The finding points the way towards a possible realization in the solid state of an exotic type of particle.

The Weyl semi-metal hosts a topologically non-trivial electronic state found as a solution of the Dirac equation for massless fermions. Such a state requires the combination of either (i) time-reversal symmetry with broken inversion symmetry, or (ii) inversion symmetry with broken time-reversal symmetry. Examples of the former type of Weyl semi-metal have been found recently, but no example of the latter has yet been confirmed.

The team, which included Marein Rahn, D. Prabhakaran and Andrew Boothroyd from Oxford, used a combination of resonant elastic and inelastic X-ray scattering on single crystal samples grown in Oxford. The results showed that the cubic pyrochlore oxide Sm2Ir2O7 transforms at a temperature of 110K into a magnetically ordered state in which the magnetic moments on the Ir atoms point either all-in or all-out of a network of tetrahedra formed by the Ir atoms. Such a magnetic structure breaks time-reveral symmetry but preserves inversion symmetry, as required for a Weyl semi-metal.

The results support a theoretical prediction that the family of pyrochlore iridates may host Weyl fermions, although electronic correlations also revealed by the study could forestall the Weyl semi-metal state. In that case, it might be necessary to tune the electronic structure by chemical modifications or external perturbations to form the Weyl semi-metal.

Reference: C. Donnerer et al., Phys. Rev. Lett. 117,037201 (2016) (arXiv:1604.06401)