Ising quasiparticles and hidden order in URu₂Si₂

Abstract

The observation of Ising quasiparticles is a signatory feature of the hidden order phase of . In this paper, we discuss its nature and the strong constraints it places on current theories of the hidden order. In the hastatic theory, such anisotropic quasiparticles are naturally described by resonant scattering between half-integer spin conduction electrons and integer-spin Ising moments. The hybridization that mixes states of different Kramers parity is spinorial; its role as a symmetry-breaking order parameter is consistent with optical and tunnelling probes that indicate its sudden development at the hidden order transition. We discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three body bound-states into integer spin fermions and half-integer spin bosons. After reviewing key features of hastatic order and their broader implications, we discuss our predictions for experiment and recent measurements. We end with challenges both for hastatic order and more generally for any theory of the hidden order state in .

Keywords:

• Ising models
• heavy-fermion metals
• order
• intermetallic compounds
• Kondo effect

Acknowledgements

We have benefitted from inspiring discussions with our colleagues who include C. Batista, C. Broholm, K. Haule, N. Harrison, G. Kotliar, G. Lonzarich, J. Mydosh, K. Ross and J. Schmalian. PC and PC are grateful to Trinity College, Cambridge and the Cavendish Laboratory where this article was completed.

Notes

This work was supported by the National Science Foundation [grant number NSF-DMR-1334428] (P. Chandra) and [grant number DMR-1309929] (P. Coleman), and by the Simons Foundation (R. Flint).