Ising Quasiparticles and Hidden Order in URu$_2$Si$_2$

TL;DR

This paper explores the nature of Ising quasiparticles and proposes the hastatic order as a new theory for the hidden order phase in URu$_2$Si$_2$, supported by experimental predictions and analysis.

Contribution

It introduces the hastatic order as a novel explanation for hidden order, describing anisotropic quasiparticles via resonant scattering and fractionalization of bound states.

Findings

Hastatic order explains the anisotropic quasiparticles in URu$_2$Si$_2$.

The hybridization acts as a spinorial symmetry-breaking order parameter.

Predictions align with optical and tunneling experiments.

Abstract

The observation of Ising quasiparticles is a signatory feature of the hidden order phase of URu$_2$Si$_2$. 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 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 an 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 URu$_2$Si$_2$.