Seeing the light: Experimental signatures of emergent electromagnetism in a quantum spin ice

Abstract

The "spin ice" state found in the rare earth pyrochlore magnets Ho2Ti2O7 and Dy2Ti2O7 offers a beautiful realisation of classical magnetostatics, complete with magnetic monopole excitations. It has been suggested that in "quantum spin ice" materials, quantum-mechanical tunnelling between different ice configurations could convert the magnetostatics of spin ice into a quantum spin liquid which realises a fully dynamical, latticeanalogue of quantum electromagnetism. Here we explore how such a state might manifest itself in experiment, within the minimal microscopic model of a such a quantum spin ice. We develop a lattice field theory for this model, and use this to make explicit predictions for the dynamical structure factor which would be observed in neutron scattering experiments on a quantum spin ice. We find that "pinch points", which are the signal feature of a classical spin ice, fade away as a quantum ice is cooled to its zero-temperature ground state. We also make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the "photons" of this emergent electromagnetism. The predictions of this field theory are shown to be in quantitative agreement with Quantum Monte Carlo simulations at zero temperature.

Quadrupole Order in the Frustrated PyrochloreTb2+xTi2−xOTakatsu1, Onoda2, Kittaka3 et al. 2016Phys. Rev. Lett.57158901View full textAdd to dashboardCite

“…An intriguing theoretical proposal for a U(1) quantum spin liquid (QSL) state [8] has been made for variants of SI endowed with quantum spin fluctuations [9][10][11][12][13][14]. The U(1) QSL state [8-10] is characterized by an emergent U(1) gauge field producing gapless fictitious photons and by gapped bosonic spinon excitations carrying the SI magnetic monopole charge [8,9,13,15]. By increasing the transverse interaction, the system can undergo a phase transition from the U(1) QSL to a long range ordered (LRO) state of transverse spins or pseudospins representing electric-quadrupole moments for non-Kramers ions [9][10][11].…”

Crystal-field states of Pr3+in the candidate quantum spin ice Pr2Sn2

“…If the nature and strength of the exchange interactions between spins is favourable, the fluctuations can become correlated allowing quantum mechanical tunneling within the ice rules manifold of states. It has been predicted that this particular spin liquid state could realise a fully dynamical, lattice analogue of quantum electromagnetism with linearly dispersing magnetic excitations exactly analogous to photons 8,9 in addition to other exotic excitations 10,11 .…”

Hydrogenic states of monopoles in diluted quantum spin ice

“…There are several candidate materials for quantum spin ice behavior, such as Tb 2 Ti 2 O 7 6-9 , Yb 2 Ti 2 O 7 10-12 and Pr 2 Zr 2 O 7 13 , but an unambiguous experimental signature of quantum spin ice has been lacking. Logically, much recent theoretical work has focused on looking for quantum generalizations of spin ice in which quantum fluctuations can lead to a fully quantum U(1) spin liquid [14][15][16][17][18][19][20] .…”