Experimental signatures of emergent quantum electrodynamics in Pr2Hf2O7

Abstract

In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical longrange order to form an exotic state that is quantum entangled and coherent over macroscopic length scales 1-2 . Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal fundamentally novel physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory 3-7 . This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr 2 Hf 2 O 7 that provide evidence for a quantum spin ice ground state. We find a quasielastic structure factor with pinch points -a signature of a classical spin ice -that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite temperature.Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr 2 Hf 2 O 7 can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a threedimensional quantum spin liquid -a topical state of matter which has so far mostly been explored in lower dimensionalities.

Screening and the pinch point paradox in spin ice

“…The pinch point Fig. 1 a is a type of singularity that is expected, and may be observed, in at least two types of system: firstly, dipolar systems, such as ferromagnets [1,2], and secondly, ice-rule systems, including hydrogen bonded ferroelectrics [3][4][5][6], water ice [7,8], spin ice [9][10][11][12][13][14], ionic ice [15,16], artificial spin ice [17][18][19], quantum spin ice [20][21][22] and antiferromagnetic spin liquids [23][24][25][26][27]. Such systems have the notable feature that they enter a highly correlated low-temperature state without any symmetry breaking.…”

Spin correlations of quantum spin liquid and quadrupole-ordered states ofTb2+xTi2−xO7+yKadowaki1, Wakita2, Fåk3 et al. 2019Phys. Rev. BSelf Cite121250View full textAdd to dashboardCite

“…Quantum effects in frustrated magnetic systems ranging from quantum annealing [11,12] to quantum spin liquid (QSL) states [13], the origin of which dates back to the proposal of the RVB state [14], have attracted much attention. Experimental challenges of finding real QSL substances [15,16] and of investigating QSL states using available techniques [17][18][19][20][21][22] have been addressed in recent years.…”

Quantum Spin Ice with Frustrated Transverse Exchange: From a π -Flux Phase to a Nematic Quantum Spin Liquid

“…Pyrochlore materials also stand at the forefront of the search for quantum spin liquids (QSL), massivelyentangled quantum phases of matter, which provide accessible examples of exotic, topological (quasi-)particles previously studied in high-energy physics [6][7][8][9]. In particular, the quantum analogue of spin ice has been shown to support a threedimensional QSL with fractional excitations, described by a U (1) lattice gauge theory [10][11][12][13][14][15], and has been vigorously pursued in experiment [16][17][18][19][20][21].…”