Spin Waves and Quantum Criticality in the Frustrated XY Pyrochlore Antiferromagnet Er2Ti2O7

TL;DR

This study investigates the magnetic phase diagram of Er2Ti2O7, revealing unconventional low energy states, coexisting orders, and a quantum critical point driven by magnetic field tuning, using neutron scattering and heat capacity measurements.

Contribution

It provides the first detailed experimental characterization of quantum criticality and spin wave behavior in the frustrated XY pyrochlore antiferromagnet Er2Ti2O7.

Findings

Identification of coexisting short and long range magnetic order.

Observation of soft collective spin excitations at zero field.

Detection of a quantum critical point at 1.5 T with vanishing spin fluctuation energy.

Abstract

We report detailed measurements of the low temperature magnetic phase diagram of Er$_2$Ti$_2$O$_7$. Heat capacity and time-of-flight neutron scattering studies of single crystals, subject to magnetic fields applied along the crystallographic [110] direction, reveal unconventional low energy states. Er$^{3+}$ magnetic ions reside on a pyrochlore lattice in Er$_2$Ti$_2$O$_7$, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of non-collinear phases, divided by a zero temperature phase transition at $\mu_0 H_c \sim$ 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.