Exploiting polarization dependence in two dimensional coherent spectroscopy: examples of Ce$_2$Zr$_2$O$_7$ and Nd$_2$Zr$_2$O$_7$

TL;DR

This paper demonstrates how polarization-dependent two-dimensional coherent spectroscopy can distinguish fractionalized excitations and conventional magnons in candidate quantum materials, providing detailed insights into their low-energy excitations and proximity to quantum critical points.

Contribution

It introduces a polarization-sensitive 2DCS method to probe and differentiate fractionalized and conventional excitations in complex quantum materials, exemplified by Ce$_2$Zr$_2$O$_7$ and Nd$_2$Zr$_2$O$_7$.

Findings

Polarization sensitivity allows distinguishing spinons and magnons.

The response from polarized spin chains reveals the dipolar-octupolar mixing angle.

$[001]$ polarization probes the spinon continuum edge and quantum criticality.

Abstract

Two dimensional coherent spectroscopy (2DCS) probes the nonlinear optical response of correlated systems. An interesting application is the study of fractionalized excitations, which are challenging to distinguish unambiguously in linear response. Here we demonstrate how the sensitivity of optical matrix elements to variations in the photon polarization allows one to probe different aspects of low lying excitations in models of the candidate fractionalized materials Ce$_2$Zr$_2$O$_7$ and Nd$_2$Zr$_2$O$_7$, which host effective one-dimensional spin chains when subjected to a [110] magnetic field. We show how both fractionalized spinon excitations or conventional magnons can be picked out in the 2DCS response, and how the response from polarized spin chains can be used to probe the dipolar-octupolar mixing angle $\theta$ through the relative intensity of one- and two-magnon signals. Further, we find that a $[001]$ polarization of the probe field is particularly sensitive to lower band edge of the spinon continuum and can be used as a measure of the proximity of a quantum critical point in Ce$_2$Zr$_2$O$_7$. 2DCS can thus be employed to provide invaluable and detailed information both on the constituent degrees of freedom of a quantum material and on their collective behaviour.