Materials perspectives on achieving pyrochlore-based quantum spin liquid ground states

Abstract

Exotic magnetic systems often stem from frustration caused by competing interactions. The epitome of magnetically frustrated system is incarnated by the quantum spin liquids. These systems are expected to evade magnetic ordering or freezing down to absolute zero Temperature. Magnetic correlations are expected to be strong and can lead to quantum entanglement over large scales as well as the emergence of exotic excitations. Stabilizing such a phase in a rare-earth pyrochlore oxide yields a so-called quantum spin ice resulting from the coherent superposition of ‘two-in-two-out’ spin ice configurations, reminiscent of the arrangement of hydrogen atoms in water ice. In some rare-earth pyrochlore oxides, magnetic rare earth ions experience a strong uniaxial anisotropy as well as frustrated ferromagnetic interactions, two of the main ingredients stabilizing a spin ice state. In particular, Ho2Ti2O7 and Dy2Ti2O7 have been thoroughly studied and are now two well established examples of classical spin ices. Owing to a reduced dipolar interaction, similar materials hosting smaller magnetic moments are thought to be ideal systems to search for quantum spin ices states. Such a state is favored by an Hamiltonian composed of a leading but not exceedingly large Ising term, forming the ’ice’ state, accompanied by smaller but non-negligible transverse terms allowing quantum fluctuations to tunnel through equivalent spin ice configurations. Additionally, other properties such as multipolar degrees of freedom, structural disorder or low-lying crystal-electric field levels are expected to have significant contributions to the magnetic ground state. Cerium-based pyrochlores are promising quantum spin liquids candidates, which received little attention until recently due to their demanding synthesis. Using thermodynamic measurements together with a detailed structural and crystal-electric field analysis, we show that both Ce2Sn2O7 and Ce2Hf2O7 display magnetic moments with an Ising character and no sign of spin ordering or freezing despite signs of magnetic correlations at low temperatures. Through various neutron experiments, we demonstrate that the compounds form a peculiar octupolar quantum spin ice state, where dominant octupolar correlations conspire to form a coherent ‘ice-like’ phase from which fractional excitations emerge. High resolution neutron backscattering spectroscopy shows convinc- ing agreement with the theoretical predictions of the quantum electrodynamics emerging from a quantum spin ice phase. Disorder is known to have a significant influence on spin liquids, typically leading to spin freezing or ordering. In particular, pyrochlore oxides containing non-Kramers rare earths are very sensitive to structural disorder, a specificity that was theoretically proposed as a way to stabilize a quantum spin liquid phase. We hereby present the study of three cases where structural disorder plays a prominent role in the low temperature magnetic properties. We start with the case of Tb2Hf2O7, a strongly disordered yet very promising spin liquid candidate with a puzzling low temperature correlated state. A detailed structural analysis, coupled with a point charge model, yields a qualitative understanding of the single-ion properties of this compound and provides new insights into the mechanisms behind its low temperature behavior. We then perform a comparative structural study of two praseodymium-based compounds, Pr2Hf2O7 and Pr2Zr2O7, which display distinct magnetic ground states in spite of their structural and chemical proximity. Finally, we show how the control over the structural disorder via chemical substitution allows the tuning of the magnetic behavior of spin ice state observed in Ho2Ti2O7