Orientation Dependence of the Magnetic Phase Diagram of Yb$_2$Ti$_2$O$_7$

TL;DR

This study explores how the magnetic phase diagram of Yb$_2$Ti$_2$O$_7$ varies with orientation, revealing complex anisotropic behaviors and critical phenomena that challenge classical models, providing insights into its magnetic properties.

Contribution

It provides a comprehensive experimental and theoretical analysis of the anisotropic magnetic phase diagram of Yb$_2$Ti$_2$O$_7$, highlighting phenomena beyond classical mean-field predictions.

Findings

Critical endpoint for B parallel to <100>

Enhanced critical temperature for B parallel to <110> and <111>

Anomalies not explained by classical mean-field theory

Abstract

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\vec{B}\,\parallel\,\langle 100\rangle$, while field parallel to $\langle 110 \rangle$ and $\langle 111 \rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.