Cascade of magnetic-field-driven quantum phase transitions in Ce3Pd20Si6

TL;DR

This study uncovers a new high-field phase in Ce3Pd20Si6 with distinct multipolar orderings, revealing complex quantum phase transitions driven by magnetic fields and advancing understanding of hidden electronic orders in f-electron systems.

Contribution

It reports the discovery of a previously unknown phase transition in Ce3Pd20Si6 at high magnetic fields, expanding the known phase diagram and providing insights into multipolar ordering phenomena.

Findings

Identification of a new phase transition above 12 T in Ce3Pd20Si6.

Observation of two distinct multipolar-ordered ground states separated by a quantum phase transition.

Correlation between collective excitations and the number of phases in the magnetic phase diagram.

Abstract

Magnetically hidden order is a hypernym for electronic ordering phenomena that are visible to macroscopic thermodynamic probes but whose microscopic symmetry cannot be revealed with conventional neutron or x-ray diffraction. In a handful of f-electron systems, the ordering of odd-rank multipoles leads to order parameters with a vanishing neutron cross-section. Among them, Ce$_3$Pd$_{20}$Si$_6$ is known for its unique phase diagram exhibiting two distinct multipolar-ordered ground states (phases II and II'), separated by a field-driven quantum phase transition associated with a putative change in the ordered quadrupolar moment from $O_2^0$ to $O_{xy}$. Using torque magnetometry at subkelvin temperatures, here we find another phase transition at higher fields above 12 T, which appears only for low-symmetry magnetic field directions $\mathbf{B} \parallel \langle11L\rangle$ with $1 < L \leq 2$. While the order parameter of this new phase II'' remains unknown, the discovery renders Ce$_3$Pd$_{20}$Si$_6$ a unique material with two field-driven phase transitions between distinct multipolar phases. They are both clearly manifested in the magnetic-field dependence of the field-induced (111) Bragg intensities measured with neutron scattering for $\mathbf{B} \parallel [112]$. We also find from inelastic neutron scattering that the number of nondegenerate collective excitations induced by the magnetic field correlates with the number of phases in the magnetic phase diagram for the same field direction. Furthermore, the magnetic excitation spectrum suggests that the new phase II'' may have a different propagation vector, revealed by the minimum in the dispersion that may represent the Goldstone mode of this hidden-order phase.