Provost Lecture Series: Yejun Feng

Speaker: Yejun Feng, Professor, Electronic and Quantum Magnetism Unit
Title: Where physics is not enough...
Abstract:
Over the past several decades, there has existed abundant amount of physics interest in materials with a pyrochlore sublattice, which can potentially host spin frustrations in the three-dimensional space and lead to exotic spin states of either trivial or non-trivial topological properties. However, experimentally, even the ground state of spinel ZnFe2O4, a classical spin system that had been studied over seventy years, had not been clearly resolved. With arduous efforts from my former Ph.D. student Margarita Dronova and many former and current Unit members, we experimentally improved the chemical quality by limiting various types of disorder in single crystal ZnFe2O4 to a total level of ~0.2% [1]. Such high-quality crystals allow a clarification of the long-range antiferromagnetic ground state with a three-dimensional checkerboard pattern [2]. This full control of materials’ chemistry enables us to further explore the experimental identity of spin glass. Here we introduce non-magnetic cations into ZnFe2O4 in a controlled manner by limiting them only to pyrochlore but no other sites. This destruction of a long-range order draws a similarity to the paradigm of quantum criticality, and the minimal amount of disorder used to drive this evolution allows one to explore spin glass in its emergent state, in the proximity of either long- or short-range order. Neutron magnetic diffuse scattering, probing at pico-second time scale, provides key separations between the entities of long-, short-range orders and spin glass [3].

Reference:
[1] Dronova et al., PNAS 119, e2208748119 (2022).  
[2] Dronova et al., PRB 109, 064421 (2024), with Editor’s Suggestion.
[3] Dronova et al., Arxiv:2507.07783.
Chair: Pinaki Chakraborty, Professor, Fluid Mechanics Unit