Pulling order back from the brink of disorder: Observation of a nodal line spin liquid and fluctuation stabilized order in K$_2$IrCl$_6$

TL;DR

This study reveals a new nodal line spin liquid in K$_2$IrCl$_6$, where spins fluctuate within a line-like manifold, and shows how quantum fluctuations can stabilize magnetic order near this spin liquid state.

Contribution

The paper experimentally demonstrates a nodal line spin liquid in a face-centered cubic iridate and elucidates how quantum fluctuations influence the emergence and stabilization of magnetic order.

Findings

Identification of a nodal line spin liquid with line-like fluctuations in reciprocal space.

Quantum fluctuations near the spin liquid enhance and stabilize magnetic order.

Presence of a large spin-wave gap influenced by proximity to the spin liquid state.

Abstract

Competing interactions in frustrated magnets can give rise to highly degenerate ground states from which correlated liquid-like states of matter often emerge. The scaling of this degeneracy influences the ultimate ground state, with extensive degeneracies potentially yielding quantum spin liquids, while sub-extensive or smaller degeneracies yield static orders. A longstanding problem is to understand how ordered states precipitate from this degenerate manifold and what echoes of the degeneracy survive ordering. Here, we use neutron scattering to experimentally demonstrate a new "nodal line" spin liquid, where spins collectively fluctuate within a sub-extensive manifold spanning one-dimensional lines in reciprocal space. Realized in the spin-orbit coupled, face-centered cubic iridate K$_2$IrCl$_6$, we show that the sub-extensive degeneracy is robust, but remains susceptible to fluctuations or longer range interactions which cooperate to select a magnetic order at low temperatures. Proximity to the nodal line spin liquid influences the ordered state, enhancing the effects of quantum fluctuations and stabilizing it through the opening of a large spin-wave gap. Our results demonstrate quantum fluctuations can act counter-intuitively in frustrated materials: instead of destabilizing ordering, at the brink of the nodal spin liquid they can act to stabilize it and dictate its low-energy physics.