Pulling Order Back from the Brink of Disorder*

Fluctuations are widely acknowledged to suppress long-range orders in systems characterized by a unique minimum ground-state energy at zero temperature. However, in frustrated magnetic systems with continuous, sub-extensive, degeneracy, fluctuations are found to lift the degeneracy and induce orders [1]. This phenomenon is commonly referred to as the “order-by-disorder" mechanism. The disorder could arise from either quantum or thermal fluctuations, or dilution, with thermal and quantum fluctuations preferring collinear states and dilution favoring noncollinear states [2,3]. Consequently, an intricate phase selection could be present in a real material. In this talk, I will present a striking dichotomy between dynamic and static spin correlations in a face-centered-cubic(FCC) iridate K₂IrCl₆, which shows a phase competition between a quantum fluctuation selected type-I state and a next-nearest-neighbor(NNN) J₂ selected type-III state. The quantum fluctuations are reflected in the dynamic correlations where a hard gap emerges at T_N and a correlated nodal-line spin-liquid state is revealed at T>T_N. While the order-by-quantum-disorder mechanism should select a type-I state, a well-ordered type-III state prevails in K₂IrCl₆ statically that is selected by the weak antiferromagnetic NNN interaction. Our findings show a phase coexistence of multiple magnetic orders that are stabilized through two different mechanisms, which manifests the complicated ground-state selection in frustrated magnets.

[1] J. Villain et al, J. Phys. France 41, 1263-1272 (1980)

[2] C. L. Henley, J. Appl. Phys. 61, 3962–3964 (1987)

[3] C. L. Henley, Phys. Rev. Lett. 62, 2056 (1989)