The nature of low-temperature spin-freezing in frustrated Kitaev magnets

Abstract: The subtle interplay between competing degrees of freedom, anisotropy, and spin correlations in frustrated Kitaev quantum materials offers an ideal platform to host non-trivial quantum states with exotic fractional excitations. The signature of spin-freezing behavior of these spin-orbit-driven frustrated magnets is characterized by a bifurcation of zero-field-cooled and field-cooled magnetic susceptibility at low temperatures much below the characteristic interaction energy scale of spins. The temperature dependence of magnetic specific heat exhibits Cm~ T^2 dependence near the freezing temperature. The field-independent behavior of Cm below the freezing temperature implies the presence of exotic low-energy excitations. The aging and memory effect experiments in the Kitaev magnets suggest a non-hierarchical free energy distribution, which differs from the hierarchical organization of conventional spin-freezing. Furthermore, the NMR spin-lattice relaxation rate follows a power law behavior below the spin-freezing temperature, suggesting the persistence of unconventional spin excitation spectra. Herein, we demonstrate that the observed low-temperature spin-freezing phenomena in a few representative Kitaev quantum materials can be effectively explained by the Halperin and Saslow (HS) hydrodynamic modes relevant for non-trivial spin glass materials. The linearly dispersive HS modes are hypothesized to account for instigating non-abelian defect propagation, thereby inducing a spin jam state in the low-temperature regime in frustrated Kitaev magnets. Our investigation reveals that HS modes capture the essence of unconventional spin-freezing ascribed to topological origin in two-dimensional (2D) Kitaev magnets decorated on a honeycomb lattice and its 3D analog hyperhoneycomb that offers a viable ground to extend this framework to a large class of frustrated quantum materials.