Disorder upon disorder: Localization effects in the Kitaev spin liquid

In recent years, several magnetic Mott insulators with strong spin-orbit couplings were suggested to be proximate to the Kitaev quantum spin liquid (QSL) whose one of the most exciting features is the fractionalization of spin excitations into itinerant Majorana fermions and static Z2 fluxes. Motivated by the emergence of this plethora of 4d and 5d transition metal Kitaev materials and by the fact that some level of disorder is inevitable in real materials, here we study how the Kitaev QSL responds to various forms of disorder, such as vacancies, impurities, and bond randomness. First, we argue that the presence of the quenched disorder in the Kitaev QSL can lead to the Anderson localization of Majorana fermions and the appearance of Lifshitz tails. We point out that the Anderson localization of low-energy states is particularly strong in the extended Kitaev model with the time reversal symmetry breaking term. Second, we show that the disorder effects on the low-energy Majorana fermion modes can be detected in thermal transport. Third, we show that at finite temperatures the Z2 fluxes become thermally excited and give rise to an additional disorder for the Majorana fermions. This source of the disorder dominates at high temperatures. Fourth, we demonstrate that both the structure of the energy spectrum and the thermal transport properties of the disordered Kitaev QSL depend strongly on the character of disorder. While we find that both the site disorder and the bond randomness suppress the longitudinal thermal conductivity, the low-energy localization is stronger in the case of the site disorder. (c) 2021 Elsevier Inc. All rights reserved.

Automated summary

This study explores the response of the Kitaev QSL to various forms of disorder, revealing that quenched disorder can lead to Anderson localization of Majorana fermions and the appearance of Lifshitz tails. Disorder effects on low-energy Majorana fermion modes can be detected in thermal transport, with Z2 fluxes becoming thermally excited at finite temperatures, dominating as a source of disorder at high temperatures. The energy spectrum and thermal transport properties of disordered Kitaev QSL strongly depend on the character of disorder, with site disorder and bond randomness suppressing longitudinal thermal conductivity while low-energy localization is stronger with site disorder.