Disorder-Induced Quantum Spin Liquid in Spin Ice Pyrochlores

We propose that in a certain class of magnetic materials, known as non-Kramers “spin ice,” disorder induces quantum entanglement. Instead of driving glassy behavior, disorder provokes quantum superpositions of spins throughout the system and engenders an associated emergent gauge structure and set of fractional excitations. More precisely, disorder transforms a classical phase governed by a large entropy, classical spin ice, into a quantum spin liquid governed by entanglement. As the degree of disorder is increased, the system transitions between (i) a “regular” Coulombic spin liquid, (ii) a phase known as “Mott glass,” which contains rare gapless regions in real space, but whose behavior on long length scales is only modified quantitatively, and (iii) a true glassy phase for random distributions with large width or large mean amplitude.

Figure 1

Phase diagram in the mean strength of disorder $\overline{h}$—disorder $\delta h$ plane. The dotted line indicates a first order transition, while the solid lines represent second order transitions or crossover (between the Coulomb QSL and Griffiths Coulomb QSL). In the disordered boson language, the paramagnet is a “superfluid” (Higgs) phase, the Griffiths phase is a Mott glass, and the Coulomb QSL is a “Mott insulator.” The clean spin ice point sits at $\overline{h}=\delta h=0$ and is represented by a white circle.