In a new study published in Physical Review Letters, researchers have shown that magnetic order can survive weak quantum fluctuations in disordered magnets that lack an energy gap. The work establishes robust ferromagnetism in the two-dimensional random-bond quantum Ising model, confirming a longstanding conjecture in quantum statistical mechanics.

Many magnets owe their order to spontaneous symmetry breaking (SSB). Physicists have long sought to prove that this order is stable against perturbations such as quantum fluctuations. Existing proofs, however, typically require the system to have an energy gap. Disordered magnets such as the random-bond Ising model are gapless, placing them outside the reach of these proofs.

The researchers developed a proof technique that does not rely on an energy gap. They adapted an argument from statistical mechanics, known as the Peierls argument, to quantum systems.