Abstract
A �Bose glass� is a novel state of matter that emerges in systems of interacting bosons in the presence of quenched disorder. At sufficiently low temperatures, disorder-free bosons are subject to so-called Bose-Einstein condensation (BEC). BEC can involve atoms in liquid 4He, laser-cooled ions in magnetic traps,2 Cooper pairs in superconductors, or magnons in magnetic systems. Due to peculiarities of Bose statistics, particles lose their individuality and occupy a unique quantum-mechanical state. The wave function of this condensate establishes long-range quantum phase coherence across a macroscopic sample. This, in turn, spawns unique quantum phenomena such as superfluity,5 Josephson effect6 and vortex matter. For repulsive bosons, quenched disorder disrupts the condensate and interferes with phase coherence. The result is a peculiar glassy state with only short-range phase correlations. While some experimental evidence of this was found in ultracold atoms,9 novel high-temperature superconductors,10 and quantum magnets,11, 12 none of the studies were direct. The key characteristic, namely the wave function of the condensate disrupted by disorder on the microscopic scale, remained inaccessible. Hereby we report a direct neutron diffraction observation of short range correlations of the BEC order parameter in a magnetic Bose glass. This phase is realized in the quantum spin ladder compound IPA-Cu(Cl0.96Br0.04)3, where disorder is induced by random chemical substitution.
