Disordered states in IPA-Cu(ClxBr1-x)3 induced by bond randomness.

The mixtures of two spin-gap compounds IPA-Cu(ClxBr1-x)3 are studied by electron paramagnetic resonance and magnetization processes [M(H)]. From electron paramagnetic resonance spectra, the symmetry of the spin-gap state breaks down, even for x=0.99. From M(H) curves for x=0.95 and 0.92, however, spin gaps survive below mu0Hc1=10+/-1 T, and the M(H) slopes bend at mu0Hc3=40+/-1 T, below the saturation field Hc2. Such a curvature suggests an exotic phase transition: Bose-Einstein condensation of spin triplets occurs at Hc1 <H <Hc3 and a Bose-glass state may be realized at Hc3 <H <Hc2, successively. As a result, the possibility of fine-tuning the density of bosons by H provides a tool for determining the role of the interactions.