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

The mixtures of two spin-gap compounds IPA-Cu(Cl(x)Br(1-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 mu(0)H(c1)=10 +/- 1 T, and the M(H) slopes bend at mu(0)H(c3)=40 +/- 1 T, below the saturation field H(c2). Such a curvature suggests an exotic phase transition: Bose-Einstein condensation of spin triplets occurs at H(c1)< H < H(c3) and a Bose-glass state may be realized at H(c3)< H < H(c2), 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.