Quantum fluctuations and frustration of the collinear phase in CaV3O7

We study a theoretical model for CaV3O7: the 14-depleted square spin-12 Heisenberg model which includes both the nearest-neighbor coupling (J) and the next-nearest-neighbor coupling (J′), where J and J′ are antiferromagnetic. Recent experiments of the neutron diffraction by Harashima et al. report a magnetic collinear ordering at low temperatures, which may be called as a stripe phase. Most remarkable feature we find in CaV3O7 is that the stripe phase, which is not stable in the classical theory, is stabilized by the quantum fluctuations. The modified spin wave theory predicts that the stripe phase is realized for J′/J > 0.69. In CaV3O7, the coupling constants are estimated as J ∼ J′ by comparing the theoretical and experimental results.