Neel-VBS phase boundary of the extended J_1-J_2 model with biquadratic   interaction
  The J_1-J_2 model with the biquadratic (plaquette-four-spin) interaction was simulated with the numerical-diagonalization method. Some limiting cases of this model have been investigated thoroughly. Taking the advantage of the extended parameter space, we survey the phase boundary separating the N'eel and valence-bond-solid phases. According to the deconfined-criticality scenario, the singularity of this phase boundary is continuous, accompanied with unconventional critical indices. Diagonalizing the finite-size cluster with N \le 36 spins, we observe a signature of continuous phase transition. Our tentative estimate for the correlation-length critical exponent is \nu=1.1(3). In order to elucidate a non-local character of criticality, we evaluated the Roomany-Wyld \beta function around the critical point. 
Yoshihiro Nishiyama (Okayama University)
cond-mat.stat-mech