| A spin-1/2 square-lattice Heisenberg model including a particular 4-spin interaction can be studied using quantum Monte Carlo simulations on large lattices without sign problems. This "J-Q" model undergoes a quantum phase transition from the Neel state into a valence-bond-solid (VBS) state, thus allowing detailed studies of this phase transition. Another very interesting aspect of the system is the nature of the VBS state. The quantum fluctuations affect not only the amplitude of the order parameter, but also its "angle" characterizing the type of VBS; columnar or plaquette. These fluctuations are surprisingly strong aven rather deep inside the VBS state, reflecting a length-scale which diverges faster than the correlation length. This is in qualitative agreement with the recent theory of "deconfined" quantum-critical points, which predicts emergence of U(1) symmetry in the VBS state close to the transition. I will discuss the nature of these fluctuations and attempts to quantitatively extract the exponent governing the emergence of U(1) symmetry. |
|