时 间: 2011年11月25日（周五）下午3:30

地 点: 清华大学高等研究院 科学馆322报告厅

题 目: Gutzwiller Projected wavefunctions in the fermonic theory of S=1 spin chains

报 告 人: Dr. 刘正鑫

Institute for Advanced Study, Tsinghua

报告摘要: Based on the fermionic mean field theory for spins, we develop the Gutzwiller projection method for the BCS-type mean field states of S = 1 systems. Through variational Monte Carlo (VMC) approach, we can use the projected wavefunctions to approximate the ground state. We study the S = 1 bilinear-biquadratic model $H=\sum_i J\mathbf S_i\cdot\mathbf S_{i+1} +K(\mathbf S_i\cdot\mathbf S_{i+1})^2$ and find that the optimized projected wavefunction is very close to the ground state when there is gap. The topology of the mean field state is important. After projection the topologically non-trivial states (weak pairing states) correspond to the Haldane phase, while the topologically trivial states (strong pairing states) fall in the dimerized phase. The transition point obtained from VMC is very close to the exact result. The correspondence between weak/strong pairing and Haldane/dimerized phase can be explained by the Z2 gauge uctuations around the mean field ground state.