Keywords: 
• 周期性各向异性XY模型 /
• 量子相变 /
• 各向同性铁磁相 /
• 冯·诺依曼熵

Abstract: The quantum phase transitions of one-dimensional period-two anisotropic XY models in a transverse field with the Hamiltonian H=N∑i=1[-J/2(-1+γi/2σxiσxi+1+1-γi/2σyiσyi+1)-1/2hσzi],where the anisotropy parameters γi take γ and αγ alternately,are studied.The Hamiltonian can be reduced to the diagonal form by Jordan-Wigner and Bogoliubov transformations.The long-range correlations Cx and Cy are calculated numerically.The phase with Cx ＞ Cy ≠ 0 (or Cy ＞ Cx ≠ 0) is referred to as the ferromagnetic (FM) phase along the x (or y) direction,while the phase with Cx =Cy =0 is the paramagnetic (PM) phase.It is found that the phase diagrams with the ratio α ≠-1 and α =-1 are different obviously.For the case with α ≠-1,the line h =hc1 =/1-[γ(1-α)/2]2 separates an FM phase from a PM phase,while the line γ =0 is the boundary between a ferromagnetic phase along the x direction (FMx) and a ferromagnetic phase along the y direction (FMy).These are similar to those of the uniform XY chains in a transverse field (i.e.,α =1).When α =-1,the FMx and FMy phases disappear and there appears a new FM phase.The line h =hc2 =√1-γ2 separates this new FM phase from the PM phase.The new phase is gapless with two zeros in single particle energy spectrum.This is due to the new symmetry in the system with α =-1,i.e.,the Hamiltonian is invariant under the transformation σx2i → σy2i+1,σy2i → σx2i+1.The correlation function between the 2i-1 and 2i lattice points along the x (y) direction is equal to that between the 2i and 2i + 1 lattice points along the y (x) direction.As a result,the long-range correlation functions along two directions are equivalent.In order to facilitate the description,we call this gapless phase the isotropic ferromagnetic (FMxx) phase.Finally,the relationship between quantum entanglement and quantum phase transitions of the system is studied.The scaling behaviour of the von Neumann entropy at each point in the FMxx phase is SL ～ 1/3 log2 L+ Const,which is similar to that at the anisotropic phase transition point of the uniform XY model in a transverse field,and different from those in the FMx and FMy phases.