We introduce a matrix product state (MPS) with an incommensurate periodicity by applying the spin-rotation operator of each site to a uniform MPS in the thermodynamic limit. The relation between the improvement of the variational method due to the spin-rotation operators and the spontaneous symmetry breaking of the MPS is studied. The optimized pitch of the rotational operator reflects the commensurate/incommensurate properties of spin-spin correlation functions in the
S=1/2 Heisenberg chain and the
S=1/2 ferromagnetic-antiferromagnetic zigzag chain.
We study the spin-
21 antiferromagnetic Heisenberg model on an $\ensuremath{\infty}\ifmmode\times\else\texttimes\fi{}N$ square lattice for even
N's up to 14. Previously, the nonlinear sigma model perturbatively predicted that its spin-rotational symmetry breaks asymptotically with $N\ensuremath{\rightarrow}\ensuremath{\infty}$, i.e., when it becomes two dimensional (2D). However, we identify a critical width
Nc=10 for which this symmetry breaks spontaneously. It shows the signature of a dimensional transition from one dimensional (1D) including quasi-1D to 2D. The finite-size effect differs from that of the $N\ifmmode\times\else\texttimes\fi{}N$ lattice. The ground-state (GS) energy per site approaches the thermodynamic limit value, in agreement with the previously accepted value, by one order of
1/N faster than when using $N\ifmmode\times\else\texttimes\fi{}N$ lattices in the literature. Methodwise, we build and variationally solve a matrix product state (MPS) on a chain, converting the
N sites in each rung into an effective site. We show that the area law of entanglement entropy does not apply when
N increases in our method and the reduced density matrix of each effective site has a saturating number of dominant diagonal elements with increasing
N. These two characteristics make the MPS rank needed to obtain a desired energy accuracy quickly saturate when
N is large, making our algorithm efficient for large
N's. Furthermore, the latter enables space reduction in MPS. Within the framework of MPS, we prove a theorem that the spin-spin correlation at infinite separation is the square of staggered magnetization and demonstrate that the eigenvalue structure of a building MPS unit of $\ensuremath{\langle}g|g\ensuremath{\rangle},|g\ensuremath{\rangle}$ being the GS is responsible for order, disorder, and quasi-long-range order.