Low-energy singlet sector in spin-$\frac{1}{2}$ $J_1$--$J_2$ Heisenberg model on square lattice

A. Yu. Aktersky, A. V. Syromyatnikov

Based on a special variant of plaquette expansion, an operator is constructed whose eigenvalues give the low-energy singlet spectrum of spin-$\frac{1}{2}$ Heisenberg antiferromagnet on…