Helical Quantum States in a Strongly Frustrated Two-Dimensional Magnet

Thermodynamic properties of the J (1)-J (2)-J (3) quantum Heisenberg model are investigated on a square lattice with spin S = 1/2. The calculation of spin-spin correlators, spin excitation spectra, susceptibility, and heat capacity within a spherically symmetric approach shows that the third exchange J (3) may qualitatively change the properties of the system. Along with standard short-range order (antiferromagnetic, ferromagnetic, and stripe) structures, various quantum helices arise. In particular, these structures may be isotropic with a local minimum of the spectrum along a circle in the Brillouin zone. The character of these states represents both ferromagnetic and antiferromagnetic twisted quantum spin ordering. Moreover, a range of parameters is determined in which heat capacity exhibits two-peak temperature behavior.