Ising versus Potts criticality in a low-temperature magneto-thermodynamics of a frustrated spin-1/2 Heisenberg triangular bilayer
Department of Theoretical Physics and Astrophysics
P. J. Šafárik University in Košice
Low-temperature magnetization curves and thermodynamics of a frustrated spin-1/2 Heisenberg triangular bilayer with the antiferromagnetic intra-dimer interaction and either ferromagnetic or antiferromagnetic inter-dimer interaction are investigated in a highly frustrated parameter region, where localized many-magnon eigenstates provide the most dominant contribution to a magnetothermodynamics. Low-energy states of the highly frustrated spin-1/2 Heisenberg triangular bilayer can be accordingly found from a mapping correspondence with the efective spin-1/2 Ising model on a triangular lattice. The description based on the effective Ising model implies that the spin-1/2 Heisenberg triangular bilayer with the ferromagnetic inter-dimer coupling displays in a zero-temperature magnetization curve discontinuous magnetization jump, which is reduced upon increasing of temperature until a continuous field-driven quantum phase transition from the Ising universality class is reached at a certain critical temperature. The spin-1/2 Heisenberg triangular bilayer with the antiferromagnetic inter-dimer coupling contrarily exhibits multistep magnetization curve with intermediate plateaus at one-third and two-thirds of the saturation magnetization, whereas discontinuous magnetization jumps observable at zero temperature change to continuous field-driven quantum phase transitions from the universality class of three-state Potts model at sufficiently low temperatures. The results obtained from Monte Carlo simulations of the effective Ising model are confronted with full exact diagonalization data for the Heisenberg triangular bilayer in order to corroborate these findings.