Séminaire Katarina KARLOVA

Mardi 28 novembre 2023, 14h00, salle E 4.13 Hybride

Katarina KARLOVA
LPTM, CY Cergy Paris University

Magnetic analogue of liquid-gas phase transition of water: case studies of spin-1/2 Ising-Heisenberg and Heisenberg models on a diamond decorated square lattice

Finite-temperature phase transitions of two geometrically frustrated spin-1/2 Ising-Heisenberg and Heisenberg models on a diamond-like decorated square lattice are investigated in a presence of the magnetic field by making use of several advanced analytical and numerical calculation schemes. The spin-1/2 Ising-Heisenberg model on the diamond-like decorated square lattice can be exactly mapped via a generalized decoration-iteration transformation onto an effective spin-1/2 Ising square lattice with temperature-dependent nearest-neighbor interaction and field. We have found a rigorous evidence that the effective field vanishes along the line of first-order phase transitions between the classical ferrimagnetic phase and monomer-dimer phase, which additionally terminates at an Ising-type critical point. An existence of discontinuous reentrant phase transitions were observed in a limited range of the interaction parameters. Exact results derived from the zero-field effective Ising square lattice are supplemented with classical Monte Carlo simulations in a particular subspace of the parameter region where an effective field becomes nonzero.

To compare with, the analogous spin-1/2 Heisenberg model on the diamond-like decorated square lattice similarly displays the line of first-order phase transitions between the quantum ferrimagnetic and monomer-dimer phases, which also turns out to terminate at the Ising-type critical point. This finding is corroborated by an advanced quantum Monte Carlo simulations performed in the dimer basis in order to avoid a notorious sign problem of this frustrated quantum spin model. The most interesting quantum spin arrangements is found within the macroscopically degenerate dimer-tetramer phase, whose residual entropy coincides with that one of hard-dimer model on square lattice.
 

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