We consider the spin-1/2 antiferromagnetic Heisenberg model on a bilayer honeycomb lattice including interlayer frustration in the presence of an external magnetic field. In the vicinity of the saturation field, we map the low-energy states of this quantum system onto the spatial configurations of hard hexagons on a honeycomb lattice. As a result, we can construct effective classical models (lattice-gas as well as Ising models) on the honeycomb lattice to calculate the properties of the frustrated quantum Heisenberg spin system in the low-temperature regime. We perform classical Monte Carlo simulations for a hard-hexagon model and adopt known results for an Ising model to discuss the finite-temperature order-disorder phase transition that is driven by a magnetic field at low temperatures. We also discuss an effective-model description around the ideal frustration case and find indications for a spin-flop-like transition in the considered isotropic spin model.
Motivated by experimental progress in pressure and strain tuning of quantum materials, we examine the thermodynamic response of frustrated magnets to uniaxial strain. Specifically, we study Ising and Heisenberg models on spatially anisotropic triangular (and, for the Ising model, also kagome) lattices. We determine the entropy as a function of temperature and strain, and use it to compute the elastic Grüneisen ratio
η. The Ising models can be strain-tuned into and out of classical spin-liquid phases, and we show that
η can become arbitrarily large at low temperature
T near the point of maximal frustration, a universal hallmark of an extensive ground-state entropy. In contrast, the spin-
1/2 Heisenberg model is moderately frustrated and displays multiple
T=0 phase transitions. These transitions dominate
η at low
T while the intermediate-
T behavior is similar to that of the Ising model. We discuss the extent to which the elastic Grüneisen ratio can be used to deduce the phase diagram, and we connect our results to recent experiments on triangular-lattice magnets.
The quantum phase transition in the spin-
21 XX Heisenberg chain model with three-spin interaction is studied. Using the Jordan-Wigner transformation, several thermodynamic, as well as thermal and spin transport quantities of the system are calculated exactly. It is shown that the three-spin interaction influences the calculated quantities and leads to characteristic features of the quantum phase transition. The effects of a finite magnetic field on the magnetic moment and magnetic susceptibility are also discussed.