Cong Thanh Bach | Searcharxiv

arXiv2022

Temperature and Field Dependence of Ferromagnetic Magnon in Monolayer Honeycomb Spin Lattice

Temperature and field dependence of collective spin excitations or magnon in monolayer honeycomb spin lattices is investigated using an anisotropic exchange XZ-Heisenberg model in an external field. Magnetic phase transition in the presence of the transverse field is the spin reorientation (SR) transition with magnon intensity existing above the SR temperature. The transverse field either decreases or sustains the spin-wave intensity in the temperature region below or above the SR temperature, respectively. The gap of the zero-momentum low-energy magnon branch closes at the SR transverse field, which is the critical quantum phase transition field at zero temperature. The application of the model to a two-dimensional CrI$_3$ explains the existence of the zero-momentum magnon mode above the Curie temperature and shows the suitable values of the exchange parameters compared with the DFT calculations. The estimated magnon velocity near the Dirac point in this material is about 1.74 km/s.

cond-mat.str-el↗

arXiv2019

Thermodynamic properties of ferroics described by the transverse Ising model and their applications for CoNb$_2$O$_6$

The temperature and transverse field dependence of the entropy and the specific heat of ferroics in the quantum paramagnetic (QPa) state is investigated using the transverse Ising model (TIM) with different spin values within mean field and Gaussian spin fluctuation approximations. A maximum peak of the temperature dependence of the specific heat curves is enhanced in the QPa state due to spin fluctuations. This peak shifts to higher temperature region and its magnitude reduces with increasing transverse field. In addition, the temperature corresponding to this maximum depends linearly on the deviation of the transverse field from its critical value. The obtained specific heat qualitatively agrees with the experimental observation for the quasi-one-dimensional (1D) Ising ferromagnet CoNb$_2$O$_6$ in the QPa phase. It is also shown that the spin-1/2 three dimensional (3D) TIM clearly describes the specific heat of CoNb$_2$O$_6$ in the QPa states near the critical temperature. However, the spin-3/2 3D-TIM is more adequate than the spin-1/2 3D-TIM for describing the thermal behavior of CoNb$_2$O$_6$ in the QPa states at high fields and at elevated temperatures.

cond-mat.str-el↗