Polarization-dependent thermal transport in 2D multiferroic InCrX3 (X = S, Se)

Regulating the intrinsic thermal transport properties of conventional monolayer ferroelectric materials by flipping the polarization orientation is a key scientific challenge due to their adherence to Neumann's principle. Introducing Cr atoms into monolayer In2X3 (X = S, Se) forms stable multiferroic InCrX3 structures. Moreover, the introduction of Cr atoms enables a drastic reconstruction of the local coordination environment upon polarization reversal, thereby potentially altering the phonon dispersion and scattering phase space in a manner that is not constrained by the higher symmetry of the pristine lattice. First-principles calculations reveal that the polarization orientation could affect the phonon thermal transport properties of InCrX3 significantly. For the polarization-up state, the coupling between acoustic and optical phonon branches becomes more pronounced in InCrX3, enhancing phonon anharmonicity in the low-frequency region. Meanwhile, numerous phonon quasi-flat bands appear in InCrX3 with polarization-up state as well, which give rise to lower group velocity and broaden anharmonic scattering channels. Therefore, the lattice thermal conductivity of InCrX3 in the polarization-up state is one order of magnitude lower than that in the polarization-down state. Given the semimetallic characteristics of the polarization-up state, the electronic thermal conductivity is further calculated. After incorporating electronic thermal conductivity, the total thermal conductivity still exhibits observable variation between the two polarization states. These findings uncover an electric-field-controllable heat transfer mechanism and provide a promising strategy for designing smart thermal management devices based on extreme two-dimensional monolayer multiferroic materials.