Ferroelectric and Room-Temperature Ferromagnetic Semiconductors in the 2D M_IM_IIGe₂X₆ Family: First-Principles and Machine Learning Investigations

Abstract

Inspired by experimentally discovering ferromagnetism and ferroelectricity in two-dimensional (2D) CrGeTe 3 and CuInP 2 S 6 with similar geometric structures, respectively, we systematically investigated ferroic properties in a large family of 2D M I M II Ge 2 X 6 (M I and M II = metal elements, X = S/Se/Te) by combining high-throughput first-principles calculations and the machine learning method. We identified 12 stable 2D multiferroics containing simultaneously ferromagnetic (FM) and ferroelectric (FE) properties and 35 2D ferromagnets without FE polarization. Particularly, the predicted FM Curie temperatures (T C ) of eight 2D FM+FE semiconductors are close to or above room temperature. The ferroelectricity originates from the spontaneous geometric symmetry breaking induced by the unexpected shift of Ge−Ge atomic pairs and the emergence of Ge lone pair electrons, which also strengthens the p−d orbital hybridization between X atoms and metal atoms, leading to enhanced super-super-exchange interactions and raising the FM T C . Our findings not only enrich the family of 2D ferroic materials and present room-temperature FM semiconductors but also disclose the mechanism of the emerging ferroelectricity and enhanced ferromagnetism, which sheds light on the realization of high temperature multiferroics as well as FM semiconductors.

Spontaneous Valley Polarization and Electrical Control of Valley Physics in Single-Layer TcIrGe2S6

“…The electric polarization in SL TcIrGe 2 S 6 is switchable, resulting in the ferroelectricity. 34 In addition, we investigate the dynamic stability of the antiferroelectric state of SL TcIrGe 2 S 6 using phonon dispersion calculations. As shown in Figure S1c, the imaginary frequency at the Γ point is small; therefore, the antiferroelectric state of SL TcIrGe 2 S 6 is also stable.…”

“…And, following the previous work, the U values of Tc and Ir atoms are chosen as 2 and 0.5 eV, respectively. 34 The vacuum space is set to 20 Å. The cutoff energy is set to 500 eV.…”

“…The valence electronic configuration for the Tc atom is 4d 5 5s 2 . 34 Based on the octahedral crystal field, the d orbitals of Tc atom split into two orbitals: the higher e g orbitals (d x y . This causes a magnetic moment of 2.14 μ B / Tc, and Ir atoms carry no magnetic moments.…”

“…To describe the strong correlated correction of 3d electrons of Tc and Ir atoms, we use the GGA+U method. And, following the previous work, the U values of Tc and Ir atoms are chosen as 2 and 0.5 eV, respectively . The vacuum space is set to 20 Å.…”

“…We next analyze the magnetic properties of SL TcIrGe 2 S 6 . The valence electronic configuration for the Tc atom is 4d 5 5s 2 . Based on the octahedral crystal field, the orbitals of Tc atom split into two orbitals: the higher orbitals ( and ) and the lower orbitals ( , and ); see Figure d.…”