Non-volatile control of topological phase transition in an asymmetric ferroelectric In2Te2S monolayer.

The coupling of topological electronic states and ferroelectricity is highly desired due to their abundant physical phenomenon and potential applications in multifunctional devices. However, it is difficult to achieve such a phenomenon in a single ferroelectric (FE) monolayer because the two polarized states are topologically equivalent. Here, we demonstrate that the symmetry of polarized states can be broken by constructing a Janus structure in a FE monolayer. We illustrate such a general idea by replacing a layer of Te atoms in the In2Te3 monolayer with S atoms. Using first-principles calculations, we show that the In2Te2S monolayer has two asymmetric polarized states, which are characterized by a metal and semiconductor, respectively. Importantly, as the spin-orbit coupling is included, a band gap (50.4 meV) is created in the metallic state, resulting in a non-trivial topological phase. Thus, it proves to be a feasible method to engineer non-volatile FE control of topological order in a single-phase system. We also demonstrate the underlying physical mechanism of topological phase transition, which is unveiled to be related to the weakened intrinsic electric field resulting from charge transfer. These interesting results provide a general way to design asymmetric FE materials and shed light on their potential application in non-volatile multifunctional devices.

Coexistence of intrinsic room-temperature ferromagnetism and piezoelectricity in monolayer BiCrX3 (X = S, Se, and Te).

Two-dimensional (2D) materials with intrinsic ferromagnetism and piezoelectricity have received growing attention due to their potential applications in nanoscale spintronic devices. However, their applications are highly limited by the low Curie temperatures (TC) and small piezoelectric coefficients. Here, using first-principles calculations, we have successfully predicted that BiCrX3 (X = S, Se, and Te) monolayers simultaneously possess ferromagnetism and piezoelectricity by replacing one layer of Bi atoms with Cr atoms in Bi2X3 monolayers. Our results demonstrate that BiCrX3 monolayers are not only intrinsic ferromagnetic semiconductors with indirect band gaps, adequate TC values of higher than 300 K, and significant out-of-plane magnetic anisotropic energies, but also exhibit appreciable in-plane and out-of-plane piezoelectricity. In particular, the in-plane piezoelectric coefficients of BiCrX3 monolayers with ABCAB configuration are up to 15.16 pm V-1, which is higher than those of traditional three-dimensional piezoelectric materials such as α-quartz. The coexistence of ferromagnetism and piezoelectricity in BiCrX3 monolayers gives them promising applications in spintronics and nano-sized sensors.

Intrinsic ferromagnetism and the quantum anomalous Hall effect in two-dimensional MnOCl2 monolayers.

Due to their potential application in spintronic devices, two-dimensional (2D) ferromagnetic materials are highly desired. We used first-principles calculations and Monte Carlo simulations to investigate the electronic structure and magnetic characteristics of the MnOCl2 monolayers. We discovered two stable monolayer structures, Pmna-MnOCl2 and Pmmn-MnOCl2. Our findings show that the Pmna-MnOCl2 monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.152 eV and a Curie temperature (TC) of 202 K, while the Pmmn-MnOCl2 monolayer is an intrinsic ferromagnetic Dirac semimetal with a high TC (910 K) and triaxial magnetic anisotropy. We also show that a Pmmn-MnOCl2 monolayer with a nontrivial band gap of 6.2 meV can achieve the quantum anomalous Hall effect (QAHE) with Chern number C = 1. Additionally, the existence of a gapless edge state can be flexibly regulated by choosing the terminal edges. Our studies reveal that the Pmmn-MnOCl2 monolayer can serve as a candidate material to achieve high-temperature QAHE.

Intrinsic half-metallicity in two-dimensional Cr2TeX2 (X = I, Br, Cl) monolayers.

Two-dimensional (2D) materials with intrinsic half-metallicity at or above room temperature are important in spin nanodevices. Nevertheless, such 2D materials in experiment are still rarely realized. In this work, a new family of 2D Cr2TeX2 (X = I, Br, Cl) monolayers has been predicted using first-principles calculations. The monolayer is made of five atomic sublayers with ABCAB-type stacking along the perpendicular direction. It is found that the energies for all the ferromagnetic (FM) half-metallic states are the lowest. The phonon spectrum calculations and molecular dynamics simulations both demonstrate that the FM states are stable, indicating the possibility of experimentally obtaining the 2D Cr2TeX2 monolayers with half-metallicity. The Curie temperatures from Monte Carlo simulations are 486, 445, and 451 K for Cr2TeI2, Cr2TeBr2, and Cr2TeCl2 monolayers, respectively, and their half-metallic bandgaps are 1.72, 1.86 and 1.90 eV. The corresponding magnetocrystalline anisotropy energies (MAEs) are about 1185, 502, 899 µeV per Cr atom for Cr2TeX2 monolayers, in which the easy axes are along the plane for the Cr2TeBr2 and Cr2TeCl2 monolayers, but being out of the plane in the Cr2TeI2. Our study implies the potential application of the 2D Cr2TeX2 (X = I, Br, Cl) monolayers in spin nanodevices.

Intrinsic ferromagnetic semiconductors in rhombohedral RMnO3 (R = Sc, Y, and Lu) with high critical temperature and large ferroelectric polarization.

Ferromagnetic (FM) semiconductors have been recognized as the cornerstone for next-generation highly functional spintronic devices. However, the development in practical applications of FM semiconductors is limited by their low Curie temperatures (T C). Here, on the basis of model analysis, we find that the FM super-exchange couplings in the d 5 - d 3 system can be significantly strengthened by reducing the virtual exchange gap (G ex) between occupied and empty e g orbitals. By first-principle calculations, we predict robust ferromagnetism in three rhombohedral RMnO3 (R = Sc, Y, and Lu) compounds with the T C that is as high as ∼1510 K (YMnO3). The oxygen breathing motions open a band gap and create an unusual Mn2+/Mn4+ charge ordering of the Mn-d electrons, which play an important role in altering the G ex. Interestingly, the rhombohedral RMnO3 compounds are also ferroelectric (FE) with a large spontaneous polarization approaching that of LiNbO3. These results not only deepen the understandings of magnetic couplings in d 5 - d 3 system, but also provide a way to design room-temperature FM-FE multiferroics.

First-principles study of the electric, magnetic, and orbital structure in perovskite ScMnO3.

Perovskite ScMnO3 has been synthesized under high temperature and high pressure. The magnetic ordering of this compound was proposed to be in the E-AFM state in previous theoretical research. Such magnetic ordering would lead the Mn3+ ions to be off-centered in the MnO6 octahedra; however, this is not detected experimentally. To address this issue, we systematically investigate the magnetic, orbital, and electric structures of perovskite ScMnO3 with first-principles calculations. It is found that its magnetic ground state is G-AFM and the magnetic ordering can explain the puzzle very well. Moreover, there is an unreported three-dimensional alternating cooperative orbital ordering in perovskite ScMnO3. The antiferromagnetic coupling between the nearest-neighbor Mn3+ ions is stabilized by the strong octahedral distortions that decrease the ferromagnetic interaction between the eg orbitals of the Mn3+ ions. In addition, we find that perovskite ScMnO3 is a bipolar antiferromagnetic semiconductor in which completely spin-polarized currents with reversible spin polarization can be tuned simply by applying a gate voltage. Such controllability of the spin polarization of the current opens up new avenues for future spintronic devices. Our results not only suggest that the G-AFM phase is the ground magnetic state for perovskite ScMnO3, but also enrich research in orbital ordering in rare-earth manganites.