Coexisting Magnetism, Ferroelectric, and Ferrovalley Multiferroic in Stacking-Dependent Two-Dimensional Materials.

Two-dimensional (2D) multiferroic materials have widespread application prospects in facilitating the integration and miniaturization of nanodevices. However, the magnetic, ferroelectric, and ferrovalley properties in one 2D material are rarely coupled. Here, we propose a mechanism for manipulating magnetism, ferroelectric, and valley polarization by interlayer sliding in a 2D bilayer material. Monolayer GdI2 is a ferromagnetic semiconductor with a valley polarization of up to 155.5 meV. More interestingly, the magnetism and valley polarization of bilayer GdI2 can be strongly coupled by sliding ferroelectricity, making these tunable and reversible. In addition, we uncover the microscopic mechanism of the magnetic phase transition by a spin Hamiltonian and electron hopping between layers. Our findings offer a new direction for investigating 2D multiferroic devices with implications for next-generation electronic, valleytronic, and spintronic devices.

Anisotropic optical property of ferroelectric Bi2O2X(X = S,Se,Te) monolayer and strain engineering.

Based on the first-principles calculations, ferroelectricBi2O2X(X=S,Se,Te)monolayers with unequivalent in-plane lattice constants are confirmed to be the ground state, which is consistent with the experiment result (Ghoshet al2019Nano Lett.195703-09), and the anisotropic optical property is firstly investigated. We find that the polarizations ofBi2O2Xmonolayers points along the direction ofa-axis, andBi2O2Temonolayer process the largest polarization. Furthermore, both the biaxial and uniaxial strains are favor for the enhancement of polarization ofBi2O2Xmonolayers. It should be mentioned that the type of band gap will convert from indirect to direct forBi2O2Temonolayer when thea-axial tensile strain is larger than 2%. At last, the optical absorption coefficient forBi2O2Xmonolayers are calculated, and we obtain thatBi2O2Temonolayer has the strongest optical absorption within the range of visible light, the anisotropy and possible strain engineering to improve the optical absorption are discussed in detail. Our findings are significant in fields of optoelectronics and photovoltaics.

Tailoring angle dependent ferroelectricity in nanoribbons of group-IV monochalcogenides.

Ferroelectricity is significant in low dimensional structures due to the potential applications in multifunctional nanodevices. In this work, the tailoring angle dependent ferroelectricity is systematically investigated for the nanoribbons and nanowires of puckered group-IV monochalcogenides MX (M =Ge,Sn; X =S,Se). Based on first-principles calculations, it is found that the ferroelectricity of nanoribbon and nanowire strongly depends on the tailoring angle. Firstly, the critical width for the bare nanoribbon of group-IV monochalcogenide is obtained and discussed. As the nanowires are concerned, the ferroelectricity will disappear when the tailoring angle becomes small. At last, H-passivation on the edge and the strain engineering are employed to improve the ferroelectricity of nanoribbon, and it is obtained that H-passivation is beneficial to the enhancement of polarization for nanoribbons tailored near the armchair direction, while the polarization of nanoribbons tailored along the diagonal direction will decrease when the edges are passivated with H atoms, and the tensile strain along the length direction always favors the improvement of ferroelectricity of the considered nanoribbons. Therefore, tailoring angle has great influence on the ferroelectricity of nanoribbons and nanowires, which may be used as an effective way to tune the ferroelectricity and further the electronic structures of nanostructures in the field of nanoelectronics.

Coexistence of polar distortion and conduction in doped 2D group-IV ferroelectrics: SiGe, SiSn, and GeSn.

Since the concept of ferroelectric metal predicted in the 1960s has been experimentally realized in the bulk Weyl semimetal WTe2 [Sharma et al 2019 Sci. Adv. 5, eaax5080], it is significant to find the ultrathin polar metal or ferroelectric metal due to the demand of miniature of electronic nanodevices. Here, 2D buckled monolayers composed of group-IV elements such as SiGe, SiSn, and GeSn are selected as prototype. Then, the stability of 2D ferroelectricity in the above monolayers are confirmed based on the results of first-principles calculations. Most interesting, a robustly metallic polar state has been found in the above 2D ferrolectrics under both the electron doping and hole doping, and the polar distortion becomes even more remarkable when the electrons are doped as compared with the undoped system. Thus, the coexistence of polar state and conduction is theoretically verified in the doped group-IV monolayers. We hope the 2D ferroelectric materials can be used as a starting point to look for the polar metals with atomic thickness, and further broaden their applications in 2D electronics or spintronics in the future.

First-principles study of interface doping in ferroelectric junctions.

Effect of atomic monolayer insertion on the performance of ferroelectric tunneling junction is investigated in SrRuO3/BaTiO3/SrRuO3 heterostrucutures. Based on first-principles calculations, the atomic displacement, orbital occupancy, and ferroelectric polarization are studied. It is found that the ferroelectricity is enhanced when a (AlO2)(-) monolayer is inserted between the electrode SRO and the barrier BTO, where the relatively high mobility of doped holes effectively screen ferroelectric polarization. On the other hand, for the case of (LaO)(+) inserted layer, the doped electrons resides at the both sides of middle ferroelectric barrier, making the ferroelectricity unfavorable. Our findings provide an alternative avenue to improve the performance of ferroelectric tunneling junctions.

Expansion dynamics in a one-dimensional hard-core boson model with three-body interactions.

Using the adaptive time-dependent density matrix renormalization group method, we numerically investigate the expansion dynamics of bosons in a one-dimensional hard-core boson model with three-body interactions. It is found that the bosons expand ballistically with weak interaction, which are obtained by local density and the radius Rn. It is shown that the expansion velocity V, obtained from Rn = Vt, is dependent on the number of bosons. As a prominent result, the expansion velocity decreases with the enhancement of three-body interaction. We further study the dynamics of the system, which quenches from the ground state with two-thirds filling, the results indicate the expansion is also ballistic in the gapless phase regime. It could help us detect the phase transition in the system.