Switchable Out‐of‐Plane Polarization in 2D LiAlTe₂

Abstract

in certain improper ferroelectrics. [3] This is important for the electronic polarizability that plays an important role in this class of materials, and underlies the hyperferroelectric behavior, specifically the resistance to depolarizing field. The key condition is the instability of longitudinal optic modes in addition to active transverse optic instability characteristic of normal ferroelectrics. This provides a mechanism for canceling the depolarizing field. As shown in Figure 1, a number of these hyperferroelectrics adopt a polar stuffed wurtzite structure which is typically formed by an ABC stoichiometry (A, B, and C represent different elements). [4] This mechanism motivates us to extend the range of materials by searching for a different class of covalent-polar compounds, ABC 2 , exemplified by LiAlTe 2 . [5] It provides opportunities for designing switchable devices since it is compatible with substrates such as germanium tellurium (GeTe), which are prototypical phase change memory materials.The ABC 2 ternary compound β-LiAlTe 2 has a layered polar covalent tetrahedral structure, space group, P3m1 (156). [5a] As shown in Figure 1a, this quadruple-layered (QL) crystal is formed by an in-plane covalent bonded AlTe 4 tetrahedral layer with Li atoms located in the distorted trigonal antiprisms, with van der Waals (vdW) bonding between the QL. This vdW bonding suggests that in addition to being polar, such material may be an amenable formation in single QL form as a 2D material. This raises the possibility of having a hyperferroelectric 2D layered material based on the phase with an out-of-plane polarization. In most case, there are two major approaches to obtain polarization in 2D materials: (a) functionalization of nonpolar 2D materials; (b) searching for 2D intrinsic ferroelectric materials. [6] The former strategy uses chemical doping or surface passivation to produce switchable dipoles. [7] The latter approach is to find materials with intrinsic 2D polarization, such as Janus 2D materials. [8] These derivative 2D materials with broken mirror symmetry attracted considerable attention because of their unique properties, such as large piezoelectric effect, Rashba spin splitting, and second-harmonic generation response.

Discovery of New Polymorphs of Gallium Oxides with Particle Swarm Optimization‐Based Structure Searches

“…[ 43 ] We calculated the spontaneous polarization of κ‐Ga 2 O 3 and Pmc 2 1 using the Berry phase approach. [ 44–46 ] Consideration of similar structures can interfere with data analysis; we, therefore, adopted bond characterization matrix (BCM) method [ 47 ] to eliminate similar structures from the structure searching and left only ≈800 structures to explore the relationship amid the energy, band gap, and coordination numbers (CNs) of Ga 2 O 3 polymorphs.…”

Insulator-to-conductor transition driven by the Rashba–Zeeman effect

“…This model realistically describes certain types of 2D materials deposited on a magnetic insulator substrate. Specifically, there exist a handful of 2D direct band-gap semiconductors, such as Ag 2 Te 26 , BiSb 27 , and LiAlTe 2 28 monolayers whose electronic structure around the conduction band minimum and the valence band maximum can be well described by the single-band Rashba model. When these monolayers are deposited on a proper magnetic insulator substrate, their electronic structure is expected to be well captured by Eq.…”

“…In addition to the Ag 2 Te/Cr 2 O 3 system, there are other potential candidates with different 2D materials and magnetic substrates to explore the insulator-to-conductor transition. For example, the above mentioned BiSb 27 and LiAlTe 2 28 monolayers have a smaller band gap and giant Rashba parameters. The magnetic insulator materials can be extended to FM EuO 41 and CrI 3 42,43 , and ferrimagnetic YIG 44 .…”

Intrinsic Dipole Coupling in 2D van der Waals Ferroelectrics for Gate‐Controlled Switchable Rectifier

“…The vdWs 2D ferroelectrics have no physical limit of critical thickness, which has been the main issue in conventional ferroelectric films . Thus the out‐of‐plane (OOP) and in‐plane (IP) ferroelectricity could be easily achieved in some atomically thin vdWs 2D ferroelectric films, such as SnTe, CuInP 2 S 6 , LiAlTe 2 , and In 2 Se 3 . Particularly, different from ultrathin SnTe and CuInP 2 S 6 , whose polarization only involves either pure IP or OOP orientations, atomically thin 2D In 2 Se 3 shows both IP and OOP ferroelectricity in its ground state of the α phase .…”

“…[8][9][10][11][12][13][14] Particularly, different from ultrathin SnTe and CuInP 2 S 6 , whose polarization only involves either pure IP or OOP orientations, atomically thin 2D In 2 Se 3 shows both IP and OOP ferroelectricity in its ground state of the α phase. [8][9][10][11][12][13][14] Particularly, different from ultrathin SnTe and CuInP 2 S 6 , whose polarization only involves either pure IP or OOP orientations, atomically thin 2D In 2 Se 3 shows both IP and OOP ferroelectricity in its ground state of the α phase.…”