Prediction of Coupled Electronic and Phononic Ferroelectricity in Strained 2D h-NbN: First-Principles Theoretical Analysis

Abstract

Using first-principles density functional theoretical analysis, we predict coexisting ferroelectric and semi-metallic states in two-dimensional monolayer of h-NbN subjected to electric field and in-plane strain ( ). At strains close to =4.85%, where its out-of-plane spontaneous polarization changes sign without inverting the structure, we demonstrate a hysteretic response of its structure and polarization to electric field, and uncover a three-state (P=±Po, 0) switching during which h-NbN passes through Dirac semi-metallic states. With first-principles evidence for a combination of electronic and phononic ferroelectricity, we present a simple model that captures the energetics of coupled electronic and structural polarization, and show that electronic ferroelectricity arises in a material which is highly polarizable (small bandgap) and exhibits a large electron-phonon coupling leading to anomalous dynamical charges. These insights will guide search for electronic ferroelectrics, and our results on 2D h-NbN will stimulate development of piezo-field effect transistors and devices based on the multi-level logic.

Two-dimensional polar metal of a PbTe monolayer by electrostatic doping

“…With increasing applied electric field, the Pb atoms with positive charge gradually move down whereas the Te atoms with negative charge move up, leading to the decrease of electric polarization. The polarization direction is reverted suddenly when the electric field is larger than 30 V/nm, which is very close to that predicted in other 2D ferroelectrics [48]. All these results demonstrate that PbTe monolayer is intrinsic ferroelectrics with switchable polarization.…”

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

“…Chanana et al investigated the electric field and strain effects on the polarization properties of h-NbN. [49] When the tensile strain is larger than 4.85% (Figure 13b), a sign change of the spontaneous polarization without a structural reverse is predicted. Under the tensile strain of ≈5%, the buckling and ionic polarization remain large with the direction unchanged; therefore, the electronic polarization must strongly cancel the ionic polarization, thereby changing the sign of net polarization.…”

“…It is possible to incorporate the electronic polarization in the LGD theory to interpret the electronic origin. [ 49 ] Moreover, the external fields can also be considered in the LGD theory to reveal their influences on the ferroelectric performances. For bulk materials, the influences of piezoelectric, flexoelectric, and electric effects on the ferroelectric phase transition as well as the formation of domain walls have been successfully revealed, based on the extended LGD theory.…”

High-efficient ab initio Bayesian active learning method and applications in prediction of two-dimensional functional materials

“…More importantly, as there is the depolarizing electrostatic field in 2D materials, the electrical polarization of 2D materials is generally low, 20,22 limiting their further applications. Recently, BN, GdAg2, GdAu2 and MgB2 monolayers have been successfully synthesized experimentally, [29][30][31] thus attracting extensive attention on the family of 2D HBCs including some potential ferroelectrics NbN, 32 CrN and CrB2. 33 We shall employ the ab initio Bayesian active learning to efficiently and accurately seek for ferroelectric materials with maximal polarizations from 2D HBCs.…”