| Novel two-dimensional ferroic materials have attracted significant attention and have been at the leading edge of materials science due to their unique physical properties and potential applications.For example,graphene,as the most prominent example of two-dimensional(2D)materials,has been attracting tremendous interest in the past decade.Many excellent properties of graphene,such as its ultra-high carrier mobility and micro-level mean free path,can be attributed to its unique 2D geometric structure.Therefore,novel 2D materials with unique properties have also attracted scientists’research interests.At the same time,the study of ferroic materials has been one of the most important topics in condensed matter physics today.The development of novel ferroic materials no only enriched theoretical and experimental research,but also had an impact on industrial applications.In addition,recently,2D ferroic materials have drawn much attention for their unique properties.A lot of candidates are proposed theoretically and are confirmed experimentally.Due to its controllable ferroic properties,it is expected to have enormous potential application in the field of spintronic devices.Applying strain and doping electron has been an effective method to tune the property of 2D ferroic materials.Based on first-principles calculations,we systematically investigate two types of novel 2D ferroic materials.Firstly,a tunable bond forming and breaking driven isostructural phase transition has been proposed in the newly discovered MoN2 2D material based on first-principles calculations.Besides the a-phase with a N-N nonbonding state and a FM ordering,we identify a second β-phase with a N-N single bond state and a AFM ordering.To our best knowledge,this is the first time that a tunable isostructural bonding-nonbonding phase transition is discovered in a 2D material.It is also the first to realize a strain-induced FM to AFM magnetic phase transition for a 2D material.We show that the transition between the two phases can be readily achieved by applying a biaxial strain.At the phase transition,the N-N distance has a sudden change associated with the bond forming or breaking,while the crystalline symmetry is preserved.The magnetic structure is totally transformed during the transition,with a transfer of magnetism from the N(2p)sublattice to the Mo(4d)sublattice and the magnetic coupling transformed from FM to AFM.We have provided physical pictures for understanding these remarkable phenomena.Secondly,we introduced the concept of elemental Te bilayers with ferroelectricity.The elemental Te bilayers exhibit spontaneous in-plane polarization due to the interlayer interaction between lone pairs.The magnitude of the polarization reaches about 2.04×10-10 C/m,which can be detected by current experimental technology.The ferroelectricity is stable above room temperature.Remarkably,the elemental Te bilayers show that switchable spin-texture can be achieved via the inherent strong spin-orbital effect of tellurium.It offers a realistic platform to explore the intriguing physics of 2D ferroelectric phases as well as promising device applications.Finally,we proposed that the multilayer Te is also a stable ferroelectric material.The magnitude of spontaneous in-plane polarization was calculated to be 1.02×1 0-10 C/m per layer.At the same time,there exists an antiferroelectric phase in multilayer Te.The spontaneous polarization in the upper and lower parts of multilayer Te antiferroelectric phase is in the opposite direction,so that the total spontaneous polarization of system is zero.Te multilayer maintain FE phase stable in the ground state,but the stable AFE phase could be induced by simple physical means of electron doping and photoexcitation.Our results broaden the field of physical means induced phase transitions.This new discovery may have potential application in electronic devices. |
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