Intrinsic Origin of Enhancement of Ferroelectricity in SnTe Ultrathin Films

Abstract

Previous studies showed that, as ferroelectric films become thinner, their Curie temperature (T_{c}) and polarization below T_{c} both typically decrease. In contrast, a recent experiment [Chang et al., Science 353, 274 (2016)SCIEAS0036-807510.1126/science.aad8609] observed that atomic-thick SnTe films have a higher T_{c} than their bulk counterpart, which was attributed to extrinsic effects. We find, using first-principles calculations, that the 0-K energy barrier for the polarization switching (which is a quantity directly related to T_{c}) is higher in most investigated defect-free SnTe ultrathin films than that in bulk SnTe, and that the 5-unit-cell (UC) SnTe thin film has the largest energy barrier as a result of an interplay between hybridization interactions and Pauli repulsions. Further simulations, employing a presently developed effective Hamiltonian, confirm that freestanding, defect-free SnTe thin films have a higher T_{c} than bulk SnTe, except for the 1-UC case. Our work, therefore, demonstrates the possibility to intrinsically enhance ferroelectricity of ultrathin films by reducing their thickness.

From an atomic layer to the bulk: Low-temperature atomistic structure and ferroelectric and electronic properties of SnTe films

“…Band bending is larger on a ferroelectrically-coupled (AA) 4 AL SnTe film when compared to an antiferroelectrically-coupled (AB ) 4 AL SnTe film, because the electric field lines cancel out at the exposed edge on the latter case. 70,71 Here, we use energetics and the experimental values of ∆α for 2 AL and 4 ALs, to demonstrate an antiferroelectric coupling on 4 AL SnTe that is at odds with previous claims of ferroelectric coupling 68,89 and consistent with experiment. 70,71 In the present calculations, the AA structure shown in Fig.…”

“…These SnTe slabs have not been created by capping a bulk sample, but grown from the bottom up [70][71][72] . And while common theoretical approaches assume a slab can be obtained by cutting two opposing surfaces of bulk rocksalt 73 or rhombic 59,68,89 bulk samples, the present work is aimed to explore the structural evolution of a freestanding SnTe slab containing 2n ALs by the successive addition of a 2 ALs in the overall lowest-energy conformation to the slab containing 2(n−1) ALs with n a positive integer, complementing the experimental results of Ref. 71.…”

“…At the same time, interest on ultrathin SnTe originated due to theoretical predictions of ferroelectricity on these films [63][64][65][66][67][68][69] and their experimental fabrication 70,71 . These SnTe slabs have not been created by capping a bulk sample, but grown from the bottom up [70][71][72] .…”

Progress and prospects in low‐dimensional multiferroic materials

“…Tensile strain can effectively enhance the Tc value while out‐of‐plane electric field can decrease the coercive field for the reversal of polarization. Liu et al explored intrinsic origin of enhancement of ferroelectricity in SnTe ultrathin films by first‐principles calculations and effective Hamiltonian simulations . They revealed that the FE switching energy barrier and Tc in freestanding, defect‐free SnTe thin films first increase with thickness when the film thickness is less than 5 UC, then decreases with thickness for thicker films.…”

Constructing realistic effective spin Hamiltonians with machine learning approaches

“…Different kinds of effective Hamiltonians were proposed to deal with different degrees of freedom [9][10][11][12][13]. In this work, we focus on the effective spin Hamiltonians of magnetic systems to illustrate the power of our method.…”