Negative Piezoelectric Coefficient in Ferromagnetic 1H-LaBr2 Monolayer

The discovery of two-dimensional (2D) magnetic materials that have excellent piezoelectric response is promising for nanoscale multifunctional piezoelectric or spintronic devices. Piezoelectricity requires a noncentrosymmetric structure with an electronic band gap, whereas magnetism demands broken time-reversal symmetry. Most of the well-known 2D piezoelectrics, e.g., 1H-MoS2 monolayer, are not magnetic. Being intrinsically magnetic, semiconducting 1H-LaBr2 and 1H-VS2 monolayers can combine magnetism and piezoelectricity. We compare piezoelectric properties of 1H-MoS2, 1H-VS2, and 1H-LaBr2 using density functional theory. The ferromagnetic 1H-LaBr2 and 1H-VS2 monolayers display larger piezoelectric strain coefficients, namely, d(11) = -4.527 pm/V for 1H-LaBr2 and d(11) = 4.104 pm/V for 1H-VS2, compared to 1H-MoS2 (d(11) = 3.706 pm/V). 1H-MoS2 has a larger piezoelectric stress coefficient (e(11) = 370.675 pC/m) than 1H-LaBr2 (e(11) = -94.175 pC/m) and 1H-VS2 (e(11) = 298.100 pC/m). The large d(11) for 1H-LaBr2 originates from the low elastic constants, C-11 = 30.338 N/m and C-12 = 9.534 N/m. The sign of the piezoelectric coefficients for 1H-LaBr2 is negative, and this arises from the negative ionic contribution of e(11), which dominates in 1H-LaBr2, whereas the electronic part of e(11) dominates in 1H-MoS2 and 1H-VS2. We explain the origin of this large ionic contribution of e(11) for 1H-LaBr2 through Born effective charges (Z(11)) and the sensitivity of the atomic positions to the strain (du/d eta). We observe a sign reversal in the Z(11) values of Mo and S compared to the nominal oxidation states, which makes both the electronic and ionic parts of e(11) positive and results in the high value of e(11). We also show that a change in magnetic order can enhance (reduce) the piezoresponse of 1H-LaBr2 (1H-VS2).

Automated summary

The discovery of two-dimensional magnetic materials with excellent piezoelectric response offers potential for nanoscale multifunctional piezoelectric or spintronic devices. Comparing the piezoelectric properties of different materials, the ferromagnetic 1H-LaBr2 and 1H-VS2 monolayers exhibit larger piezoelectric strain coefficients than 1H-MoS2, while 1H-MoS2 has a larger piezoelectric stress coefficient. The origin of the large ionic contribution to piezoelectric coefficients in 1H-LaBr2 is explained through Born effective charges and the sensitivity of atomic positions to strain.