Hidden phases with neuromorphic responses and highly enhanced piezoelectricity in an antiferroelectric prototype

Designing polar textures with ultrafast neuromorphic features from atomistic simulations

“…Thirdly, the parameters quantifying the strength of such terms are then derived from first-principle calculations on small cells [36]. Fourthly and finally, molecular dynamics (MD) simulations are conducted on relatively large supercells, namely 18 × 18 × 18 in [5] and 12 × 12 × 12 in [6,7], using these degrees of freedom, analytical expressions and parameters, in order to obtain time-dependent quantities at finite temperature. It is important to further know that these simulations also incorporate the influence of an electric field on physical properties by simply adding, within the effective Hamiltonian method, an energy −Z * ∑ i u i • E, where E is the electric field, u i the local mode at each site i, and Z * the dynamical charge of the local mode.…”

“…Let us now focus on NaNbO 3 bulk at 100 K and subject to a train of pulses equally separated by a time of 8 ps and given by equation (1) (this time is chosen such as to allow NNO bulk to relax within the reached hidden states after each pulse is applied and died out), with the field being applied along the pseudo-cubic [111] direction, E 0 having a magnitude of 2.2 √ 3 × 10 8 V m −1 , δ = 0.4 ps, and ν = ω/2π = 6 THz [6]. It is important to know that we numerically found that NNO does not precisely possess for some temperatures the (antiferroelectric) orthorhombic Pbcm symmetry but rather an orthorhombic Pca2 1 space group that is deduced from Pbcm by adding a rather small polarization along the pseudo-cubic [001] direction [41].…”

“…They have been found in superconductors [13], colossal magnetoresistance manganites [14], charge density wave materials [15], and incipient ferroelectrics [16,17]. Such states were also found [5,6] to have properties distinct from those of the thermodynamically stable ground state in relaxors.…”

“…In this review, we present three different model polar compounds that, according to our large-scale atomistic simulations, exhibit neuromorphic signatures, when subject to THz electric pulses [5][6][7]. The first polar system is lead-magnesium-niobate (Pb(Mg 1/3 Nb 2/3 )O 3 , denoted as PMN), which is a representative of relaxor ferroelectrics which are characterized by intrinsic short-correlated dipolar textures, the so-called polar nanoregions [8][9][10].…”

“…This review is organized as follows. Section 2 provides some details about the numerical scheme used in the three aforementioned studies [5][6][7]. Sections 3-5 present and emphasize some results of such scheme for PMN, NNO and PZT systems, respectively.…”