Quantum Cafe: Roberto Car

Title: Studies of the ferroelectric phase transition with the Deep Potential model

Abstract: In the Deep Potential (DP) framework, the dependence on the atomic coordinates of the potential energy and of the dielectric polarization is represented by deep neural networks trained on density functional theory (OFT) data. The model retains quantum mechanical accuracy at nearly the cost of empirical force fields, opening the way to large scale molecular dynamics (MD) simulations with ab-initio predictive power. The approach can be used to study the ferroelectric phase transition beyond mean field and reduced models. Here, I report recent results for three prototypical ferroelectric materials at ambient pressure: lead titanate (PTO), potassium dihydrogen phosphate (KDP), and lead magnesium niobate (PMN). PTO is a perovskite crystal in which the phase transition exhibits both order-disorder and soft mode features. KDP is a hydrogen bonded crystal in which the phase transition is greatly affected by deuteration, as the latter changes Tc by more than 100K and the spontaneous polarization by ~25 percent. PMN is a chemically disordered perovskite material whose dielectric response exhibits glassy behavior, called relaxor, that bears analogies with the magnetic response of spin glasses. After empirically correcting for DFT deficiencies, such as excessive tetragonality in PTO and excessive quantum delocalization of the protons in KDP, classical and path integral DPMD simulations predict in the three cases finite temperature properties in close agreement with experiment, providing fresh insight into the microscopic mechanisms of the transition. In PTO, the simulations confirm the dominant order- disorder character inferred from x-ray and neutron diffraction experiments but also predict the dynamic soft mode behavior observed in optical experiments. In KDP, path integral simulations indicate that the large isotope effects originate from dipolar defects induced by quantum fluctuations. These defects are always present in ferroelectric KDP but disappear at low temperature in deuterated KDP (DKDP), which behaves much more classically than its protonated counterpart. Finally, in PMN, the calculated static and dynamic susceptibilities reproduce the glassy behavior found in experiments, casting light on the similarity in the relaxation dynamics of relaxors, magnetic spin glasses, and fragile structural glasses.