Giant piezoelectric effects of topological structures in stretched ferroelectric membranes

©️ Copyright 2024 @ Yihao Hu (胡逸豪)
Author： Yihao Hu (胡逸豪) 📨 Jiyuan Yang (杨季元) 📨 Shi Liu (刘仕) ^†📨
Date：2024-01-11 (The last update was on 2024-07-29)
Lisence：This document is licensed under Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0) license.
📖 Citing in your papers
We shall greatly appreciate if scientific work done using the published deep potential (DP) and/or the associated database and scripts for data analysis will contain an acknowledgment to the following references
[1] Giant piezoelectric effects of topological structures in stretched ferroelectric membranes, Yihao Hu, Jiyuan Yang and Shi Liu*, Phys. Rev. Lett. 133, 046802 (2024) (Editors’ Suggestion)
[2] Modular development of deep potential for complex solid solutions, Jing Wu+, Jiyuan Yang+, Liyang Ma, Linfeng Zhang, and Shi Liu*, Phys. Rev. B 107, 144102 (2023)
[3] Deep Potential Molecular Dynamics: A Scalable Model with the Accuracy of Quantum Mechanics, Linfeng Zhang, Jiequn Han, Han Wang*, Roberto Car, and Weinan E†, Phys. Rev. Lett. 120, 143001 (2018)
[4] End-to-end symmetry preserving inter-atomic potential energy model for finite and extended systems, Linfeng Zhang, Jiequn Han, Han Wang*, Wissam A. Saidi†, Roberto Car, Weinan E‡, 32nd NeurIPS (2018)

1. Introduction

We share the force field model, essential input files for density functional theory (DFT) calculations and molecular dynamics (MD) simulations, data analysis scripts, and selected original MD trajectories, as detailed in our paper [1]. The model file for PbTiO$_3$ , together with the complete training database and testing data, can be found in our GitHub repository.

The directory is organized as illustrated in the following diagram:

• train: Contains the training dataset and the input.json file which holds the training metadata. Refer to Section 2. Includes a representative INCAR file for DFT SCF calculations that were used to construct training database. Refer to Section 2.3.
• model: Stores the force field file compress01.pb.
• paper:
  • DFT_phase_diagram: We have shared the CONTCAR files and their energies for all ≈700 configurations. Refer to Section 3.1.
  • dipole_spiral: Offers selected MD trajectories and scripts to implement calculations (Refer to Section 3.2.) and demonstrate the robustness of dipole spiral Refer to (Section 3.3).
  • Piezoelectric: Dedicated to piezoelectric coefficient $d_{33}$ calculations via finite-field MD simulations. Refer to Section 3.4.
  • other_domain: Offers selected MD trajectories and scripts to implement calculations of various ferroelectric domains. Refer to Section 3.5.

2. Database Construction

2.1. Training database

The force field of PbTiO$_3$ utilized in this work is a deep neural network-based model potential, referred to as deep potential (DP).

Details regarding the construction of the training database, DFT calculations, and metadata of the DP model were documented in our previous work [2]. Specifically, we adopted the DP-GEN, a concurrent learning procedure, to construct the training database (see details in Section 2.1). The initial training database contains DFT energies and atomic forces for structures derived from random perturbations of ground-state structures of $P4mm$ (tetragonal) and $Pm3m$ (cubic) phases of PbTiO$_3$ . The final training database comprises 13021 PbTiO$_3$ configurations. You can access the training database in Spiral/train/PSTO-data.zip.

2.2. DP-GEN

We employ the Deep Potential Generator (DP-GEN) to construct the training database. DP-GEN is a concurrent learning procedure consisting of three stages: labeling, training, and exploration, which together form a closed loop. Starting with an initial training database that contains DFT energies and forces for a few configurations, four DP models with distinct random initializations of neural networks are trained. In the exploration phase, one of these models is employed for MD simulations to explore the configuration space. Predictions (energies and atomic forces) are generated using all four models for each new configuration sampled from MD. For configurations that are well represented by the current training database, these four models should display nearly identical predictive accuracy. However, for those not well-represented, we expect the four models to produce scattered predictions with significant deviations. The maximum standard deviation of predictions from the four models serves as a criterion for labeling: configurations from MD with significant model deviation are labeled. The energies and atomic forces of these labeled configurations, as computed using DFT, are subsequently integrated into the training database for the next training cycle. Here, the maximum atomic force standard deviation, denoted as ε, is used as the labeling criterion. We introduce two thresholds, ε_lo and ε_hi ; only configurations for which ε_lo < ε < ε_hi are labeled for DFT calculations. We set ε_lo = 0.12 and ε_hi = 0.25. The introduction of ε_hi is to handle the exceptions due to highly distorted configurations resulting from low-quality DP models (especially in the first few cycles of DP-GEN) or unconverged DFT calculations. The iteration stops when all configurations sampled from MD simulations satisfy a predefined accuracy across all four models.  A primary advantage of the DP-GEN approach is its streamlined and largely autonomous data generation, minimizing human intervention.

2.3. DFT calculations

We employ 2x2x2 supercells of 40 atoms for first-principles DFT calculations using the Vienna Ab initio Simulation (VASP) package. The projected augmented wave method is employed, and the generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) type is chosen as the exchange-correlation functional. The energy cutoff is set at 800 eV, and the k-spacing is set at 0.3 Å^-1 . A sample INCAR file for the self-consistent field (SCF) calculations can be found in the Spiral/train/ directory.

2.4. Deep Potential

The DP model, based on a deep neural network with the number of learnable parameters on the order of 10$^{6}$, offers a robust mathematical structure to represent highly nonlinear and complex interatomic interactions while bypassing the need to handcraft descriptors that represent local atomic environments. Specifically, the DP model features a symmetry-preserving embedding network that maps an atom's local environment to inputs for a fitting neural network which then outputs the atomic energy; the sum of atomic energies yields the total energy. The original references to the DP model can be found in [3] and [4].

In this study, we utilized the smooth version of the DP model and employed the DEEPMD-KIT package for training. The cutoff radius is set to 6 Å, with smoothing starting at 0.5 Å. The embedding network follows a ResNet-like architecture with dimensions (25, 50, 100). The fitting network consists of three layers, each containing 240 nodes. The loss function is defined as:

$$L({p}_\epsilon, {p}_f, {p}_\xi) = {p}_\epsilon \Delta{\epsilon}^2 + \frac{p_f}{3N} \sum_i \left| \Delta{{{\mathbf{F}_i}}} \right|^2 + \frac{p_\xi}{9} \left | \Delta \xi \right |^2$$

Here, $\Delta$ represents the difference between DP predictions and training data, $N$ is the number of atoms, $\epsilon$ is the energy per atom, and $\mathbf{F}_i$ is the atomic force of atom $i$. The prefactors ${p}_\epsilon$ , ${p}_f$, and ${p}_\xi$ are adjustable parameters. We increased ${p}_\epsilon$ from 0.02 to 1, while reducing ${p}_f$ from 1000 to 1.

The input.json file for training is located in the Spiral/train directory.

2.5. Fitting perfomance

Phonon spectra of tetragonal PTO, and cubic PTO. Temperature dependence of spontaneous polarization and local atomic displacements of Pb and Ti ($D_{\rm Pb}$ and $D_{\rm Ti}$) in PTO from DPMD simulations.

Here we compare the energies and atomic forces predicted by DFT and DP for all the structures in the final training database.

The final training database comprises 13021 PbTiO$_3$ configurations and we calculated 231 cases in our paper, as indicated by the red dots in the graph.

The following graph can be plotted using data files Spiral/paper/DFT_phase_diagram/model-error.dat and Spiral/paper/DFT_phase_diagram/Energy-min1st.

3. Results

3.1. DFT phase diagram

For each strain condition, we used 4 initial configurations as POSCAR for the DFT calculations. These 4 configurations have different initial polarizations: A represents polarization along [110], B represents polarization along [011], C represents polarization along [001], and D represents polarization along [111]. For each strain condition, we performed in-plane constrained, out-of-plane fully relaxed structural optimizations on these 4 structures. The optimized structures obtained may have the same polarization directions. Then we need to analyze whether these 4 structures have the same polarization direction, and finally screened out 4 or fewer unique configurations. We then compared their energy magnitudes and obtained Fig.1.

The INCAR file for optimizing the structure is located in Spiral/paper/DFT_phase_diagram/IO/.

All the CONTCAR are in these directories e.g., Spiral/paper/DFT_phase_diagram/IO/a3.932b3.954/iniA:

1. a3.932b3.954 represent the strain condition: $a_{\rm IP}$=3.932 Å, $b_{\rm IP}$=3.954 Å.

2. iniA represent the initial configuration whose polarization along [110]; iniB represent the initial configuration whose polarization along [101]; iniC represent the initial configuration whose polarization along [001]; iniD represent the initial configuration whose polarization along [111].

The energy of all structures is recorded in the data file Spiral/paper/DFT_phase_diagram/energy_all.txt. The displacement of all structures is recorded in the data file Spiral/paper/DFT_phase_diagram/displacement_all.txt.

Next, let's explain the label in the first row of the data file:

1. a: in-plane lattice parameter a. Unit is angstrom.

2. b: in-plane lattice parameter b. Unit is angstrom.

3. E1min: the 1st lowest energy configuration among the four

4. E2min: the 2nd lowest energy configuration among the four

5. E3min: the 3rd lowest energy configuration among the four

6. E4min: the 4th lowest energy configuration among the four

7. energy: Unit is eV/u.c.

8. dx: The x-component of the Ti displacement. Unit is angstrom.

9. dy: The y-component of the Ti displacement. Unit is angstrom.

10. dz: The z-component of the Ti displacement. Unit is angstrom.

If the displacement is (0,0,0), it indicates that the polarization of the stable configuration obtained after equilibration starting from the initial configuration is the same as the polarization of another stable configuration obtained after equilibration starting from another initial configuration. Therefore, the two will be merged into a single unique state.

Displacements of Ti greater than 0.14 Å are classified as 1, those smaller than 0.03 are classified as 0, and those in between are denoted as u. By using a plotting script, Fig.1 in [1] can be obtained.

The following LAMMPS input file settings are applied to all MD cases mentioned in the articles.

3.2. dipole spiral

Here is an example about dipole spiral，The LAMMPS initial structure file Spiral/paper/dipole_spiral/PTO_C.data (representing the $c$ domain) is used to submit the computation task to obtain the equilibrium structure. The trajectories can be saved in the traj folder. Therefore, it is necessary to manually create the folder traj before the computation. We also performed calculations for the case where the initial structure is in the R phase.

mkdir traj

The required LAMMPS input files have been placed in the folder Spiral/paper/dipole_spiral/continue/. For the already equilibrated structure (uncertain) PTO_C.restart, submit the computation task again to obtain the equilibrium structure.

cd continue; cp ../RunscriptDP ./ ; cp ../PTO_C.restart ./ ; mkdir traj 

The original trajectories file is located in folder Spiral/paper/dipole_spiral/example_Spiral/15_15_15/traj/ (supercell $15\times15\times15$)，We performed calculations for over 3000 ps, but due to the large file size (more than 2GB), only the data for the last 100 ps is provided here.

Top view (XY plane) of Spiral: (XY cross-sectional plots of different layers in the z-direction)

Side view (XZ/YZ plane) of Spiral:

3.3. Robustness of dipole spiral

There are also results of Spiral under different supercell sizes in the folder Spiral/paper/dipole_spiral/example_Spiral/: 15_15_13/, represents supercell $15\times15\times13$，15_15_25/ represents supercell $15\times15\times25$ , 242415/ represents supercell $24\times24\times15$ , 101015/ represents supercell $10\times10\times15$. Due to the large number of files, only the data for 50 ps is provided here. In folder B_average_disp.dat_XY, B_average_disp.dat_YZ, B_average_disp.dat_XZ, you will find cross-sectional plots of the equilibrium structure.

3.4. Piezoelectric coefficient $d_{33}​$

Apply an out-of-plane external electric field to the structure after re-equilibration using the file Spiral/paper/dipole_spiral/continue/PTO_C.restart.continue and submit the computation task to obtain the equilibrium structure. The required LAMMPS input files with the applied electric field are already placed in folder Spiral/paper/dipole_spiral/continue/E-0.001/, with an electric field intensity of -100 kV/cm. We can obtain the equilibrium structures of dipole spiral under various electric fields.

cd E-0.001; cp ../RunscriptDP ./ ; cp ../PTO_C.restart.continue ./; mkdir traj

When the calculations complete, retrieving the lattice from the last 1500 lines of log.lammps to calculate the mean and standard deviation, plot the strain as a function of electric field. This reproduces Fig.2c in [1].

grep -B1500 'Loop time' log.lammps | grep -v Loop | awk '{c+=$10/15.0;csq+=$10*$10/225.0} END {print c/NR, sqrt(csq/NR - (c/NR)**2)}'

Once we calculate and obtain the equilibrium structures under different electric field intensities, we can plot the variation of strain with electric field intensity. The slope of the line obtained through linear fitting represents the piezoelectric coefficient.

3.5. Other domain

$c/a$ doamin: Due to the large number of files, only the data for 25 ps is provided here Spiral/paper/other_domain/example_ca/4_40_40.

$a_1/a_2$ doamin: Due to the large number of files, only the data for 25 ps is provided here Spiral/paper/other_domain/example_a1a2/40_40_4.

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