The thermal properties of β-Ga₂O₃ can significantly affect the performance and reliability of high-power electronic devices. To date, due to the absence of a reliable interatomic potential, first-principles calculations based on density functional theory (DFT) have been routinely used to probe the thermal properties of β-Ga₂O₃. DFT calculations can only tackle small-scale systems due to the huge computational cost, while the thermal transport processes are usually associated with large time and length scales. In this work, we develop a machine learning based Gaussian approximation potential (GAP) for accurately describing the lattice dynamics of perfect crystalline β-Ga₂O₃ and accelerating atomic-scale simulations. The GAP model shows excellent convergence, which can faithfully reproduce the DFT potential energy surface at a training data size of 32 000 local atomic environments. The GAP model is then used to predict ground-state lattice parameters, coefficients of thermal expansion, heat capacity, phonon dispersions at 0 K, and anisotropic thermal conductivity of β-Ga₂O₃, which are all in excellent agreement with either the DFT results or experiments. The accurate predictions of phonon dispersions and thermal conductivities demonstrate that the GAP model can well describe the harmonic and anharmonic interactions of phonons. Additionally, the successful application of our GAP model to the phonon density of states of a 2500-atom β-Ga₂O₃ structure at elevated temperature indicates the strength of machine learning potentials to tackle large-scale atomic systems in long molecular simulations, which would be almost impossible to generate with DFT-based molecular simulations at present.

Two-dimensional (2D) GaS, GaSe, and InSe were reported to be semiconductors and have been recently fabricated with potential applications in photoelectrics, where in-depth understanding from electronic structure is necessary. In addition, the thermal transport properties play a key role as to the thermal stability and the efficient heat dissipation for device operation, which are also necessary to be addressed. In this paper, we present a systematic first-principles study on the electronic, optical, and thermal transport properties for the representative group III–VI monolayer GaS, GaSe, and InSe. Our results indicate that monolayer GaS, GaSe, and InSe are semiconductors with an indirect bandgap. The predominant influence of interband transitions due to the large bandgap causes monolayer GaSe to possess the highest absorptivity along both “in-plane” and “out-of-plane” directions compared to the other two systems. Moreover, the lattice thermal conductivities (κ_L ) of these materials are found to be inversely proportional to their average atomic mass, but the decrease in thermal conductivity from GaS to GaSe is negligible in comparison to that of GaSe to InSe with a nearly equivalent mass difference. It is found that the underlying mechanism lies in the larger phonon relaxation time of GaSe caused by weaker anharmonicity. Our study provides a comprehensive understanding of the inherent physical properties of monolayer GaS, GaSe, and InSe, which would benefit their future applications in photoelectrics.

Auxetic materials are characterized by a negative Poisson’s ratio that they expand laterally in the directions perpendicular to the applied stretching stress and vice versa. Piezoelectrics will change their dimensions when exposed to an external electric field. Here we introduce the concept of the “electric auxetic effect”: electric auxetic materials will contract or expand in all dimensions in response to an electric field. Such unusual piezoelectric response driven by an electric field is a close analogy to the auxetic effect driven by a stress field. A key feature of electric auxetic materials is that their longitudinal and transverse piezoelectric coefficients are of the same sign. We demonstrate using first-principles calculations that the $Pca{2}_1$ orthorhombic phase of ferroelectric ${\mathrm{HfO}}_2$ exhibits both the negative longitudinal piezoelectric effect and the electric auxetic effect. The unusual negative longitudinal piezoelectric effect arises unexpectedly from the domination of the negative internal-strain contribution over the positive clamped-ion contribution, a character often found in van der Waals solids. We confirm a few more electric auxetic materials with finite electric field calculations by screening through a first-principles-based database of piezoelectrics.

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells as an unprecedented photovoltaic technology with both relatively low cost and high-power conversion. The fascinating physical and chemical properties of perovskites are benefited from their unique crystal structures represented by the general chemical formula $AM{X}_3$, where the $A$ cations occupy the hollows formed by the $M{X}_3$ octahedra and thus balance the charge of the entire network. Despite a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite $\left({\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3\right)$ with direct nonequilibrium ab initio molecular dynamics simulation. Resorting to full first-principles calculations, we illustrate the details of the mysterious vibration of the methylammonium cluster $\left({\mathrm{CH}}_3{\mathrm{NH}}_3^{+}\right)$ and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultralow thermal conductivity of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. We also reveal the strongly localized phonons associated with the internal motions of the ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cluster, which contribute little to the total thermal conductivity. The importance of the ${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$ cluster to the structural instability is also discussed in terms of the unconventional dispersion curves by freezing the partial freedoms of the organic cluster. These results provide more quantitative description of organic-inorganic interaction and coupling dynamics from accurate first-principles calculations, which are expected to underpin the development of emerging photovoltaic devices.

The macroscopic thermal transport is fundamentally determined by the intrinsic interactions among microscopic electrons and phonons. In conventional insulators and semiconductors, phonons dominate the thermal transport, and the contribution of electron-phonon interaction (EPI) is negligible. However, in polar semiconductors, the Fröhlich electron-phonon coupling is strong and its influence on phononic thermal transport is of great significance. In this work, the effect of EPI on phonon dispersion and lattice thermal conductivity of wurtzite gallium nitride (GaN) is comprehensively investigated from the atomistic level by performing first-principles calculations. Due to the existence of relatively large electronegativity difference between Ga and N atoms, the Fröhlich coupling in wurtzite GaN is remarkably strong. Consequently, the lattice thermal conductivity of natural wurtzite GaN at room temperature is reduced by ∼24%–34% when including EPI, and the resulted thermal conductivity value is in better agreement with experiments. Furthermore, the scattering rate of phonons due to EPI, the intrinsic phonon-phonon interaction (PPI) as well as isotope disorder is computed and analyzed. It shows that the EPI scattering rate is comparable to PPI for low-frequency heat-carrying phonons. This work attempts to explore the mechanism of thermal transport beyond intrinsic PPI for polar semiconductors, with a great potential of thermal conductivity engineering for desired performance.