Density Functional Perturbation Theory Calculations Research Articles

Can Pressure be a Good Strategy for Optimizing Thermoelectric Performance of SnPS3${\rm SnPS}_3$?

AbstractExternal pressure can significantly alter the transport coefficients, power factor, and figure of merit because of its direct influence on the electronic structure, electron–phonon, and phonon–phonon couplings. This study delves into the electronic and thermal transport properties of at external pressures up to 30 GPa using first‐principles calculations and Boltzmann transport theory. The electron–phonon relaxation time is computed within the electron–phonon‐averaged (EPA) approximation, enabling exploration beyond the constant relaxation time approximation. The first‐principles calculations reveal an indirect bandgap of 1.76 (without pressure) and 0.12 eV (30 GPa). The density functional perturbation theory calculations confirm the dynamic stability of at external pressure up to 30 GPa. The electronic transport properties are improved by more than one order of magnitude at 30 GPa, consistent with experimental observations. The Peierls–Boltzmann transport calculations demonstrate the room temperature lattice thermal conductivity of 0.22 (without pressure) and 7.4 (at 30 GPa). The results emanate that exhibits of 0.71 at 900 K at a hole doping of 2 at zero pressure, which decreases with increasing pressure. The findings explore the effect of external pressure on both electronic and thermal transport properties of , warranting further experimental exploration of thermal transport properties at higher pressures.

InAs–InP Superlattice Nanowires with Tunable Phonon Frequencies

AbstractThe control of heat conduction through the manipulation of phonons in solids is of fundamental interest and can be exploited in applications for thermoelectric conversion. In this context, the advent of novel semiconductor nanomaterials with high surface‐to‐volume ratio, e.g. nanowires, offer exciting perspectives, leading to significant leaps forwarding the efficiency of solid‐state thermoelectric converters after decades of stagnation. Beyond the high aspect ratio, the nanowire geometry offers unprecedented possibilities of materials combination and crystal phase engineering not achievable with 2D counterparts. In this work, the growth of long (up to 100 repetitions) wurtzite InAs/InP superlattice nanowires with homogeneous segment thicknesses is reported, with control down to the single digit of nanometer. By means of Raman scattering experiments, clear modifications of the phonon dispersion in superlattice nanowires are found, where both InAs‐like and InP‐like modes are present. The experimentally measured modes are well reproduced by density functional perturbation theory calculations. Remarkably, it is found that the phonon frequencies can be tuned by the superlattice periodicity, opening exciting perspectives for phonon engineering and thermoelectric applications.

Demagnetizing Ferromagnetic Catalysts to the Sabatier Optimal of Haber-Bosch Process.

Achieving the Sabatier optimal of a chemical reaction has been the central topic in heterogeneous catalysis for a century. However, this ultimate goal was greatly hindered in previous catalyst design strategies since the active sites indeed changed. Fortunately, the magneto-catalytic effect (MCE) provides a promising solution to this long-standing challenge. Recent research suggests that the performance of ferromagnetic catalysts is capable to be promoted without changing its chemical structure. Herein, we use time-dependent density functional perturbation theory (TDDFPT) calculations to elucidate that a partially demagnetized (DM) ferromagnet could be a Sabatier optimal catalyst. Using ammonia synthesis as the model reaction, we determined the activity of Cobalt at each DM state by including the magnetic thermal excitations via magnon analysis, making the 55% DM Co to the genuine Sabatier optimal. As an essential but underexcavated phenomenon in heterogeneous catalysis, the MCE will open a new avenue to design high-performance catalysts.

Electronic and vibrational properties of MgCu2O3

AbstractThe vibrational properties of orthorhombic MgCu2O3 compound have been investigated within the framework of the density functional perturbation theory (DFPT) as well as experimentally, to validate the computational results. MgCu2O3 was synthesized by solid‐state reaction and characterized by synchrotron‐based X‐ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The DFT + U‐based first principle calculations were performed to obtain the correct electronic ground state and the band structure of this compound. The same DFT + U methodology was employed along with DFPT calculations for obtaining vibrational properties: phonon density of states and phonon band structure. The atomic vibrations for each mode were also analyzed, and the Raman and the IR active modes are identified. Experimentally observed Raman and infrared (IR) spectra agree well with the computed ones.

Machine Learning K-Means Clustering in Interpolative Separable Density Fitting Algorithm: Advancing Accurate and Efficient Cubic-Scaling Density Functional Perturbation Theory Calculations within Plane Waves.

Density functional perturbation theory (DFPT) is a crucial tool for accurately describing lattice dynamics. The adaptively compressed polarizability (ACP) method reduces the computational complexity of DFPT calculations from O(N4) to O(N3) by combining the interpolative separable density fitting (ISDF) algorithm. However, the conventional QR factorization with column pivoting (QRCP) algorithm, used for selecting the interpolation points in ISDF, not only incurs a high cubic-scaling computational cost, O(N3), but also leads to suboptimal convergence. This convergence issue is particularly pronounced when considering the complex interplay between the external potential and atomic displacement in ACP-based DFPT calculations. Here, we present a machine learning K-means clustering algorithm to select the interpolation points in ISDF, which offers a more efficient quadratic-scaling O(N2) alternative to the computationally intensive cubic-scaling O(N3) QRCP algorithm. We implement this efficient K-means-based ISDF algorithm to accelerate plane-wave DFPT calculations in KSSOLV, which is a MATLAB toolbox for performing Kohn-Sham density functional theory calculations within plane waves. We demonstrate that this K-means algorithm not only offers comparable accuracy to QRCP in ISDF but also achieves better convergence for ACP-based DFPT calculations. In particular, K-means can remarkably reduce the computational cost of selecting the interpolation points by nearly 2 orders of magnitude compared to QRCP in ISDF.

Deep-Learning Density Functional Perturbation Theory.

Calculating perturbation response properties of materials from first principles provides a vital link between theory and experiment, but is bottlenecked by the high computational cost. Here, a general framework is proposed to perform density functional perturbation theory (DFPT) calculations by neural networks, greatly improving the computational efficiency. Automatic differentiation is applied on neural networks, facilitating accurate computation of derivatives. High efficiency and good accuracy of the approach are demonstrated by studying electron-phonon coupling and related physical quantities. This work brings deep-learning density functional theory and DFPT into a unified framework, creating opportunities for developing abinitio artificial intelligence.

Bistability between π-diradical open-shell and closed-shell states in indeno[1,2-a]fluorene.

Indenofluorenes are non-benzenoid conjugated hydrocarbons that have received great interest owing to their unusual electronic structure and potential applications in nonlinear optics and photovoltaics. Here we report the generation of unsubstituted indeno[1,2-a]fluorene on various surfaces by the cleavage of two C-H bonds in 7,12-dihydroindeno[1,2-a]fluorene through voltage pulses applied by the tip of a combined scanning tunnelling microscope and atomic force microscope. On bilayer NaCl on Au(111), indeno[1,2-a]fluorene is in the neutral charge state, but it exhibits charge bistability between neutral and anionic states on the lower-workfunction surfaces of bilayer NaCl on Ag(111) and Cu(111). In the neutral state, indeno[1,2-a]fluorene exhibits one of two ground states: an open-shell π-diradical state, predicted to be a triplet by density functional and multireference many-body perturbation theory calculations, or a closed-shell state with a para-quinodimethane moiety in the as-indacene core. We observe switching between open- and closed-shell states of a single molecule by changing its adsorption site on NaCl.

Electron-phonon coupling from GW perturbation theory: Practical workflow combining BerkeleyGW, ABINIT, and EPW

We present a workflow of practical calculations of electron-phonon (e-ph) coupling with many-electron correlation effects included using the GW perturbation theory (GWPT). This workflow combines BerkeleyGW, ABINIT, and EPW software packages to enable accurate e-ph calculations at the GW self-energy level, going beyond standard calculations based on density functional theory (DFT) and density-functional perturbation theory (DFPT). This workflow begins with DFT and DFPT calculations (ABINIT) as starting point, followed by GW and GWPT calculations (BerkeleyGW) for the quasiparticle band structures and e-ph matrix elements on coarse electron k- and phonon q-grids, which are then interpolated to finer grids through Wannier interpolation (EPW) for computations of various e-ph coupling determined physical quantities such as the electron self-energies or solutions of anisotropic Eliashberg equations, among others. A gauge-recovering symmetry unfolding technique is developed to reduce the computational cost of GWPT (as well as DFPT) while fulfilling the gauge consistency requirement for Wannier interpolation.

GaAs/GaP Superlattice Nanowires for Tailoring Phononic Properties at the Nanoscale: Implications for Thermal Engineering.

The possibility to tune the functional properties of nanomaterials is key to their technological applications. Superlattices, i.e., periodic repetitions of two or more materials in one or more dimensions, are being explored for their potential as materials with tailor-made properties. Meanwhile, nanowires offer a myriad of possibilities to engineer systems at the nanoscale, as well as to combine materials that cannot be put together in conventional heterostructures due to the lattice mismatch. In this work, we investigate GaAs/GaP superlattices embedded in GaP nanowires and demonstrate the tunability of their phononic and optoelectronic properties by inelastic light scattering experiments corroborated by ab initio calculations. We observe clear modifications in the dispersion relation for both acoustic and optical phonons in the superlattices nanowires. We find that by controlling the superlattice periodicity, we can achieve tunability of the phonon frequencies. We also performed wavelength-dependent Raman microscopy on GaAs/GaP superlattice nanowires, and our results indicate a reduction in the electronic bandgap in the superlattice compared to the bulk counterpart. All of our experimental results are rationalized with the help of ab initio density functional perturbation theory (DFPT) calculations. This work sheds fresh insights into how material engineering at the nanoscale can tailor phonon dispersion and open pathways for thermal engineering.

Investigation of the electronic, optical, elastic, mechanical and vibrational properties of CuI using HSE03

Ab Initio density functional theory (DFT) simulations have been employed to systematically explore the electronic, optical, elastic, mechanical and vibrational properties. In this study, we revealed that [Formula: see text]-CuI has a wide direct bandgap energy of 3.21 eV, is pure covalent and brittle. We also found that the core level is made up of I s electron orbitals, the valence band is constructed with I p orbitals, and the Cu s orbital states mainly contribute to the conduction band minimum (CBM). The reflectivity of CuI is reported to be low (35.9% for the light reflected), showing high material absorption. A high absorption coefficient of [Formula: see text] is also reported. The elastic and mechanical properties can further confirm the mechanical stability of the CuI system, derived from DFT-calculated elastic constants and phonon dispersion from density functional perturbation theory (DFPT) calculations.

Temperature dependence of Raman scattering in CdO: Insights into phonon anharmonicity and plasmon excitations

We present a temperature-dependent Raman scattering study of CdO in the temperature range between 80 and 500 K. Distinctive features present in the second-order Raman spectra of CdO are identified as overtones and combination modes with the help of density functional perturbation theory calculations. A wide, broad band in the region between 250 and $470 {\mathrm{cm}}^{\ensuremath{-}1}$ is attributed to Raman scattering by charge-density fluctuations in the high-density overdamped electron gas present in the sample. Very good agreement is found between the Raman spectra and the expected scattering arising from the overtone density of states and plasmon excitations. The temperature dependence of the modes yields information about the anharmonic phonon interactions. Four-phonon processes arising from quartic anharmonicity play a significant role in the observed temperature dependence.

Ab-initio calculations of temperature dependent electronic structures of inorganic halide perovskite materials.

Despite wide interest in halide perovskite materials, it is still challenging to accurately calculate their electronic structure and its temperature dependence. In this work, we present ab-initio calculations of the temperature dependence of the electronic structure of CsPbX3 materials (X = Cl, Br or I) in the cubic form and of the zero temperature electronic structure of the orthorhombic phase of these materials. Phonon-induced temperature dependent band energy renormalization was calculated within the framework of Allen-Heine-Cardona theory, where we exploited the self-consistent procedure to determine both the energy level shifts and their broadenings. The phonon spectrum of the materials was obtained using the self-consistent phonon method since standard density functional perturbation theory calculations in harmonic approximation yield phonon modes with imaginary frequencies due to the fact that the cubic structure is not stable at zero temperature. Our results suggest that low energy phonon modes mostly contribute to phonon-induced band energy renormalization. The calculated values of the band gaps at lowest temperature where the material exhibits a cubic structure are in good agreement with experimental results from the literature. The same is the case for the slope of the temperature dependence of the band gap for the CsPbI3 material where reliable experimental data are available in the literature. We also found that phonon-induced temperature dependence of the band gap is most pronounced for the conduction band minimum and valence band maximum, while other bands exhibit a weaker dependence.

Significantly anisotropic Raman response of the (110) surface in quasi-one-dimensional system (TaSe4)2I by polarized Raman spectroscopy.

Quasi-one-dimensional materials are usually characterized by optical response spectroscopy methods, which show significant polarization dependence. Herein, we report a systematical investigation of polarized Raman scattering on the (110) crystal surface of the layered (TaSe4)2I compound. Taking into account group theory analysis of the crystal structure and the Raman tensor transformation technique, the vibrational mode of the Raman peaks can be differentiated by the angular dependence of the Raman peak intensity in parallel and vertical polarization Raman scattering tests. Moreover, density functional perturbation theory (DFPT) calculation confirmed the form of the Raman tensor of the (110) crystal surface, which was consistent with the result of the Raman tensor transformation technique, and the Raman spectrum and phonon dispersion curve calculations were also performed based on the Vienna ab initio simulation package (VASP). This new method provides useful insight for accurately identifying the lattice vibration behavior in new 2D layered structures.

Vibrational instabilities in multilayer graphene and graphite: Effects of strain and number of layers

We study the rippling instabilities of multilayer graphene and graphite as a function of strain and number of layers using density-functional theory and density-functional perturbation theory calculations. We find that, for multilayer graphene, a small but finite compressive strain is needed to induce a rippling instability, signaled by the onset of imaginary frequencies near the center of the Brillouin zone. For monolayer graphene, this critical strain is zero within numerical uncertainty. Using insights from continuum elasticity theory, we fit the phonon dispersion of the ZA mode near the zone center to a function ${\ensuremath{\omega}}^{2}(q)={C}_{2}{q}^{2}+{C}_{4}{q}^{4}$ and determine the ${C}_{2}$ and ${C}_{4}$ coefficients as a function of strain and number of layers. In particular, the ${C}_{2}$ coefficient is positive for multilayers in the absence of strain and shows a linear dependence with strain. It assumes negative values for compressive strains beyond a critical value and it is responsible for the appearance of imaginary frequencies in the ZA mode. We discuss these results in light of both continuum and atomistic approaches. Critical parameters vary smoothly with the number of layers from monolayer graphene (two-dimensional) to graphite (three-dimensional).

Tailorable magnetocrystalline anisotropy at the MnPt3(001) surface

We report results of the density functional theory and density functional perturbation theory calculations on thermodynamic stability, magnetic structure, and magnetocrystalline anisotropy (MCA) of a ${\mathrm{MnPt}}_{3}(001)$ thin film. It is predicted that the magnetic ground state of the Pt-terminated ${\mathrm{MnPt}}_{3}$(001) thin films is the ferromagnetic as found in its bulk structure, while the MnPt termination favors the A-type antiferromagnetic order at the surface. Even more drastic effect of the surface termination on MCA is revealed; in contrast to an in-plane magnetization preferred for the MnPt termination, the Pt-terminated ${\mathrm{MnPt}}_{3}$(001) thin films exhibit an extremely large perpendicular MCA (PMCA) up to an order of 10 erg/${\mathrm{cm}}^{2}$ regardless of its film thickness. Moreover, from detailed single-particle energy spectra analyses, this magnetization reversal is mainly determined by an interplay between two in-plane orbital states, ${d}_{\text{xy}}$ and ${d}_{{\text{x}}^{2}\ensuremath{-}{\text{y}}^{2}}$ states, of the Pt atom with induced magnetism at the surface, as a result of their energy level changes associated with the Mn $3d\ensuremath{-}\mathrm{Pt}\phantom{\rule{4pt}{0ex}}5d$ hybridization. This reported thin-film system can act as a prototype for the in-depth study of the importance of the surface termination with large perpendicular magnetocrystalline anisotropy in spintronic applications.

Optical properties of corundum-structured In2O3

The optical properties of a single-phase corundum-structured In2O3 epilayer grown by a mist chemical vapor deposition method have been studied. Raman scattering measurements on a c face and on a lateral face reveal all of the seven Raman-active modes of the corundum structure, with good adherence to the Raman selection rules. Three out of the four infrared-active modes are observed in the spectroscopic ellipsometry measurements. The phonon frequencies obtained from Raman and ellipsometry measurements are in excellent agreement with density functional perturbation theory calculations. No trace of bixbyite phase was detected in the spectra. In the UV region, the imaginary part of the dielectric function shows two distinct onsets of strong absorption associated with direct band-to-band transitions at 3.38 and 3.86 eV.

Unified ab initio description of Fröhlich electron-phonon interactions in two-dimensional and three-dimensional materials

\textit{Ab initio} calculations of electron-phonon interactions including the polar Fr\"ohlich coupling have advanced considerably in recent years. The Fr\"ohlich electron-phonon matrix element is by now well understood in the case of bulk three-dimensional (3D) materials. In the case of two-dimensional (2D) materials, the standard procedure to include Fr\"ohlich coupling is to employ Coulomb truncation, so as to eliminate artificial interactions between periodic images of the 2D layer. While these techniques are well established, the transition of the Fr\"ohlich coupling from three to two dimensions has not been investigated. Furthermore, it remains unclear what error one makes when describing 2D systems using the standard bulk formalism in a periodic supercell geometry. Here, we generalize previous work on the \textit{ab initio} Fr\"ohlich electron-phonon matrix element in bulk materials by investigating the electrostatic potential of atomic dipoles in a periodic supercell consisting of a 2D material and a continuum dielectric slab. We obtain a unified expression for the matrix element, which reduces to the existing formulas for 3D and 2D systems when the interlayer separation tends to zero or infinity, respectively. This new expression enables an accurate description of the Fr\"ohlich matrix element in 2D systems without resorting to Coulomb truncation. We validate our approach by direct \textit{ab initio} density-functional perturbation theory calculations for monolayer BN and MoS$_2$, and we provide a simple expression for the 2D Fr\"ohlich matrix element that can be used in model Hamiltonian approaches. The formalism outlined in this work may find applications in calculations of polarons, quasiparticle renormalization, transport coefficients, and superconductivity, in 2D and quasi-2D materials.

Nonadiabatic Born Effective Charges in Metals and the Drude Weight.

In insulators, Born effective charges describe the electrical polarization induced by the displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where the macroscopic polarization is ill defined. Here we show that, in clean conductors, going beyond the adiabatic approximation results in nonadiabatic Born effective charges that are well defined in the low-frequency limit. In addition, we find that the sublattice sum of the nonadiabatic Born effective charges does not vanish as it does in the insulating case, but instead is proportional to the Drude weight. We demonstrate these formal results with density functional perturbation theory calculations of Al and electron-doped SnS_{2} and SrTiO_{3}.

Piezoelectricity in two-dimensional aluminum, boron and Janus aluminum-boron monochalcogenide monolayers

Piezoelectricity is a property of a material that converts mechanical energy into electrical energy or vice versa. It is known that group-III monochalcogenides, including GaS, GaSe, and InSe, show piezoelectricity in their monolayer form. Piezoelectric coefficients of these monolayers are the same order of magnitude as the previously discovered two-dimensional (2D) piezoelectric materials such as boron nitride and molybdenum disulfide (MoS2) monolayers. Considering a series of monolayer monochalcogenide structures including boron and aluminum (MX, M = B, Al, X = O, S, Se, Te), we design a series of derivative Janus structures (AlBX2, X = O, S, Se, Te). Ab-initio density functional theory and density functional perturbation theory calculations are carried out systematically to predict their structural, electronic, electromechanical and phonon dispersion properties. The electronic band structure analysis indicate that all these 2D materials are semiconductors. The absence of imaginary phonon frequencies in phonon dispersion curves demonstrate that the systems are dynamically stable. In addition, this study shows that these materials exhibit outstanding piezoelectric properties. For AlBO2 monolayer with the relaxed-ion piezoelectric coefficients, d 11 = 15.89(15.87) pm V−1 and d 31 = 0.52(0.44) pm V−1, the strongest piezoelectric properties were obtained. It has large in-plane and out-of-plane piezoelectric coefficients that are comparable to or larger than those of previously reported non-Janus monolayer structures such as MoS2 and GaSe, and also Janus monolayer structures including: In2SSe, Te2Se, MoSeTe, InSeO, SbTeI, and ZrSTe. These results, together with the fact that a lot of similar 2D systems have been synthesized so far, demonstrate the great potential of these materials in nanoscale electromechanical applications.

Raman Spectra of Bulk and Few-Layer GeSe From First-Principles Calculations

Raman spectra play a significant role in the study of polar materials. Herein, we report the influence of strain and interlayer shift on vibration responses in bulk and few-layer ferrovalley material GeSe in different polarization states (ferroelectric/FE and antiferroelectric/AFE) based on density functional theory and density functional perturbation theory calculations. We find Ag1 mode shifts by about 10 cm−1 from monolayer to bilayer and trilayer due to the interlayer coupling. The Ag3 mode on behalf of FE mode is observed that is consistent with the experiments in bulk and few-layer GeSe. Meanwhile, in our calculations, with the transition between AFE and FE state in the bilayer and trilayer, the Raman frequency of Ag2 and Ag3 mode decrease obviously whereas that of Ag1 mode increases. Interestingly, the Raman peaks shifted a lot due to the strain effect. We expect these variations in the Raman spectroscopy can be employed to identify the status of GeSe films, e.g., the AFE or FE state, and the number of layers in experiments.