Improving the precision of forces in real-space pseudopotential density functional theory.

The high-order finite difference real-space pseudopotential density functional theory (DFT) approach is a valuable method for large-scale, massively parallel DFT calculations. A significant challenge in the approach is the oscillating "egg-box" error introduced by aliasing associated with a coarse grid spacing. To address this issue while minimizing computational cost, we developed a finite difference interpolation (FDI) scheme [Roller et al.

Phonon-Phonon Interactions in the Polarization Dependence of Raman Scattering.

We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank tensor formalism. This generalization of the second-rank Raman tensor stems from the effect of off-diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon-phonon interaction in inelastic light scattering, markedly separate from the better-known phonon lifetime.

Finite Difference Interpolation for Reduction of Grid-Related Errors in Real-Space Pseudopotential Density Functional Theory.

The real-space pseudopotential approach is a well-known method for large-scale density functional theory (DFT) calculations. One of its main limitations, however, is the introduction of errors associated with the positioning of the underlying real-space grid, a phenomenon usually known as the "egg-box" effect. The effect can be controlled by using a finer grid, but this raises the cost of the calculations or even undermines their feasibility altogether.

Diverging Expressions of Anharmonicity in Halide Perovskites.

Lead-based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs AgBiBr , has been recently studied as a lead-free alternative for optoelectronic applications.

Unraveling Heat Transport and Dissipation in Suspended MoSe from Bulk to Monolayer.

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe .

Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites.

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs_{2}AgBiBr_{6}, the prototypical compound in this class and show that the lattice dynamics of Cs_{2}AgBiBr_{6} is highly anharmonic, largely in regards to tilting of AgBr_{6} and BiBr_{6} octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch.

Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO_{3} Perovskite from First Principles.

Structural phase transitions and soft phonon modes pose a long-standing challenge to computing electron-phonon (e-ph) interactions in strongly anharmonic crystals. Here we develop a first-principles approach to compute e-ph scattering and charge transport in materials with anharmonic lattice dynamics. Our approach employs renormalized phonons to compute the temperature-dependent e-ph coupling for all phonon modes, including the soft modes associated with ferroelectricity and phase transitions.

Crystal Chemistry and Phonon Heat Capacity in Quaternary Honeycomb Delafossites: Cu[LiSn]O and Cu[NaSn]O.

This work presents an integrated approach to study the crystal chemistry and phonon heat capacity of complex layered oxides. Two quaternary delafossites are synthesized from ternary parent compounds and copper monohalides via a topochemical exchange reaction that preserves the honeycomb ordering of the parent structures. For each compound, Rietveld refinement of the powder X-ray diffraction patterns is examined in both monoclinic C2/ c and rhombohedral R3̅ m space groups.

Lattice Thermal Conductivity of Polyethylene Molecular Crystals from First-Principles Including Nuclear Quantum Effects.

Molecular crystals such as polyethylene are of intense interest as flexible thermal conductors, yet their intrinsic upper limits of thermal conductivity remain unknown. Here, we report a study of the vibrational properties and lattice thermal conductivity of a polyethylene molecular crystal using an ab initio approach that rigorously incorporates nuclear quantum motion and finite temperature effects. We obtain a thermal conductivity along the chain direction of around 160  W m^{-1} K^{-1} at room temperature, providing a firm upper bound for the thermal conductivity of this molecular crystal.

Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles.

The interest in improving the thermoelectric response of bulk materials has received a boost after it has been recognized that layered materials, in particular SnSe, show a very large thermoelectric figure of merit. This result has received great attention while it is now possible to conceive other similar materials or experimental methods to improve this value. Before we can now think of engineering this material it is important we understand the basic mechanism that explains this unusual behavior, where very low thermal conductivity and a high thermopower result from a delicate balance between the crystal and electronic structure.

Lattice Vibrations Change the Solid Solubility of an Alloy at High Temperatures.

We develop a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti_{1-x}Al_{x}N alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy corresponding to the true equilibrium state of the system. We demonstrate that the vibrational contribution including anharmonicity and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti_{1-x}Al_{x}N alloy, lowering the maximum temperature for the miscibility gap from 6560 to 2860 K.

Highly Efficient Free Energy Calculations of the Fe Equation of State Using Temperature-Dependent Effective Potential Method.

Free energy calculations at finite temperature based on ab initio molecular dynamics (AIMD) simulations have become possible, but they are still highly computationally demanding. Besides, achieving simultaneously high accuracy of the calculated results and efficiency of the computational algorithm is still a challenge. In this work we describe an efficient algorithm to determine accurate free energies of solids in simulations using the recently proposed temperature-dependent effective potential method (TDEP).