Phonons and related properties of extended systems from density-functional perturbation theory

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudo-potential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long wave-length vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic

A universal equivariant graph neural network for the elasticity tensors of any crystal system

The elasticity tensor that describes the elastic response of a material to external forces is among the most fundamental properties of materials. The availability of full elasticity tensors for inorganic crystalline compounds, however, is limited due to experimental and computational challenges. Here, we report the materials tensor (MatTen) model for rapid and accurate estimation of the full fourth-rank elasticity tensors of crystals. Based on equivariant graph neural networks, MatTen satisfies the two essential requirements for elasticity tensors: independence of the frame of reference and preservation of material symmetry. Consequently, it provides a universal treatment of elasticity tensors for all crystal systems across diverse chemical spaces. MatTen was trained on a dataset of first-principles elasticity tensors garnered by the Materials Project over the past several years (we are releasing the data herein) and has broad applications in predicting the isotropic elastic properties of polycrystalline materials, examining the anisotropic behavior of single crystals, and discovering new materials with exceptional mechanical properties. Using MatTen, we have discovered a hundred new crystals with extremely large maximum directional Young's modulus and eleven polymorphs of elemental cubic metals with unconventional spatial orientation of Young's modulus

Electronic Functionalities in Two-Dimensional Layered Materials for Device Applications

The rise of two-dimensional (2D) materials has enabled the realization of ultra-thin electronic devices with a broad range of applications in transistors, memory devices, photodetectors, chemical sensors, and electronic displays. The optoelectronic functionality displayed by this unique material class is determined by the underlying phenomena relating to their crystal structure, quantum confinement, and heterogeneities such as defects, dopants, and atomic interface in their heterostructures.The first part of this thesis highlights the effects of heterogeneities in tuning the electronic functionalities of 2D materials. For example, the presence of sharp atomic interfaces could introduce p-n junction rectifying behavior which is the fundamental unit of electronic devices.The second part introduces a novel 2D anisotropic material, palladium diselenide (PdSe2), with a unique pentagonal, puckered structure unlike most other 2D materials with hexagonal building blocks. PdSe2 displays a strong layer-dependent optical and electronic properties. Density Functional Theory (DFT) calculations and absorption spectroscopy reveal that PdSe2 exhibit a wide-tunable indirect bandgap from ~0 eV in bulk to 1.3 eV in monolayer. Also, the anomalous layer-dependent Raman peak shifts around 5 – 9 cm-1 from bulk to monolayer for PdSe2 confirms the strong interlayer coupling in PdSe2.The third section discusses the field-effect transistor (FET) device performance of PdSe2, which shows a characteristic high carrier mobility as high as 158 cm2V-1s-1 and air stability for wide-tunable electronic applications. Also, PdSe2 devices show temperature-dependent conductivity with observed metal to insulator (MIT) transition.Finally, through plasma treatment, a new complementing metallic phase can be achieved from PdSe2 that forms a sharp atomic interface with negligible Schottky barrier heights. The phase transformation process is understood to be induced by the removal of selenium atoms. The entirely new material, Pd17Se15, with an electrically-conducting property, is used as a contact for PdSe2 devices which resulted in the reduction of the Schottky barrier present at the metal-semiconductor interface. This realization is an important step in the quest to eliminate contact resistance in 2D electronic devices.The ease of manipulating the structure of 2D materials, coupled with ample device engineering opportunities, makes 2D materials viable candidates for future nano-electronics

The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics

A longstanding limitation of first-principles calculations of substitutional alloy phase diagrams is the difficulty to account for lattice vibrations. A survey of the theoretical and experimental literature seeking to quantify the impact of lattice vibrations on phase stability indicates that this effect can be substantial. Typical vibrational entropy differences between phases are of the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of configurational entropy differences in binary alloys (at most 0.693 k_B/atom). This paper describes the basic formalism underlying ab initio phase diagram calculations, along with the generalization required to account for lattice vibrations. We overview the various techniques allowing the theoretical calculation and the experimental determination of phonon dispersion curves and related thermodynamic quantities, such as vibrational entropy or free energy. A clear picture of the origin of vibrational entropy differences between phases in an alloy system is presented that goes beyond the traditional bond counting and volume change arguments. Vibrational entropy change can be attributed to the changes in chemical bond stiffness associated with the changes in bond length that take place during a phase transformation. This so-called ``bond stiffness vs. bond length'' interpretation both summarizes the key phenomenon driving vibrational entropy changes and provides a practical tool to model them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure