Advanced materials with extreme thermal conductivity are critically important for various technological applications including energy conversion, storage, and thermal management. High thermal conductivity is desirable for efficient heat spreading in electronics, and low thermal conductivity is needed for thermal insulation and thermoelectric energy harvesting. However, practical application deployments are usually limited by the materials availability and understanding the fundamental origins for extreme thermal conductivity remains challenging. My PhD research focuses on applying and developing first-principles computations to understand the microscopic thermal transport mechanisms of the emerging materials and to discover new materials with ultrahigh and ultralow thermal conductivity. My dissertation is composed of three themes. The first theme is focused on understanding the fundamental origins and transport mechanisms for a group of high thermal conductivity semiconductors that were discovered recently by our group. In particular, boron phosphide (BP) and boron arsenide (BAs) crystals have been synthesized and measured with thermal conductivities of 460 and 1300 W/mK respectively, representing the best thermal conductor among common bulk metals and semiconductors. I have conducted ab initio calculations based on density functional theory to investigate phonon anharmonicity, size-dependent transport from diffusive to ballistic regime, as well as the effect from defect scattering. Our study shows that, unlike the commonly accepted rule for most materials near room temperature, high-order anharmonicity through the four-phonon process is significant in BA because of its unique band structure. In addition, I have performed multiscale Monte Carlo simulations to solve phonon Boltzmann transport equations to compute heat dissipation in three-dimensional practical measurement samples and electronic devices, which quantitively determines temperature distributed resulted by non-equilibrium phonon transport and underscores the promise of our developed BP and BAs for the next generation of thermal management technologies. The second theme of my thesis is to theoretical search for new ultra-high thermal conductivity materials, with the aim to push the limit of existing materials database. We have calculated the thermal conductivity of several B-C-X ternary compounds and found the R3m-BNC2 has ultrahigh thermal conductivity at ~2200 W/mK, which is comparable with the existing highest thermal conductivity materials, diamond. We also calculate the thermal conductivity of single-layer boron compounds in III-V group, and find high thermal conductivity of single-layer h-BAs at around 400 W/K. My computational studies enable atomistic understanding through their phonon band structures, scattering spaces, lifetimes, etc. The third theme of my thesis is to investigate phonon transport in ultralow thermal conductivity materials with a focus on tin selenide (SnSe). SnSe is a recently discovered high performance thermoelectric material, but its intrinsic low thermal conductivity remains debating in recent literature. In collaboration with my labmates, we combine phonon theory and experiments to investigate phonon softening physics. In particular, my calculated phonon frequencies of SnSe under varying temperatures indicate strong phonon renormalization due to higher-order anharmonicity. The comparison of my theory results with experiments indicates that the widely used harmonic model fails to descript the phonon renormalization and thus thermal conductivity of SnSe. Instead, I have developed self-consistent phonon theory to capture the higher order interactions and provided very good agreement with the experimentally measured ultralow thermal conductivity and thermophysical properties of SnSe.

This is a classic text of its time in condensed matter physics.

The first volume of this treatment, Phonons: Theory and Experiments I, was devoted to the basic concepts of the physics of phonons and to a study of models for interatomic forces. The second volume, Phonons: Theory and Experiments II, contains a study of experimental techniques and the inter pretation of experimental results. In the present third volume we treat a number of phenomena which are directly related to phonons. The aim of this book is to bridge the gap between theory and experi ment. An attempt has been made to present the descriptive as well as the analytical aspects of the topics. Although emphasis is placed on the role of phonons in the different topics, most chapters also contain a general intro duction into the specific subject. The book is addressed to experimentalists and to theoreticians working in the vast field of dynamical properties of solids. It will also prove useful to graduate students starting research in this or related fields. The choice of the topics treated was partly determined by the author's own activity in these areas. This is particularly the case for the chapters dealing with phonons in one-dimensional metals, disordered systems, super ionic conductors and certain newer aspects of ferroelectricity and melting. I am very grateful to my colleagues J. Bernasconi, V.T. Hochli and 1.

Maintaining a practical perspective, Electronic Transport Theories: From Weakly to Strongly Correlated Materials provides an integrative overview and comprehensive coverage of electronic transport with pedagogy in view. It covers traditional theories, such as the Boltzmann transport equation and the Kubo formula, along with recent theories of transport in strongly correlated materials. The understood case of electronic transport in metals is treated first, and then transport issues in strange metals are reviewed. Topics discussed are: the Drude-Lorentz theory; the traditional Bloch-Boltzmann theory and the Grüneisen formula; the Nyquist theorem and its formulation by Callen and Welton; the Kubo formalism; the Langevin equation approach; the Wölfle-Götze memory function formalism; the Kohn-Luttinger theory of transport; and some recent theories dealing with strange metals. This book is an invaluable resource for undergraduate students, post-graduate students, and researchers with a background in quantum mechanics, statistical mechanics, and mathematical methods.