Summary

These lecture notes explain the thermal physics of solids from the microscopic foundations upward. Lecture 1  Fundamental Concepts of Solids begins with the basic question of what a solid is, distinguishing crystalline, polycrystalline, and amorphous matter and showing how thermal behaviour is rooted in atomic-scale correlations, bonding, and structure. Then Lecture 2  Interatomic Bonding establishes the link between cohesive forces, lattice stiffness, characteristic vibrational frequencies, and thermal energy scales. These first lectures make clear that the thermal properties of solids cannot be understood without first understanding how atoms are arranged, how they are bound, and which microscopic energy scales govern their motion.

Building on this, the lecture notes introduce the structural language of solid-state physics in real and reciprocal space. The real-space description of crystal structures in Lecture 3  Fundamentals of Crystal Structure, the symmetry principles and reciprocal-space framework of periodic solids in Lecture 4  Symmetry and Reciprocal Space, and the diffraction methods that make lattice order experimentally visible in Lecture 5  Diffraction and Structure Factor together provide the conceptual and mathematical basis for everything that follows. These chapters establish that periodicity is not merely a structural feature: it organizes the allowed excitations of the system, determines how waves propagate through a crystal, and governs how structure becomes experimentally accessible.

The central part of the lecture notes develops lattice dynamics in a systematic way. Starting from the Born-Oppenheimer and harmonic approximation in Lecture 6  Lattice Dynamics I, they derive the vibrational modes of crystals, from the general dynamical-matrix description of three-dimensional solids to the explicit dispersions of monatomic and diatomic chains discussed in Lecture 7  Lattice Dynamics II. The quantization of these normal modes is performed in Lecture 8  Phonons I which leads naturally to phonons, whose statistics and density of states provide the bridge from microscopic motion to macroscopic thermodynamics. This framework is subsequently used in Lecture 9  Phonons II to explain the heat capacity of solids, including the Einstein and Debye models, the low-temperature \(T^3\) law, and the high-temperature Dulong-Petit limit.

The later chapters show how departures from perfect harmonicity give rise to the most characteristic thermal phenomena of real materials. Lecture 10  Anharmonicity and Thermal Expansion explains thermal expansion, connects phonon frequencies to volume through Grüneisen parameters, and gives phonons finite lifetimes. This, in turn, leads to a microscopic treatment of lattice thermal conductivity in Lecture 11  Thermal Transport by Phonons in terms of phonon heat capacity, group velocity, and mean free path, together with the roles of boundary, defect, alloy, and umklapp scattering. In Lecture 12  Electronic Contributions to Thermal Properties, this treatment is then extended beyond purely lattice contributions to include the electronic heat capacity of metals, thermoelectric effects, and the Wiedemann-Franz law. The role of electron-phonon coupling in resistivity, thermal transport, and phonon linewidths is discussed in Lecture 13  Electron–Phonon Coupling (optional).

The lectures aim to present a unified picture of thermal properties in solids: structure determines bonding, bonding determines vibrational scales, vibrations become phonons, and phonons govern the observable thermodynamic and transport behaviour of materials. The lecture notes close with Lecture 14  Experimental Methods for Thermal Properties (optional) by connecting the theoretical framework to experiments, showing how infrared, Raman, Brillouin, and neutron scattering reveal phonon energies, dispersions, occupations, and lifetimes.