# Condensed Matter Physics
Condensed matter physics asks what happens when you bring 10²³
atoms together and let them settle. The answer is almost never the
sum of the parts — order, rigidity, conduction, and magnetism are
_collective_ properties that no single atom possesses.
_Figure 001 — A crystal is a lattice: identical atoms repeat with perfect periodicity,
and one unit cell tiles the whole solid.
_
_Figure 002 — Band theory: a filled valence band and an empty conduction band, split by a
forbidden gap Eg.
_
The starting point is structure. Most solids are **crystals** —
atoms arranged on a periodic lattice — and that periodicity is the single
fact from which nearly everything else follows. Choose a unit cell and a
basis, and the reciprocal lattice hands you the natural language for waves
inside the solid.
Periodicity reshapes the electrons. In a periodic potential the allowed
energies collect into **bands** separated by _gaps_, and
where the last electrons land — mid-band or at a band edge — decides whether
a material is a metal, an insulator, or a semiconductor.
_Figure 003 — A phonon: a quantized lattice vibration travelling through the atoms, each
bobbing about its rest position.
_
The lattice itself is never still. Its atoms vibrate in collective normal
modes, and quantizing those modes gives **phonons** — the
particle-like carriers of sound and heat that also scatter electrons and set
a metal's resistance.
Before bands, the crudest model already carries a metal a long way: treat the
conduction electrons as a _free gas_. Pauli exclusion stacks them into
momentum space up to the Fermi energy, and the heat capacity, magnetism, and
transport of a metal all trace back to what happens at that surface.
_Figure 004 — The free-electron Fermi sphere: states fill momentum space up to kF,
and the surface governs the metal.
_
From this foundation the subject fans out: **semiconductors**
and the doped junctions that make devices, dielectrics and ferroelectrics,
and the several distinct origins of **magnetism** in solids.
_Figure 005 — Superconductivity: a superconductor expels magnetic flux (B = 0 inside),
levitating a magnet by the Meissner effect.
_
The dramatic finale is **superconductivity** — below a critical
temperature, electrons bind into Cooper pairs, resistance vanishes, and the
material expels magnetic flux entirely, a purely quantum effect made visible
at human scale.
Throughout, one move recurs: find the symmetry, count the states, and let
the collective behavior emerge. The course follows Kittel, Ashcroft &
Mermin, and Simon, ending in the low-dimensional physics of nanostructures
and graphene.
---
## Contents
### 1. Molecules and Chemical Bonding
1. [Bonding Mechanisms](/condensed-matter/molecules-and-bonding/bonding-mechanisms)
2. [The Molecular-Orbital Method and H₂⁺](/condensed-matter/molecules-and-bonding/molecular-orbitals-and-h2-plus)
3. [The Hydrogen Molecule, Exchange, and Hybridization](/condensed-matter/molecules-and-bonding/hydrogen-molecule-and-exchange)
4. [Van der Waals Forces](/condensed-matter/molecules-and-bonding/van-der-waals-forces)
### 2. Molecular Spectra
1. [Rotational and Vibrational Spectra of Molecules](/condensed-matter/molecular-spectra/rotational-vibrational-spectra)
2. [Anharmonicity and Rovibrational Structure](/condensed-matter/molecular-spectra/anharmonicity-and-rovibrational-structure)
3. [Raman Scattering and Electronic Bands](/condensed-matter/molecular-spectra/raman-and-electronic-bands)
4. [Lasers, Masers, and Stimulated Emission](/condensed-matter/molecular-spectra/lasers-and-masers)
### 3. Crystal Structure
1. [The Structure of Solids](/condensed-matter/crystal-structure/structure-of-solids)
2. [Bravais Lattices, Bases, and Crystal Structures](/condensed-matter/crystal-structure/bravais-lattices-and-crystal-systems)
3. [The Reciprocal Lattice and Brillouin Zones](/condensed-matter/crystal-structure/reciprocal-lattice-and-brillouin-zones)
4. [X-ray and Neutron Diffraction](/condensed-matter/crystal-structure/diffraction-and-structure-factors)
### 4. Lattice Dynamics
1. [The Harmonic Crystal and Phonon Dispersion](/condensed-matter/lattice-dynamics/phonon-dispersion)
2. [Phonons, Density of States, and Crystal Momentum](/condensed-matter/lattice-dynamics/phonons-quantization-and-dos)
3. [Thermal Properties — Einstein and Debye Models](/condensed-matter/lattice-dynamics/debye-einstein-heat-capacity)
4. [Anharmonicity, Thermal Expansion, and Heat Conduction](/condensed-matter/lattice-dynamics/anharmonicity-and-thermal-transport)
### 5. Free-Electron Fermi Gas
1. [Conduction and the Free-Electron Gas](/condensed-matter/free-electron-fermi-gas/free-electron-gas-and-conduction)
2. [The Sommerfeld Model: Ground State and Heat Capacity](/condensed-matter/free-electron-fermi-gas/sommerfeld-model-and-heat-capacity)
3. [Transport, Wiedemann–Franz, and the Hall Effect](/condensed-matter/free-electron-fermi-gas/transport-and-the-hall-effect)
4. [Screening, Plasmons, and the Limits of Free Electrons](/condensed-matter/free-electron-fermi-gas/screening-and-plasmons)
### 6. Band Theory
1. [Bloch's Theorem and Energy Bands](/condensed-matter/band-theory/blochs-theorem-and-energy-bands)
2. [The Nearly-Free-Electron Model](/condensed-matter/band-theory/nearly-free-electron-model)
3. [The Tight-Binding Method](/condensed-matter/band-theory/tight-binding-method)
4. [Fermi Surfaces, Effective Mass, and Metals vs Insulators](/condensed-matter/band-theory/fermi-surfaces-and-semiclassical-dynamics)
### 7. Semiconductors
1. [Band Theory and Semiconductors](/condensed-matter/semiconductors/semiconductor-bands-and-junctions)
2. [Carrier Statistics: Intrinsic and Extrinsic Semiconductors](/condensed-matter/semiconductors/intrinsic-and-extrinsic-semiconductors)
3. [Carrier Transport and Recombination](/condensed-matter/semiconductors/carrier-transport-and-recombination)
4. [The p-n Junction in Depth](/condensed-matter/semiconductors/the-pn-junction)
5. [Transistors and Optoelectronic Devices](/condensed-matter/semiconductors/transistors-and-optoelectronics)
### 8. Dielectrics and Ferroelectrics
1. [Dielectrics, Polarization, and the Local Field](/condensed-matter/dielectrics-and-ferroelectrics/dielectrics-and-polarization)
2. [Ferroelectrics, Piezoelectrics, and Structural Transitions](/condensed-matter/dielectrics-and-ferroelectrics/ferroelectrics-and-piezoelectrics)
### 9. Magnetism in Solids
1. [Diamagnetism and Paramagnetism](/condensed-matter/magnetism/diamagnetism-and-paramagnetism)
2. [Exchange and Ferromagnetism](/condensed-matter/magnetism/exchange-and-ferromagnetism)
3. [Antiferromagnetism, Ferrimagnetism, and Domains](/condensed-matter/magnetism/antiferromagnetism-and-domains)
4. [Spin Waves and Magnons](/condensed-matter/magnetism/spin-waves-and-magnons)
### 10. Superconductivity
1. [Superconductivity: Phenomenology and BCS](/condensed-matter/superconductivity/superconductivity-phenomenology)
2. [London Theory and the Meissner Effect](/condensed-matter/superconductivity/london-theory-and-the-meissner-effect)
3. [Ginzburg–Landau Theory, Vortices, and Type-II](/condensed-matter/superconductivity/ginzburg-landau-theory)
4. [Microscopic BCS Theory](/condensed-matter/superconductivity/bcs-theory)
5. [Josephson Effects and Unconventional Superconductors](/condensed-matter/superconductivity/josephson-and-high-tc)
### 11. Nanostructures
1. [Quantum Wells, Wires, and Dots](/condensed-matter/nanostructures/quantum-wells-wires-and-dots)
2. [The 2D Electron Gas and the Integer Quantum Hall Effect](/condensed-matter/nanostructures/integer-quantum-hall-effect)
3. [The Fractional Quantum Hall Effect and Topological Order](/condensed-matter/nanostructures/fractional-quantum-hall-and-topology)
4. [Graphene and Dirac Materials](/condensed-matter/nanostructures/graphene-and-dirac-materials)