Condensed matter physics

Crystals, band theory, semiconductors, and superconductivity

What is condensed matter physics?

Condensed matter physics studies the physical properties of matter in its solid and liquid forms — how huge numbers of atoms, packed closely together, behave collectively. It's the branch of physics closest to the materials we touch and use every day.

Why solids hold their shape

In a solid, atoms are packed closely together and held in relatively fixed positions by the electric forces between them (covered in the Electromagnetism article), which is why solids keep a definite shape rather than flowing like a liquid or spreading out like a gas. In many solids, called crystals, the atoms arrange themselves in a highly regular, repeating pattern — table salt, diamond, and metals are all examples of crystalline solids, each with its own characteristic repeating structure.

Conductors, insulators, and semiconductors

Materials differ enormously in how easily electricity flows through them. In a conductor (like copper), some electrons are only loosely attached to their atoms and can move freely through the material, carrying electric current easily. In an insulator (like rubber or glass), electrons are tightly bound and cannot move freely, so current barely flows at all. Semiconductors (like silicon) sit in between: they conduct poorly on their own, but their conductivity can be carefully controlled — this controllability is exactly what makes semiconductors the basis of essentially all modern electronics, from computer chips to solar panels.

Everyday states and changes of matter

Condensed matter physics also explains the everyday behavior of materials: why some metals bend easily while others are brittle, why ice floats on water, why heating a solid eventually melts it into a liquid, and how a material's structure at the atomic level determines its properties at the scale we can see and touch.

Why condensed matter physics matters

Nearly every material technology in daily use — computer chips, batteries, LEDs, superconducting MRI magnets, and the metals and plastics that make up most manufactured objects — is built on an understanding of condensed matter physics.