Exploring solid-state physics

In this article, our focus is on so-called solid-state physics. In the video above, Kate explains what a solid is and why our ability to understand them is so crucial.

Let’s look at what a solid is in a bit more detail.

What is a solid?

A solid is a material in which the atoms are held together in a rigid arrangement. It takes some external force, often a very large one, to change this arrangement and deform the solid.

Nearly every property of a solid can be understood in terms of how the atoms it is composed of are arranged. We call this the material’s structure.

A unit cell

The materials we’ll focus on will be crystals. By this, we don’t mean rubies or diamonds (although these are crystals). We mean materials that are composed of an arrangement of atoms, known as a unit cell, that repeats regularly to build up the solid. Examples include metals, salt (that you put on food) and graphite (in your pencil).

This unit cell might just contain a single atom or, for complex solids, could house many hundreds of atoms. But what is remarkable is that no matter how big our solid, it is just made up of perfect copies of this unit cell, stuck together in a repeating pattern.

Crystals

Because the crystal is just built up of copies of the unit cell, all the information about the solid’s fundamental properties is contained within just a single unit cell. Adding more cells just makes the crystal bigger – it doesn’t change its nature.

Whether a material is hard, soft, electrically conducting or insulating, opaque or translucent, colourful or dull – everything is set by the properties of the unit cell.

Impurities

The real world is never so perfect. Real crystals tend to have the occasional error in their repeating pattern. Perhaps an atom is missing here or there: we call these vacancies.

Perhaps the crystal has grown with the wrong type of atom in one position: these are known as impurities. These so-called defects can modify the properties of materials, meaning our simple idea of repeating unit cells is a great start but needs a little refinement to explain the full range of material properties we see around us.

How big are these unit cells?

Below is a table of the SI prefixes applied to lengths. Each row of the table gives a length a thousand times smaller than in the previous row. Alongside each is an example of something with about that length.

Note that we said about that length. Not all people are a metre tall, but we are (mostly) somewhere around 0.5m to 2m, so about a metre is a pretty good estimate when we are jumping about by factors of 1000 between rows of the table!

A unit cell is of order nanometres (usually written as nm), or one thousand millionth of a metre. A nanometre is an exceptionally small distance. In fact, as you might imagine, it is only a little larger than the size of an atom (which is about 10^-10m across).

So, when we talk about exploring the nanoscale, we really mean understanding and manipulating solids on their smallest length scales – the size of the unit cell that defines their properties.