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Exploring solid-state physics

In this video, Dr Kate Lancaster explains what a solid is, and why our ability to understand them is so crucial to society.
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Welcome to week 2 of the course. Last week we focussed on the so-called fourth state of matter – plasma. This week we will move our attention to seemingly more familiar territory by thinking about the world of solid materials. You might think that the physics of materials would be mundane compared to the exotic world of plasmas, but it turns out that even everyday materials have some surprising secrets locked within them. We should start this week by thinking about what makes solids unique. A solid is a material whose atoms are locked into some rigid arrangement. To understand the properties of a solid is really to understand this atomic arrangement. A large class of materials have a regular arrangement to their atoms.
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You can imagine them as being made up of a huge number of identical cells, each with the same atomic arrangement, stacked together. These solids are known as crystals. Familiar examples are gemstones such as diamonds and rubies, but crystalline order also exists in the vast majority of metals, and even within our own bones. Some materials lack this ordering, displaying a seemingly random arrangement of atoms. Glass, many plastics and wax are all examples of these so-called amorphous materials. Some interesting properties of materials, like the huge mechanical strength of diamonds or the unusual optical properties of some crystals such as calcite, can be traced directly back to their arrangement of atoms.
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An ability to combine these materials and to modify their shape in inventive ways is one of the key things which has set humans apart from other animals and enabled the many technological revolutions which have shaped our world. Our earliest advances saw us move from using and shaping naturally occurring rocks to form tools and weapons, to learning how to extract and purify the metals locked within. Through the bronze and iron ages we refined our abilities, ultimately enabling us to create ever larger buildings, printing presses, compasses, clocks and powered vehicles. In the late 1940’s we harnessed the power of a new type of crystal, the semiconductor, and so began the information age.
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Basically, we can track back huge portions of our development as a species to advances in our mastery of materials. But this development is far from over. As we will see this week we are now able to view and manipulate the structure of materials on the atomic level. We can use supercomputers to design innovative materials, use electrons to probe and understand their structure, and modify their shape at the nanoscale to enable exciting new applications. This ability to design the world we live in makes solid state physics one of the largest and most exciting communities in modern research.

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!

length Example
metre (m) Height of a person
millimetre (mm), 10-3m Length of a flea
micrometre, or micron ((mu)m) 10-6m Length of a bacterium
nanometre (nm) 10-9m A unit cell of a solid

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.

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Frontier Physics, Future Technologies

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