Condensed Matter Physics by Ross H. McKenzie

Author:Ross H. McKenzie [McKenzie, Ross]
Language: eng
Format: epub
ISBN: 9780192584137
Publisher: OUP Oxford
Published: 2023-04-19T00:00:00+00:00

Creating Flatland in a laboratory

As a result of the painstaking work and ingenuity of many scientists, it is possible to fabricate materials that are essentially one-dimensional or two-dimensional. For example, the electrons in these materials can only move in one or two directions. We have already come across one example of a two-dimensional material: graphene, a single layer of carbon atoms.

Suppose a perfect crystal is cut in two. The surface of a piece is two-dimensional. It can be used as a substrate to create a two-dimensional system. For example, liquid helium placed on a graphite surface can be adjusted in quantity to create a single layer of helium atoms. This has led to studies of superfluids in two dimensions showing that the nature of the superfluid state and the transition to it are different from that in three dimensions.

It is also possible to make layered materials to which layers of atoms are added one layer at a time. By varying the chemical composition of different layers, interfaces can be created with unique two-dimensional properties. A specific example of this is called a semiconductor heterojunction, in which a gas of electrons is trapped at the interface between two different semiconductor materials. The electrons can then only move in directions parallel, but not perpendicular, to the interface. These two-dimensional electron gases exhibit new states of matter, known as quantum Hall states, which we will come to later.

Another class of materials is described as quasi-two-dimensional. They are three-dimensional crystals composed of layers of atoms in which there are only weak interactions between the neighbouring layers. Graphite is an example; it is a crystal composed of layers of graphene. If the interactions between the layers are weak enough, many of the properties of the material may be similar to those of a single layer. Later in this chapter, we will look at a class of superconducting materials that are quasi-two-dimensional. Chemists have also made crystals that consist of chains of atoms and are classified as quasi-one-dimensional materials.

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