Condensed-Matter Physics

Condensed-Matter Physics, the study of liquids and solids, including crystalline solids, and irregularly structured materials such as glasses, ceramics, organics, polymers, and composite materials. Mainly a 20th-century discipline, research in condensed-matter physics has included the use of X-ray diffraction to study crystals from about 1910, the discovery of semiconductors in the 1920s, and a microscopic theory of superconductivity in the 1950s. This research has led to the development of important devices such as transistors, optical fibres, and semiconductor lasers. A hallmark of 21st-century technology will probably be the use of new materials with novel properties, discovered by research in condensed-matter physics.

One of the most important insights of condensed-matter physics is the energy-band theory of solids. An electron bound to an isolated atom can exist only in a distinct set of atomic energy levels (see Atom). However, in a crystalline solid, which has many identical atoms in a regular array, these energy levels merge into bands of allowed energies separated by bands of forbidden energies (See Metals). Because the band structure is a property of the crystal as a whole, each atom may contribute an outer (valence) electron to fill the allowed bands. According to the exclusion principle developed by Wolfgang Pauli, the electrons fill each of the energy levels two at a time, one of the electrons with spin up and one with spin down. At absolute zero, all the lowest energy levels in the crystal are filled. At higher temperatures, the electrons become more energetic, and some will move to higher energy levels. The energy level known as the Fermi level—which is the dividing line above which energy levels tend to be empty, and below which energy levels tend to be full—helps define the insulation and conduction properties of materials.

If the Fermi level occurs in the middle of an allowed band, then the solid is a conductor. Some examples of conductors are the metals silver and copper. With the Fermi level in the middle of an allowed band, even small energies can excite electrons in the highest filled level to the lowest unoccupied level. This mobility of the electrons gives conductors their defining qualities, such as their ability to conduct electricity and heat, their ability to absorb light, and their opacity. If the Fermi level occurs at the top of an allowed band, and there is a relatively large gap between this and the next higher-energy allowed band, then the solid is an insulator, like diamond or quartz. Because only large energies can excite the electrons, an insulator is a poor conductor of heat and electricity, cannot absorb light, and is transparent. Finally, if the Fermi level is near the top of a band but there is only a narrow forbidden energy gap above it, then the solid is a semiconductor, like silicon.

An area of active research in condensed-matter physics is the study of superconductors. The electrical resistance of metals typically decreases steadily as temperature falls; upon reaching extremely low temperatures a few degrees above absolute zero, however, many metals suddenly lose all electrical resistance. When the electrical resistance of a metal disappears, it is called a superconductor. A current flowing in a superconducting ring will persist indefinitely. Electrical currents have been observed to circulate in such superconducting rings for months with no noticeable reduction of the current.

For most of the 20th century, superconducting phenomena were observable only at the frigid temperatures of liquid helium (below -268.9° C/-452.0° F). However, the discovery in the 1980s of a class of ceramic copper-oxide materials that become superconducting at the much higher temperatures of liquid air (about -200° C or -328° F) has opened up new possibilities for the application of superconducting materials, perhaps in high-speed electronic devices, or more efficient power-transmission lines.