The serious study of solid state physics began with the discovery of x-ray
diffraction by crystals and the publication of a series of simple calculations of
the properties of crystals and of electrons in crystals. Why crystalline solids
rather than noncrystalline solids? The important electronic properties of solids
are best expressed in crystals. Thus the properties of the most important semiconductors
depend on the crystalline structure of the host, essentially because
electrons have short wavelength components that respond dramatically to the
regular periodic atomic order of the specimen. Noncrystalline materials, notably
glasses, are important for optical propagation because light waves have a
longer wavelength than electrons and see an average over the order, and not
the less regular local order itself.
We start the book with crystals. A crystal is formed by adding atoms in a
constant environment, usually in a solution. Possibly the first crystal you ever
saw was a natural quartz crystal grown in a slow geological process from a silicate
solution in hot water under pressure. The crystal form develops as identical
building blocks are added continuously. Figure 1 shows an idealized picture of
the growth process, as imagined two centuries ago. The building blocks here
are atoms or groups of atoms. The crystal thus formed is a three-dimensional
periodic array of identical building blocks, apart from any imperfections and
impurities that may accidentally be included or built into the structure.
The original experimental evidence for the periodicity of the structure
rests on the discovery by mineralogists that the index numbers that define the
orientations of the faces of a crystal are exact integers. This evidence was supported
by the discovery in 1912 of x-ray diffraction by crystals, when Laue developed
the theory of x-ray diffraction by a periodic array, and his coworkers
reported the first experimental observation of x-ray diffraction by crystals.
The importance of x-rays for this task is that they are waves and have a wavelength
comparable with the length of a building block of the structure. Such
analysis can also be done with neutron diffraction and with electron diffraction,
but x-rays are usually the tool of choice.
The diffraction work proved decisively that crystals are built of a periodic
array of atoms or groups of atoms. With an established atomic model of a crystal,
physicists could think much further, and the development of quantum theory
was of great importance to the birth of solid state physics. Related studies
have been extended to noncrystalline solids and to quantum fluids. The wider
field is known as condensed matter physics and is one of the largest and most
vigorous areas of physics.
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