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In thewe discussed in detail the effects of lattice periodicity on the single-particle wavefunctions and the energy eigenvalues. We also touched on the notion that a crystal can have symmetries beyond the translational periodicity, such as rotations around axes, reflections on planes, and combinations of these operations with translations by vectors that are not lattice vectors, called “non-primitive” translations. All these symmetry operations are useful in calculating and analyzing the physical properties of a crystal. There are two basic advantages to using the symmetry operations of a crystal in describing its properties. First, the volume in reciprocal space for which solutions need to be calculated is further reduced, usually to a small fraction of the first Brillouin zone, called the “irreducible” part; for example, in the FCC crystals with one atom per unit cell, the irreducible part is of the full BZ.
Up to this point we have been dealing with the ground-state properties of electrons in solids. Even in the case of doped semiconductors, the presence of extra electrons or of holes relative to the undoped ideal crystal was due to the introduction of additional electrons (or removal of some electrons) as a result of the presence of impurities, with the additional charges still corresponding to the ground state of the solid. Some of the most important applications of materials result from exciting electrons out of the ground state. These include the optical properties of solids and the dielectric behavior (shielding of external electric fields). These phenomena are also some of the more interesting physical processes that can take place in solids when they interact with external electromagnetic fields. We turn our attention to these issues next.
Materials exhibit an extremely wide range of properties, which is what makes them so useful and indispensable to humankind. The extremely wide range of the properties of materials is surprising, because most of them are made up from a relatively small subset of the elements in the Periodic Table: about 20 or 30 elements, out of more than 100 total, are encountered in most common materials. Moreover, most materials contain only very few of these elements, from one to half a dozen or so. Despite this relative simplicity in composition, materials exhibit a huge variety of properties over ranges that differ by many orders of magnitude. It is quite extraordinary that even among materials composed of single elements, physical properties can differ by many orders of magnitude.