A particularly important role in the development of modern technology is played by solid-state physics, which studies the properties of solids in relation to their intimate structure both from the crystallographic point of view (type of elementary cell, lattice parameters, position in the cell of atoms of different species), and with regard to the forces that bind the atoms together in the crystal lattice.
Solid state physics is the broadest branch of condensed matter physics and concerns the study of the properties of solids, both electronic, mechanical, optical and magnetic. The bulk of theoretical and experimental research in solid-state physics is focused on crystals, both because of their characteristic periodic atomic structure, which facilitates their mathematical modeling, and because of their wide technological use.
Solid state physics and the physics of liquids and soft (or soft) matter constitute condensed matter physics. It seeks to relate to the types of bonds and their intensities the specific properties of solids, such as those of resistance to mechanical stress, deformability, electrical and thermal conductivity, and others.
The physics of solids, although having originated in previous centuries, has had its own characteristic line of development since the end of the nineteenth century, with the discovery of X-rays (W.K. Röntgen, 1895) and with the demonstration, by M. von Laue (1912), that they were electromagnetic waves of small wavelength and that they could be diffracted by the atoms of a crystal.
These experiments showed that crystalline solids are composed of an atomic lattice and X-rays became an irreplaceable tool for the study of geometric arrangement of atoms in the solid. M. Born, later, developed a classical theory of the binding energy of some types of crystals and a theory on lattice vibrations, predicting proper modes of vibration of crystals, unifying the classical theory of static elasticity and sound waves of large wavelength.
Also at the beginning of twentieth century was introduced the concept of free electron in metals: this model allowed to explain the electrical resistance as due to diffusion of these electrons following collisions on atomic nuclei of the lattice. The attempts of theoretical explanation of several experimental phenomena (the photoelectric effect (A. Einstein, 1905), superconductivity (K. Onnes, 1911), the anomalous behavior of the heat capacity of solids with respect to the classical value predicted by Dulong and Petit’s law (Einstein, 1907 and P.J.W. Debye, 1912) required also in the physics of solids the introduction of concepts at the base of quantum theory, such as the photon, or quantum of light radiation, introduced by Einstein to explain the photoelectric effect, and the phonon, or quantum of excitation of normal modes of vibration, introduced by Debye quantizing Born’s model for lattice modes.
The modern development of solid physics occurred after 1928 with the application of the laws of quantum mechanics (Schrödinger equation) and statistics, that is, when, with the determination of definitive theoretical bases, a process of conceptual unification of the mechanical, electrical, magnetic, dielectric, optical, and thermal properties of solids began.
The boundaries of solid physics with other branches of science, such as metallurgy, electronics, chemistry, etc., dealing with solid materials cannot be precisely defined. The need to find solutions to technological problems has always been a major practical motivation for developing research in the field of solid physics.
The study of X-ray diffraction through solids has shown that most solids have a crystalline structure; that is, they are composed of atoms arranged in space according to a lattice. Crystalline lattices can present various symmetries: cubic, tetragonal, etc.. To the crystalline state are opposed, in small percentage, different amorphous states, in which the matter does not present the intimate regularity of structure of the crystalline structure.
The existence of a spatial symmetry of atoms is fundamental for a rigorous theoretical treatment of the solid state, from which are deduced schemes that allow the interpretation of fundamental properties (such as band theory for the distinction between conductors and insulators).
In crystals sometimes there are imperfections or irregularities in the lattice, which have great importance because they are related to particular properties, such as those of semiconductors, and characteristic optical phenomena (luminescence, laser effect), plastic, crystal growth, diffusion, etc..
The study of ideal crystals, i.e. those imagined without imperfections, deals with lattice vibrations, electron motion, interactions with radiation, ferromagnetism, superconductivity and defines the so-called intrinsic properties of the solid depending only on its chemical nature and its ideal crystal structure.
The discoveries and studies on semiconductors, which have revolutionized electronics, allowing the miniaturization of circuits, represent one of the best known chapters of the physics of solids, but the deepening and the extension of the subject have progressively become such as to influence and be influenced by more and more research fields (nuclear physics, nuclear energy, space science, electronics, etc.) that require particular developments in materials technology.
One field of research of great interest is the physics of surfaces and interfaces of solids. Considerable progress has been made mainly due to the refinement of vacuum techniques, which have made it possible to obtain increasingly pure surfaces, and to various techniques for deposition and manipulation of atoms and molecules, particle beam techniques, and chemical etching.
With the use of the tunnel effect microscope in the 1980s, it was possible to control surfaces to an accuracy of 0.1 Å (the angstrom unit, still used in crystallography, is defined as 1 Å = 10-8 cm), and also to place, literally, one atom at a time in the desired positions on the surface. The study of the interaction between atoms or molecules and surfaces (collisions, adsorption, desorption along with various physical and chemical reactions) is a topic of research in this field.