Solid-state physics is the study of rigidmatter, orsolids, through methods such assolid-state chemistry,quantum mechanics,crystallography,electromagnetism, andmetallurgy. It is the largest branch ofcondensed matter physics. Solid-state physics studies how the large-scale properties of solid materials result from theiratomic-scale properties. Thus, solid-state physics forms a theoretical basis ofmaterials science. Along withsolid-state chemistry, it also has direct applications in the technology oftransistors andsemiconductors.
Solid materials are formed from densely packed atoms, which interact intensely. These interactions produce the mechanical (e.g.hardness andelasticity),thermal,electrical,magnetic andoptical properties of solids. Depending on the material involved and the conditions in which it was formed, the atoms may be arranged in a regular, geometric pattern (crystalline solids, which includemetals and ordinarywater ice) or irregularly (anamorphous solid such as common windowglass).
The bulk of solid-state physics, as a general theory, is focused oncrystals. Primarily, this is because the periodicity ofatoms in a crystal — its defining characteristic — facilitates mathematical modeling. Likewise, crystalline materials often haveelectrical,magnetic,optical, ormechanical properties that can be exploited forengineering purposes.
The forces between the atoms in a crystal can take a variety of forms. For example, in a crystal ofsodium chloride (common salt), the crystal is made up ofionicsodium andchlorine, and held together withionic bonds. In others, the atoms shareelectrons and formcovalent bonds. In metals, electrons are shared amongst the whole crystal inmetallic bonding. Finally, the noble gases do not undergo any of these types of bonding. In solid form, the noble gases are held together withvan der Waals forces resulting from the polarisation of the electronic charge cloud on each atom. The differences between the types of solid result from the differences between their bonding.
The physical properties of solids have been common subjects of scientific inquiry for centuries, but a separate field going by the name of solid-state physics did not emerge until the1940s, in particular with the establishment of the Division of Solid State Physics (DSSP) within theAmerican Physical Society. The DSSP catered to industrial physicists, and solid-state physics became associated with the technological applications made possible by research on solids. By the early 1960s, the DSSP was the largest division of the American Physical Society.[1][2]
Large communities of solid state physicists also emerged inEurope afterWorld War II, in particular inEngland,Germany, and theSoviet Union.[3] In the United States and Europe, solid state became a prominent field through its investigations intosemiconductors,superconductivity,nuclear magnetic resonance, and diverse other phenomena. During the early Cold War, research in solid state physics was often not restricted to solids, which led some physicists in the 1970s and 1980s to found the field ofcondensed matter physics, which organized around common techniques used to investigate solids, liquids, plasmas, and other complex matter.[1] Today, solid-state physics is broadly considered to be the subfield of condensed matter physics, often referred to as hard condensed matter, that focuses on the properties of solids with regular crystal lattices.
Many properties of materials are affected by theircrystal structure. This structure can be investigated using a range ofcrystallographic techniques, includingX-ray crystallography,neutron diffraction andelectron diffraction.
The sizes of the individual crystals in a crystalline solid material vary depending on the material involved and the conditions when it was formed. Most crystalline materials encountered in everyday life arepolycrystalline, with the individual crystals being microscopic in scale, but macroscopicsingle crystals can be produced either naturally (e.g.diamonds) or artificially.
Real crystals featuredefects or irregularities in the ideal arrangements, and it is these defects that critically determine many of the electrical and mechanical properties of real materials.
Properties of materials such aselectrical conduction andheat capacity are investigated by solid state physics. An early model of electrical conduction was theDrude model, which appliedkinetic theory to theelectrons in a solid. By assuming that the material contains immobile positive ions and an "electron gas" of classical, non-interacting electrons, the Drude model was able to explain electrical andthermal conductivity and theHall effect in metals, although it greatly overestimated the electronic heat capacity.
Arnold Sommerfeld combined the classical Drude model withquantum mechanics in thefree electron model (or Drude-Sommerfeld model). Here, the electrons are modelled as aFermi gas, a gas of particles which obey the quantum mechanicalFermi–Dirac statistics. The free electron model gave improved predictions for the heat capacity of metals, however, it was unable to explain the existence ofinsulators.
Thenearly free electron model is a modification of the free electron model which includes a weak periodicperturbation meant to model the interaction between the conduction electrons and the ions in a crystalline solid. By introducing the idea ofelectronic bands, the theory explains the existence ofconductors,semiconductors andinsulators.
The nearly free electron model rewrites theSchrödinger equation for the case of a periodicpotential. The solutions in this case are known asBloch states. SinceBloch's theorem applies only to periodic potentials, and since unceasing random movements of atoms in a crystal disrupt periodicity, this use of Bloch's theorem is only an approximation, but it has proven to be a tremendously valuable approximation, without which most solid-state physics analysis would be intractable. Deviations from periodicity are treated by quantum mechanicalperturbation theory.
Modern research topics in solid-state physics include:
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