Editing Solid-state physics

{{short description|Branch of physics focused on matter in the solid state}}
{{redirect|State theory|theories in political science|State (polity)}}
'''Solid-state physics''' is the study of rigid [[matter]], or [[solid]]s, through methods such as [[solid-state chemistry]], [[quantum mechanics]], [[crystallography]], [[electromagnetism]], and [[metallurgy]]. It is the largest branch of [[condensed matter physics]]. Solid-state physics studies how the large-scale properties of solid materials result from their [[atom]]ic-scale properties. Thus, solid-state physics forms a theoretical basis of [[materials science]]. Along with [[solid-state chemistry]], it also has direct applications in the technology of [[transistor]]s and [[semiconductor]]s.

==Background==
Solid materials are formed from densely packed atoms, which interact intensely. These interactions produce the mechanical (e.g. [[hardness]] and [[Elasticity (physics)|elasticity]]), [[Heat conduction|thermal]], [[Electrical conduction|electrical]], [[Magnetism|magnetic]] and [[Crystal optics|optical]] 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 ([[crystal|crystalline solids]], which include [[metal]]s and ordinary [[Ice|water ice]]) or irregularly (an [[amorphous solid]] such as common window [[glass]]).

The bulk of solid-state physics, as a general theory, is focused on [[crystal]]s. Primarily, this is because the periodicity of [[atom]]s in a crystal — its defining characteristic — facilitates mathematical modeling. Likewise, crystalline materials often have [[electrical engineering|electrical]], [[magnetism|magnetic]], [[optics|optical]], or [[mechanical engineering|mechanical]] properties that can be exploited for [[engineering]] purposes.

The forces between the atoms in a crystal can take a variety of forms. For example, in a crystal of [[sodium chloride]] (common salt), the crystal is made up of [[ion]]ic [[sodium]] and [[chlorine]], and held together with [[ionic bond]]s. In others, the atoms share [[electron]]s and form [[covalent bond]]s. In metals, electrons are shared amongst the whole crystal in [[metallic bond]]ing. Finally, the noble gases do not undergo any of these types of bonding. In solid form, the noble gases are held together with [[van der Waals force]]s 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.

==History==
{{See also|Timeline of condensed matter physics}}
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 the [[1940s]], in particular with the establishment of the Division of Solid State Physics (DSSP) within the [[American 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.<ref name="martin-pip">{{cite journal|last=Martin|first=Joseph D. |title=What's in a Name Change? Solid State Physics, Condensed Matter Physics, and Materials Science|journal=Physics in Perspective|date=2015|volume=17|issue=1|doi=10.1007/s00016-014-0151-7|pages=3–32|bibcode = 2015PhP....17....3M |s2cid=117809375 |url=http://dro.dur.ac.uk/29168/1/29168.pdf |archive-url=https://web.archive.org/web/20191214125047/http://dro.dur.ac.uk/29168/1/29168.pdf |archive-date=2019-12-14 |url-status=live }}</ref><ref name="Hoddeson-1992">{{cite book|last=Hoddeson|first=Lillian|title=Out of the Crystal Maze: Chapters from The History of Solid State Physics|year=1992|publisher=Oxford University Press|isbn=9780195053296|url=https://books.google.com/books?id=WCpPPHhMdRcC&pg=PA29|display-authors=etal}}</ref>

Large communities of solid state physicists also emerged in [[Europe]] after [[World War II]], in particular in [[England]], [[Germany]], and the [[Soviet Union]].<ref name="hoffmann-pss">{{cite journal|last=Hoffmann|first=Dieter|title=Fifty Years of ''Physica Status Solidi'' in Historical Perspective|journal=Physica Status Solidi B|date=2013|volume=250|issue=4|doi=10.1002/pssb.201340126|pages=871–887|bibcode=2013PSSBR.250..871H|s2cid=122917133 }}</ref> In the United States and Europe, solid state became a prominent field through its investigations into [[semiconductor]]s, [[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 of [[condensed matter physics]], which organized around common techniques used to investigate solids, liquids, plasmas, and other complex matter.<ref name="martin-pip"/> 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.

==Crystal structure and properties==
[[File:Fcc lattice 4.jpg|190px|thumb|An example of a [[cubic crystal system|cubic lattice]]]]
Many properties of materials are affected by their [[crystal structure]]. This structure can be investigated using a range of [[Crystallography|crystallographic]] techniques, including [[X-ray crystallography]], [[neutron diffraction]] and [[electron 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 are [[polycrystal]]line, with the individual crystals being microscopic in scale, but macroscopic [[single crystal]]s can be produced either naturally (e.g. [[diamond]]s) or artificially.

Real crystals feature [[Crystallographic defect|defects]] or irregularities in the ideal arrangements, and it is these defects that critically determine many of the electrical and mechanical properties of real materials.

==Electronic properties==
Properties of materials such as [[electrical conduction]] and [[heat capacity]] are investigated by solid state physics. An early model of electrical conduction was the [[Drude model]], which applied [[kinetic theory of gases|kinetic theory]] to the [[electron]]s 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 and [[thermal conductivity]] and the [[Hall effect]] in metals, although it greatly overestimated the electronic heat capacity.

[[Arnold Sommerfeld]] combined the classical Drude model with [[quantum mechanics]] in the [[free electron model]] (or Drude-Sommerfeld model). Here, the electrons are modelled as a [[Fermi gas]], a gas of particles which obey the quantum mechanical [[Fermi–Dirac statistics]]. The free electron model gave improved predictions for the heat capacity of metals, however, it was unable to explain the existence of [[Insulator (electricity)|insulators]].

The [[nearly free electron model]] is a modification of the free electron model which includes a weak periodic [[perturbation theory (quantum mechanics)|perturbation]] meant to model the interaction between the conduction electrons and the ions in a crystalline solid. By introducing the idea of [[Electronic band structure|electronic bands]], the theory explains the existence of [[electrical conductor|conductors]], [[semiconductor]]s and [[Insulator (electricity)|insulators]].

The nearly free electron model rewrites the [[Schrödinger equation]] for the case of a periodic [[potential]]. The solutions in this case are known as [[Bloch state]]s. Since [[Bloch'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 mechanical [[perturbation theory (quantum mechanics)|perturbation theory]].

==Modern research==
Modern research topics in solid-state physics include:
*[[High-temperature superconductivity]]
*[[Quasicrystal]]s
*[[Spin glass]]
*[[Strongly correlated material]]s
*[[Two-dimensional materials]]
*[[Nanomaterials]]

==See also==
{{portal|Physics}}
*[[Condensed matter physics]]
*[[Crystallography]]
*[[Nuclear spectroscopy]]
*[[Solid mechanics]]

==References==
{{reflist}}

==Further reading==
*[[Neil Ashcroft|Neil W. Ashcroft]] and [[N. David Mermin]], ''Solid State Physics'' (Harcourt: Orlando, 1976).
*[[Charles Kittel]], ''[[Introduction to Solid State Physics]]'' (Wiley: New York, 2004).
*H. M. Rosenberg, ''The Solid State'' (Oxford University Press: Oxford, 1995).
*[[Steven H. Simon]], ''The Oxford Solid State Basics'' (Oxford University Press: Oxford, 2013).
*''Out of the Crystal Maze. Chapters from the History of Solid State Physics'', ed. Lillian Hoddeson, Ernest Braun, Jürgen Teichmann, Spencer Weart (Oxford: Oxford University Press, 1992).
*M. A. Omar, ''Elementary Solid State Physics'' (Revised Printing, Addison-Wesley, 1993).
*{{cite book |last=Hofmann |first=Philip |date= 2015-05-26|title=Solid State Physics |edition=2 |url=https://philiphofmann.net/solid-state-physics-book/ |publisher=Wiley-VCH |isbn=978-3527412822 }}

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[[Category:Condensed matter physics]]
[[Category:Metallurgy]]