Plasmons

Type of physical science: Condensed matter physics

Field of study: Solids

A plasmon is a quantum, or discrete amount, of energy that moves through the interatomic electron plasma of a crystal. Plasmons transfer energy through the crystalline lattice structures of many materials, especially metals and semiconductors.

Overview

The matter of which the entire universe is composed can exist in four distinct phases or states: solid, liquid, gas, and plasma. The last phase, the plasma, is rarely found in large quantities on Earth, but it probably comprises the bulk of the universe's matter. A plasma is a gaseous cloud of charged particles, usually negatively charged subatomic particles called electrons, positively charged subatomic protons, positively charged atoms or molecules (cations), and negatively charged atoms or molecules (anions). Examples of plasmas include the earth's thermosphere and any star such as the sun.

A plasmon, however, is a quantum (discrete amount) of energy that moves longitudinally through the interatomic spaces of a solid crystal. Plasmons are transmitted through the electron plasma, which is located in between the atoms of the crystalline solid. As such, it is important to note that, while the four states of matter are distinct in terms of their respective physical properties, the four phases are interdependent and can coexist simultaneously for any type of matter.

The matter of the universe consists of ninety-two naturally occurring elements, any one of which can exist in any of the four phases, depending upon the amount of energy that is available in the immediate environment. Each element consists of atoms, all of which are identical for that particular element. An atom consists of specific numbers of three types of subatomic particles: positively charged protons, negatively charged electrons, and zero-charged neutrons. Protons and neutrons mass at the center, or nucleus, of the atom. Electrons orbit the nucleus in distinct energy levels, each energy level possessing precise quanta, or amounts, of energy. For any atom, a maximum of two electrons can occupy the first, innermost energy level that is located near the nucleus, according to quantum mechanical theory. A maximum of eight electrons can occupy the second energy level. A maximum of eighteen electrons can occupy the third energy level, and so forth.

When atomic energy levels are unfilled, when they lack the maximum number of electrons, they become slightly unstable. To overcome this instability, atoms share electrons and, therefore, covalently bond together to produce molecules and compounds which number in the millions of different types. Two atoms of nitrogen (N) covalently bond to produce molecular nitrogen (N₂). Two hydrogen (H) atoms and one oxygen (O) atom covalently bond to produce water (H₂O). One carbon (C) atom and four hydrogen atoms covalently bond to produce methane (CH₄). Six carbon atoms, twelve hydrogen atoms, and six oxygen atoms covalently bond to produce glucose [H-O-H]. The list of such substances is enormous.

Some substances consist of atoms that are covalently bonded together into regular, patterned arrangements, forming sheets or rows of repeating atoms. Such substances usually exist in the solid state and form what is called a crystal. The patterned arrangement of atoms forming the crystal is skeleton-like and is appropriately called the crystalline lattice. Often, the rows of atoms will not be perfectly aligned or impurity atoms of other elements will be present, thus creating a lattice defect.

Within the lattice structure of a crystal, neighboring atoms are covalently bonded to one another, thereby filling their respective outermost energy levels with needed electrons and thereby becoming structurally stable. From the periodic table of the elements, group IVA elements (carbon, silicon, germanium, tin, and lead) tend to form very stable, ordered crystals that exist as solids at room temperature. Atoms of each of these elements share one major characteristic: They always form four covalent bonds, the maximum possible number of bonds for most elements. Atoms of group IVA elements always form four covalent bonds because they all lack four electrons in their outermost energy levels.

Examples of crystals include the carbon-based diamond and graphite, sheets of any metal (for example, copper, iron, gold), minerals (for example, silicon dioxide, also known as quartz), semiconductors (silicon, germanium), and superconductors (yttrium-barium-copper oxide). Thousands of atoms covalently bond to form extensive, orderly crystalline lattices in each of these substances.

Both graphite and diamond crystals consist of endless arrays of covalently bonded carbon atoms. In graphite, the carbon atoms form a repeating hexagonal (six-sided) pattern within a plane. In diamond, the arrangement of carbon atoms is shifted slightly into a seemingly more disordered three-dimensional pattern, with every carbon atom being at the center of a tetrahedron (pyramid). The two-carbon crystals are interconvertible based upon experimental changes in temperature and pressure.

Metals, which are excellent electrical conductors with low resistance, and semiconductors, which also make fine conductors although with higher resistance, both consist of relatively pure concentrations of one element. Atoms of this element are covalently bonded together to form crystals, which resemble the carbon-based diamond and graphite crystal lattices.

The covalent bonding patterns between the atoms of any crystal are critical in establishing energy transmission for crystalline lattices of the solid substance.

If the covalent bonding between the atoms of a crystal is incomplete because of the shortage of needed outer-energy-level electrons, then "holes" develop in the crystal. A hole is an electronless failed covalent bond between two atoms. The presence of holes in a crystal will give the solid a net positive (p) charge since negatively charged electrons are missing. On the other hand, if crystalline covalent bonding is complete and excess electrons are migrating through the crystal lattice, then the solid has a net negative (n) charge. These extra electrons are termed "carriers" because they can assist electrical conductivity through the crystal lattice. Both holes and excess electron carriers contribute to electrical conductivity.

On the quantum mechanical level, plasmons and phonons are the units of energy that are associated with the electron plasma transmission of electricity. Plasmons, phonons, and the identical photons each are distinct quanta (amounts) of energy. A quantum of energy is defined by the equation E = hf, where E is the quantum of energy being measured, h is Planck's constant (6.626196 x 10^-34 Joule per second), and f is the vibrational frequency of the energy quantum.

When a solid crystal is placed within an electric field (that is, negative to positive pole orientation) or is bombarded with electromagnetic radiation, such as light or X rays, the crystal lattice vibrates. The quantum of energy that is associated with each crystalline vibration is called a phonon. If the energy that is imparted to the crystalline lattice is directed, then the quantum of energy that is associated with each longitudinal wave of energy/vibration through the lattice is called a plasmon.

Like all quanta of energy, the plasmon exhibits both particulate and wavelike properties, the particle-wave duality of nature. As particles, plasmons are propagated through the crystalline lattice via electron carriers and holes. Electron carriers move rapidly through the interatomic spaces of the crystal, in the process carrying the energy quanta of the plasmons.

Therefore, plasmons are intricately associated with the electron plasma phase that exists within the interatomic spaces of the crystalline solid phase. Likewise, holes will propagate plasmons as electrons occupy former holes, thereby creating new holes elsewhere within the interatomic spaces of the crystalline lattice. Still, even with holes, the mechanism of particulate plasmon transfer is the electron plasma.

From a wavelike interpretation, plasmons can be appreciated as distinct energy-carrying units. The transmission of energy through a crystal can be imagined as a series of longitudinal waves much like ocean waves. For ocean waves or for electromagnetic waves such as plasmons, the high point of a wave is the crest, whereas the low point is the trough.

When a wave moves forward through water, it is energy that is being carried forward, not water.

The water is simply displaced up and down in the same area. The water is only the medium for energy transferrance.

The same phenomenon is true for crystalline solids. The solid atoms are the physical medium, whereas the plasmons of the electron plasma are the energy that is being transferred through the physical medium of the crystal. A plasmon is the quantum energy content of any longitudinal wave (disturbance or vibration) that is moving through the crystal lattice via electron carriers and electron holes. The primary determinant of the plasmon's energy is the frequency of the longitudinal wave. Frequency is defined as the number of longitudinal waves passing a given point in space per second. The unit of frequency is hertz. Another means of looking at frequency is the speed of a wave. The faster the movement of a plasmon wave, or the more waves passing a point per second, the higher the energy content of each wave. The physical conducting properties of a crystalline solid determine the energy content of plasmon transmission.

Applications

The phenomenon of energy transmission by plasmons is the primary basis of electrical conduction. Consequently, plasmons occur in every type of electrical phenomenon, in particular the energy-demanding machines devised by humans. All power lines, electrical devices, and machines incorporate crystalline solids for electrical conductivity. These items include the copper found in household electrical wiring, the transistors in radios and televisions, and the silicon chips used to manufacture computer microprocessors. When these and other devices are operational, plasmons flow through them. Plasmons move through any substance that is capable of conducting electricity. This includes shafts of metal such as golf clubs and communication/ electrical towers. It also includes living organisms. In most animal species, electrical conduction along millions of specialized cells called neurons employ plasmons, thus contributing to a multitude of essential bodily functions, including movement, pain, sight, hearing, smell, memory, and intelligence.

Plasmon transmissibility through all of these materials depends upon the crystalline lattice arrangement of the atoms composing the material. In most cases, the crystal is a metal having excellent conductivity and very little resistance. In many machines, however, conducting materials with properties in between those of metals and nonmetals are desired. For computer microprocessors and transistors, the semiconducting periodic group IVA elements have proved to be very valuable plasmon-conducting crystals. Composites of group IIIA and group VA elements, such as gallium arsenide, also make excellent semiconductors.

One of the principal components of metal conductors and group IVA semiconductors is the p-n junction, where positively charged (p) and negatively charged (n) crystal lattices come into contact. The p-n junction is one of the fundamental components of the transistor, a device first developed in 1947 by physicists William Shockley, John Bardeen, and Walter H. Brattain of Bell Telephone Laboratories. These three scientists shared the 1956 Nobel Prize in Physics.

In a p-n junction, two crystals of, say, silicon are placed next to each other. One silicon crystal is a p-type semiconductor because it has a net positive charge caused by the presence of periodic group IIIA (boron, aluminum, gallium, indium, tellurium) impurities that are covalently bonded within the silicon crystalline lattice. As a result, there will be electron holes in this p-type semiconductor. The other silicon crystal is an n-type semiconductor because it has a net negative charge caused by the presence of periodic group VA (nitrogen, phosphorus, arsenic, antimony, bismuth) impurities that are covalently bonded within the silicon crystalline lattice. As a result, there will be excess electron carriers in this n-type semiconductor. When an electric field is applied to the p-n junction, the electron flow, and hence the plasmon flow, will be from n-type semiconductor to p-type semiconductor because negatively charged electrons are attracted to positively charged materials.

The understanding of plasmon transmission and crystalline transistors replaced vacuum tubes as electrically conducting devices in many devices such as radios and televisions.

Miniaturization of transistors resulted in integrated circuits, where hundreds of thousands of transistors could be imprinted on a single coin-sized silicon chip, thus creating the microprocessors used in personal computers, supercomputers, typewriters, watches, video cameras, and the like. These crystalline solid and plasmonrelated breakthroughs have allowed a reduction in size and an increase in speed and efficiency for many appliances.

Research into ceramic crystalline solids has led to the development of superconductors, complex composite materials that are powerful conductors, that have negligible resistance, and that can conduct electricity at temperatures well above absolute zero (for example, at 125 Kelvins for tellurium-calcium-barium-copper oxide). Physicists Karl Alexander Muller and J. Georg Bednorz of IBM received the 1987 Nobel Prize in Physics for their discovery of high-temperature ceramic superconductors. Superconductors, with their plasmon-conducting ceramic crystalline lattices, have tremendous potential, particularly in the storage of energy, although much work still needs to be done on these materials before their impact can be fully realized.

Context

For all practical purposes, a plasmon is a photon. A photon is a quantum of electromagnetic energy that is moving through the vacuum of space or through a gas such as the atmosphere. Depending upon its frequency, the photon could be one of several types of electromagnetic radiation such as radio, television, visible light, or X rays. When the photon strikes a solid surface, it may be reflected or absorbed by that surface. If the photon is absorbed by a solid, its energy will be imparted to the crystalline lattice. Electrons orbiting contact atoms of the crystalline lattice will be energized by absorbed photons to higher-energy levels or to escape velocity. Escaping electrons carry the absorbed energy from atom to atom or from hole to hole through the crystalline lattice. It is here that the imparted energy behaves as plasmons.

Therefore, plasmons are quanta of energy that are passing through the electron plasma of the interatomic spaces of the solid crystalline lattice. Furthermore, plasmons are the continuation of the photons and phonons that initially were transferred to the crystal.

Energy has always dominated the universe and is transmitted everywhere. Virtually every solid absorbs energy when it is heated. On a hot summer day, one can imagine plasmons flowing through the crystal lattices of such materials as roofing, pavement, and the metal framework of automobiles, although such flow is extremely slow in the absence of an electric field.

For the conducting materials (for example, copper and aluminum) found within household appliances, radios, televisions, generators, power lines, and major power stations, the application of electric fields accelerates plasmon flow through the conducting crystalline lattices at speeds approaching that of light (300 million meters per second). Metals make excellent conductors, whereas nonmetals are insulators (that is, nonconductors because of electrical resistivity). Periodic group IVA semi- metal elements, such as silicon and germanium, and composites of group IIIA metals/group VA nonmetals make good conductors, although they are not as good as true metal conductors because of their higher electrical resistivity. Semiconductors have revolutionized electrical conductivity and the miniaturization of various high technological appliances, most notably computers since the 1960's.

The immediate future potential of plasmons may be within ceramics, the crystalline composites of superconductors. In superconductors, the longitudinal wave propagation of plasmons through the interatomic spaces of the crystalline lattice occurs rapidly with essentially no resistance. The original superconductors required supercooling to temperatures near absolute zero. Beginning in the 1980's, physicists began developing relatively high-temperature superconductors, ceramic materials that can superconduct plasmons at temperatures above 100 Kelvins. High-temperature superconducting ceramics are part of continuous advances in energy transfer physics.

Within living organisms and their by-products, research is ongoing into organic conductors and superconductors. Organic conductors include the molecules involved in the electrical transmission of nerve impulses along billions of neurons running throughout the animal body. Plasmon flow is found here as well as in cellular energy generating processes. In plants, the chlorophyll molecule is a beautiful example of a natural semiconductor that utilizes plasmon flow. Some biophysicists are exploring the possibility of interfacing natural and synthetic semiconductors such as the growth of human brain neurons on silicon chips/microprocessors.

Advances in this field could have tremendous medical applications, particularly in the areas of neurophysiology and cardiology.

Plasmons represent a phenomenon of quantum mechanical theory that describes the flow of energy through the crystalline solid state. They greatly resemble the longitudinal flow of energy that occurs in ocean waves or any waves moving through water in a given direction, a situation in which the translational medium (for example, water, solid crystalline lattice) vibrates up and down but remains in the same final resting place as it started. Plasmons flow through all things.

Principal terms

COVALENT BOND: an attractive force between two atoms that is brought about by the sharing of outermost-energy-level electrons between the atoms

CRYSTAL: an orderly arrangement of atoms or molecules to produce a regular solid having definite dimensions and physical properties

ELECTRON: a negatively charged, spin 1/2 subatomic particle that usually orbits the nucleus of an atom within distinct quantized energy levels

HOLE: a gap or electronless region between two atoms of a crystal caused by a shortage of electrons in the covalent bonding structure of the crystal

LATTICE DEFECT: an imperfection in the orderly arrangement of the atoms comprising a crystal; often caused by impurities and/or the staggered alignment of rows of atoms

PHONON: a quantum of energy associated with the disturbance, or vibration, of a crystalline lattice

PLASMA: a gaseous fourth state of matter that is a cloud of ionized particles, usually electrons and ions

PLASMON: a quantum of energy moving longitudinally through a crystal lattice as a wave

P-N JUNCTION: the contact area between two regions of a semiconducting material, a positively charged region (p) and a negatively charged region (n)

SEMICONDUCTOR: a material, usually a periodic group IVA element (for example, silicon, germanium) or an equivalent composite, that conducts energy at a level in between metal conductors and nonmetal resistors

Bibliography

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Jerome, D. "The Physics of Organic Superconductors." SCIENCE 252 (June 14, 1991): 1509-1514. This detailed review article is an excellent discussion of solid crystal lattices and electron plasmas between crystal atoms. The quantum mechanical properties of wave propagation through various semiconducting and superconducting materials are considered, with special emphasis being placed upon organic solids. Crystal structures are described, and an up-to-date reference list is provided for further in-depth research.

Mulligan, Joseph F. INTRODUCTORY COLLEGE PHYSICS. 2d ed.

New York: McGraw-Hill, 1991. This excellent introduction to physics for the beginning student provides clear discussions of key concepts, historical sketches, numerous examples and illustrations, and many problem sets for practice. Mathematical explanations involve simple algebra and trigonometry. Chapter 22, "Electromagnetic Waves," and chapter 30, "Solid State Physics," describe energy transference and the properties of semiconducting crystals. Several other chapters are devoted to electrical properties and measurement.

Serway, Raymond A., and Jerry S. Faughn. COLLEGE PHYSICS.

Philadelphia: Saunders College Publishing, 1989. This strong introduction to physics is designed for the beginning student. All major topics are discussed in great detail, with many useful illustrations and examples. Numerous problem sets are provided for practice; the mathematics is very manageable. Chapter 29, "Quantum Physics," describes the dual wave-particle properties of electromagnetic energy. Chapter 31, "Molecules and Solids,"

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Princeton, N.J.: D. Van Nostrand, 1950. In this very detailed work, the inventor of the transistor, Nobel laureate William Shockley, describes the theories and properties behind electrical conductance through semiconductors. The early chapters of the book provide a strong introduction, whereas later chapters become very specialized. Shockley considers crystal structures, electron plasmas, and quantum mechanics in this exhaustive treatment of the subject.

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New York: McGraw-Hill, 1961. In this well-organized introduction to the theory and properties of transistors, a leading semiconductor expert describes all aspects of transistor structure and function. Part 1, "Introductory Concepts," contains four outstanding chapters in which transistor types, crystal structures, hole and electron conduction, and p-n junctions are explained. Later chapters deal with more specialized aspects of transistors.

Electrons and Atoms

Radio and Television

Defects in Solids

Thermal Properties of Solids