An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids; the exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge. An exciton can form; this excites an electron from the valence band into the conduction band. In turn, this leaves behind a positively charged electron hole; the electron in the conduction band is effectively attracted to this localized hole by the repulsive Coulomb forces from large numbers of electrons surrounding the hole and excited electron. This attraction provides a stabilizing energy balance; the exciton has less energy than the unbound electron and hole. The wavefunction of the bound state is said to be hydrogenic, an exotic atom state akin to that of a hydrogen atom. However, the binding energy is much smaller and the particle's size much larger than a hydrogen atom.
This is because of both the screening of the Coulomb force by other electrons in the semiconductor, the small effective masses of the excited electron and hole. The recombination of the electron and hole, i.e. the decay of the exciton, is limited by resonance stabilization due to the overlap of the electron and hole wave functions, resulting in an extended lifetime for the exciton. The electron and hole may have anti-parallel spins; the spins are coupled by the exchange interaction. In periodic lattices, the properties of an exciton show momentum dependence; the concept of excitons was first proposed by Yakov Frenkel in 1931, when he described the excitation of atoms in a lattice of insulators. He proposed that this excited state would be able to travel in a particle-like fashion through the lattice without the net transfer of charge. Excitons may be treated in two limiting cases, depending on the properties of the material in question. In materials with a small dielectric constant, the Coulomb interaction between an electron and a hole may be strong and the excitons thus tend to be small, of the same order as the size of the unit cell.
Molecular excitons may be located on the same molecule, as in fullerenes. This Frenkel exciton, named after Yakov Frenkel, has a typical binding energy on the order of 0.1 to 1 eV. Frenkel excitons are found in alkali halide crystals and in organic molecular crystals composed of aromatic molecules, such as anthracene and tetracene. In semiconductors, the dielectric constant is large. Electric field screening tends to reduce the Coulomb interaction between electrons and holes; the result is a Wannier exciton. Small effective mass of electrons, typical of semiconductors favors large exciton radii; as a result, the effect of the lattice potential can be incorporated into the effective masses of the electron and hole. Because of the lower masses and the screened Coulomb interaction, the binding energy is much less than that of a hydrogen atom on the order of 0.01eV. This type of exciton was named for Nevill Francis Mott. Wannier-Mott excitons are found in semiconductor crystals with small energy gaps and high dielectric constants, but have been identified in liquids, such as liquid xenon.
They are known as large excitons. In single-wall carbon nanotubes, excitons have both Wannier-Mott and Frenkel character; this is due to the nature of the Coulomb interaction between holes in one-dimension. The dielectric function of the nanotube itself is large enough to allow for the spatial extent of the wave function to extend over a few to several nanometers along the tube axis, while poor screening in the vacuum or dielectric environment outside of the nanotube allows for large binding energies. There is more than one band to choose from for the electron and the hole leading to different types of excitons in the same material. High-lying bands can be effective as femtosecond two-photon experiments have shown. At cryogenic temperatures, many higher excitonic levels can be observed approaching the edge of the band, forming a series of spectral absorption lines that are in principle similar to hydrogen spectral series. An intermediate case between Frenkel and Wannier excitons, charge-transfer excitons occur when the electron and the hole occupy adjacent molecules.
They occur in ionic crystals. Unlike Frenkel and Wannier excitons they display a static electric dipole moment. At surfaces it is possible for so called image states to occur, where the hole is inside the solid and the electron is in the vacuum; these electron-hole pairs can only move along the surface. Alternatively, an exciton may be an excited state of an atom, ion, or molecule, the excitation wandering from one cell of the lattice to another; when a molecule absorbs a quantum of energy that corresponds to a transition from one molecular orbital to another molecular orbital, the resulting electronic excited state is properly described as an exciton. An electron is said to be found in the lowest unoccupied orbital and an electron hole in the highest occupied molecular orbital, since they are found within the same molecular orbital manifold, the electron-hole state is said to be bound. Molecular excitons have characteristic lifetimes on the order of nanoseconds, after which the ground electronic state is restored and the m
Superconductivity is a phenomenon of zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials, called superconductors, when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 1911, in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon, it is characterized by the Meissner effect, the complete ejection of magnetic field lines from the interior of the superconductor during its transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood as the idealization of perfect conductivity in classical physics; the electrical resistance of a metallic conductor decreases as temperature is lowered. In ordinary conductors, such as copper or silver, this decrease is limited by impurities and other defects. Near absolute zero, a real sample of a normal conductor shows some resistance.
In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current through a loop of superconducting wire can persist indefinitely with no power source. In 1986, it was discovered that some cuprate-perovskite ceramic materials have a critical temperature above 90 K; such a high transition temperature is theoretically impossible for a conventional superconductor, leading the materials to be termed high-temperature superconductors. The cheaply-available coolant liquid nitrogen boils at 77 K, thus superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures. There are many criteria; the most common are: A superconductor can be Type I, meaning it has a single critical field, above which all superconductivity is lost and below which the magnetic field is expelled from the superconductor. These points are called vortices. Furthermore, in multicomponent superconductors it is possible to have combination of the two behaviours.
In that case the superconductor is of Type-1.5. It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional, otherwise. A superconductor is considered high-temperature if it reaches a superconducting state when cooled using liquid nitrogen – that is, at only Tc > 77 K) – or low-temperature if more aggressive cooling techniques are required to reach its critical temperature. Superconductor material classes include chemical elements, ceramics, superconducting pnictides or organic superconductors. Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature, critical field, critical current density at which superconductivity is destroyed. On the other hand, there is a class of properties. For instance, all superconductors have zero resistivity to low applied currents when there is no magnetic field present or if the applied field does not exceed a critical value; the existence of these "universal" properties implies that superconductivity is a thermodynamic phase, thus possesses certain distinguishing properties which are independent of microscopic details.
The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample. The resistance of the sample is given by Ohm's law as R = V / I. If the voltage is zero, this means. Superconductors are able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe, depending on the wire geometry and the temperature. In practice, currents injected in superconducting coils have persisted for more than 23 years in superconducting gravimeters. In such instruments, the measurement principle is based on the monitoring of the levitation of a superconducting niobium sphere with a mass of 4 grams.
In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice. The electrons are colliding with the ions in the lattice, during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat, the vibrational kinetic energy of the lattice ions; as a result, the energy carried by the current is being dissipated. This is the phenomenon of electrical Joule heating; the situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs; this pairing is caused by an attractive force between electrons from the exchange of phonons. Due to quantum mechanics, the energy spectr
Plasma is one of the four fundamental states of matter, was first described by chemist Irving Langmuir in the 1920s. Plasma can be artificially generated by heating or subjecting a neutral gas to a strong electromagnetic field to the point where an ionized gaseous substance becomes electrically conductive, long-range electromagnetic fields dominate the behaviour of the matter. Plasma and ionized gases have properties and display behaviours unlike those of the other states, the transition between them is a matter of nomenclature and subject to interpretation. Based on the surrounding environmental temperature and density ionized or ionized forms of plasma may be produced. Neon signs and lightning are examples of ionized plasma; the Earth's ionosphere is a plasma and the magnetosphere contains plasma in the Earth's surrounding space environment. The interior of the Sun is an example of ionized plasma, along with the solar corona and stars. Positive charges in ions are achieved by stripping away electrons orbiting the atomic nuclei, where the total number of electrons removed is related to either increasing temperature or the local density of other ionized matter.
This can be accompanied by the dissociation of molecular bonds, though this process is distinctly different from chemical processes of ion interactions in liquids or the behaviour of shared ions in metals. The response of plasma to electromagnetic fields is used in many modern technological devices, such as plasma televisions or plasma etching. Plasma may be the most abundant form of ordinary matter in the universe, although this hypothesis is tentative based on the existence and unknown properties of dark matter. Plasma is associated with stars, extending to the rarefied intracluster medium and the intergalactic regions; the word plasma comes from Ancient Greek πλάσμα, meaning'moldable substance' or'jelly', describes the behaviour of the ionized atomic nuclei and the electrons within the surrounding region of the plasma. Each of these nuclei are suspended in a movable sea of electrons. Plasma was first identified in a Crookes tube, so described by Sir William Crookes in 1879; the nature of this "cathode ray" matter was subsequently identified by British physicist Sir J.
J. Thomson in 1897; the term "plasma" was coined by Irving Langmuir in 1928. Lewi Tonks and Harold Mott-Smith, both of whom worked with Irving Langmuir in the 1920s, recall that Langmuir first used the word "plasma" in analogy with blood. Mott-Smith recalls, in particular, that the transport of electrons from thermionic filaments reminded Langmuir of "the way blood plasma carries red and white corpuscles and germs."Langmuir described the plasma he observed as follows: "Except near the electrodes, where there are sheaths containing few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons." Plasma is a state of matter in which an ionized gaseous substance becomes electrically conductive to the point that long-range electric and magnetic fields dominate the behaviour of the matter. The plasma state can be contrasted with the other states: solid and gas.
Plasma is an electrically neutral medium of unbound negative particles. Although these particles are unbound, they are not "free" in the sense of not experiencing forces. Moving charged particles generate an electric current within a magnetic field, any movement of a charged plasma particle affects and is affected by the fields created by the other charges. In turn this governs collective behaviour with many degrees of variation. Three factors define a plasma: The plasma approximation: The plasma approximation applies when the plasma parameter, Λ, representing the number of charge carriers within a sphere surrounding a given charged particle, is sufficiently high as to shield the electrostatic influence of the particle outside of the sphere. Bulk interactions: The Debye screening length is short compared to the physical size of the plasma; this criterion means that interactions in the bulk of the plasma are more important than those at its edges, where boundary effects may take place. When this criterion is satisfied, the plasma is quasineutral.
Plasma frequency: The electron plasma frequency is large compared to the electron-neutral collision frequency. When this condition is valid, electrostatic interactions dominate over the processes of ordinary gas kinetics. Plasma temperature is measured in kelvin or electronvolts and is, informally, a measure of the thermal kinetic energy per particle. High temperatures are needed to sustain ionisation, a defining feature of a plasma; the degree of plasma ionisation is determined by the electron temperature relative to the ionization energy, in a relationship called the Saha equation. At low temperatures and electrons tend to recombine into bound states—atoms—and the plasma will become a gas. In most cases the electrons are close enough to thermal equilibrium that their temperature is well-defined; because of the large difference in ma
Peter Joseph William Debye was a Dutch-American physicist and physical chemist, Nobel laureate in Chemistry. Born Petrus Josephus Wilhelmus Debije in Maastricht, Debye enrolled in the Aachen University of Technology in 1901. In 1905, he completed his first degree in electrical engineering, he published his first paper, a mathematically elegant solution of a problem involving eddy currents, in 1907. At Aachen, he studied under the theoretical physicist Arnold Sommerfeld, who claimed that his most important discovery was Peter Debye. In 1906, Sommerfeld received an appointment at Munich and took Debye with him as his assistant. Debye got his Ph. D. with a dissertation on radiation pressure in 1908. In 1910, he derived the Planck radiation formula using a method which Max Planck agreed was simpler than his own. In 1911, when Albert Einstein took an appointment as a professor at Prague, Debye took his old professorship at the University of Zurich, Switzerland; this was followed by moves to Utrecht in 1912, to Göttingen in 1913, to ETH Zurich in 1920, to University of Leipzig in 1927, in 1934 to Berlin, succeeding Einstein, he became director of the Kaiser Wilhelm Institute for Physics whose facilities were built only during Debye's era.
He was awarded the Lorentz Medal in 1935. From 1937 to 1939 he was the president of the Deutsche Physikalische Gesellschaft. In May 1914 he became member of the Royal Netherlands Academy of Arts and Sciences and in December of the same year he became foreign member. Debye was described as a martinet when it came to scientific principles, yet was always approachable and made time for his students, his personal philosophy emphasized a fulfillment of enjoyment in one's work. Debye was an avid trout fisherman and gardener, collector of cacti, was "always known to enjoy a nice cigar". While in Berlin as an assistant to Arnold Sommerfeld, Debye became acquainted with Mathilde Alberer. Mathidle was the daughter of the proprietor of the boarding house. Malthide would soon change her citizenship and in 1913, Debye married Mathilde Alberer. Debye would enjoy working in his rose garden with Mathilde Albere late into his years, they had a son, Peter P. Debye, a daughter, Mathilde Maria. Peter became a physicist and collaborated with Debye in some of his researches, had a son, a chemist.
His first major scientific contribution was the application of the concept of dipole moment to the charge distribution in asymmetric molecules in 1912, developing equations relating dipole moments to temperature and dielectric constant. In consequence, the units of molecular dipole moments are termed debyes in his honor. In 1912, he extended Albert Einstein's theory of specific heat to lower temperatures by including contributions from low-frequency phonons. See Debye model. In 1913, he extended Niels Bohr's theory of atomic structure, introducing elliptical orbits, a concept introduced by Arnold Sommerfeld. In 1914–1915, Debye calculated the effect of temperature on X-ray diffraction patterns of crystalline solids with Paul Scherrer. In 1923, together with his assistant Erich Hückel, he developed an improvement of Svante Arrhenius' theory of electrical conductivity in electrolyte solutions. Although an improvement was made to the Debye–Hückel equation in 1926 by Lars Onsager, the theory is still regarded as a major forward step in our understanding of electrolytic solutions.
In 1923, Debye developed a theory to explain the Compton effect, the shifting of the frequency of X-rays when they interact with electrons. From 1934 to 1939 Debye was director of the physics section of the prestigious Kaiser Wilhelm Institute in Berlin. From 1936 onwards he was professor of Theoretical Physics at the Frederick William University of Berlin; these positions were held during the years that Adolf Hitler ruled Nazi Germany and, from 1938 onward, Austria. In 1939 Debye traveled to the United States to deliver the Baker Lectures at Cornell University in Ithaca, New York. After leaving Germany in early 1940, Debye became a professor at Cornell, chaired the chemistry department for 10 years, became a member of Alpha Chi Sigma. In 1946 he became an American citizen. Unlike the European phase of his life, where he moved from city to city every few years, in the United States Debye remained at Cornell for the remainder of his career, he continued research until his death. Much of Debye's work at Cornell concerned the use of light-scattering techniques to determine the size and molecular weight of polymer molecules.
This started as a result of his research during World War II on synthetic rubber, but was extended to proteins and other macromolecules. In April 1966, Debye suffered a heart attack, in November of that year a second one proved fatal, he is buried in the Pleasant Grove Cemetery. In January 2006, a book appeared in The Netherlands, written by Sybe Rispens, entitled Einstein in the Netherlands. One chapter of this book discusses the relationship between Albert Debye. Rispens discovered documents that, as he believed, were new and proved that, during his directorship of the Kaiser Wilhelm Society, Debye was involved in cleansing German science institutions of Jewish and other "non-Aryan elements". Rispens records that on December 9, 1938, Debye wrote in his capacity as chairman of the Deutsche Physikalische Gesellschaft to all the members of the DPG: In light of the current situation, membership by German Jews as stipulated by the Nuremberg laws, of the Deutsche Physikalische G
Heike Kamerlingh Onnes
Professor Heike Kamerlingh Onnes FRSFor HFRSE FCS was a Dutch physicist and Nobel laureate. He exploited the Hampson–Linde cycle to investigate how materials behave when cooled to nearly absolute zero and to liquefy helium for the first time, in 1908, he was the discoverer of superconductivity in 1911. Kamerlingh Onnes was born in Netherlands, his father, Harm Kamerlingh Onnes, was a brickworks owner. His mother was Anna Gerdina Coers of Arnhem. In 1870, Kamerlingh Onnes attended the University of Groningen, he studied under Robert Bunsen and Gustav Kirchhoff at the University of Heidelberg from 1871 to 1873. Again at Groningen, he obtained his masters in 1878 and a doctorate in 1879, his thesis was Nieuwe bewijzen voor de aswenteling der aarde. From 1878 to 1882 he was assistant to Johannes Bosscha, the director of the Delft Polytechnic, for whom he substituted as lecturer in 1881 and 1882, he had one child, named Albert. His brother Menso Kamerlingh Onnes was a well known painter, while his sister Jenny married another famous painter, Floris Verster.
From 1882 to 1923 Kamerlingh Onnes served as professor of experimental physics at the University of Leiden. In 1904 he founded a large cryogenics laboratory and invited other researchers to the location, which made him regarded in the scientific community; the laboratory is known now as Kamerlingh Onnes Laboratory. Only one year after his appointment as professor he became member of the Royal Netherlands Academy of Arts and Sciences. On 10 July 1908, he was the first to liquefy helium, using several precooling stages and the Hampson–Linde cycle based on the Joule–Thomson effect; this way he lowered the temperature to the boiling point of helium. By reducing the pressure of the liquid helium he achieved a temperature near 1.5 K. These were the coldest temperatures achieved on earth at the time; the equipment employed is at the Boerhaave Museum in Leiden. In 1911 Kamerlingh Onnes measured the electrical conductivity of pure metals at low temperatures; some scientists, such as William Thomson, believed that electrons flowing through a conductor would come to a complete halt or, in other words, metal resistivity would become infinitely large at absolute zero.
Others, including Kamerlingh Onnes, felt that a conductor's electrical resistance would decrease and drop to nil. Augustus Matthiessen said that when the temperature decreases, the metal conductivity improves or in other words, the electrical resistivity decreases with a decrease of temperature. On 8 April 1911, Kamerlingh Onnes found that at 4.2 K the resistance in a solid mercury wire immersed in liquid helium vanished. He realized the significance of the discovery, he reported that "Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state". He published more articles about the phenomenon referring to it as "supraconductivity" and, only adopting the term "superconductivity". Kamerlingh Onnes received widespread recognition for his work, including the 1913 Nobel Prize in Physics for "his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium"; some of the instruments he devised for his experiments can be seen at the Boerhaave Museum in Leiden.
The apparatus he used to first liquefy helium is on display in the lobby of the physics department at Leiden University, where the low-temperature lab is named in his honor. His student and successor as director of the lab Willem Hendrik Keesom was the first person, able to solidify helium, in 1926; the former Kamerlingh Onnes laboratory building is the Law Faculty at Leiden University and is known as "Kamerlingh Onnes Gebouw" shortened to "KOG". The current science faculty has a "Kamerlingh Onnes Laboratorium" named after him, as well as a plaque and several machines used by Kamerling Onnes in the main hall of the physics department; the Kamerlingh Onnes Award was established in his honour, recognising further advances in low-temperature science. The Onnes effect referring to the creeping of superfluid helium is named in his honor; the crater Kamerlingh Onnes on the Moon is named after him. Onnes is credited with coining the word "enthalpy". Onnes' discovery of superconductivity was named an IEEE Milestone in 2011.
Matteucci Medal Rumford Medal Nobel Prize in Physics Franklin Medal Kamerlingh Onnes, H. "Nieuwe bewijzen voor de aswenteling der aarde." Ph. D. dissertation. Groningen, Netherlands, 1879. Kamerlingh Onnes, H. "Algemeene theorie der vloeistoffen." Amsterdam Akad. Verhandl. Kamerlingh Onnes, H. "On the Cryogenic Laboratory at Leyden and on the Production of Very Low Temperature." Comm. Phys. Lab. Univ. Leiden. Kamerlingh Onnes, H. "Théorie générale de l'état fluide." Haarlem Arch. Neerl.. Kamerlingh Onnes, H. "Further experiments with liquid helium. C. On the change of electric resistance of pure metals at low temperatures, etc. IV; the resistance of pure mercury at helium temperatures." Comm. Phys. Lab. Univ. Leiden. 120b, 1911. Kamerlingh Onnes, H. "Further experiments with liquid helium. D. On the change of electric resistance of pure metals at low temperatures, etc. V; the disappearance of the resistance of mercury." Comm. Phys. Lab. Uni