Bronze is an alloy consisting of copper with about 12–12.5% tin and with the addition of other metals and sometimes non-metals or metalloids such as arsenic, phosphorus or silicon. These additions produce a range of alloys that may be harder than copper alone, or have other useful properties, such as stiffness, ductility, or machinability; the archeological period in which bronze was the hardest metal in widespread use is known as the Bronze Age. The beginning of the Bronze Age in India and western Eurasia is conventionally dated to the mid-4th millennium BC, to the early 2nd millennium BC in China; the Bronze Age was followed by the Iron Age starting from about 1300 BC and reaching most of Eurasia by about 500 BC, although bronze continued to be much more used than it is in modern times. Because historical pieces were made of brasses and bronzes with different compositions, modern museum and scholarly descriptions of older objects use the generalized term "copper alloy" instead; the word bronze is borrowed from French bronze, itself borrowed from Italian bronzo "bell metal, brass" from either, bróntion, back-formation from Byzantine Greek brontēsíon from Brentḗsion ‘Brindisi’, reputed for its bronze.
Bronze tools, weapons and building materials such as decorative tiles were harder and more durable than their stone and copper predecessors. Bronze was made out of copper and arsenic, forming arsenic bronze, or from or artificially mixed ores of copper and arsenic, with the earliest artifacts so far known coming from the Iranian plateau in the 5th millennium BC, it was only that tin was used, becoming the major non-copper ingredient of bronze in the late 3rd millennium BC. Tin bronze was superior to arsenic bronze in that the alloying process could be more controlled, the resulting alloy was stronger and easier to cast. Unlike arsenic, metallic tin and fumes from tin refining are not toxic; the earliest tin-alloy bronze dates to 4500 BC in a Vinča culture site in Pločnik. Other early examples date to the late 4th millennium BC in Egypt and some ancient sites in China and Mesopotamia. Ores of copper and the far rarer tin are not found together, so serious bronze work has always involved trade.
Tin sources and trade in ancient times had a major influence on the development of cultures. In Europe, a major source of tin was the British deposits of ore in Cornwall, which were traded as far as Phoenicia in the eastern Mediterranean. In many parts of the world, large hoards of bronze artifacts are found, suggesting that bronze represented a store of value and an indicator of social status. In Europe, large hoards of bronze tools socketed axes, are found, which show no signs of wear. With Chinese ritual bronzes, which are documented in the inscriptions they carry and from other sources, the case is clear; these were made in enormous quantities for elite burials, used by the living for ritual offerings. Though bronze is harder than wrought iron, with Vickers hardness of 60–258 vs. 30–80, the Bronze Age gave way to the Iron Age after a serious disruption of the tin trade: the population migrations of around 1200–1100 BC reduced the shipping of tin around the Mediterranean and from Britain, limiting supplies and raising prices.
As the art of working in iron improved, iron improved in quality. As cultures advanced from hand-wrought iron to machine-forged iron, blacksmiths learned how to make steel. Steel holds a sharper edge longer. Bronze was still used during the Iron Age, has continued in use for many purposes to the modern day. There are many different bronze alloys, but modern bronze is 88% copper and 12% tin. Alpha bronze consists of the alpha solid solution of tin in copper. Alpha bronze alloys of 4–5% tin are used to make coins, springs and blades. Historical "bronzes" are variable in composition, as most metalworkers used whatever scrap was on hand; the proportions of this mixture suggests. The Benin Bronzes are in fact brass, the Romanesque Baptismal font at St Bartholomew's Church, Liège is described as both bronze and brass. In the Bronze Age, two forms of bronze were used: "classic bronze", about 10% tin, was used in casting. Bladed weapons were cast from classic bronze, while helmets and armor were hammered from mild bronze.
Commercial bronze and architectural bronze are more properly regarded as brass alloys because they contain zinc as the main alloying ingredient. They are used in architectural applications. Bismuth bronze is a bronze alloy wit
A quantum point contact is a narrow constriction between two wide electrically conducting regions, of a width comparable to the electronic wavelength. Quantum point contacts were first reported in 1988 by a Dutch group and, independently, by a British group, they are based on earlier work by the British group which showed how split gates could be used to convert a two-dimensional electron gas into one-dimension, first in silicon and in gallium arsenide There are several different ways of fabricating a QPC. It can be realized in a break-junction by pulling apart a piece of conductor; the breaking point forms the point contact. In a more controlled way, quantum point contacts are formed in a two-dimensional electron gas, e.g. in GaAs/AlGaAs heterostructures. By applying a voltage to suitably shaped gate electrodes, the electron gas can be locally depleted and many different types of conducting regions can be created in the plane of the 2DEG, among them quantum dots and quantum point contacts. Another means of creating a QPC is by positioning the tip of a scanning tunneling microscope close to the surface of a conductor.
Geometrically, a quantum point contact is a constriction in the transverse direction which presents a resistance to the motion of electrons. Applying a voltage V across the point contact induces a current to flow, the magnitude of this current is given by I = G V, where G is the conductance of the contact; this formula resembles Ohm's law for macroscopic resistors. However, there is a fundamental difference here resulting from the small system size which requires a quantum mechanical analysis. At low temperatures and voltages and untrapped electrons contributing to the current have a certain energy/momentum/wavelength called Fermi energy/momentum/wavelength. Much like in a waveguide, the transverse confinement in the quantum point contact results in a "quantization" of the transverse motion—the transverse motion cannot vary continuously, but has to be one of a series of discrete modes; the waveguide analogy is applicable as long as coherence is not lost through scattering, e.g. by a defect or trapping site.
The electron wave can only pass through the constriction if it interferes constructively, which for a given width of constriction, only happens for a certain number of modes N. The current carried by such a quantum state is the product of the velocity times the electron density; these two quantities by themselves differ from one mode to the other, but their product is mode independent. As a consequence, each state contributes the same amount e 2 / h per spin direction to the total conductance G = N G Q; this is a fundamental result. The integer number N is determined by the width of the point contact and equals the width divided by half the electron wavelength; as a function of the width of the point contact, the conductance shows a staircase behavior as more and more modes contribute to the electron transport. The step-height is given by G Q. An external magnetic field applied to the quantum point contact lifts the spin degeneracy and leads to half-integer steps in the conductance. In addition, the number N of modes that contribute becomes smaller.
For large magnetic fields, N is independent of the width of the constriction, given by the theory of the quantum Hall effect. An interesting feature, not yet understood, is a plateau at 0.7 G Q, the so-called 0.7-structure. Apart from studying fundamentals of charge transport in mesoscopic conductors, quantum point contacts can be used as sensitive charge detectors. Since the conductance through the contact depends on the size of the constriction, any potential fluctuation in the vicinity will influence the current through the QPC, it is possible to detect single electrons with such a scheme. In view of quantum computation in solid-state systems, QPCs can be used as readout devices for the state of a quantum bit. In device physics, the configuration of QPCs is used for demonstrating a ballistic field-effect transistor. C. W. J. Beenakker and H. van Houten. "Quantum Transport in Semiconductor Nanostructures". Solid State Physics. 44: 1–228. ArXiv:cond-mat/0412664. Bibcode:2004cond.mat.12664B. Doi:10.1016/s0081-194760091-0.
ISBN 9780126077445. K. J. Thomas. "Possible spin polarization in a one-dimensional electron gas". Physical Review Letters. 77: 135–138. ArXiv:cond-mat/9606004. Bibcode:1996PhRvL..77..135T. Doi:10.1103/PhysRevLett.77.135. PMID 10061790. Nicolás Agraït. "Quantum properties of atomic-sized conductors". Physics Reports. 377: 81. ArXiv:cond-mat/0208239. Bibcode:2003PhR...377...81A. Doi:10.1016/S0370-157
Calcium channel, voltage-dependent, L type, alpha 1C subunit is a protein that in humans is encoded by the CACNA1C gene. Cav1.2 is a subunit of L-type voltage-dependent calcium channel. This gene encodes an alpha-1 subunit of a voltage-dependent calcium channel. Calcium channels mediate the influx of calcium ions into the cell upon membrane polarization; the alpha-1 subunit consists of 24 transmembrane segments and forms the pore through which ions pass into the cell. The calcium channel consists of a complex of alpha-1, alpha-2/delta and beta subunits in a 1:1:1 ratio; the S3-S4 linkers of Cav1.2 determine the gating phenotype and modulated gating kinetics of the channel. Cav1.2 is expressed in the smooth muscle, pancreatic cells and neurons. However, it is important and well known for its expression in the heart where it mediates L-type currents, which causes calcium-induced calcium release from the ER Stores via ryanodine receptors, it depolarizes at -30mV and helps define the shape of the action potential in cardiac and smooth muscle.
The protein is inhibited by dihydropyridine. In the arteries of the brain, high levels of calcium in mitochondria elevates activity of nuclear factor kappa B NF-κB and transcription of CACNA1c and functional Cav1.2 expression increases. Cav1.2 regulates levels of osteoprotegerin. CaV1.2 is inhibited by the action of STIM1. The activity of CaV1.2 channels is regulated by the Ca2+ signals they produce. An increase in intracellular Ca2+ concentration implicated in Cav1.2 facilitation, a form of positive feedback called Ca2+-dependent facilitation, that amplifies Ca2+ influx. In addition, increasing influx intracellular Ca2+ concentration has implicated to exert the opposite effect Ca2+ dependent inactivation; these activation and inactivation mechanisms both involve Ca2+ binding to calmodulin in the IQ domain in the C-terminal tail of these channels. Cav1.2 channels are arranged on average, in the cell membrane. When calcium ions bind to calmodulin, which in turn binds to a Cav1.2 channel, it allows the Cav1.2 channels within a cluster to interact with each other.
This results in channels working cooperatively when they open at the same time to allow more calcium ions to enter and close together to allow the cell to relax. Mutation in the CACNA1C gene, the single-nucleotide polymorphism located in the third intron of the Cav1.2 gene, are associated with a variant of Long QT syndrome called Timothy's syndrome and with Brugada syndrome. Large-scale genetic analyses have shown the possibility that CACNA1C is associated with bipolar disorder and subsequently with schizophrenia. A CACNA1C risk allele has been associated to a disruption in brain connectivity in patients with bipolar disorder, while not or only to a minor degree, in their unaffected relatives or healthy controls. Click on genes and metabolites below to link to respective Wikipedia articles. Calcium channel GeneReviews/NIH/NCBI/UW entry on Brugada syndrome CACNA1C+protein,+human at the US National Library of Medicine Medical Subject Headings GeneReviews/NIH/NCBI/UW entry on Timothy SyndromeThis article incorporates text from the United States National Library of Medicine, in the public domain