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Opportunities in High Magnetic Field Science C Glossary antiferromagnetism: In substances known as antiferromagnets, the magnetic moments of adjacent atoms tend to line up antiparallel, yielding an ordered state with no net magnetic moment. Consequently, such materials display almost no response to an external magnetic field at low temperatures. band structure, bandgap: Theory describing the collective organization and interaction of atoms (and notably their valence electrons) in a solid. The band structure of a solid is the continuous range of states with different energies that are filled by the charge carriers in an extended solid. In insulators and semiconductors, there is a bandgap that separates the last filled state from the first excited state, unoccupied at zero temperature. The electrical properties of a material at room temperature can be influenced by the (temperature-dependent) population of charge carriers near the bandgap. For instance, materials without a bandgap (i.e., metals) or with a bandgap comparable to the thermal energy (i.e., semiconductors) are typically conductors, while materials with a very wide bandgap are typically insulators. Bitter magnet: Design for DC resistive magnets invented by Francis Bitter in the late 1930s. Bitter’s design meets the high conductivity and cooling requirements of high-field resistive magnets using perforated copper plates that are sandwiched between insulating layers, a small region of electrical contact being allowed between plates so that current can flow from one plate to the next. Electrical current flows
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Opportunities in High Magnetic Field Science through the resulting copper spiral, and coolant flows through perforations in the conductors, which are aligned vertically. bore, magnet: Inner diameter of a cylindrical magnet where the magnetic field is generated. The bore of a magnet constrains the volume available for experimental use. coil, magnet(ic): Electric current in most electromagnets passes through coils of wire. Since the coils of all such magnets are their active component, the terms coil and magnet are often used as synonyms. coherence length: Characteristic scale of a Cooper pair in a superconducting material. The coherence length effectively represents the longest distance over which the two electrons of the Cooper pair act in tandem and is typically on the order of 1.5 nm for high-field materials. Cooper pair: Entity believed to explain the superconductivity of many materials. A Cooper pair consists of two electrons that are paired together into a new state with zero net charge and angular momentum. Below the superconducting transition temperature, Cooper pairs form a condensate—a macroscopically occupied single quantum state—in which current flows without resistance. conductor: Material such as Cu or Al in which charge carriers can move under the influence of an electrical voltage. Unlike superconductors, conductors have finite, nonzero resistance. correlated electron systems: Also strongly correlated electron systems; a many-particle system in which strong interactions between electrons play a crucial role in determining fundamental properties. Electronic correlations can cause striking many-body effects like superconductivity, electronic localization, magnetism, and charge ordering, which cannot be described using the simpler independent particle picture. These properties and dynamics arise from the collective interactions of the electrons with one another. critical current density (Jc): At a certain temperature, the maximum electrical current density that a superconductor can carry before it quenches and enters the normal state. In general, as the current flowing through a superconductor increases, the Tc (see below) will usually decrease. critical field (Hc): At zero applied current, the maximum magnetic field (at a given temperature) that a superconductor can transport before it quenches and returns
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Opportunities in High Magnetic Field Science to a nonsuperconducting state. Typically, a higher Tc (see below) is associated with a higher Hc. cryogenically cooled probe: Device installed in the bore of an NMR magnet that carries the samples to be studied as well as the electronics necessary both for perturbing the orientation of nuclear spins in samples and for detecting the consequences of those perturbations electromagnetically. Probes may include additional devices for controlling the sample environment. In a cryogenically cooled probe, in order to improve signal to noise, electronic components are cooled to liquid helium temperatures, which minimizes shot noise. cyclotron: Device for experimental particle physics that uses an oscillating electric field to accelerate charged particles and a magnetic field to control particle trajectories. Charged carriers of all kinds follow cyclotron-like spiral trajectories in magnetic fields. An important quantity in high magnetic field studies is the cyclotron frequency, which is a measure of the density of mobile carriers in doped semiconductors and metals. DC magnet: Steady-state magnet. DC stands for direct current, meaning that the flow of current in the magnet’s coils is constant in time. electromagnet: Device designed to generate a magnetic field by having electric current passed through it. electron paramagnetic resonance (EPR): Electron paramagnetic resonance (EPR), also known as electron spin resonance (ESR) or electron magnetic resonance (EMR), is the resonant absorption of microwave radiation by paramagnetic ions or molecules with at least one unpaired electron spin in the presence of a static magnetic field. It has a wide range of applications in chemistry, physics, biology, and medicine. For example, it may be used to probe the static structure of solid and liquid systems and is also very useful in investigating dynamic processes. ferromagnetism: In substances known as ferromagnets, the magnetic moments of adjacent atoms tend to line up in parallel, yielding an ordered state that has a macroscopic magnetic moment. Once the magnetic domains in a ferromagnetic substance have become aligned by a small field, the magnetic moment of the bulk material will persist even in the absence of an external magnetic field, a property unique to ferromagnets. Consequently, ferromagnetic materials can be used to make permanent magnets that deliver fields as large as 1-2 T. Elements such as iron, nickel, and cobalt are ferromagnetic at room temperature.
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Opportunities in High Magnetic Field Science fusion: Nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy. gauss (G): Unit of measure for magnetic field strength in the cgs system of units. Earth’s magnetic field is about 0.5 G. One G is equal to 0.0001 tesla (T), the mks unit of magnetic field. hybrid magnet: In a hybrid magnet system, resistive and superconducting magnet technologies are combined. The superconducting magnet takes the place of the outer portion of the resistive coil. The resistive portion operates as an insert to the superconducting magnet and produces the portion of the field that exceeds the critical current and field limits of the superconducting magnet. hybrid magnet, series-connected: Hybrid magnet system where the current supplied to the resistive insert travels first through the superconducting outer magnet. ion cyclotron resonance (ICR) mass spectroscopy (ICRMS): Method for precisely measuring the mass of a collection of ions originating from the chemical dissociation of complex molecules and solids; it depends on cyclotron resonance. Ions with a range of mass-to-charge ratios are exposed to a high-frequency electric field in the presence of a constant magnetic field perpendicular to the varying electric field. Maximum energy is gained by the ions that satisfy the cyclotron resonance condition and that can be separated on that basis from ions that have only a slightly different mass-to-charge ratio. Jc: See critical current linewidth: Energy resolution of a feature in an experimental measurement; typically, a peak observed in a spectrum. Los Alamos National Laboratory (LANL): National laboratory in northern New Mexico operated by the University of California for the Department of Energy. macromolecule: Molecule of high relative molecular mass the structure of which usually consists of multiply repeated units that are derived—actually or conceptually—from molecules of low relative molecular mass; particularly a molecule of this kind that is of biological origin. magnetic field: Modification of free space or vacuum caused by the presence of moving charges that results in a force being exerted on other moving charges.
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Opportunities in High Magnetic Field Science Magnetic fields are caused by electrical currents, which may be either microscopic (i.e., orbital motion in atoms) or macroscopic, as in the case of an electromagnet. magnetic moment: Property of a magnetic dipole that determines the amount of torque exerted on it when it is placed in a magnetic field. magnetic order: Systematic arrangement of magnetic moments in a material that forms a long-range pattern. magnetic resonance imaging (MRI): Noninvasive technique based on nuclear magnetic resonance for imaging the interior of objects that is often used medically. The sample to be imaged is placed in a strong magnetic field that varies across its volume in a known manner and is then exposed to electromagnetic radiation of appropriate frequency. In this environment the frequency of the NMR signals generated by all the magnetically active atoms in the sample will vary with location in the sample. The 3D distributions of molecules of a particular type in a sample can be reconstructed from the frequency distributions. magnetism: The attractive and repulsive forces magnets exert on each other. Commonly taken as synonymous with ferromagnetism—that is, the intrinsic magnetic fields characteristic of ferromagnetic materials. More generally, magnetism spans the whole range of phenomena displayed by materials with constituents having magnetic dipoles, including antiferromagnetism and other forms of short-range permanent or ephemeral order. magnetoresistance: In some materials, electrical resistance depends dramatically on external magnetic field. Antiferromagnetically coupled magnetic layers separated by nonmagnetic spacers can display an extreme form of magnetoresistance called giant magnetoresistance, which is taken advantage of in the magnetic sensors used in high-density disk drives. MgB2: Magnesium diboride is a superconductor that has conventional superconducting properties despite having two types of electrons that participate in its superconductivity. Its critical temperature (about 39 K) is the highest of all known phonon-mediated superconductors. This relatively inexpensive material was first synthesized in 1953, but its superconducting properties were not discovered until 2001. National High Magnetic Field Laboratory (NHMFL): National laboratory for the production of high and specialized magnetic fields for scientific research. It is
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Opportunities in High Magnetic Field Science operated by the National Science Foundation. Its steady-state magnetic field facility is located in Tallahassee, Florida; its pulsed-field facility is based at LANL in New Mexico; and it has an MRI facility and a high field-to-temperature ratio experimental facility at Gainesville, Florida. The NHMFL develops and operates high magnetic field facilities that scientists use for research in physics, biology, bioengineering, chemistry, geochemistry, biochemistry, materials science, and engineering. It is the only facility of its kind in the United States and one of about a dozen in the world. Nb3Sn: Niobium-tin (Tc of about 18 K) is a superconducting compound that has been widely used for the construction of high-field magnets with field greater than 10 T or so. Nb-Ti: Niobium-titanium (Tc of about 9 K) is the workhorse superconducting material in the high-field magnet industry. neutron source: A nuclear reactor or an accelerator-based facility that generates beams of neutrons of (usually) modest energy that have high intensity and flux. Neutrons are important probes of the microstructure of matter because (1) having no net charge, they interact with the nuclei of atoms, not their electron clouds; (2) they interact weakly with matter and therefore can probe deeply into the interior of samples; and (3) their energies are well suited to the scales of electronic and atomic processes. Moreover, because they are spin-1/2 particles having a magnetic moment, they can be used to study the magnetic microstructure of matter. NMR spectrometer: Instrument used to measure the frequencies of NMR transitions. A modern NMR spectrometer usually includes (1) a superconducting magnet; (2) a probe for holding the sample in the magnet that includes coils for irradiating it with electromagnetic radiation and detecting the electromagnetic radiation emitted by the sample; and (3) a console, which contains the electronics necessary to operate the probe and a computer to control what happens in the probe and analyze the data returned from the probe. nuclear magnetic resonance (NMR): When an atomic nucleus in a magnetic field is exposed to photons that have an energy corresponding to the difference in energy between two possible orientations of its magnetic moment, it will resonate—that is, its magnetic moment will rapidly change orientation, in the process first absorbing energy and then radiating it. Only a finite number of different orientations are possible for the magnetic moments of any such nucleus in a magnetic field, each orientation having its own characteristic energy. This behavior
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Opportunities in High Magnetic Field Science is efficient enough that it can be detected over only a narrow range of photon energies (frequencies). The frequencies at which resonances are seen in some specified magnetic field not only identify the kinds of atom responsible for them but can also provide valuable information about the molecular environment in which the atoms are found. organic superconductors: Class of organic conductors that superconduct at low temperature. They include molecular salts, polymers, and even pure carbon systems—for example, carbon nanotubes and C60 compounds. They are also sometimes called molecular superconductors. They are typically large, carbon-based molecules of 20 or more atoms and consist of a planar organic molecule and a nonorganic anion. pulsed magnet: Resistive magnet designed to provide transient magnetic fields, often for durations as short as microseconds but occasionally for as long as several seconds. Because it is active for only short times, a pulsed magnet uses less power and needs less cooling than a DC magnet of similar bore and maximum field strength. Today, research magnets with the highest fields are pulsed magnets. quantum critical point: Phase transitions of any sort that occur at absolute zero; thought to be a characteristic feature of all strongly correlated electron systems. The novel behaviors that are observed signal the dominance of quantum fluctuations over the thermal fluctuations that are characteristic of phase transitions at finite temperatures. Many believe that the unconventional properties of high-temperature superconductors may be related to a hidden quantum critical point in these materials. quantum Hall effect (QHE): When a magnetic field is applied perpendicularly to a thin metal film or a semiconductor film that is conducting an electric current, a voltage will be observed that is perpendicular to the axis of both the film and the magnetic field. This voltage is proportional to the strength of the applied field. Discovered in 1879, this phenomenon was named for its discoverer, Edwin H. Hall. It was later observed that certain superconducting devices display steps in their Hall resistance—that is, the ratio of Hall voltage to current—that reflect the tuning of charge carrier occupancy states by the external magnetic field. K. Von Klitzing was awarded the Nobel prize in physics in 1985 for his demonstration of this phenomenon, which is known as the integer quantum Hall effect. quantum Hall effect, fractional (FQHE): Fractional version of QHE, in which the Hall resistance progresses in fractions of integer quanta, was discovered in 1982 by
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Opportunities in High Magnetic Field Science D. Tsui and H. Störmer in experiments performed on gallium arsenide heterostructures. This behavior was explained by R. Laughlin in 1983 in terms of a novel quantum liquid phase that accounts for the effects of interactions between electrons. The three were awarded the 1998 Nobel prize in physics for this work. quench: The transition, often sudden, in a superconducting material from its superconducting state to its normal, resistive conducting state. It occurs when either the critical current density (Jc) of the material or its critical temperature (Tc) is exceeded. resistive magnet: Electromagnet that generates a magnetic field by the passage of electric current through resistive conductors. resistivity: Property of a material that inhibits the flow of electricity, usually because of collisions between the charge carriers and the material’s internal lattice structure. Spallation Neutron Source (SNS): Large research facility under construction at the Oak Ridge National Laboratory. It is expected to be the most powerful pulsed neutron source in the world when completed. The neutrons produced by a spallation source are knocked out of a target (spalled), which is usually a mass of some high atomic weight metal, by high-energy protons generated by an accelerator of some kind. solenoid: Magnetic solenoid; the most common type of magnet, formed by wrapping coils of conductor around a central cylindrical volume. spectroscopy: (Usually) the experimental study of the energy levels of materials. More generally, a spectrum is a display of the dependence of some property of a sample as a function of some other parameter—for example, energy absorption versus energy or abundance versus molecular mass. Any experimental activity that generates such plots can be described as spectroscopy. stored energy: Potential energy; energy that can be released to do work, as in an electric motor. A magnet’s energy is stored in its magnetic field. superconducting magnet: Electromagnet whose conductor is made of superconducting material.
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Opportunities in High Magnetic Field Science superconductivity: Phenomenon that occurs in certain materials at low temperatures. It is characterized by the complete loss of electrical resistance and the complete expulsion of externally applied (weak) magnetic fields (the Meissner effect). superconductor: Any material that will conduct electricity without resistance. superconductor, high-temperature (HTS): Superconducting material that has a high critical temperature. There is no specific temperature separating HTS from LTS materials. HTS now normally also means a CuO2-based superconductor. superconductor, low-temperature (LTS): Superconducting materials whose Tc is below about 30 K, though many now call MgB2 a low-temperature superconductor even though its Tc can be as high as 40 K. See HTS. synchrotron light source: Charged particles traveling in circular trajectories emit electromagnetic radiation. This phenomenon results in a serious loss of ion energy from the circular accelerators used by the high-energy physics community, and some of the radiation emitted can be x rays. A synchrotron light source is an accelerator designed and operated for the electromagnetic radiation it produces rather than for beams of high-energy ions. Tc: Scientific notation for the critical transition temperature (at zero applied magnetic field and current) below which a material begins to superconduct. tesla: Unit of measure for magnetic field strength in the SI system of units, abbreviated as T. One tesla is equivalent to 10,000 gauss. transition temperature: See Tc. YBCO: Acronym for a well-known ceramic superconductor composed of yttrium, barium, copper, and oxygen.
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