E

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, a bandgap 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 direct current 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 through the resulting copper spiral, and coolant flows through perforations in the conductors, which are aligned vertically.



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E 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 inter- action 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, a bandgap 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 direct current 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. Electri- cal current flows through the resulting copper spiral, and coolant flows through perforations in the conductors, which are aligned vertically. 185

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186 High Magnetic Field Science and I t s A pp l i c a t i o n in the US bore, magnet: Inner diameter of a cylindrical magnet where the magnetic field is available for use. The bore of a magnet constrains the volume that can be utilized for experimental measurements. 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 mate- rial. 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. 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. 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 transi- tion temperature, Cooper pairs form a condensate—a macroscopically occupied single quantum state—in which current flows without resistance. correlated electron systems: A many-particle system in which strong interac- tions between electrons play a crucial role in determining fundamental properties. Electronic correlations can cause striking many-body effects like superconduc- tivity, 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. Also “strongly correlated electron systems.” 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 to a nonsuperconducting state. Typically, a higher Tc (see below) is associated with a higher Hc. (For the superconductors of interest to this report, the critical field is technically the upper critical field, properly denoted as Hc2.)

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A pp e n d i x E 187 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 per- turbing the orientation of nuclear spins in samples and for detecting the conse- quences 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 ratio, electronic components are cooled to liquid helium temperatures, which minimizes shot noise. cuprates (copper oxides): Materials or chemical compounds that contain copper anions. cyclotron: Device for experimental particle physics that uses an oscillating elec- tric field to accelerate charged particles and a magnetic field to control particle trajectories. cyclotron frequency: Charged carriers of all kinds follow cyclotron-like spiral trajectories in strong magnetic fields, with a frequency proportional to the field strength. In doped semiconductors and metals, measurements of the cyclotron frequency give information about the effective mass of the mobile carriers. dc magnet: A steady-state magnet; dc stands for direct current, meaning that the flow of current in the magnet’s coils is constant in time. diamagnetism: The property of some materials that causes them to create a magnetic field that opposes an externally applied magnetic field and therefore are slightly repelled by that external field. dynamic nuclear polarization (DNP): DNP is a process in which the large polar- izations of electron spins in a strong magnetic field are partially transferred to nuclear spins by irradiation of EPR (see below) transitions, resulting in large enhancements of nuclear spin polarizations and hence NMR signals. electromagnet: Device designed to generate a magnetic field by having electric current passed through it. electron paramagnetic resonance (EPR): EPR, also known as electron spin reso- nance (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 appli- cations in chemistry, physics, biology, and medicine. For example, it may be used

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188 High Magnetic Field Science and I t s A pp l i c a t i o n in the US to probe the static structure of solid and liquid systems and is also very useful in investigating dynamic processes. Faraday rotation: An interaction between light and a magnetic field by which the plane of polarization of the light rotates. Fermi surface: Separates occupied electron states, whose energies lie below a cut- off (the Fermi energy), from unoccupied or empty states, whose energies lie above the cutoff. Electrons close to the Fermi surface are of particular interest because they play a dominant role in electrical transport and many other properties of the material. ferromagnetism/ferromagnetic materials: 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 may persist even in the absence of an exter- nal 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. Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry: FT- ICR is a technique in which the mass-to-charge ratios of charged particles, espe- cially molecular ions, are measured from the frequencies of their cyclotron motions in a strong magnetic field. Compared with other forms of mass spectrometry, FT- ICR has the highest demonstrated precision and resolution, making it the method of choice for complex mixtures of molecules and for distinguishing among chemi- cal species with nominally identical masses but different elemental compositions. functional magnetic resonance imaging (fMRI): A form of magnetic resonance imaging that registers changes in blood oxygenation and flow to areas of the brain, thereby measuring brain activity. 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.

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A pp e n d i x E 189 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 sup- plied to the resistive insert travels first through the superconducting outer magnet. ion cyclotron resonance mass spectroscopy (ICRMS): Method for precisely mea- suring 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 con- dition and that can be separated on that basis from ions that have only a slightly different mass-to-charge ratio. ITER: ITER is a large-scale scientific experiment being built in the south of France that aims to demonstrate the feasibility of producing fusion power in a sustained burning plasma, while also demonstrating some of the key technology components. Jc: See critical current density linewidth: Energy resolution of a feature in an experimental measurement; typi- cally, 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 mol- ecule of this kind that is of biological origin. magnetic confinement fusion reactors: One of the two major avenues along which fusion energy research is proceeding (the other being inertial confinement fusion), magnetic confinement fusion relies on magnetic fields to confine a plasma at suf- ficient densities and appropriate conditions so that fusion reactions take place.

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190 High Magnetic Field Science and I t s A pp l i c a t i o n in the US 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. Magnetic fields are caused by electrical currents, which may be either microscopic (e.g., due to electron spins or orbital motion within atoms) or macroscopic, as in case of currents in the coils 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 medical technique based on nuclear magnetic resonance for imaging the interior of objects. The sample to be imaged is placed in a strong magnetic field that varies across its volume in a known manner; it 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 three-dimensional distributions of molecules of a particular type in a sample can be reconstructed from the frequency distributions. magnetic susceptibility: a dimensionless proportionality constant that indicates the extent to which a material is magnetized in response to a given, external mag- netic field. magnetism: The attractive and repulsive forces magnets exert on each other. Com- monly 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 sepa- rated 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. megavolt ampere (MVA): measures the apparent power in a power supply.

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A pp e n d i x E 191 Meissner effect: The active exclusion of magnetic fields from the interior of materi- als as they transition from the normal to a superconducting state. MgB2: Magnesium diboride is a superconductor that has conventional supercon- ducting 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. molecular beam epitaxy (MBE): A process widely used in the semiconductor industry for growing single crystals on a heated crystalline substrate. nanophotonics: The study and use of light and optics at the nanometer scale. National High Magnetic Field Laboratory (NHMFL): National laboratory for the production of high and specialized magnetic fields for scientific research. It is operated by the National Science Foundation. Its steady-state magnetic field facil- ity is located in Tallahassee, Florida, its pulsed-field facility is based at Los Alamos National Laboratory 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, materi- als 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 tesla or so. Nb-Ti: Niobium-titanium (Tc of about 9 K) is the workhorse superconducting material in the high-field-magnet industry. Neel temperature: The temperature at which the thermal energy is large enough above which an antiferromagnetic or ferrimagnetic material becomes paramagnetic. neutron source: A nuclear reactor or 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

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192 High Magnetic Field Science and I t s A pp l i c a t i o n in the US samples; and (3) their energies are well suited to the scales of electronic and atomic processes. Moreover, because they are spin-½ 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 tran- sitions. 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 that 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 absorb- ing 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 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 sys- tems—for example, carbon nanotubes and C60 compounds. They are also some- times 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-field 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 fluctua- tions over the thermal fluctuations that are characteristic of phase transitions at

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A pp e n d i x E 193 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 cur- rent, a voltage will be observed that is perpendicular to the axis of both the film and the magnetic field. Discovered in 1879, this phenomenon was named for its discoverer, Edwin H. Hall. Classically, this voltage is proportional to the strength of the applied magnetic field. However, it was later observed that certain two- dimensional semiconducting devices display precisely quantized plateaus in their Hall resistance—that is, the ratio of Hall voltage to current—which 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): The fractional version of QHE, in which the Hall resistance progresses in fractions of integer quanta, was discovered in 1982 by 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: Transition, often sudden, in a superconducting material from its supercon- ducting state to its normal, resistive conducting state. It occurs when either the criti- cal 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. scanning tunneling microscopy (STM): An instrument that uses quantum tun- neling to image surfaces at the atomic level. soft condensed matter: Encompasses a variety of physical systems that are soft in the sense that they can be easily deformed by mechanical or thermal stress, or by electric and magnetic fields. Such systems include polymers, gels, colloids, mem- branes, and biological cells or organisms. The binding between molecules in these

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194 High Magnetic Field Science and I t s A pp l i c a t i o n in the US mostly organic or biological materials (hydrogen bonding, van der Waals, or π-π bonding), is much weaker than in normal solids. High magnetic fields can be used to assemble and align functional, organic or inorganic, nano- and microstructures, and to probe their structures, properties, and dynamics, with potential applications in drug delivery, optics, sensors, and nanoelectronics. solenoid: Magnetic solenoid; the most common type of magnet, formed by wrap- ping coils of conductor around a central cylindrical volume. Spallation Neutron Source (SNS): Large research facility at the Oak Ridge National Laboratory, completed in 2006, which provides the most powerful pulsed neutron source in the world. 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. 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 supercon- ducting material. superconductivity: Phenomenon that occurs in certain materials at low tem- peratures. It is characterized by the complete loss of electrical resistance and the complete expulsion of weak externally applied 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, typically above 30 K. However, there is no specific tem- perature separating HTS from low-temperature superconducting materials. HTS is now frequently used to mean superconductors where the pairing mechanism is believed to arise from electron-electron interactions, such as the copper-oxide- and iron-pnictide-based superconductors, as opposed to materials where the supercon- ductivity is believed due to phonon-mediated interactions.

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A pp e n d i x E 195 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, because its superconductivity is believed to arise from conventional phonon-mediated pairing. See preceeding entry, HTS. synchrotron light source: Relativistic charged particles traveling in circular trajec- tories emit electromagnetic radiation, known as synchrotron radiation. Although this phenomenon can result in a serious loss of particle energy in circular accel- erators used by the high-energy physics community, the emitted radiation can be a powerful source of photons, with energies ranging from the infrared to X-rays. A synchrotron light source is an electron storage ring, typically fed by a separate accelerator, which is designed and operated for the electromagnetic radiation it produces. Tc: Scientific notation for the critical transition temperature (at zero applied mag- netic field and current) below which a material begins to superconduct. tesla (T): Unit of measure for magnetic field strength in the SI system of units. One tesla is equivalent to 10,000 gauss. THz radiation: Electromagnetic radiation at frequencies around 1 terahertz (1012 Hz), with the upper and lower boundaries not clearly designated. A range of 0.1 THz (100 gigahertz) to 10 THz has a corresponding wavelength range of 3 mm to 0.03 mm (or 30 µm). topological insulator: A material whose interior behaves as an insulator but whose surface contains conducting states. transition temperature: See Tc. van der Waals bond: The weak attractive or repulsive forces between atoms or molecules that arise principally from induced or permanent dipoles existing in the particles. YBCO: Acronym for a well-known high-temperature ceramic superconductor composed of yttrium, barium, copper, and oxygen.