Superconducting devices

Devices that perform functions in the superconducting state that would be difficult or impossible to perform at room temperature, or that contain components which perform such functions. The superconducting state involves a loss of electrical resistance and occurs in many metals and alloys at temperatures near absolute zero. An enormous impetus was provided by the discovery in 1986 of a new class of ceramic, high-transition-temperature (T_c) superconductors, which has resulted in a new superconducting technology at liquid nitrogen temperature. Superconducting devices may be conveniently divided into two categories: small-scale thin-film devices, and large-scale devices which employ zero-resistance superconducting windings made of type II superconducting materials. See Superconductivity

Small-scale devices

A variety of thin-film devices offer higher performance than their nonsuperconducting counterparts. The prediction and discovery in the early 1960s of the Josephson effects introduced novel opportunities for ultrasensitive detectors, high-speed switching elements, and new physical standards. Niobium-based devices, patterned on silicon wafers using photolithographic techniques taken over from the semiconductor industry, have reached a high level of development, and a variety of such devices are commercially available. These devices operate at or below 4.2 K (-452°F), the temperature of liquid helium boiling under atmospheric pressure. See Liquid helium

The discovery of the high-transition-temperature superconductors has enabled the operation of devices in liquid nitrogen at 77 K (-321°F). Not only is liquid nitrogen much cheaper and more readily available than liquid helium, but it also boils away much more slowly, enabling the use of simpler and more compact dewars or simpler, relatively inexpensive refrigerators. Of the new ceramic superconductors, only YBa₂Cu₃O_7-x (YBCO) has been developed in thin-film form to the point of practical applications, and several devices are available. Intensive materials research has resulted in techniques, notably laser-ablation and radio-frequency sputtering, for the epitaxial growth of high-quality films with their crystalline planes parallel to the surface of the substrate. Most of the successful Josephson-junction devices have been formed at the interface between two grains of YBCO. These so-called grain-boundary junctions are made by depositing the film either on a bicrystal in which the two halves of the substrate have a carefully engineered in-plane misalignment of the crystal axes, or across a step-edge patterned in the substrate. See Cryogenics, Grain boundaries

Low-transition-temperature SQUIDs are widely used to measure the magnetic susceptibility of tiny samples over a wide temperature range. Another application is a highly sensitive voltmeter, used in measurements of the Hall effect and of thermoelectricity. Low-transition-temperature SQUIDs are used as ultrasensitive detectors of nuclear magnetic and nuclear quadrupole resonance, and as transducers for gravitational-wave antennas. So-called scanning SQUIDs are used to obtain magnetic images of objects ranging from single-flux quanta trapped in superconductors to subsurface damage in two metallic sheets riveted together. See Hall effect, Magnetic susceptibility, Thermoelectricity, Voltmeter

Perhaps the single largest area of application is biomagnetism, notably to image magnetic sources in the human brain or heart. In these studies an array of magnetometers or gradiometers is placed close to the subject, both generally being in a magnetically shielded room. The fluctuating magnetic signals recorded by the various channels are analyzed to locate their source. These techniques have been used, for example, to pinpoint the origin of focal epilepsy and to determine the function of the brain surrounding a tumor prior to its surgical removal.

The most sensitive detector available for millimeter and submillimeter electromagnetic radiation is the superconductor-insulator-superconductor (SIS) quasiparticle mixer. In this tunnel junction, usually niobium–aluminum oxide–niobium, the Josephson supercurrent is quenched and only single-electron tunneling occurs. The current-voltage characteristic exhibits a very sharp onset of current at a voltage 2Δ/e, where Δ is the superconducting energy gap. The mixer is biased near this onset where the characteristics are highly nonlinear and used to mix the signal frequency with the frequency of a local oscillator to produce an intermediate frequency that is coupled out into a low-noise amplifier. These mixers are useful at frequencies up to about 750 GHz (wavelengths down to 400 micrometers). Such receivers are of great importance in radio astronomy, notably for airborne, balloon-based, or high-altitude, ground-based telescopes operating above most of the atmospheric water vapor.

The advent of high-transition-temperature superconductors stimulated major efforts to develop passive radio-frequency and microwave components that take advantage of the low electrical losses offered by these materials compared with normal conductors in liquid nitrogen. The implementation of thin-film YBCO receiver coils has improved the signal-to-noise ratio of nuclear magnetic resonance (NMR) spectrometers by a factor of 3 compared to that achievable with conventional coils. This improvement enables the data acquisition time to be reduced by an order of magnitude. These coils also have potential applications in low-frequency magnetic resonance imaging (MRI). High-transition-temperature bandpass filters have application in cellular communications.

Large-scale devices

Large-scale applications of superconductivity comprise medical, energy, transportation, high-energy physics, and other miscellaneous applications such as high-gradient magnetic separation. When strong magnetic fields are needed, superconducting magnets offer several advantages over conventional copper or aluminum electromagnets. Most important is lower electric power costs because once the system is energized only the refrigeration requires power input, generally only 5–10% that of an equivalent-field resistive magnet. Relatively high magnetic fields achievable in unusual configurations and in smaller total volumes reduce the costs of expensive force-containment structures. See Magnet

Niobium-titanium (NbTi) has been used most widely for large-scale applications, followed by the A15 compounds, which include niobium-tin (Nb₃Sn), niobium-aluminum (Nb-Al), niobium-germanium (Nb-Ge), and vanadium-gallium (Va₃Ga). Niobium-germanium held the record for the highest critical field (23 K; -418.5°F) until the announcement of high-temperature ceramic superconductors. See A15 phases

Significant advances have been made in high-temperature superconducting wire development. Small coils have been wound that operate at 20 K (-410°F). Current leads are in limited commercial use. Considerable development remains necessary to use these materials in very large applications.

MRI dominates superconducting magnet systems applications. Most of the MRI systems are in use in hospitals and clinics, and incorporate superconducting magnets.

Some of the largest-scale superconducting magnet systems are those considered for energy-related applications. These include magnetic confinement fusion, superconducting magnetic energy storage, magnetohydrodynamic electrical power generation, and superconducting generators. See Nuclear fusion

In superconducting magnetic energy storage superconducting magnets are charged during off-peak hours when electricity demand is low, and then discharged to add electricity to the grid at times of peak demand. The largest systems would require large land areas, for example, an 1100-m-diameter (3600-ft) site for a 5000-MWh system. However, intermediate-size systems are viable. A 6-T peak-field solenoidal magnet system designed for the Alaskan power network stores 1800 megajoules (0.5 MWh). High-purity-aluminum-stabilized niobium-titanium alloy conductor carrying 16 kiloamperes current is used for the magnet winding.

Superconducting magnets have potential applications for transportation, such as magnetically levitated vehicles. In addition, superconducting magnets are used in particle accelerators and particle detectors. See Particle accelerator, Particle detector

Superconducting devices

Small-scale devices

Two types of superconducting quantum interference device (SQUID) detect changes in magnetic flux: the dc SQUID and the rf SQUID. The dc SQUID, which operates with a dc bias current, consists of two Josephson junctions incorporated into a superconducting loop. The maximum dc supercurrent, known as the critical current, and the current-voltage (I-V) characteristic of the SQUID oscillate when the magnetic field applied to the device is changed. The oscillations are periodic in the magnetic flux &PHgr; threading the loop with a period of one flux quantum, &PHgr;₀ = h/2e ≈ 2.07 × 10^-15 weber, where h is Planck's constant and e is the magnitude of the charge of the electron. Thus, when the SQUID is biased with a constant current, the voltage is periodic in the flux. The SQUID is almost invariably operated in a flux-locked loop. A change in the applied flux gives rise to a corresponding current in the coil that produces an equal and opposite flux in the SQUID. The SQUID is thus the null detector in a feedback circuit, and the output voltage is linearly proportional to the applied flux. See SQUID

The rf SQUID consists of a single Josephson junction incorporated into a superconducting loop and operates with an rf bias. The SQUID is coupled to the inductor of an LC-resonant circuit excited at its resonant frequency, typically 30 MHz. The characteristics of rf voltage across the tank circuit versus the rf current depends on applied flux. With proper adjustment of the rf current, the amplitude of the rf voltage across the tank circuit oscillates as a function of applied flux. The rf SQUID is also usually operated in a feedback mode.

SQUIDs are mostly used in conjunction with an input circuit. For example, magnetometers are made by connecting a superconducting pickup loop to the input coil to form a flux transformer. A magnetic field applied to the pickup loop induces a persistent current in the transformer and hence a magnetic flux in the SQUID. These magnetometers have found application in geophysics, for example, in magnetotellurics.

The most sensitive detector available for millimeter and submillimeter electromagnetic radiation is the superconductor-insulator-superconductor (SIS) quasiparticle mixer. In this tunnel junction, usually niobium–aluminum oxide–niobium, the Josephson supercurrent is quenched and only single electron tunneling occurs. The current-voltage characteristic exhibits a very sharp onset of current at a voltage 2Δ/e, where Δ is the superconducting energy gap. The mixer is biased near this onset where the characteristics are highly nonlinear and used to mix the signal frequency with the frequency of a local oscillator to produce an intermediate frequency that is coupled out into a low-noise amplifier. These mixers are useful at frequencies up to about 750 GHz (wavelengths down to 400 micrometers). Such receivers are of great importance in radio astronomy, notably for airborne, balloon-based, or high-altitude, ground-based telescopes operating above most of the atmospheric water vapor.

Large-scale devices

MRI dominates superconducting magnet systems applications. Most of the MRI systems are in use in hospitals and clinics, and incorporate superconducting magnets.

单词	superconducting devices
释义	Superconducting devices Superconducting devices Devices that perform functions in the superconducting state that would be difficult or impossible to perform at room temperature, or that contain components which perform such functions. The superconducting state involves a loss of electrical resistance and occurs in many metals and alloys at temperatures near absolute zero. An enormous impetus was provided by the discovery in 1986 of a new class of ceramic, high-transition-temperature (T_c) superconductors, which has resulted in a new superconducting technology at liquid nitrogen temperature. Superconducting devices may be conveniently divided into two categories: small-scale thin-film devices, and large-scale devices which employ zero-resistance superconducting windings made of type II superconducting materials. See Superconductivity Small-scale devices A variety of thin-film devices offer higher performance than their nonsuperconducting counterparts. The prediction and discovery in the early 1960s of the Josephson effects introduced novel opportunities for ultrasensitive detectors, high-speed switching elements, and new physical standards. Niobium-based devices, patterned on silicon wafers using photolithographic techniques taken over from the semiconductor industry, have reached a high level of development, and a variety of such devices are commercially available. These devices operate at or below 4.2 K (-452°F), the temperature of liquid helium boiling under atmospheric pressure. See Liquid helium The discovery of the high-transition-temperature superconductors has enabled the operation of devices in liquid nitrogen at 77 K (-321°F). Not only is liquid nitrogen much cheaper and more readily available than liquid helium, but it also boils away much more slowly, enabling the use of simpler and more compact dewars or simpler, relatively inexpensive refrigerators. Of the new ceramic superconductors, only YBa₂Cu₃O_7-x (YBCO) has been developed in thin-film form to the point of practical applications, and several devices are available. Intensive materials research has resulted in techniques, notably laser-ablation and radio-frequency sputtering, for the epitaxial growth of high-quality films with their crystalline planes parallel to the surface of the substrate. Most of the successful Josephson-junction devices have been formed at the interface between two grains of YBCO. These so-called grain-boundary junctions are made by depositing the film either on a bicrystal in which the two halves of the substrate have a carefully engineered in-plane misalignment of the crystal axes, or across a step-edge patterned in the substrate. See Cryogenics, Grain boundaries Two types of superconducting quantum interference device (SQUID) detect changes in magnetic flux: the dc SQUID and the rf SQUID. The dc SQUID, which operates with a dc bias current, consists of two Josephson junctions incorporated into a superconducting loop. The maximum dc supercurrent, known as the critical current, and the current-voltage (I-V) characteristic of the SQUID oscillate when the magnetic field applied to the device is changed. The oscillations are periodic in the magnetic flux &PHgr; threading the loop with a period of one flux quantum, &PHgr;₀ = h/2e ≈ 2.07 × These magnetometers have found application in geophysics, for example, in magnetotellurics. Low-transition-temperature SQUIDs are widely used to measure the magnetic susceptibility of tiny samples over a wide temperature range. Another application is a highly sensitive voltmeter, used in measurements of the Hall effect and of thermoelectricity. Low-transition-temperature SQUIDs are used as ultrasensitive detectors of nuclear magnetic and nuclear quadrupole resonance, and as transducers for gravitational-wave antennas. So-called scanning SQUIDs are used to obtain magnetic images of objects ranging from single-flux quanta trapped in superconductors to subsurface damage in two metallic sheets riveted together. See Hall effect, Magnetic susceptibility, Thermoelectricity, Voltmeter Perhaps the single largest area of application is biomagnetism, notably to image magnetic sources in the human brain or heart. In these studies an array of magnetometers or gradiometers is placed close to the subject, both generally being in a magnetically shielded room. The fluctuating magnetic signals recorded by the various channels are analyzed to locate their source. These techniques have been used, for example, to pinpoint the origin of focal epilepsy and to determine the function of the brain surrounding a tumor prior to its surgical removal. The most sensitive detector available for millimeter and submillimeter electromagnetic radiation is the superconductor-insulator-superconductor (SIS) quasiparticle mixer. In this tunnel junction, usually niobium–aluminum oxide–niobium, the Josephson supercurrent is quenched and only single-electron tunneling occurs. The current-voltage characteristic exhibits a very sharp onset of current at a voltage 2Δ/e, where Δ is the superconducting energy gap. The mixer is biased near this onset where the characteristics are highly nonlinear and used to mix the signal frequency with the frequency of a local oscillator to produce an intermediate frequency that is coupled out into a low-noise amplifier. These mixers are useful at frequencies up to about 750 GHz (wavelengths down to 400 micrometers). Such receivers are of great importance in radio astronomy, notably for airborne, balloon-based, or high-altitude, ground-based telescopes operating above most of the atmospheric water vapor. The advent of high-transition-temperature superconductors stimulated major efforts to develop passive radio-frequency and microwave components that take advantage of the low electrical losses offered by these materials compared with normal conductors in liquid nitrogen. The implementation of thin-film YBCO receiver coils has improved the signal-to-noise ratio of nuclear magnetic resonance (NMR) spectrometers by a factor of 3 compared to that achievable with conventional coils. This improvement enables the data acquisition time to be reduced by an order of magnitude. These coils also have potential applications in low-frequency magnetic resonance imaging (MRI). High-transition-temperature bandpass filters have application in cellular communications. Large-scale devices Large-scale applications of superconductivity comprise medical, energy, transportation, high-energy physics, and other miscellaneous applications such as high-gradient magnetic separation. When strong magnetic fields are needed, superconducting magnets offer several advantages over conventional copper or aluminum electromagnets. Most important is lower electric power costs because once the system is energized only the refrigeration requires power input, generally only 5–10% that of an equivalent-field resistive magnet. Relatively high magnetic fields achievable in unusual configurations and in smaller total volumes reduce the costs of expensive force-containment structures. See Magnet Niobium-titanium (NbTi) has been used most widely for large-scale applications, followed by the A15 compounds, which include niobium-tin (Nb₃Sn), niobium-aluminum (Nb-Al), niobium-germanium (Nb-Ge), and vanadium-gallium (Va₃Ga). Niobium-germanium held the record for the highest critical field (23 K; -418.5°F) until the announcement of high-temperature ceramic superconductors. See A15 phases Significant advances have been made in high-temperature superconducting wire development. Small coils have been wound that operate at 20 K (-410°F). Current leads are in limited commercial use. Considerable development remains necessary to use these materials in very large applications. MRI dominates superconducting magnet systems applications. Most of the MRI systems are in use in hospitals and clinics, and incorporate superconducting magnets. Some of the largest-scale superconducting magnet systems are those considered for energy-related applications. These include magnetic confinement fusion, superconducting magnetic energy storage, magnetohydrodynamic electrical power generation, and superconducting generators. See Nuclear fusion In superconducting magnetic energy storage superconducting magnets are charged during off-peak hours when electricity demand is low, and then discharged to add electricity to the grid at times of peak demand. The largest systems would require large land areas, for example, an 1100-m-diameter (3600-ft) site for a 5000-MWh system. However, intermediate-size systems are viable. A 6-T peak-field solenoidal magnet system designed for the Alaskan power network stores 1800 megajoules (0.5 MWh). High-purity-aluminum-stabilized niobium-titanium alloy conductor carrying 16 kiloamperes current is used for the magnet winding. Superconducting magnets have potential applications for transportation, such as magnetically levitated vehicles. In addition, superconducting magnets are used in particle accelerators and particle detectors. See Particle accelerator, Particle detector Superconducting devices Devices that perform functions in the superconducting state that would be difficult or impossible to perform at room temperature, or that contain components which perform such functions. The superconducting state involves a loss of electrical resistance and occurs in many metals and alloys at temperatures near absolute zero. An enormous impetus was provided by the discovery in 1986 of a new class of ceramic, high-transition-temperature (T_c) superconductors, which has resulted in a new superconducting technology at liquid nitrogen temperature. Superconducting devices may be conveniently divided into two categories: small-scale thin-film devices, and large-scale devices which employ zero-resistance superconducting windings made of type II superconducting materials. Small-scale devices A variety of thin-film devices offer higher performance than their nonsuperconducting counterparts. The prediction and discovery in the early 1960s of the Josephson effects introduced novel opportunities for ultrasensitive detectors, high-speed switching elements, and new physical standards. Niobium-based devices, patterned on silicon wafers using photolithographic techniques taken over from the semiconductor industry, have reached a high level of development, and a variety of such devices are commercially available. These devices operate at or below 4.2 K (-452°F), the temperature of liquid helium boiling under atmospheric pressure. The discovery of the high-transition-temperature superconductors has enabled the operation of devices in liquid nitrogen at 77 K (-321°F). Not only is liquid nitrogen much cheaper and more readily available than liquid helium, but it also boils away much more slowly, enabling the use of simpler and more compact dewars or simpler, relatively inexpensive refrigerators. Of the new ceramic superconductors, only YBa₂Cu₃O_7-x (YBCO) has been developed in thin-film form to the point of practical applications, and several devices are available. Intensive materials research has resulted in techniques, notably laser-ablation and radio-frequency sputtering, for the epitaxial growth of high-quality films with their crystalline planes parallel to the surface of the substrate. Most of the successful Josephson-junction devices have been formed at the interface between two grains of YBCO. These so-called grain-boundary junctions are made by depositing the film either on a bicrystal in which the two halves of the substrate have a carefully engineered in-plane misalignment of the crystal axes, or across a step-edge patterned in the substrate. See Cryogenics, Grain boundaries Two types of superconducting quantum interference device (SQUID) detect changes in magnetic flux: the dc SQUID and the rf SQUID. The dc SQUID, which operates with a dc bias current, consists of two Josephson junctions incorporated into a superconducting loop. The maximum dc supercurrent, known as the critical current, and the current-voltage (I-V) characteristic of the SQUID oscillate when the magnetic field applied to the device is changed. The oscillations are periodic in the magnetic flux &PHgr; threading the loop with a period of one flux quantum, &PHgr;₀ = h/2e ≈ 2.07 × 10^-15 weber, where h is Planck's constant and e is the magnitude of the charge of the electron. Thus, when the SQUID is biased with a constant current, the voltage is periodic in the flux. The SQUID is almost invariably operated in a flux-locked loop. A change in the applied flux gives rise to a corresponding current in the coil that produces an equal and opposite flux in the SQUID. The SQUID is thus the null detector in a feedback circuit, and the output voltage is linearly proportional to the applied flux. See SQUID The rf SQUID consists of a single Josephson junction incorporated into a superconducting loop and operates with an rf bias. The SQUID is coupled to the inductor of an LC-resonant circuit excited at its resonant frequency, typically 30 MHz. The characteristics of rf voltage across the tank circuit versus the rf current depends on applied flux. With proper adjustment of the rf current, the amplitude of the rf voltage across the tank circuit oscillates as a function of applied flux. The rf SQUID is also usually operated in a feedback mode. SQUIDs are mostly used in conjunction with an input circuit. For example, magnetometers are made by connecting a superconducting pickup loop to the input coil to form a flux transformer. A magnetic field applied to the pickup loop induces a persistent current in the transformer and hence a magnetic flux in the SQUID. These magnetometers have found application in geophysics, for example, in magnetotellurics. Low-transition-temperature SQUIDs are widely used to measure the magnetic susceptibility of tiny samples over a wide temperature range. Another application is a highly sensitive voltmeter, used in measurements of the Hall effect and of thermoelectricity. Low-transition-temperature SQUIDs are used as ultrasensitive detectors of nuclear magnetic and nuclear quadrupole resonance, and as transducers for gravitational-wave antennas. So-called scanning SQUIDs are used to obtain magnetic images of objects ranging from single-flux quanta trapped in superconductors to subsurface damage in two metallic sheets riveted together. See Thermoelectricity, Voltmeter Perhaps the single largest area of application is biomagnetism, notably to image magnetic sources in the human brain or heart. In these studies an array of magnetometers or gradiometers is placed close to the subject, both generally being in a magnetically shielded room. The fluctuating magnetic signals recorded by the various channels are analyzed to locate their source. These techniques have been used, for example, to pinpoint the origin of focal epilepsy and to determine the function of the brain surrounding a tumor prior to its surgical removal. The most sensitive detector available for millimeter and submillimeter electromagnetic radiation is the superconductor-insulator-superconductor (SIS) quasiparticle mixer. In this tunnel junction, usually niobium–aluminum oxide–niobium, the Josephson supercurrent is quenched and only single electron tunneling occurs. The current-voltage characteristic exhibits a very sharp onset of current at a voltage 2Δ/e, where Δ is the superconducting energy gap. The mixer is biased near this onset where the characteristics are highly nonlinear and used to mix the signal frequency with the frequency of a local oscillator to produce an intermediate frequency that is coupled out into a low-noise amplifier. These mixers are useful at frequencies up to about 750 GHz (wavelengths down to 400 micrometers). Such receivers are of great importance in radio astronomy, notably for airborne, balloon-based, or high-altitude, ground-based telescopes operating above most of the atmospheric water vapor. The advent of high-transition-temperature superconductors stimulated major efforts to develop passive radio-frequency and microwave components that take advantage of the low electrical losses offered by these materials compared with normal conductors in liquid nitrogen. The implementation of thin-film YBCO receiver coils has improved the signal-to-noise ratio of nuclear magnetic resonance (NMR) spectrometers by a factor of 3 compared to that achievable with conventional coils. This improvement enables the data acquisition time to be reduced by an order of magnitude. These coils also have potential applications in low-frequency magnetic resonance imaging (MRI). High-transition-temperature bandpass filters have application in cellular communications. See Mobile radio Large-scale devices Large-scale applications of superconductivity comprise medical, energy, transportation, high-energy physics, and other miscellaneous applications such as high-gradient magnetic separation. When strong magnetic fields are needed, superconducting magnets offer several advantages over conventional copper or aluminum electromagnets. Most important is lower electric power costs because once the system is energized only the refrigeration requires power input, generally only 5–10% that of an equivalent-field resistive magnet. Relatively high magnetic fields achievable in unusual configurations and in smaller total volumes reduce the costs of expensive force-containment structures. See Magnet Niobium-titanium (NbTi) has been used most widely for large-scale applications, followed by the A15 compounds, which include niobium-tin (Nb₃Sn), niobium-aluminum (Nb-Al), niobium-germanium (Nb-Ge), and vanadium-gallium (Va₃Ga). Niobium-germanium held the record for the highest critical field (23 K; -418.5°F) until the announcement of high-temperature ceramic superconductors. Significant advances have been made in high-temperature superconducting wire development. Small coils have been wound that operate at 20 K (-410°F). Current leads are in limited commercial use. Considerable development remains necessary to use these materials in very large applications. MRI dominates superconducting magnet systems applications. Most of the MRI systems are in use in hospitals and clinics, and incorporate superconducting magnets. Some of the largest-scale superconducting magnet systems are those considered for energy-related applications. These include magnetic confinement fusion, superconducting magnetic energy storage, magnetohydrodynamic electrical power generation, and superconducting generators. See Magnetohydrodynamic power generator, Nuclear fusion In superconducting magnetic energy storage superconducting magnets are charged during off-peak hours when electricity demand is low, and then discharged to add electricity to the grid at times of peak demand. The largest systems would require large land areas, for example, an 1100-m-diameter (3600-ft) site for a 5000-MWh system. However, intermediate-size systems are viable. A 6-T peak-field solenoidal magnet system designed for the Alaskan power network stores 1800 megajoules (0.5 MWh). High-purity-aluminum-stabilized niobium-titanium alloy conductor carrying 16 kiloamperes current is used for the magnet winding. Superconducting magnets have potential applications for transportation, such as magnetically levitated vehicles. In addition, superconducting magnets are used in particle accelerators and particle detectors. See Magnetic levitation
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