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Magnetic Levitation Using High-Temperature Superconductors

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Magnetic Levitation Using High-Temperature Superconductors A Train Without Wheels: Magnetic Levitation Using High‐Temperature Superconductors Nicholas R. Anderson and Morten Ring Eskildsen Department of Phyy,sics, University of Notre Dame, Notre Dame, Indiana 46656, USA What is a Superconductor? History of Superconductivity A superconductor is a material which conduct an electric current without The history of superconductivity is rich in discoveries, both of new pheno‐ of superconductivity: The phenomenological Ginzburg‐Landau (GL) theory resistance. Superconductivity is observed in a variety of materials such as tin, mena, new superconducting materials and breakthroughs in our and the microscopic Bardeen‐Cooper‐Schrieffer (BCS) model. Furthermore, in aluminum, metallic alloys, and even some ceramics. A common property of understanding of the fundamental 1957, Alexei Abrikosov predicted the superconductors, is that the zero resistance state only occur below their so‐ physics behind it all. Many of these existence of a second (type‐II) class of called critical temperature which varies for different materials. In a properly discoveries have earned Nobel prices. superconductors and the formation cooled loop of superconducting material, a current can flow indefinitely. The figure shows a timeline of the of magnetic vortices in such materials. Practical applications of superconductors have become more common as history of superconductivity. In 1986 Johannes Bednorz and progress has been made in discovering materials with increased critical Superconductivity was discovered in K. Alex Müller discovered the first temperature and thereby reducing the cost of cooling. Currently, the highest 1911 by Heike Kamerlingh‐Onnes who ceramic, high temperature supercon‐ known critical temperature is 138 K (‐211 F). used liquid helium to cool mercury to ductor, with transition temperatures Superconductors can be used to make extremely effective electromagnets, 4.19 K ( ‐452° F) and observed a total which soon exceeded the boiling which are commonly used in equipment such as MRI scanners, NMR loss of electrical resistance. In 1912 he point of liquid Nitrogen at 77 K machines, and mass spectrometers. They are also used to guide the beams in showed that a current running in a (‐321 F). In recent years superconduc‐ particle accelerators such as the Large Hadron Collider at CERN. superconductor does not diminish over tivity have been found in MgB2 as time. For his discoveries, well as the iron‐arsenic pnictides. Kammerlingh‐Onnes was awarded the Current research includes both 1913 Noble Prize in Physics. studies of the fundamental properties In 1933 Walter Meissner and Robert of superconductors as well as the Ochsenfeld found that superconducting potential for practical applications. materials expelled magnetic fields. At Notre Dame several groups are Together with zero resistance, this involved in such research, including Perfect diamagnetism constitutes the the synthesis of novel superconduc‐ two hallmark properties of a ting materials, studies of the superconductor. properties of exotic superconductors The 1950‐ies saw the emergence of the and their vortices, and the Loss free energy transport in superconduc‐ 36.5 MW ship propulsion motor manufac‐ two most successful theoretical models applications in e.g. single‐photon ting cable (physicsweb.org). tured by American Superconductor. sensors. Superconducting Vortices How Does Levitation Work? Real Levitating Trains In addition to their reakableemarkable elec tr ic proetieoperties, sueuperc odutoonductors als o Our maetiagnetic lev itation demo nst ratio n Magneti c lev itation (ale)(maglev) taitrains aeare in ueuse in seealeveral plac es thr oug hout behave in a curious fashion when they are exposed to a magnetic field. If the consists of two parts: A magnetic track and the world. The first levitating passenger train operated in Hamburg, Germany magnetic field is weak it is completely expelled from the superconductor YBCO superconductors with a critical tem‐ in 1979, although only permanent magnets were used. While wide‐spread use which is known as the Meissner‐Ochsenfeld effect. perature of 93 K (‐292 F) cooled by liquid is still uncommon, there are plans for levitating trains in place in many At higher fields the superconductor is unable to expel nitrogen. The track (1) is made up of several countries. it, and in stead vortices (or electric tornadoes) appear. hundred individual magnets organized in By using superconductors, levitating trains at test tracks in Japan have been The magnetic field is funneled through the vortices, three parallel rows, oriented such that the able to reach a speed of 360 mph. which are surrounded by superconducting electric magnetic field is directed up at the center currents. The vortices each carry one quantum of and down at the edges. magnetic flux (h/2e) and therefore a change of the field When the superconductors is cooled below the will cause the vortex density to increase or decrease. critical temperature in the presence of the track’s Manned MLX01‐901 train at the Yamanashi Depending on the quality of the superconductor the vortices may either be magnetic field (red lines), vortices are nucleated in Maglev Test Line in Japan. This train have free to move (clean samples) or they may be strongly pinned to defects (dirty the YBCO (2). Due to the large degree of reached a speed of 360 mph and sustained a samples). In the latter case the vortices will oppose any attempt to change the imperfections in the YBCO material the vortices two‐train crossing test at a relative speed of magnetic field since it would require them to move in or out of the are strongly pinned, creating a stable imprint of 637 mph. superconductor. the magnetic field (black lines) at the time of cooling (3). Picture from the Railway Technical Research Institute, Japan. A slight displacement of the superconductor will now lead to a mismatch of the magnetic field and the “frozen in” vortices, and give rise to a restoring force which seeks to realign the two (4). The Acknowledgment restoring force acts both perpendicular to the track as well as in the vertical direction. Only along the This demonstration was developed as a part of the project Magnetic field lines confined to vortices as direction of the track is the magnetic field entitled “Vortices and the Interplay between Superconductivity they pass through a superconductor. Vortices emerging at the surface of a super‐ uniform (1) and the superconductor is free to move. and Magnetism” and funded by the National Science conductor, imaged by Scanning Tunneling Foundation through grant no. DMR‐0804887. Spectroscopy..
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