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MATERIALS RESEARCH
Two independent research teams have succeeded in growing high-quality, oriented thin films of magnesium diboride (MgB2), an advance that is seen as crucial for the development of a new, more efficient generation of superconducting electronic devices. Current niobium-based superconductor circuits must operate at temperatures near 4.2 K, which requires heavy cryocoolers. MgB2-based circuits would operate in the 20–25-K range, which is achievable using a compact cryocooler with roughly one-tenth the mass and power consumption of a 4.2-K cooler with the same cooling capacity. The two teams used different approaches. One, led by Chang-Beom Eom, a professor of materials science and engineering (MS&E) at the University of Wisconsin, Madison, deposited boron films using radio-frequency magnetron sputtering and then annealed the films at 850 °C in magnesium vapor [Appl. Phys. Lett., 81, 1851 (2002)]. The other team, led by Xiaoxing Xi, associate professor of physics and MS&E at Pennsylvania State University, University Park, grew MgB2 films from diborane (B2H6) and magnesium vapor at temperatures up to 760 °C [Nat. Mater., 1, 35 (2002)]. Darrell G. Schlom, a Penn State MS&E professor who is a coauthor on both papers, says the films of Eom and Xi are of comparable structural quality. These films are superior to earlier MgB2 films in that they are epitaxial--all the crystal grains are lined up with the substrate's. Eom made his epitaxial MgB2 films before Xi, but they contain MgO impurities, which Xi's films do not. As a result, Xi's films superconduct at a higher temperature--39.3 K versus 35 K. "It's difficult to make clean films," Schlom notes, and Xi's films "are the cleanest in the world." Xi believes that his group's technique will be more suitable for the fabrication of multilayer devices. Indeed, superconductivity expert John M. Rowell of Northwestern University, commenting in Nature Materials, notes that Xi's approach to film synthesis "promises to allow a breakthrough" in the fabrication of multilayer devices based on MgB2. | |||||||||
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