Direct comparisons of quantized Hall resistances

The quantized Hall resistances (QHRs) of three GaAs/Al/sub x/Ga/sub 1-x/As heterostructures were directly compared. Two of the three samples were grown by metalorganic chemical vapor deposition. and the third was produced by molecular beam epitaxy. A cryogenic current comparator bridge was used for the measurements. The provisional results show a residual difference lower than a few parts in 10/sup 9/ with an uncertainty of the same order.<
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