When two conductors are brought so close together that wavefunctions of electrons in them begin to overlap appreciably, electrons in one of the conductors may tunnel into the other. The magnitude of this tunneling current depends, if the current flows, say, from a probe tip to a surface to be probed, primarily on the probability that electron states having energy and momentum similar to those of electrons in the tip exist and are unoccupied in the surface to be probed. The scanning tunneling microscope, in which a metal probe scans a surface and the tunneling current at various positions is measured, therefore yields information about the nature and density of electron states on the scanned surface. STM studies of a clean surface of 2HNbSe2 when it is superconducting have now revealed new and unexpected features in the spectrum of electron states in magnetic flux lines in superconductors. The remarkable spatial resolution of the STM, which has steadily yielded new and interesting results about physics at surfaces since the instrument was invented, once again underlies the new discoveries: Without the STM it might not have been possible to probe electronic states on the scale of a few angstroms.

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