Mind the Gap

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Science  20 Dec 2013:
Vol. 342, Issue 6165, pp. 1419
DOI: 10.1126/science.342.6165.1419-a

One of the basic characteristics of a superconductor is the energy needed to break up Cooper pairs, which form the superfluid flow in these materials. In conventional superconductors such as Nb, this energy, twice the size of the so-called superconductor “gap,” does not depend on the momenta of the electrons forming the pair; however, in cuprates, for example, the gap disappears entirely at certain points in momentum space. The symmetry of the gap in iron-based superconductors is still under debate, and there are indications that it might not be the same for all of them. Yang et al. used scanning tunneling spectroscopy to observe the evolution of the local density of states (LDOS) near a nonmagnetic impurity dopant (Cu) in the material Na(Fe 0.96-xCo0.03Cu x)As. The effect of a Cu impurity on LDOS appeared to be a pronounced enhancement near 2 meV, inside the superconductor gap. Such in-gap states near nonmagnetic impurities are consistent with the gap symmetry, where the electron and hole pockets of the Fermi surface have opposite signs of the gap function; the authors performed magnetization measurements to demonstrate that the Cu impurities are indeed nonmagnetic. Comparison to theory indicated that the results were incompatible with other proposed gap symmetries, which may have implications for the mechanism of superconductivity in these materials.

Nat. Comm. 4, 2749 (2013).

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