Physics

Comeback Superconductivity

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Science  16 Mar 2012:
Vol. 335, Issue 6074, pp. 1280
DOI: 10.1126/science.335.6074.1280-b

The properties of materials at low temperatures are often tuned by varying chemical composition or pressure. In iron-based superconductors, the transition temperature Tc at which their electrical resistivity plummets to zero exhibits a maximum as a function of both composition and pressure; beyond these optimal settings, Tc monotonically falls to zero. Sun et al. report radically different behavior in some representatives of a family described by the formula A1-xFe2-ySe2 (where A = K, Rb, or Cs, with possible Tl substitution). In these materials, an increase in pressure beyond the optimal pressure is accompanied by the usual monotonic decrease of Tc down to zero; however, superconductivity unexpectedly and abruptly reappears at even higher pressures and then disappears just as abruptly. Notably, the reemergent Tc is substantially higher than the maximum Tc in the conventional part of the phase diagram (48.7 K for the compound K0.8Fe1.7Se2, approaching the highest Tc for an iron-based superconductor of 55 K). The explanation for this unusual effect awaits further structural and magnetic characterization of the samples under pressure, although preliminary results indicate that the tetragonal crystal structure is preserved throughout.

Nature 483, 67 (2012).