Enhancement of $T_c$ by disorder in underdoped iron pnictides

Abstract

We analyze how disorder affects the transition temperature $T_c$ of the $s^{+-}$superconducting state in the iron pnictides. The conventional wisdom is that $T_c$ should rapidly decrease with increasing inter-band non-magnetic impurity scattering, but we show that this behavior holds only in the overdoped region of the phase diagram. In the underdoped regime, where superconductivity emerges from a pre-existing magnetic state, disorder gives rise to two competing effects: breaking of the Cooper pairs, which tends to reduce $T_c$, and suppression of the itinerant magnetic order, which tends to bring $T_c$ up. We show that for a wide range of parameters the second effect wins, leading to an increase of $T_c$ with disorder in the coexistence state. Our results explain several recent experimental findings and provide another evidence for $s^{+-}$-pairing in the iron pnictides.

5 pages, 3 figures; revised version accepted in PRB-RC

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