Enhancement of TcT_{c} by disorder in underdoped iron pnictides

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.Comment: 5 pages, 3 figures; revised version accepted in PRB-R

Specific heat jump at superconducting transition in the presence of Spin-Density-Wave in iron-pnictides

We analyze the magnitude of the specific heat jump \Delta C at the superconducting transition temperature T_c in the situation when superconductivity develops in the pre-existing antiferromagnetic phase. We show that \Delta C/T_c differs from the BCS value and is peaked at the tri-critical point where this coexistence phase first emerges. Deeper in the magnetic phase, the onset of coexistence, T_c, drops and \Delta C/T_c decreases, roughly as \Delta C/T_c \propto T^2_c at intermediate T_c and exponentially at the lowest T_c, in agreement with the observed behavior of \Delta C/T_c in iron-based superconductors.Comment: 4+ pages, 3 figure

Magnetic penetration depth in disordered iron-based superconductors

We study the effect of disorder on the London penetration depth in iron-based superconductors. The theory is based on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering. Within the quasiclassical approximation we derive and solve Eilenberger's equations, which include a weak external magnetic field, and provide analytical expressions for the penetration depth in the various limiting cases. A complete numerical analysis of the doping and temperature dependence of the London penetration depth reveals the crucial effect of disorder scattering, which is especially pronounced in the coexistence phase. The experimental implications of our results are discussed.Comment: 10 pages, 6 figure