Volume 86, Number 4, May 2009
Article Number 47001
Number of page(s) 6
Section Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties
Published online 26 May 2009
EPL, 86 (2009) 47001
DOI: 10.1209/0295-5075/86/47001

Superfluid density of the $\pm $s-wave state for the iron-based superconductors

Yunkyu Bang

Department of Physics, Chonnam National University - Kwangju 500-757, Korea and Asia Pacific Center for Theoretical Physics - Pohang 790-784, Korea

received 14 April 2009; accepted in final form 29 April 2009; published May 2009
published online 26 May 2009

We study the superfluid density of the $\pm $s-wave state of the minimal two-band model for the Fe-based superconductors and its evolution with impurity concentration. We show that the impurity scattering of the strong-coupling limit induces the self-energy of a generic form ${\rm Im}\Sigma _{imp}(\omega )\approx i\gamma +i\beta \omega $ beyond a critical impurity concentration $\Gamma _{imp} > \Gamma _{crit}$. This form of ${\rm Im}\Sigma _{imp}(\omega)$ causes the temperature dependence of the superfluid density $[\rho _{s}(T)-\rho _{s}(0)]\approx -\gamma T^{2}-\beta T^{3}$. Combined with the full gap behavior of $\rho _{s}(T)$ for lower impurity concentration $\Gamma _{imp} < \Gamma _{crit}$, the $\pm $s-wave state produces a continuous evolution of $\Delta \lambda (T)$: exponentially flat $\rightarrow T^{3}\rightarrow T^{2}$ with increasing impurity concentration that is consistent with the measurements of the Fe pnictide superconductors such as M-1111 (M = La, Nd, Sm, Pr) and Ba-122 with various dopings, except LaFePO which shows $\Delta \lambda (T)\propto T^{1.2}$ at low temperatures by a recent experiment. Our results also demonstrate that the density of states (DOS) measured by thermodynamic properties and the DOS measured by transport properties can in general be different.

74.20.-z - Theories and models of superconducting state.
74.20.Rp - Pairing symmetries (other than s-wave).
74.25.Nf - Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.).

© EPLA 2009