Microwave Surface-Impedance Measurements of the Magnetic Penetration Depth in Single CrystalBa1−xKxFe2As2Superconductors: Evidence f

Abstract

We report high-sensitivity microwave measurements of the in-plane penetration depth λ ab and quasiparticle scattering rate 1/τ in several single crystals of hole-doped Fe-based superconductor Ba1−xKxFe2As2 (x ≈ 0.55). While power-law temperature dependence of λ ab with the power ∼ 2 is found in crystals with large 1/τ , we observe exponential temperature dependence of superfluid density consistent with the existence of fully opened two gaps in the cleanest crystal we studied. The difference may be a consequence of different level of disorder inherent in the crystals. We also find a linear relation between the low-temperature scattering rate and the density of quasiparticles, which shows a clear contrast to the case of d-wave cuprate superconductors with nodes in the gap. These results demonstrate intrinsically nodeless order parameters in the Fe-arsenides. The discovery of high-T c superconductivity in Fepnictides [1] has attracted tremendous interests both experimentally and theoretically. The 'mother' materials have antiferromagnetic spin-density-wave order [2] and the superconductivity appears by doping charge carriers, either electrons or holes. Such carrier doping induced superconductivity resembles high-T c cuprates, but one of the most significant differences is the multiband electronic structure having electron and hole pockets in the Fe-based superconductors. Unconventional superconducting pairings, most notably the sign-reversing s ± state, have been suggested by several theories [3] featuring the importance of the nesting between the hole and electron bands. This is also in sharp contrast to other multiband superconductors such as MgB 2 , where the coupling between the different bands is very weak. Thus the most crucial is to clarify the novel multiband nature of superconductivity in this new class of materials.

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

“…These were placed in a BN crucible, sealed in a Mo capsule under Ar atmosphere, heated up to 1190 • C, and then cooled down at a rate of 4 • C/hours, followed by a quench at 850 • C. Energy dispersive Xray (EDX) analysis reveals the doping level x = 0.55(2) [36], which is consistent with the c-axis lattice constant c = 1.341(3) nm determined by X-ray diffraction [42]. Bulk superconductivity is characterized by the magnetization measurements using a commercial magnetometer [36]. We find that the superconducting transition temperature varies slightly from sample to sample [see Ta- ble I].…”

“…However, the reliable measurement of the lower critical field is a difficult task, in particular when strong vortex pinning is present as in the case of Fe-arsenides. We also point out that to date the reported values of anisotropy parameter strongly vary [31,32,33,34] Here we review our microwave surface impedance Z s measurements in single crystals of electrondoped PrFeAsO 1−y (y ∼ 0.1) [35] and hole-doped Ba 1−x K x Fe 2 As 2 (x ≈ 0.55) [36]. By using a sensitive superconducting cavity resonator, we can measure both real and imaginary parts of Z s pricesely in tiny single crystals, from which we can extract the in-plane penetration depth λ ab (T ) as well as quasiparticle conductivity σ 1 (T ) that yields information on the quasiparticle dynamics.…”

Reply to “Comment on ‘Precision global measurements of London penetration depth in FeTe0.58Se0.42’”

“…Yet, even this difference has resulted only in a factor of two, which is very far from a factor of five claimed by Klein et al). We also note the earlier study of the superfluid density in (Ba 1−x K x )Fe 2 As 2 by the same Kyoto group, in which the Authors explained the difference between different samples to be due to different scattering rates [10].Puzzled by Klein et al claim of such a profound effect of the surface roughness, we studied its effect experimentally. We used the same crystal to deliberately damage and re-measure λ(T ) after each damage.…”

“…Yet, even this difference has resulted only in a factor of two, which is very far from a factor of five claimed by Klein et al). We also note the earlier study of the superfluid density in (Ba 1−x K x )Fe 2 As 2 by the same Kyoto group, in which the Authors explained the difference between different samples to be due to different scattering rates [10].…”

Spontaneous Stoichiometry Change in Single Crystals of Superconducting (Ba1−x K x )Fe2As2 Grown by a Rapid-Heating Sn-Flux Method

“…Similar to high-T C oxocuprates the growth of homogeneous, and impurity free single crystals of the FeAs-based compounds was found to be difficult. So far, rather small single crystals of the layered oxopnictides LnFeAs(O 1−x F x ) (Ln = Pr, Nd, Sm) were grown by using a NaCl/KCl flux method [4,5]. Larger crystals of AE 1−x K x Fe 2 As 2 (AE = Ba, Sr) were obtained by using either a Sn-flux or a self-flux method [6][7][8][9].…”