# Contrasting impurity scattering and pair breaking effects by doping Mn and Zn in Ba$_{0}$$_{.}$$_{5}$K$_{0}$$_{.}$$_{5}$Fe$_{2}$As$_{2}$ 

$^{2}$Department of Physics, Purdue University, West Lafayette, Indiana 47907, U.S.A. 

Peng Cheng $^{1}$, Bing Shen$^{1}$, Jiangping Hu1 , 2 and Hai-Hu Wen 1 ⋆ 

$^{1}$National Laboratory for Superconductivity, Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China and 

Resistivity, Hall effect, magnetoresistance and DC magnetization were measured in Mn and Zn doped Ba 0 . 5 K 0 . 5 Fe 2 As 2 samples. It is found that the Mn-doping can depress the superconducting transition temperature drastically with a rate of Δ T c /Mn-1% = -4.2 K, while that by Zn-doping is negligible. Detailed analysis reveals that the Mn-doping enhances the residual resistivity ( ρ 0 ) significantly, and induces strong local magnetic moments ( ∼ 2.58 µ B ) which play as pair breakers. While the impurity scattering measured by ρ 0 in the Zn-doped samples is much weaker, accompanied by a negligible pair breaking effect. A possible explanation is that the impurity scattering by the Zn impurities are mainly small angle scattering (or small momentum transfer), therefore it cannot break the pairing induced by the interpocket scattering and thus affect the superconducting transition temperature weakly. 

The discovery of superconductivity above 50 K in iron pnictides has posed a strong impact in the community of condensed matter physics. 1 One of the key issues here is about the superconducting pairing mechanism. Theoreti- cally it was suggested that the pairing may be established via inter-pocket scattering of electrons between the hole pockets (around Γ point) and electron pockets (around M point), leading to the pairing manner of an isotropic gap on each pocket but with opposite signs between them (the so-called S $^{±}$). 2–5 Meanwhile other models adopt the S ± pairing gap but assume that the pairing interaction is established via the local magnetic super-exchange. 6,7 Besides, by varying the height of the pnictogen to the Fe planes, it was argued that the pairing symmetry may be switched from S ± to d-wave pairing, 8 as corroborated by the data in LaFePO where a nodal gap was inferred from the penetration depth measurements. 9,10 Similarly, experimental results about the pairing symmetry remain highly controversial leaving the perspectives ranging from S $^{++}$-wave, to S ± and to d-wave. 9,11–20 Further compre- hension to this essential topic is highly desired. 

In a superconductor, the disorder induced impurity scattering and pair breaking strongly depend on the very details of the pairing gap, therefore it is informative to detect the disorder scattering effect in the superconduct- ing state. According to the Anderson’s theorem, 21 in a conventional s-wave superconductor, nonmagnetic im- purities will not lead to apparent pair-breaking effect. However, a magnetic impurity, owing to the effect of breaking the time reversal symmetry, can break Cooper pairs easily. In sharp contrast, in a d-wave superconduc- tor, nonmagnetic impurities can significantly alter the pairing interaction and induce a high density of states (DOS) due to the sign change of the gap on a Fermi sur- face. This was indeed observed in cuprate superconduc- 

PACS numbers: 74.20.Rp, 74.70.Dd, 74.62.Dh, 65.40.Ba 

## I. INTRODUCTION 

tors where Zn-doping induces T$_{c}$-suppression as strong as other magnetic disorders, such as Mn and Ni. 22 As for the pairing through exchanging the AF spin fluctua- tions between different Fermi pockets with the S ± pair- ing, it has been pointed out that nonmagnetic impurities could severely suppress T$_{c}$ and the gap. 23–31 In this pa- per, we report the doping effect of Mn and Zn to the Fe sites of superconductor Ba$_{0}$$_{.}$$_{5}$K$_{0}$$_{.}$$_{5}$Fe$_{2}$As$_{2}$. We found that the Mn-doping (leading to the magnetic impurities) de- presses T$_{c}$ drastically, while the impurity scattering and suppression to T$_{c}$ by Zn-doping (nonmagnetic scattering centers) is negligible. The contrasting impurity scatter- ing effects as revealed by our results need to be reconciled with the theoretical expectations of the picture of inter- pocket scattering via exchanging AF spin fluctuations. 

The Mn-doped and Zn-doped polycrystalline samples Ba$_{0}$$_{.}$$_{5}$K$_{0}$$_{.}$$_{5}$(Fe$_{1}$ − $_{x}$TM$_{x}$ )$_{2}$As$_{2}$ (TM = Mn and Zn) were fab- ricated by solid state reaction method $^{32}$. Powders K$_{3}$ As, FeAs, BaAs, ZnAs and MnAs were prepared previously as precursors. The samples with different doping concen- trations were pressed into pellets under the same pres- sure, wrapped in Ta foils and sintered under exactly the same conditions to eliminate the possible errors in the sample-making process. The x-ray diffraction (XRD) measurement was performed using an MXP18A-HF-type diffractometer with Cu-K$_{α}$ radiation. The analysis of x-ray diffraction data was done by using the softwares POWDER-X and Fullprof, the obtained results are con- sistent with each other. The compositions of the samples were examined by scanning electron microscopy (SEM, Hitachi S-4200) and the energy dispersive x-ray anal- ysis (EDX, Oxford-6566). The AC susceptibility mea- surements were carried out through an Oxford cryogenic 

## II. EXPERIMENTAL 

