実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report

課題番号 Project No.

装置責任者 Name of responsible person

2017A0074

Toshiya Otomo

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Relationship between the local lattice distortion with Aem2 symmetry

NOVA (BL21)

in parent compound and higher Tc in iron-based superconductor

実施日 Date of Experiment

実験責任者名 Name of principal investigator

2017/6/18-21

Katsuaki Kodama
所属 Affiliation
Japan Atomic Energy Agency
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.

Parent compounds of iron-based superconductors, LaFeAsO and NdFeAsO powders

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

We have observed the local lattice distortion in the parent compound of iron-based superconductor PrFeAsO
in the previous measurement (2015A0061). The broad Pr-As atomic correlation peak observed in the atomic
pair distribution function (PDF) at 20 K cannot be reproduced by the averaged structure with space group Cmma,and can be reproduced by the structure model with space group Aem2, indicating the existence of the local lattice
distortion with symmetry Aem2.

This space group is the same as the space group of the overdoped

antiferromagnetic phase, AF2, in LaFeAsO1-xHx (x>0.4) [1]. In LaFeAsO1-xHx system, two superconducting
phases SC1 and SC2 appear at x～0.1 and x～0.33[2]. The AF2 phase is regarded as the parent phase of theSC2 phase. The maximum value of Tc of the SC2 phase (～36 K) is larger than the value of the SC1 phase (～26 K). The maximum Tc of the superconducting phase of Pr1111 systems whose parent compound PrFeAsO
has the local lattice distortion with Aem2 symmetry, is about 50 K which is one of the highest values amongiron-based superconductors. These results suggest that the long and short-range lattice distortions with Aem2
symmetry in the parent phase advantageous to higher Tc in the superconducting phase.

To reveal therelationship between the Aem2 lattice distortion in the parent phase and Tc-value in the superconducting phase,the local structures of the parent1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
parent compounds LaFeAsO and NdFeAsO
whosemaximum

Tc-values

inthesuperconducting phases are about 26 K and 50
K, respectively, are studied by PDF analysis.
The powder diffraction measurements havebeen performed at NOVA.

The data werecollected at 5, 20 and 160 K, for both samples.
To obtain the PDF, the structure function S(Q)obtained at 90 degree bank with 1.1 < Q < 40Å-1 is connected with S(Q) obtained at 45
degree bank with 0.6 < Q < 1.1Å-1. In Fig. 1,PDF of LaFeAsO at 20 K is shown by opencircle. The blue line is the PDF calculated

Fig. 1 PDF of LaFeAsO obtained at 20 K

by using the average structure with space(open circle). Blue and red lines are the PDFs

groupCmma.

Thelinealmostcalculated by using the structure models withreproduces the observed PDF.

The redspace groups Cmma and Aem2, respectively

line is the PDFs calculated by using thestructure models with space group Aem2. The red line almost overlap with the blue line. The
estimated values of the atomic shifts of Fe and As along a-direction giving the breaking theinversion symmetry are much smaller than the atomic shifts in PrFeAsO.

These indicate thatthe local lattice distortion with Aem2 symmetry is negligibly small in LaFeAsO, supporting theabove suggestion on the relationship between the local lattice distortion and maximum Tc-value.
The local structural analysis on NdFeAsO is being performed to further reveal the validity of theabove suggestion.
references[1] M. Hiraishi et al. Nature Physics 10, 300 (2014).
[2] S. Iimura et al., Nature Commun. 3, 943 (2012).

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