⽬的Purpose

⽅法Method

展望Prospect

つくば­柏­本郷超伝導かけはしプロジェクトTsukuba-Kashiwa-HongoSuperconductivity Kakehashi Project

つくば­柏­本郷 超伝導かけはしプロジェクトNIMS 寺嶋太⼀TERASHIMA.Taichi@nims.go.jp

1144-type iron-pnictide superconductors

Physical properties

物性評価

1144型鉄系超伝導体

超伝導物質の応⽤可能性は、超伝導転移温度だけでなく、上部臨界磁場，臨界電流密度など種々の因⼦に左右される。それらの迅速な評価のため参画機関の連携を強化し、新しい超伝導物質を基礎研究の場から応⽤へ繋ぐ「かけはし」を築く。

・参画機関で協⼒し1144型鉄系超伝導体単結晶を合成し、基礎物性を評価する。・超伝導体の開発、評価をテーマにワークショップを開催する。

https://sites.google.com/view/tia-sc-workshop

・本連携により1144型鉄系超伝導体の単結晶合成に成功し、臨界電流密度などの評価を開始した。今後も5機関が連携して研究を進める。・産総研においては類縁超伝導体の発⾒、あるいは新たに単結晶化に成功するなどしており、それらについてもあわせて研究を進めていく。

1144型鉄系超伝導体とは• 産総研伊豫らが2016年に報告 (JACS(2016)、特許出願

中)• 固溶体でなく定⽐化合物であり線材応⽤に向いている

CaRbFe4As4

1144-typeP4/mmm

Ca

As(1)

As(2)Fe

Rb

Ae/A Na+ K+ Rb+ Cs+

Ca2+ ○36.5 K

◎33.1 K

◎35.0 K

◎31.6 K

(La3+,Na+) ○27.0 K 調査中

◎25.5 K

◎24.0 K

Sr2+ ○35.5 K

○36 K

◎34.9 K

◎36.4 K

Eu2+ ○34.7 K

○33 K

◎36.2 K

◎35.1 K

Ba2+ ○34 K

○38 K

○36.9 K

○26.1 K

○ 122 type（固溶体） ◎1144 type（定比化合物）

（右図）CaKFe4As4単結晶の臨界電流密度Jc。⾼磁場下でのJcが⾼温で増⼤する特異な振る舞い。 S. Ishida et al., npJ QM (2019),AIST、NIMS。また、S. Pyon et al., PRB (2019), 東⼤、AIST、NIMSも参照されたし。

contour plot in Fig. 4a. Several characteristic T and H correspond-ing to the two types of peak effect observed in Jc

H//c–T and MHLsare also marked. For comparison, the corresponding data forBaK122 (x= 0.3) which possesses the highest Jc among the 122materials27 are shown in Fig. 4b. The colour distribution forCaK1144 is characterized by the hot-colour region in theintermediate T range. It is found that the peak in Jc

H//c–T (Tp) isalmost H-independent, suggestive of a unique origin of theenhanced pinning with increasing T. At high T region (approxi-mately above Tp), the peak in MHLs (Hp) appears in the observable

H range (<7 T) similarly to BaK122, which is in general associatedwith the order-disorder transition of the vortex lattice. It is evidentthat Tp and Hp are well-separated in the H–T phase diagram,suggestive of the different mechanisms underlying the two typesof peak effect.Now we return to the defect structure in CaK1144 to under-

stand the anomalous Jc properties. The Ca122 intergrowthsobserved by the STEM are schematized in Figs. 1f, g. The colourgradation indicates the difference in Tc between the matrix andthe defects. The intergrowths are considered to be categorized

Fig. 3 Critical current properties of CaK1144 single crystal for H // ab in comparison with BaK122 (x= 0.4). a, b MHLs at T= 3–34 K andmagnetic !eld dependence of Jc for CaK1144. c, d Same data set as in a, b for BaK122. e Temperature dependence of Jc for CaK1144 (red) andBaK122 (blue) under H // ab (!lled) and c (open). Jc data of Co-Ba12220 and Nd111133 thin !lms are plotted for comparison. Note that Jc of Co-Ba122 !lm is almost isotropic. f Temperature dependence of Jc anisotropy de!ned as Jc

H//ab/JcH//c

Fig. 4 Vortex phase diagrams under H // c for a CaK1144 and b BaK122 (x= 0.3). H and T dependences of Jc are shown in the form of contourplots. Hot (cold) colours indicate high (low) Jc region. Several characteristic T and H are plotted: Ton, the onset of the peak effect in Jc–T curvesde!ned by the local minimum (purple squares); Tp, the peak position in Jc–T (blue diamonds); Hon, the onset of the second magnetization peakin M–H curves de!ned by the local minimum (open reversed triangles); Hp, the second peak position in M–H (open triangles); Hirr, irreversibility!eld de!ned by a criterion of Jc < 100 A/cm2 (open circles); and Hc2, the upper critical !eld along the c axis obtained from the resistivitymeasurements (orange circles). The dashed lines are guide for the eye

S. Ishida et al.

4

npj Quantum Materials (2019) ���27� Published in partnership with Nanjing University

（右図）CaKFe4As4単結晶の電気抵抗の温度変化（上）と弾性抵抗（下）。鉄系超伝導体における弾性抵抗の顕著な増⼤は超伝導機構解明に関連して興味が持たれている。T. Terashima et al, 投稿中。NIMS、AIST。