Strengthened proximity effect at grain boundaries to enhance inter -grain supercurrent in Ba 1-xKxFe2As2 superconductor s Zhe Cheng1 Chiheng Dong14 Huan Yang2 Qinghua Zhang3 Satoshi Awaji5 Lin Gu3

2025-05-02 0 0 1.28MB 22 页 10玖币

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Strengthened proximity effect at grain boundaries to enhance

inter-grain supercurrent in Ba1-xKxFe2As2 superconductors

Zhe Cheng1, Chiheng Dong1,4,*, Huan Yang2, Qinghua Zhang3, Satoshi Awaji5, Lin Gu3,

Hai-Hu Wen2,*, Yanwei Ma1,4,*

1Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese

Academy of Sciences, Beijing 100190, China

2Center for Superconducting Physics and Materials, National Laboratory of Solid State

Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China

3Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy

of Sciences, Beijing 100190, China

4University of Chinese Academy of Science, Beijing, 100049, China

5High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku

University, Sendai 980-8577, Japan

* E-mail: dongch@mail.iee.ac.cn, hhwen@nju.edu.cn, ywma@mail.iee.ac.cn

ABSTRACT

Iron-based superconductors have great potential for high-power applications due to

their prominent high-field properties. One of the central issues in enhancing the critical

current density of iron-based superconducting wires is to reveal the roles and limitations

of grain boundaries in supercurrent transport. Here, we finely tuned the electronic

properties of grain boundaries by doping Ba1-xKxFe2As2 superconductors in a wide

range (0.25≤x≤0.598). It is found that the intra-grain Jcintra peaks near x0.287, while

the inter-grain Jcinter has a maximum at about x0.458. Remarkably, the grain boundary

transparency parameter defined as =Jcinter/Jcintra rises monotonically with doping.

Through detailed microscopic analysis, we suggest that the FeAs segregation phase

commonly existing at grain boundaries and the adjacent grains constitute

superconductor-normal metal-superconductor (SNS) Josephson junctions which play a

key role in transporting supercurrent. A sandwich model based on the proximity effect

and the SNS junction is proposed to well interpret our data. It is found that overdoping

in superconducting grains largely strengthens the proximity effect and consequently

enhances the intergrain supercurrent. Our results will shed new insights and inspirations

for improving the application parameters of iron-based superconductors by grain

boundary engineering.

Keywords: Grain boundary engineering, Critical current density, Iron-based superconductors,

Josephson junction

1. Introduction

Today’s high-field superconducting equipments require kilometer-long

superconducting wires with a prominent ability to carry loss-less current. The physical

properties of the wires determined by the inherent crystal and electronic structures of

the inner superconducting materials lay a fundamental restriction on their practical

applications. Iron-based superconductors [1] (IBSC) discovered in 2008 perfectly meet

the basic application requirements and may provide a new opportunity to fill the gap in

current superconducting technology [2–5]. After decades of research and development,

iron-based superconducting long tapes fabricated by the powder-in-tube (PIT) method

have already been made into superconducting coils, indicating a great potential to be

applied in high field magnets [6–8]. However, the highest critical current density Jc ever

reached in Ba1-xKxFe2As2 PIT tapes is still much smaller than the Ba1-xKxFe2As2 films.

[9]. The key factors controlling the inter-grain transport of supercurrent continue to be

controversial and subjects of intense interests. Nowadays, great endeavors are mainly

contributed to increasing the compactness of grains [10–12] and suppressing the out-

of-plane misorientations [13]. Remarkable improvement of the Jctransport surpassing the

level for practical applications has been made by varieties of methods [14]. However,

the limitation of these techniques becomes more evident in recent years when we try to

enhance the Jc to a new level. It is intriguing to reconsider the key obstacles to the

improvement of Jctransport from different prospects.

As a model system for IBSC conductors, the Ba0.6K0.4Fe2As2 is widely accepted as

the promising one in making superconducting tapes with the highest Tc to achieve the

best performance [15,16]. However, recent studies show that the highest Jc of the Ba1-

xKxFe2As2 (BaK122) single crystals appears at the slightly underdoped level [17]. The

orthorhombic/antiferromagnetic (AFM) domain boundaries that may still exist in the

underdoped samples are considered to be the main source of the enhanced flux pinning

[18]. This result strongly implies that the BaK122 PIT tapes could achieve higher Jc if

we finely tune the chemical composition to the underdoped side. However, the

supercurrent crossing the interface between adjacent crystallites is largely complicated

by the microscopic electronic, structural and chemical variations at grain boundaries

[19]. It is therefore intriguing to perform an extensive research on the doping

dependence of Jc of the BaK122 tapes to reveal whether a high intragrain supercurrent

can be maintained in polycrystalline tapes and gain a detailed understanding of the

doping induced evolution of grain boundary characteristics.

In this work, we fabricated stainless steel (SS)/silver sheathed Ba1-xKxFe2As2

composite tapes (Supplementary materials, Fig. S1) with K content in region

0.25≤x≤0.598 by the PIT method and study the doping dependence of superconductivity.

It is found that the intragrain critical current density, Jcintra, follows the same doping

dependence as that of the Ba1-xKxFe2As2 single crystals. Unexpectedly, the intergrain

critical current density Jcinter reaches the maximum in the slightly overdoped region.

Moreover, the grain boundary transparency parameter ε=Jcinter/Jcintra increases

monotonically with x, which can be described by the proximity-induced SNS Josephson

junction model. We suggest that the FeAs segregation phase at grain boundaries is the

main factor governing the transport critical current density of the Ba1-xKxFe2As2

superconducting tapes.

2. Materials and Methods

2.1 Sample preparation.

The precursor powders were synthesized by the solid-state-reaction method. High-

purity Ba (99 wt.%), K (99.95 wt.%), Fe (99.95 wt.%) and As (99.9999 wt.%) were

used as the raw materials. The previously prepared BaAs and KAs intermediates were

thoroughly mixed with Fe and As powders via a ball milling machine according to the

nominal compositions Ba1-xKxFe2As2 (x=0.25, 0.3, 0.4, 0.46, 0.48, 0.5, 0.56, 0.6). All

the raw materials were handled in an Ar filled glove box (< 0.1 ppm O2 and < 0.1 ppm

H2O) to reduce contamination. The mixture was sintered at 900 ℃ for 35 hours. The

precursor bulks were ground into powders and filled into a Ag tube (outer-diameter

8mm, inner-diameter 5 mm). The Ag tube was then drawn and flat-rolled into tapes

which can be inserted into the pre-rolled stainless tube. The composite was flat rolled

into the tapes with thickness t~0.8 mm. The tapes were finally sintered at 750 oC for

half an hour. This fabrication procedure was applied to all the tapes with different

doping levels. The thoroughly mixed powders of Fe and As with a ratio of 1:1 were

pressed into a pellet and sintered at 700 oC for 24 hours to obtain the FeAs

polycrystalline bulks.

2.2 Characterization methods.

The crystal structure was characterized by X-ray diffraction (Bruker D8 Advance)

measurements at room temperature. The actual chemical composition of the tape was

examined on at least 10 points of the sample by the electron probe microanalysis

(EPMA; JEOL JXA8230). We utilized the electron backscattered diffraction orientation

imaging microscopy (EBSD-OIM) to examine the grain texture of the superconducting

core. Microscopic characterization of the grain boundaries was performed on a

spherical aberration corrected transmission electron microscope (JEOL JEM-

ARM200F). The composition variation across the grain boundary was examined by the

energy dispersive spectroscopy (EDS) equipped on the transmission electron

microscope. The transport critical current Ic was measured in liquid helium (4.2 K) up

to 14 T by the standard four-probe method with the criterion of 1 μV/cm. The field is

applied parallel to the tape surface during the Ic measurements. Magnetic and electrical

properties of the superconducting cores were measured on a Physical Property

Measurement System (PPMS-9, Quantum Design) after peeling off the metal sheaths.

The Hall effect is measured by the van der Pauw method. The superconducting cores

were shaped into rectangular slabs before the resistivity and Hall effect measurements.

3. Results and Discussion

Fig. 1a shows the powder X-ray diffraction (XRD) patterns of the samples with

different doping levels. All the diffraction peaks can be indexed with the I4/mmm space

group. No sign of impurity phase is observed. To obtain detailed crystal structure

information, we performed Rietveld refinements to the XRD patterns (Fig. S2). The

lattice parameters from fitting are shown in Fig. 1b. Through K doping, the c-axis lattice

constant elongates, while the parameters in the ab-plane decrease. The lattice

parameters depend linearly on the K content. These results are similar to that of the

published papers [20]. The actual K contents measured by electron probe microanalysis

(EPMA) are close to the nominal ones, as shown in Table.S1 (Supplementary materials).

We will use the measured K contents in the following part of the paper.

The metal sheaths were peeled off to obtain the superconducting cores as the

studied samples. The temperature dependence of the normalized susceptibility of the

superconducting core is depicted in Fig. 1c. All samples are measured under a zero-

field-cooling (ZFC) procedure with H=5 Oe parallel to the tape plane. The Tcmag is

determined from the point at which the magnetization starts to deviate from the normal-

state linear background. The sharp superconducting transition near Tcmag indicates a

homogeneous superconducting state. As we expect, the sample with x=0.397 possesses

the highest Tc=38 K. Under- or over-doping suppresses Tcmag but causes no conspicuous

broadened superconducting transition. Fig. 1d and e shows the temperature dependence

of resistivity of the superconducting core. Clearly, all the samples show a metallic

behavior above Tc. In addition, the resistivity at a fixed temperature in the normal state

gradually shifts to the lower values with the increased K content. The doping

dependence of the measured Tc is summarized in the phase diagram shown in Fig. S3.

The consistency between our data and the published ones (blue line) again validates the

accuracy of the measured K content [21].

Fig.2 shows the field dependence of the critical current density at liquid helium

temperature. We choose 4.2 K as the studied temperature instead of the reduced

temperature t=T/Tc because the variation of Tc is small in the doping range, e.g. Tcmag

ranges from 32.5 K to 37.6 K within 0.287<x<0.551. In order to study the influence of

doping on the intra-grain critical current density Jcintra, we thoroughly ground the

superconducting cores into powders and measured the isothermal magnetization

hysteresis loops at 4.2 K. The average size of the powder particles is evaluated by the

SEM. According to the Bean critical state model, the Jcintra can be estimated by

Jcintra=30ΔM/d, where ΔM is the magnetization difference (emu/cm3) between the field

ascending and descending branches of the loop, and d is the average grain size in cm

[22]. Fig. 2a shows the field dependence of Jcintra. Similar to the results obtained in the

BaK122 single crystals [17], the sample with x=0.287 achieves the highest Jcintra. It

indicates that the flux pinning force is stronger in the grains with x=0.287. On the over-

doped side where x＞0.5, the Jcintra is considerably depressed.

For the measurements of the transport critical current Ic, at least four samples were

examined for each doping level to guarantee reproducibility. The good protection

provided by the strong SS sheath and the homogeneous cold deformation procedures

cooperatively lead to the uniform and close values of Ic for the tapes with the same

composition. We denote the transport Jc as the average inter-grain critical current

density, Jcinter. Fig. 2b summarizes the field dependence of Jcinter at 4.2 K for the best

tapes at different doping levels. One can see that Jcinter presents a disparate x dependence

from Jcintra. The tape with x=0.287 which achieves the best Jcintra no longer exhibits the

highest Jcinter. The optimally doped tape (x=0.397) achieves a Jcinter=7.3 × 104 A/cm2 at

10 T, which is close to the reported value [23], indicating the good reliability of the

fabrication technology. Surprisingly, the tape with x=0.458 achieves the best

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Strengthenedproximityeffectatgrainboundariestoenhanceinter-grainsupercurrentinBa1-xKxFe2As2superconductorsZheCheng1,ChihengDong1,4,*,HuanYang2,QinghuaZhang3,SatoshiAwaji5,LinGu3,Hai-HuWen2,*,YanweiMa1,4,*1KeyLaboratoryofAppliedSuperconductivity,InstituteofElectricalEngineering,ChineseAcademyofScienc...

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Strengthened proximity effect at grain boundaries to enhance inter -grain supercurrent in Ba 1-xKxFe2As2 superconductor s Zhe Cheng1 Chiheng Dong14 Huan Yang2 Qinghua Zhang3 Satoshi Awaji5 Lin Gu3

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