Enhanced upper critical field in Co-doped Ba122 superconductors by lattice defect tuning

Abstract

Nanoscale defects in superconductors play a dominant role in enhancing superconducting properties through electron scattering, modulation of coherence length, and correlation with quantized magnetic flux. For ironbased superconductors (IBSCs) that are expected to be employed in high-field magnetic applications, a fundamental question is whether such defects develop an upper critical field (Hc2) similar to that of conventional BCS-type superconductors. Herein, we report the first demonstration of a significantly improved Hc2 in a 122-phase IBSC by introducing defects through high-energy milling. Co-doped Ba122 polycrystalline bulk samples (Ba(Fe,Co)2As2) were prepared by sintering powder which was partially mechanically alloyed through high-energy milling. A remarkable increase in full-width at half maximum of X-ray powder diffraction peaks, anomalous shrinkage in the a-axis, and elongation in the c-axis were observed. When lattice defects are introduced into the grains, semiconductor behavior of the electric resistivity at low temperature (T < 100 K), slight decrease in transition temperature (Tc), upturn of Hc2(T) near Tc, and a large increase in Hc2(T) slope were observed. The slope of Hc2(T) increased approximately by 50%, i.e., from 4 to 6 T/K, and exceeded that of single crystals and thin films. Defect engineering through high-energy milling is expected to facilitate new methods for the designing and tuning of Hc2 in 122-phase IBSCs.

Thermal response of the iron-based Ba122 superconductor to in situ and ex situ processes

“…Therefore, these powders are suitable for ex situ synthesis route. Transport and magnetic measurements confirmed that Tc of the Co-Ba122 sample is 26.3 K [25,34].…”

Enhanced critical current density in K-doped Ba122 polycrystalline bulk superconductors via fast densification

“…Based on the lattice parameters, we estimate that the K-doping levels of the samples sintered at 600°C and 650°C, which showed nearly single phase, were ∼42%. Note that x a and x c are rough estimations because lattice parameters of the samples prepared via high-energy ball milling are affected by lattice defects ( Tokuta and Yamamoto, 2019 ). Based on the EDX, the K-doping levels of the samples sintered at 600°C, 700°C, and 900°C were ∼38%, which is close to the nominal composition.…”

“…At 5 K under self-field conditions, the J c value of these bulks improved to a value approximately three times that reported in a previous study ( Tokuta et al., 2020 ). Furthermore, the H c2 value of Co-doped Ba122 polycrystalline bulks can be artificially enhanced and adjusted by introducing lattice defects into the grains via high-energy ball milling ( Tokuta and Yamamoto, 2019 ).…”

“…In this study, K-doped Ba122 polycrystalline bulks were synthesized via spark plasma sintering (SPS), a pressure-assisted sintering method expected to achieve fast densification, using a mechanically alloyed precursor powder prepared via high-energy ball milling ( Maltsev et al., 2019 ; Pervakov and Vlasenko, 2020 ; Tokuta et al., 2020 ; Tokuta and Yamamoto, 2019 ; Weiss et al., 2013 ). The density, microstructure, phase composition, and superconducting properties of the fabricated bulk samples were evaluated as a function of the sintering temperature.…”

Effect of heat treatments on superconducting properties and connectivity in K-doped BaFe2As2

“…One takes it to be an intrinsic degradation due to an suppression of superfluid density and J c at misoriented grain boundaries (GBs) 27 – 29 , while the other invokes various extrinsic factors like cracks or impurities segregated to the GBs 30 , 31 . Since there is evidence for both effects, the importance of careful synthesis and understanding of the processing conditions is becoming increasingly important 8 , 32 – 34 .…”