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Polymorphism and superconductivity in the V-Nb-Mo-Al-Ga high-entropy alloys

V-Nb-Mo-Al-Ga高熵合金的多晶型性与超导性

Abstract

High-entropy alloys (HEAs) are the focus of current research for their diverse properties, including superconductivity and structural polymorphism. However, the polymorphic transition has been observed only in non-superconducting HEAs mostly under high pressure. Here we report the discovery of the superconductivity and temperature-driven polymorphism in (V0.5Nb0.5)3−xMoxAl0.5G0.5 (0.2 ≤ x ≤ 1.4) HEAs, which are of a single body-centered cubic (bcc) structure for x = 0.2 and a mixture of the bcc and A15 structures for higher x values. Upon annealing, the bcc structure undergoes a polymorphic transformation to the A15 one and all HEAs exhibit bulk superconductivity. For the sample with x = 0.2, the bcc polymorph is not superconducting down to 1.8 K, whereas the A15 polymorph has a superconducting transition temperature Tc of 10.2 K and estimated zero-temperature upper critical field Bc2(0) of 20.1 T, both of which are the highest among HEA superconductors. With increasing Mo content x, both Tc and Bc2(0) of the A15-type HEAs decrease, yet the large ratio of Bc2(0)/Tc signifies a disorder-induced enhancement of the upper critical field over a wide x range. The decrease in Tc is attributed to the decrease in both the electronic specific-heat coefficient and electron-phonon coupling strength. Furthermore, the valence electron count dependence of Tc, which is different from both the binary A15 and other structurally different HEA superconductors, suggests that Tc may be increased further by reducing the number of valence electrons. Our results not only uncover HEA superconductors of a new structural type, but also provide the first example of polymorphism-dependent superconductivity in HEAs.

摘要

高熵合金具有结构多晶型性和超导性, 是当前研究的重点. 然而, 多晶型转变仅在非超导的高熵合金中被观察到, 且大多是在高压条件下. 本文报道了(V0.5Nb0.5)3−xMoxAl0.5Ga0.5 (0.2 ≤ x ≤ 1.4) 高熵合金中的超导和温度驱动多晶型性. 实验结果表明当x = 0.2时铸态高熵合金具有单一的体心立方(bcc)结构, 而当x值更高时则为bcc和A15的混合结构. 经高温退火后, bcc结构向A15结构进行多晶型转变, 且所有高熵合金均表现出块体超导电性. 对于x = 0.2的组分,其bcc晶型直到1.8 K仍不具备超导性, 但其A15晶型却在10.2 K表现出超导性, 估算零温上临界磁场Bc2(0)为20.1 T, 该超导温度 (Tc)和磁场强度在已知的高熵合金超导体中均为最高. 随着Mo含量x的增加, A15型高熵合金的TcBc2(0)均降低, 但Bc2(0)/Tc比值表明在宽的x范围内存在无序诱导的上临界磁场增加. Tc的降低归因于电子比热系数和电声子耦合强度的减小. 此外, 该高熵合金的Tc对价电子数依赖关系与二元A15超导体和其他结构高熵合金超导体均不同, 且表明可以通过降低价电子数目来进一步提高Tc. 本文不仅揭示了一类新结构类型的高熵合金超导体, 而且提供了高熵合金中依赖于多晶型性超导的首个示例.

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Acknowledgements

This work was financially supported by the National Key Research & Development Program of China (2017YFA0303002).

Author information

Author contributions Wu J conceived the idea, synthesized the samples and did the physical property measurements with the assistance from Liu B, Cui Y, Zhu Q, Xiao G. Wang H helped in the EDX analyses. Wu S and Cao G contributed to the magnetic susceptibility measurements. Ren Z supervised the project and wrote the paper.

Correspondence to Zhi Ren 任之.

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Conflict of interest The authors declare no conflict of interest.

Additional information

Jifeng Wu is currently a PhD student in the Department of Physics at Fudan University. He received his bachelor degree from Liaoning Shihua University in 2013, and then received his master degree from Xiamen University in 2016. He worked at GCL Nano for perovskite solar cells, China, from 2016 to 2017. His research interests include the syntheses and characterizations of novel superconductors and super-capacitor materials.

Zhi Ren is currently a research fellow in the School of Science at Westlake University since 2017. He received his bachelor and doctoral degrees from Zhejiang University in 2004 and 2009, respectively. He was a specially appointed researcher in Osaka University from 2009 to 2012 and a postdoctoral assistant in the University of Geneva from 2013 to 2017. His research interests include superconductivity and topological quantum state of materials.

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Wu, J., Liu, B., Cui, Y. et al. Polymorphism and superconductivity in the V-Nb-Mo-Al-Ga high-entropy alloys. Sci. China Mater. (2020). https://doi.org/10.1007/s40843-019-1237-5

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Keywords

  • polymorphsim
  • superconductivity
  • V-Nb-Mo-Al-Ga high-entropy alloys