Department of Physics, Tokyo Metropolitan University, 1-1, Minami-Osawa, Hachioji, 192-0397, Japan.
Department of Physics and Materials Science, Shimane University, Matsue, Shimane, 690-8504, Japan.
Sci Rep. 2023 Jan 18;13(1):1008. doi: 10.1038/s41598-023-28291-y.
Recently, c-axis negative thermal expansion (NTE) was observed in a CoZr superconductor and related transition-metal zirconides. Here, we investigated the structural, electronic, and superconducting properties of CoNiZr to achieve systematic control of c-axis NTE and switching from NTE to positive thermal expansion (PTE) by Ni substitution. At x ≤ 0.3, c-axis NTE was observed, and the thermal expansion constant α approached zero with increasing x. At x = 0.4-0.6, c-axis thermal expansion close to zero thermal expansion (ZTE) was observed, and PTE appeared for x ≥ 0.7. On the superconducting properties, we observed bulk superconductivity for x ≤ 0.6, and bulk nature of superconductivity is suppressed by Ni heavy doping (x ≥ 0.7). For x ≤ 0.6, the evolution of the electronic density of states well explains the change in the superconducting transition temperature (T), which suggests conventional phonon-mediated superconductivity in the system. By analyzing the c/a ratio, we observed a possible collapsed transition in the tetragonal lattice at around x = 0.6-0.8. The lattice collapse would be the cause of the suppression of superconductivity in Ni-rich CoNiZr and the switching from NTE to PTE.
最近，在 CoZr 超导体和相关的过渡金属锆化物中观察到了 c 轴负热膨胀（NTE）。在这里，我们通过 Ni 取代研究了 CoNiZr 的结构、电子和超导性能，以实现对 c 轴 NTE 的系统控制，并从 NTE 转变为正热膨胀（PTE）。在 x ≤ 0.3 时，观察到 c 轴 NTE，随着 x 的增加，热膨胀系数 α 接近零。在 x = 0.4-0.6 时，观察到接近零热膨胀（ZTE）的 c 轴热膨胀，并且在 x ≥ 0.7 时出现 PTE。在超导性能方面，我们观察到 x ≤ 0.6 时存在体超导性，而体超导性被 Ni 重掺杂（x ≥ 0.7）所抑制。对于 x ≤ 0.6，电子态密度的演化很好地解释了超导转变温度（T）的变化，这表明该体系中存在常规声子介导的超导性。通过分析 c/a 比，我们在 x = 0.6-0.8 左右观察到四方晶格的可能崩塌转变。晶格崩塌可能是 Ni 富 CoNiZr 中超导性被抑制以及从 NTE 到 PTE 的转变的原因。
