[论文解读] Absence of superconductivity in bulk Nd1-xSrxNiO2 (x = 0.2, 0.4)

Qing Li, Chengping He+4 more|arXiv (Cornell University)|Nov 6, 2019

Magnetic and transport properties of perovskites and related materials被引用 4

一句话总结

本研究采用三步法合成块体Nd1-xSrxNiO2（x = 0.2, 0.4），尽管其结构与超导薄膜相似，但未发现超导性。电阻率和磁性测量结果证实其为绝缘行为及顺磁性磁矩，高压研究虽抑制了绝缘特性但未诱导出超导性，表明薄膜中的超导性可能源于界面或应变效应，而非块体Ni2+ 3d9态。

ABSTRACT

Recently superconductivity at 9 - 15 K was discovered in an infinite-layer nickelate (Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ films), which has received enormous attention. Since the $Ni^{1+}$ ionic state in NdNiO$_2$ may have the $3d^9$ outer-shell electronic orbit which resembles that of the cuprates, it is very curious to know whether superconductivity discovered here has similar mechanism as that in the cuprates. By using a three-step method, we successfully synthesize the bulk samples of Nd$_{1-x}$Sr$_x$NiO$_2$ (x=0, 0.2, 0.4). The X-ray diffractions reveal that all the samples contain mainly the infinite layer phase of 112 with some amount of segregated Ni. This has also been well proved by the SEM image and the EDS composition analysis. The resistive measurements on the Sr doped samples show insulating behavior without the presence of superconductivity. Temperature dependence of the magnetic moment under high magnetic fields exhibits a Curie-Weiss law feature with the paramagnetic moment of about 2$\mu_B$/f.u.. By applying pressure on Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ up to about 50.2 GPa, we find that the strong insulating behavior at ambient pressure is significantly suppressed, but superconductivity has not been observed either. Since the lattice constants derived from our XRD data are very close to those of the reported superconducting films, we argue that the superconductivity in the reported film may not originate from the expected Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, but arise from the interface or the stress effect.

研究动机与目标

• 调查块体Nd1-xSrxNiO2（x = 0.2, 0.4）是否如最近报道的无限层镍酸盐薄膜一样表现出超导性。
• 确定薄膜中超导机制是否源于NdNiO2相的本征性质，还是受应变或界面等外部因素影响。
• 利用电阻率、磁化率及高压测量，研究块体Nd1-xSrxNiO2的电子与磁性性质。
• 比较块体样品与超导薄膜的晶格常数和相纯度，评估结构一致性。

提出的方法

• 采用三步固态合成法制备相纯的块体Nd1-xSrxNiO2（x = 0.2, 0.4）样品。
• 通过X射线衍射（XRD）确认无限层112相的存在，并检测镍富集相的副产物。
• 进行扫描电子显微镜（SEM）和能谱分析（EDS）以验证相组成与显微结构。
• 测量电阻率随温度的变化，以探测金属态或超导行为。
• 在高磁场下获取温度依赖的磁化率，以评估顺磁行为及居里-外斯特性。
• 施加静水压至约50.2 GPa，测试是否诱导出高压超导性。

实验结果

研究问题

• RQ1 块体Nd1-xSrxNiO2（x = 0.2, 0.4）是否表现出与报道的超导薄膜类似的超导性？
• RQ2 块体Nd1-xSrxNiO2的电子性质是否与超导薄膜一致，特别是在晶格常数和相纯度方面？
• RQ3 施加高压是否能抑制常压下块体样品中观察到的绝缘行为并诱导出超导性？
• RQ4 报道薄膜中的超导性是否源于外部效应（如应变或界面耦合），而非本征的块体Ni2+ 3d9态？

主要发现

• 块体Nd1-xSrxNiO2（x = 0.2, 0.4）表现出绝缘性电阻行为，且在2 K以下无任何超导迹象。
• XRD、SEM和EDS分析证实存在无限层112相，伴有少量镍富集的副相。
• 在高场下测得的磁化率遵循居里-外斯定律，顺磁磁矩约为2 μB/f.u.，表明存在局域磁矩。
• 高压研究在Nd0.8Sr0.2NiO2中将绝缘行为抑制至约50.2 GPa，但未出现超导性。
• 由XRD得出的晶格常数与超导薄膜极为接近，表明结构相似，但不足以引发块体超导性。
• 块体样品中未观察到超导性，表明薄膜中的超导性可能源于非本征效应，如界面工程或应变。