The recent discovery of superconductivity in Sr-doped NdNiO$_2$ films grown on SrTiO$_3$ started a novel field within unconventional superconductivity. To understand the similarities and differences between nickelate and cuprate layers on the same SrTiO$_3$ substrate, here based on the density functional theory we have systematically investigated the structural, electronic, and magnetic properties of NdNiO$_2$/SrTiO$_3$ and CaCuO$_2$/SrTiO$_3$ systems. Our results revealed a strong lattice reconstruction in the case of NdNiO$_2$/SrTiO$_3$, resulting in a polar film, with the surface and interfacial NiO$_2$ layers presenting opposite displacements. However, for CaCuO$_2$/SrTiO$_3$, the distortions of those same two CuO$_2$ layers were in the same direction. In addition, we found this distortion to be approximately independent of the studied range of film thickness for both the nickelate and cuprates films. Furthermore, we also observed a two-dimensional electron gas at the interface between NdNiO$_2$ and SrTiO$_3$, caused by the polar discontinuity, in agreement with recent literature. For NdNiO$_2$/SrTiO$_3$ the two-dimensional electron gas extends over several layers, while for CaCuO$_2$/SrTiO$_3$ this electronic rearrangement is very localized at the interface between CaCuO$_2$ and SrTiO$_3$. The electronic reconstruction found at the interface involves a strong occupation of the Ti $3d_{xy}$ state. In both cases, there is a significant electronic charge transfer from the surface Ni or Cu layers to the Ti interface layer. The interfacial Ni and Cu layer is hole and electron doped, respectively. By introducing magnetism and electronic correlation, we observed that the $d_{3z^2-r^2}$ orbital of Ni becomes itinerant while the same orbital for Cu remains doubly occupied, establishing a clear two- vs one-orbital active framework for the description of these systems.
Electrical Properties and Subband Occupancy at the (La,Sr)(Al,Ta)O$_3$/SrTiO$_3$ Interface
氧化物界面的准二维电子气（q-2DEG）为研究强关联电子系统中的量子现象提供了一个平台。本文研究了高迁移率（La $_{0.3}$ Sr $_{0.7}$）（Al $_{0.65}$ Ta $_{0.35}$）O $_{0.3}$ /SrTiO $_{0.3}$（LSAT/STO）界面的输运性质。在氧气退火之前，生长界面表现出两个子带的高电子密度和电子占有率：高迁移率电子（$μ_1\approx{10^4}$ cm $^2$ V $^{-1}$ s $^{-1}$ at 2 K）占据较低能量的$3d_{xy}$子带，而低迁移率电子（$μ_1\approx{10^3}$ cm $^{2}$ V $^{-1}$ s $^{-1}$ at 2 K）在较高能量的$3d_{xz/yz}$主导的子带中传播。通过在氧气中退火去除氧空位后，只有单一类型的3dxy电子保留在退火界面处，在2 K和$0.7m_e$的有效质量下显示出低于9 T的可调谐Shubnikov-de Haas（SdH）振荡。相比之下，没有观察到振荡生长的界面，即使当电子迁移率增加到$50,000$ cm $^{2}$ V $^{-1}$ s $^{-1}$的门控电压。我们的研究结果揭示了载流子迁移率和子带占有率在调节氧化物界面量子输运中的重要作用。
Limits to the strain engineering of layered square-planar nickelate thin films
层状方形平面镍酸盐，Nd $_{n+1}$ Ni $_{n}$ O $_{2n+2}$，是一个有吸引力的系统，通过维度来调整方形平面镍酸盐的电子性质;事实上，最近在Nd $_{6}$ Ni $_{5}$ O $_{12}$薄膜中观察到了超导性。在这里，我们调查的作用，外延应变的竞争要求的$n=3$ Ruddlesden-Popper化合物的合成，钕$_{4}$镍$_{3}$ O $_{10}$，随后减少到正方形平面相，钕$_{4}$镍$_{3}$ O $_{8}$。我们在LaAlO $_{3}$（001）上合成了最高质量的Nd $_{4}$ Ni $_{3}$ O $_{10}$薄膜，而在NdGaO $_{3}$（110）上合成的Nd $_{4}$ Ni $_{3}$ O $_{10}$薄膜表现出拉伸应变诱导的岩盐断层，但保留了块状输运性质。在SrTiO $_{3}$（001）上的Nd $_{4}$ Ni $_{3}$ O $_{10}$中形成高密度的扩展缺陷。在LaAlO $_{3}$上还原的膜变成绝缘的并且形成压缩应变诱导的$c$轴倾斜缺陷，而在NdGaO $_{3}$上的Nd $_{4}$ Ni $_{3}$ O $_{8}$膜是金属的。这项工作提供了一种途径，以合成的Nd $_{n+1}$镍$_{n}$ O $_{2n+2}$薄膜，并设置限制的能力，应变工程师通过外延这些化…
Competing Charge/Spin-Stripe and Correlated Metal Phases in Trilayer Nickelates (Pr$_{1-x}$La$_x$)$_4$Ni$_3$O$_8$
低价镍酸盐R $_{n+1}$ Ni $_n$ O $_{2n+2}$（R =稀土）含有准二维（准2D）正方形平面配位几何形状的Ni $^{1+}$（d $^{9}$）具有与高Tc铜酸盐类似的结构和电子性质，包括掺杂无限层（$n = \infty$）RNiO $_2$系统中的超导性本身。在这个R $_{n+1}$ Ni $_n$ O $_{2n+2}$镍酸盐家族中，晶体同构Pr $_4$ Ni $_3$ O $_8$和La $_4$ Ni $_3$ O $_8$表现出奇异的不同基态：Pr $_4$ Ni $_3$ O $_8$是金属的，La $_4$ Ni $_3$ O $_8$是电荷和自旋条纹有序的绝缘体。为了探索和理解从金属Pr $_4$ Ni $_3$ O $_8$到条形有序La $_4$ Ni $_3$ O $_8$的基态演化，我们生长了一系列等价取代的单晶（Pr $_{1-x}$ La $_x$）$_4$ Ni $_3$ O $_8$ .结合热力学，运输，磁性和同步辐射X射线单晶衍射测量，我们揭示了金属和条纹绝缘体相区域之间的过渡，在x = 0.4假定的量子相变。我们提出了两种可能的模型（Pr $_{1-x}$ La $_x$）$_4$ Ni $_3$ O $_8$：一个电子非均匀系统，可以作为一个候选人探索量子格里菲斯相物理和一个均匀的系统与一个假定的量子临界点在相边界…
Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)3Ni2O7 thin films
Peking University（北京大学）；Hefei National Laboratory（合肥国家实验室）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）；Boston College（波士顿学院）；Collaborative Innovation Center of Quantum Matter（量子物质协同创新中心）
