SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO$_3$ thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped SrTiO$_3$ films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in SrTiO$_3$, but call into question the role played by ferroelectric fluctuations.
Enhanced superconductivity in the compressively strained bilayer nickelate thin films by pressure
Nanjing University（南京大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Ruhr-University Bochum（波鸿鲁尔大学）
Epitaxial Growth of Perovskite SrBiO$_3$ Film on SrTiO$_3$ by Oxide Molecular Beam Epitaxy
空穴掺杂的钙钛矿铋酸盐例如Ba $_{1-x}$ K $_x$ Bi O $_3$和Sr $_{1-x}$ K $_x$ Bi O $_3$是公知的铋基氧化物高转变温度超导体。报道的铋酸盐薄膜显示出相对低的质量，可能是由于它们与衬底的大的晶格失配和Bi在高温下的低粘附系数。本文报道了用分子束外延法在SrO封端的SrTiO $_3$（001）衬底上成功地外延生长了铋酸锶SrBi O $_3$薄膜。两种不同的生长方法，高温共沉积或再结晶循环的低温沉积加高温退火，被开发来改善外延生长。SrBiO $_3$具有赝立方晶格常数$\sim$ 4.25 μ m，在SrTiO $_3$衬底上的$\sim$ 8.8\%的晶格失配，导致在前几个晶胞中有大的应变。通过这两种方法制备的厚度大于6个晶胞的膜完全弛豫到体晶格常数，并且具有相似的质量。与高温共沉积相比，再结晶方法可以产生更高质量的相干或部分应变的1-6个晶胞膜。光电子能谱实验表明，由于Bi和O的杂化作用，在费米能级附近存在成键和反键态，这与密度泛函理论的计算结果一致.该工作为高质量钙钛矿铋酸盐薄膜的合成提供了一般指导。
Bridging ambient- and high-pressure superconductivity in La$_2$LnNi$_2$O$_7$ films
The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所 emergent matter science 中心）；Institute for Solid State Physics, The University of Tokyo（东京大学固体物理研究所）；Institute for Materials Research (IMR), Tohoku University（东北大学材料科学研究所）；Advanced Institute for Materials Research (WPI-AIMR), Tohoku University（东北大学先进材料研究高等研究院）；Tokyo College, The University of Tokyo（东京大学东京学院）
Reversible tuning of magnetic order and intrinsic superconductivity in strained FeTe films via stoichiometry control
Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Renmin University of China（中国人民大学）；Shanxi Normal University（山西师范大学）；Tsinghua University（清华大学）
Superconducting Dome in $\mathrm{La}_{3-x}\mathrm{Sr}_{x}\mathrm{Ni}_{2}\mathrm{O}_{7-δ}$ Thin Films
National Laboratory of Solid State Microstructures, Jiangsu Key Laboratory of Artificial Functional Materials, College of Engineering and Applied Sciences, Nanjing University（南京大学工程与应用科学学院，固体微结构物理国家重点实验室，人工功能材料江苏省重点实验室）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）
Coexistence of Antiferromagnetic Spin Fluctuations and Superconductivity in La2SmNi2O7 Thin Films
ShanghaiTech University（上海科技大学）；Shanghai Advanced Research Institute, Chinese Academy of Sciences（中国科学院上海高等研究院）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Department of Physics, Stanford University（斯坦福大学物理系）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；School of Applied and Engineering Physics, Cornell University（康奈尔大学应用与工程物理学院）；Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory（斯坦福同步辐射光源，SLAC国家加速器实验室）；Max Planck Institute for Chemical Physics of Solids（马克斯·普朗克固体化学物理研究所）；Kavli Institute at Cornell for Nanoscale Technology, Cornell University（康奈尔大学卡弗里纳米技术研究所）；Geballe Laboratory for Advanced Materials, Stanford University（斯坦福大学盖巴尔先进材料实验室）；State Key Laboratory of Surface Physics and Department of Physics, Fudan University（复旦大学表面物理国家重点实验室和物理系）
Universal Substrate Effect on the Superconductivity of FeSe Monolayer Films
为了阐明单层（1 ML）FeSe在SrTiO $_3$（STO）上的$T_c$增强的机制，我们在金红石TiO $_2$的矩形（100）面上生长了高应变的1 ML FeSe，并在63 K的$T_c$下观察到了复制带和超导电性的共存。从该系统与STO（001）上的1 ML FeSe之间的相似的$T_c$，我们得出结论，在这些系统中，应变和介电常数可能对增强的$T_c$不重要。一个系统的比较1 ML FeSe的TiO $_2$与其他系统中的FeSe家庭表明，虽然单独的电荷转移可以提高$T_c$，它只是与界面的电子-声子耦合，$T_c$可以增加到在STO上的1 ML FeSe的水平。
