[论文解读] High-throughput investigation of tunable superconductivity in FeSe films

Zhongpei Feng, Jie Yuan+20 more|arXiv (Cornell University)|Jul 3, 2018

Iron-based superconductors research参考文献 38被引用 7

一句话总结

本研究采用高通量脉冲激光沉积技术，在FeSe薄膜中实现从2 K到12 K的连续超导转变温度（Tc）梯度，从而系统研究结构与电子态的调控。研究发现，增强的超导性源于通过减少晶格畸变对dxy轨道能带色散的选择性调制，而非化学势移动，为FeSe中可调超导性的统一机制提供了依据。

ABSTRACT

There is an ongoing debate about the relative importance of structural change versus doping charge carriers on the mechanism of superconductivity in Fe-based materials. Elucidating this issue is a major challenge since it would require a large number of samples where structure properties or the carrier density is systematically varied. FeSe, with its structural simplicity, is an ideal platform for addressing this question. It has been demonstrated that the superconductivity in this material can be controlled through crystal lattice tuning, as well as electronic structure manipulation. Here, we apply a high-throughput methodology to FeSe to systematically delineate the interdependence of its structural and electronic properties. Using a dual-beam pulsed laser deposition, we have generated FeSe films with a marked gradient in the superconducting transition temperature (below 2 K < Tc < 12 K) across 1 cm width of the films. The Tc gradient films display ~ 1% continuous stretch and compression in the out-of-plane and in-plane lattice constants respectively, triggering the continuous enhancement of superconductivity. Combining transport and angular-resolved photoemission measurements on uniform FeSe films with tunable Tc from 3 K to 14 K, we find that the electron carrier density is intimately correlated with Tc, i.e., it increases by a factor of 6 and ultimately surpasses the almost constant hole concentration. Our transmission electron microscope and band structure calculations reveal that rather than by shifting the chemical potential, the enhanced superconductivity is linked to the selective adjustment of the dxy band dispersion across the Fermi level by means of reduced local lattice distortions. Therefore, such novel mechanism provides a key to understand discrete superconducting phases in FeSe.

研究动机与目标

• 解决长期以来关于结构变化与载流子掺杂在铁基超导体中哪个占主导地位的争议。
• 系统解耦FeSe中晶格应变与载流子浓度对Tc的影响，FeSe是典型的铁硒族超导体。
• 开发一种高通量平台，实现复杂氧化物薄膜中超导特性的连续调控。
• 阐明FeSe中增强超导性的微观起源，超越简单的载流子浓度效应。

提出的方法

• 采用双束脉冲激光沉积技术，在1 cm宽的样品上生长出具有连续面内与面外晶格应变梯度的FeSe薄膜。
• 通过薄膜上空间分辨的输运测量，测量Tc分布，揭示Tc从2 K到12 K的连续变化。
• 对均匀调控的FeSe薄膜进行角分辨光电子能谱（ARPES）测量，绘制电子结构随Tc的演化。
• 通过透射电子显微镜（TEM）将局部晶格畸变与电子能带结构相关联。
• 进行第一性原理能带结构计算，将晶格畸变与dxy轨道色散及超导配对关联起来。
• 将电子载流子密度与Tc相关联，结果显示电子浓度增加了六倍，而空穴浓度几乎保持不变。

实验结果

研究问题

• RQ1 连续晶格应变调控如何影响FeSe薄膜中的超导转变温度（Tc）？
• RQ2 在多大程度上，FeSe中的超导性由载流子浓度决定，而非晶格畸变效应？
• RQ3 FeSe中增强超导性的微观起源是什么——化学势移动还是能带结构调制？
• RQ4 局部晶格畸变如何影响费米能级附近dxy轨道的色散？
• RQ5 高通量平台能否实现复杂氧化物中超导特性的系统性、连续调控？

主要发现

• 通过受控晶格应变，在1 cm宽的FeSe薄膜中实现了从2 K到12 K的连续Tc梯度。
• 在Tc梯度范围内，电子载流子浓度增加了六倍，而空穴浓度几乎保持不变。
• 增强的超导性主要由局部晶格畸变的减少引起，后者选择性地调制了费米能级附近的dxy轨道能带色散。
• 能带结构计算证实，dxy轨道的色散对晶格畸变最敏感，从而解释了Tc的提升。
• 未观察到显著的化学势移动，排除了简单掺杂效应的可能性，表明轨道选择性能带工程是主导机制。
• 本研究建立了晶格畸变抑制与Tc升高的直接关联，为铁基材料中超导性的工程化提供了新途径。