FeSe单晶的高压研究进展孙建平王铂森程金光中国科学院物理研究所中国科学院大学物理学院摘要：FeSe及其衍生化合物是近年来铁基高温超导领域的研宄热点.在常压下，FeSe 会首先在1>90 K形成非磁性的电子向列序，然后在T(:~8 K出现超导电性;对 FeSe施加压力会诱导长程反铁磁序，在更高压力下实现TO37 K的高温超导. 澄清高温超导与正常态电子向列序和长程反铁磁序的关系对阐明高温超导机理 具有重要意义.针对这一问题，利用立方六面砧高压装置对FeSe单晶开展了较 详细的高压电输运性质测量，取得了重要进展.首先，绘制了 FeSe单晶完整的 温度-压力相图，具体展示了电子向列序、压致长程反铁磁序和超导相随压力的 演化关系，观察到圆拱形的反铁磁相界K (P)，并揭示3种电子序之间的竞争 关系;对应电子向列序和反铁磁序被压制的临界压力，FeSe的超导转变温度T 呈台阶式升高，最终在Pc^6 GPa实现了 Tcmax=38. 5 K的高温超导.高压霍尔效应 测试表明，在2 GPa附近向列序的消失和反铁磁序的出现伴随着费米面重构，正 常态从电子型载流子主导转变为空穴型主导，而iL霍尔系数在P(.(即TTX附近出 现显著增强，表明临界反铁磁涨落对实现高温超导具有重要作用.结合第一性原 理计算，进一步证实FeSe在高压下存在电子型与空穴型费米而，支持与FeAs 基高温超导体系类似的费米面嵌套机制，对统一理解FeSe基和EeAs基高温超导 机理提供了重要依据.关键词：FeSe;高压调控;非常规超导;磁性量子临界点;作者简介：程金光 E-mail: jgcheng@收稿日期：2017-06-28基金：国家自然科学基金(11574377)Recent progress on the high-pressure studies of FeSe single crystalSUN JianPing WANG BoSen CHENG JinGuangInstitute of Physics， Chinese Academy ofSciences;Abstract：Among the iron-based superconductors, the structurally simplest EeSe and FeSe-derived materials have attracted enormous research interest recently due to its unusual electronic properties and the great tunability of the superconducting transition temperature Tc. For the bulk FeSe single crystal，it shows a relatively low Tc^8 K within the unusual nonmagnetic nematic phase below T产90 K. It has been reported that the application of high pressure can induce a long-range antiferromagnetic order below Tn-^20 K at P^l GPa, and then achieve a high Tc^37 K at P^8 GPa. It remains unclear how the high~Tc superconductivity is achieved in FeSe under pressure. In this regard， a detailed temperature-pressure phase diagram mapping out the exact relationship between the high-Tc superconductivity and the normal-state nematic order and the pressure-induced antiferromagnetic order is indispensable for achieving a better understanding. By taking advantage of the cubic anvil cel 1 apparatus that can maintain an excellent hydrostatic pressure condition up to 15 GPa, we recently undertook a comprehensive high-pressure magnetotransport study on high-quality FeSe single crystals and obtained important results. In this article, we give a brief review on the recent progresses about the high-pressure studies of FeSe. By tracking the characteristic resistivity anomalies at the nematic order， the pressure-induced antiferromagnetic order, and the superconductingtransition, we constructed for FeSe a comprehensive temperature-pressure phase diagram, which depicts explicitly the detailed evolution of these competing electronic orders. We observed a dome-shaped antiferro腿gnetic phase boundary L (P) , and revealed the competitive relationship between these electronic orders. The superconducting Tc experiences two successive step increases at the critical pressures when the nematic order and the anti ferromagnetic order are respectively suppressed, and finally the highest 17^=38. 5 K is realized near Pc^6 GPa when theantiferromagnetic order just collapses. We further measured the Hall effect of FeSe to extract the electronic structure information under high pressure. Our Hall data show that the Fermi surface of FeSe in the normal state undergoes a reconstruction at P^2 GPa, when the nematic order is suppressed and the anti ferromagnetic order just emerges. The dominant charge carriers for the transport properties in the normal state change from the electron to the hole type. In addition, in the high-Tc phase near the optimal Tcmax at Pc^6 GPa, the Hall coefficient experiences a dramatic enhancement and the magnetoresistance exhibits anomalous scaling behaviors, signaling the presence of strongly enhanced inter-band spin fluctuations. These results suggest that the critical anti ferromagnetic fluctuations play an important role for achieving the high-Tc superconductivity in F^eSe. Aided by the first-principles calculations, we further confirmed that both electron-like and hole-like Fermi surfaces are present in FeSe under high pressures, in favor of the Fermi surface nesting scenario as in the FeAs^based high-Tc superconductors. These above results on FeSe suggest great similari ties with other FeAs~based superconductors, and thus constitute an important step toward a unified understanding of iron-based superconductivity.Keyword：FeSe; high pressure magnetotransport measurements; unconventional superconductivity; magnetic quantum critical point;Received： 2017-06-28引用格式:孙建平，王钔森，程金光.Fe So单晶的高压研宂进展.科学通报， 2017, 62:3925 - 3934Sun J P, Wang B S, Cheng J G. Recent progress on the high-pressure studies of Fe Se single crystal (in Chinese) • Chin Sci Bull, 2017, 62:3925 - 3934,自从2008年在F掺杂的La OFe As中报道了 Tc^26 K的高温超导电性UL全 球范围内掀起了继铜氧化物高温超导体之后新一轮的铁基高温超异研宄热潮.人 们通过稀土离子替代很快将其转变温度提升到40 K以上，在高压下合成的Sm OnxF.Fe As的更是达到了 55 K［5］，极大地促进了铁基超导体的研宄.在铁基超导体系中，Fe Se具奋最简单的晶体结构，却表现出诸多奇特反常的物 理性质，成为近年来的研究热点.随着温度降低，Fe Se首先在Ts~90 K发生了 四方-正交结构相变，形成具有各向异性电子结构的电子向列序，造成图1所示 的电阻率P (T)曲线在Ts处出现上翘。
［6, 7］.但是，与其他Fe As基超导体系不 同的是，Fe Se在.电子向列相内并没有形成长程反铁磁序^1.通过对Fe Se施。