In this letter we show that superconducting Fe1.01Se undergoes a structural transition at 90 K from a tetragonal to an orthorhombic phase but that non-superconducting Fe1.03Se does not. Further, high resolution electron microscopy study at low temperatures reveals an unexpected additional modulation of the crystal structure of the superconducting phase involving displacements of the Fe atoms, and that the non-superconducting material shows a distinct, complex nanometer-scale structural modulation. Finally, we show that magnetism is not the driving force for the phase transition in the superconducting phase.
FeSe, despite being the structurally simplest compound in the family of iron-based superconductors, shows an astoundingly rich interplay of physical phenomena including nematicity and pressure-induced magnetism. Here, we present a microscopic study of these two phenomena by high-energy x-ray diffraction and time-domain Mössbauer spectroscopy on FeSe single crystals over a wide temperature and pressure range. The topology of the pressure-temperature phase diagram is a surprisingly close parallel to the well-known doping-temperature phase diagram of BaFe2As2 generated through partial Fe/Co and B…
In a material prone to a nematic instability, anisotropic strain in principle provides a preferred symmetry-breaking direction for the electronic nematic state to follow. This is consistent with experimental observations, where electronic nematicity and structural anisotropy typically appear hand-in-hand. In this work, we discover that electronic nematicity can be locally decoupled from the underlying structural anisotropy in strain-engineered iron-selenide (FeSe) thin films. We use heteroepitaxial molecular beam epitaxy to grow FeSe with a nanoscale network of modulations that give rise to sp…
High-resolution time- and angle-resolved photoemission measurements were made on FeSe superconductors. With ultrafast photoexcitation, two critical excitation fluences that correspond to two ultrafast electronic phase transitions were found only in the $d_{yz}$-orbit-derived band near the Brillouin-zone center within our time and energy resolution. Upon comparison to the detailed temperature dependent measurements, we conclude that there are two equilibrium electronic phase transitions (at approximately 90 and 120 K) above the superconducting transition temperature, and an anomalous contributi…
Importance of structural distortions in enhancement of transition temperature in FeSe$_{1-x}$Te$_{x}$ superconductors
利用扩展X射线吸收精细结构（EXAFS）研究了FeSe $_{1-x}$ Te $_x$（x = 0，0.5和1.0）化合物中Fe K边的温度（12 K $\le$ T $\le$ 300 K）依赖的扩展X射线吸收精细结构，以了解Te掺杂FeSe时T $_C$增加的原因.虽然FeSe和FeSe $_{0.5}$ Te $_{0.5}$在超导起始附近存在局部畸变，但它们在非超导FeTe中似乎不存在。至关重要的是阴离子高度的变化。在FeSe $_{0.5}$ Te $_{0.5}$中，在超导起始附近，两个高度，$h_{Fe-Se}$和$h_{Fe-Te}$表现出几乎相反的行为。这些变化表明铁硫族化合物的铁硫族杂化和超导转变温度之间可能存在相关性。
Structural and electronic properties of ultrathin FeSe films grown on Bi$_2$Se$_3$(0001) studied by STM/STS
本文报道了在Bi $_2$ Se $_3$（0001）衬底上生长的单晶胞和双晶胞高FeSe薄膜的扫描隧道显微镜和光谱学（STM/STS）研究。在我们的薄膜中，我们发现FeSe的四方相和沿Se-Se键方向取向的哑铃形缺陷。此外，我们观察到条纹莫尔图案的周期性（$7.3\pm 0.1$）nm之间的失配FeSe晶格和Bi $_2$ Se $_3$晶格产生的。在6.5 K下，在一个和两个UC厚的FeSe岛表面上测量的隧道谱中，我们没有发现任何超导能隙的特征。光谱，而不是显示出一个不对称的行为和有限的状态密度在费米能级（$E_F$）类似于在正常状态的散装FeSe。
Tuning electronic properties of FeSe$_{0.5}$Te$_{0.5}$ thin flakes by a novel field effect transistor
利用场效应晶体管（FET）结构和固体锂离子导体（SIC）作为栅介质，我们成功地调谐了FeSe $_{0.5}$ Te $_{0.5}$薄片中的载流子密度，并绘制了电子相图。结果表明，由SIC-FET控制的电子掺杂抑制了FeSe $_{0.5}$ Te $_{0.5}$的超导相，最终使其处于绝缘态。在选通过程中，XRD图谱中的（001）峰停留在相同的位置，没有新的衍射峰出现，表明没有明显的Li $^+$离子嵌入到FeSe $_{0.5}$ Te $_{0.5}$中。结果表明，在器件中，由于Li $^+$离子在FeSe $_{0.5}$ Te $_{0.5}$与固体离子导体界面处的积累，导致了FeSe $_{0.5}$ Te $_{0.5}$电子性质的系统变化.令人惊讶的是，这些发现与使用相同的SIC-FET在FeSe薄片中观察到的结果截然不同，其中$T_c$从8 K增强到大于40 K，然后系统进入绝缘相并伴有结构转变。
Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；University of the Basque Country (UPV/EHU)（巴斯克大学）；Goethe-Universität Frankfurt（法兰克福歌德大学）；Université de Sherbrooke（舍布鲁克大学）；RIKEN iTHEMS（理化学研究所 iTHEMS）；Center for Emergent Matter Science, RIKEN（理化学研究所 emergent matter science 中心）；Regroupement Québécois sur les Matériaux de Pointe (RQMP)（魁北克前沿材料研究联盟）
We report on the structural phase transitions in the S doped FeSe superconductor by powder synchrotron X ray diffraction at high pressures up to 18.5 GPa under compression and decompression modes. In order to create high quasi hydrostatic pressures, diamond anvil cells filled with helium as a pressure transmitting medium were used. It was found that at ambient pressure and room temperature, S doped FeSe has a tetragonal structure. Under compression, in the region of 10 GPa, a phase transition from the tetragonal into the orthorhombic structure is observed, which persists up to 18.5 GPa. Our re…
The structurally simplest high-temperature superconductor FeSe exhibits an intriguing superconducting nematic paramagnetic phase with unusual spin excitation spectra that are different from typical spin waves; thus, determining its effective magnetic exchange interactions is challenging. Here we report neutron scattering measurements of spin fluctuations of FeSe in the tetragonal paramagnetic phase. We show that the equal-time magnetic structure factor, $\mathcal{S}(\textbf{Q})$, can be effectively modeled using the self-consistent Gaussian approximation calculation with highly frustrated near…
We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the distortion at temperatures where the average observed symmetry is orthorhombic. Our results suggest that the low-temperature macroscopic nematic state in FeSe forms from an imperfect ordering of orbital-degenerac…
Pressure Induced Stripe-order Antiferromagnetism and First-order Phase Transition in FeSe
为了阐明FeSe的磁结构和向列性的起源，我们对FeSe单晶进行了高压$^{77}$ Se NMR研究。我们发现，抑制的结构转变温度与压力高达约2 GPa的各向异性的奈特位移。在2GPa以上，打破空间四重旋转对称性的条序反铁磁性由不同场取向下的NMR谱确定，温度低至50 mK。磁相变是一级相变，这意味着存在一个伴随的结构转变通过自旋-晶格耦合。条纹型自旋波动在高温下观察到，并保持强大的压力。这些结果为FeSe中向列性和磁性之间的强耦合提供了明确的证据，从而支持了铁基超导体中磁驱动向列性的普遍假设。
We performed systematic studies on the transport properties of FeSe thin films with controlled degrees of in-plane lattice strain, including both tensile and compressive strains. The superconducting transition temperature, $T_{\mathrm c}$, increases up to 12 K for films with compressive strain while the superconductivity disappears for films with large tensile strains. On the other hand, the structural (nematic) transition temperature, $T_{\mathrm s}$, slightly decreases as the in-plane strain is more compressive. This suggests that the structural transition can be extinguished by a smaller am…
The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive th…
We comparatively investigated the transport properties for S- and Te-substituted FeSe thin films under magnetic fields to clarify the origin of the contrasting behavior of the superconducting transition temperature in S and Te substitution. A classical two carrier analysis revealed that the carrier densities of the films increased with increasing Te content, while no significant change was observed for the S-substitution. This composition dependence of the carrier density well corresponds to the $T_{\mathrm c}$ behavior. The clear correlation between $T_{\mathrm c}$ and the carrier densities s…
Ab-initio Studies of (Li$_{0.8}$Fe$_{0.2}$)OHFeSe Superconductors: Revealing the Dual Roles of Fe$_{0.2}$ in Structural Stability and Charge Transfer
最近发现的（Li $_{0.8}$ Fe $_{0.2}$）OHFeSe超导体为探索FeSe衍生体系中高$T_c$超导性的微观机制提供了一个新的平台。使用密度泛函理论计算，我们首先表明，由Fe取代Li不仅显着增强（Li $_{0.8}$ Fe $_{0.2}$）OH间隔层和FeSe超导层之间的吸引力沿\emph{c}轴，但也最大限度地减少了两者之间的晶格失配在\emph{ab}平面，都有利于稳定的整体结构。接下来，我们探讨了从间隔层到FeSe的电子注入，并明确地确定Fe $_{0.2}$组分是显著增强的层间电荷转移的主要原子来源。我们进一步揭示，该系统强烈有利于共线反铁磁有序的FeSe层，但间隔层可以是反铁磁或铁磁的Fe $_{0.2}$的空间分布而定。基于这些理解，我们还预测（Li $_{0.8}$ Co $_{0.2}$）OHFeSe具有更大的电子注入和潜在的更高的$T_c$的结构稳定性。
Electronic properties of FeSe can be tuned by various routes. Here, we present a comprehensive study on the evolution of the superconductivity and nematicity in FeSe with thickness from bulk single crystal down to bilayer ($\sim$ 1.1 nm) through exfoliation. With decreasing flake thickness, both the structural transition temperature $T_{\rm s}$ and the superconducting transition temperature $T_{\rm c}^{\rm zero}$ are greatly suppressed. The magnetic field ($B$) dependence of Hall resistance $R_{xy}$ at 15 K changes from $B$-nonlinear to $B$-linear behavior up to 9 T, as the thickness ($d$) is …
The local electronic and magnetic properties of superconducting FeSe have been investigated by K$β$ x-ray emission (XES) and simultaneous x-ray absorption spectroscopy (XAS) at the Fe K-edge at high pressure and low temperature. Our results indicate a sluggish decrease of the local Fe spin moment under pressure up to 7~GPa, in line with previous reports, followed by a sudden increase at higher pressure which has been hitherto unobserved. The magnetic surge is preceded by an abrupt change of the Fe local structure as observed by the decrease of the XAS pre-edge region intensity and corroborated…
We have investigated the correlation between structural and transport properties in sputtered $β$-FeSe films grown onto SrTiO$_3$ (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced $T_{\rm c}\sim12\,$K, a relatively high $H_{\rm c2}$ and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed t…
Our study of FeSe$ _ {1-x}$Te$ _ {x}$ crystals with x $<$ 0.15 shows that the phase separation in these compositions occurs into phases with a different stoichiometry of iron. This phase separation may indicate structural instability of the iron plane in the studied range of compositions. To explain it, we discuss the bond polarity and the peculiarity of the direct $d$ exchange in the iron plane in the framework of the basic phenomenological description such as the Bethe-Slater curve. With this approach, when the distance between iron atoms is close to the value at which the sign of the magnet…
