We performed systematic transport measurements on FeSe single crystals with applying in-plane biaxial strain $\varepsilon$ ranging from -0.96% to 0.23%. Biaxial strain was introduced by firmly gluing samples to various substrate materials with different thermal expansion. With increasing $\varepsilon$, structural and superconducting transition temperatures monotonically increased and decreased, respectively. We analyzed magneto-transport results using a compensated three-carrier model. The evaluated densities of hole and electron carriers systematically changed with strain. This indicates that we succeeded in controlling the band structure of single-crystalline FeSe.
Type-II Weyl semimetal candidate MoTe$_2$, which superconducts at T_c~0.1 K, is one of the promising candidates for realizing topological superconductivity. However, the exceedingly low $T_c$ is associated with a small upper critical field ($H_{c2}$), implying a fragile superconducting phase that only exists on a small region of the $H$-$T$ phase diagram. Here, we describe a simple and versatile approach based on the differential thermal expansion between dissimilar materials to subject a thin single crystalline MoTe$_2$ to biaxial strain. With this approach, we successfully enhance the $T_c$ …
Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band structure calculations suggest that under strain the electron pockets develop a large in-plane…
Interface-induced superconductivity and strain-dependent spin density wave in FeSe/SrTiO3 thin films
铁基高温超导体（Fe-HTS's）的超导转变温度（Tc）一直保持在56 K。最近，在SrTiO_3衬底上生长的单层FeSe薄膜中，有新的65 K Tc记录的迹象。在这里，与原位光电子能谱测量，我们证实了自旋密度波（SDW）在FeSe薄膜，Fe-HTS的一个关键成分，在FeSe之前，这是错过了减弱与厚度增加或应变减少。我们表明，超导性发生时，从氧空位基板转移的电子抑制，否则最明显的SDW在单层FeSe。除了提供对FeSe薄膜的全面理解和进一步提高其Tc的方向外，我们还建立了FeSe与晶格常数的相图，其中包含了Fe-HTS的所有基本物理特性。它具有结构最简单、成分最纯净、调谐参数单一等优点，是铁基高温超导理论测试的理想设备。
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…
Understanding the interplay of structural and electronic symmetry breaking in Fe-based high temperature superconductors remains of high interest. In this work we grow strain-patterned multilayer FeSe thin films in a range of thicknesses using molecular beam epitaxy. We study the formation of electronic nematic domains and spatially-varying strain using scanning tunneling microscopy and spectroscopy. We directly visualize the formation of edge dislocations that give rise to a two-dimensional edge dislocation network in the films. Interestingly, we observe a 45 degree in-plane rotation of the di…
We study the effect of applied strain as a physical control parameter for the phase transitions of Ca(Fe1-xCox)2As2 using resistivity, magnetization, x-ray diffraction and 57Fe Mössbauer spectroscopy. Biaxial strain, namely compression of the basal plane of the tetragonal unit cell, is created through firm bonding of samples to a rigid substrate, via differential thermal expansion. This strain is shown to induce a magneto-structural phase transition in originally paramagnetic samples; and superconductivity in previously non-superconducting ones. The magneto-structural transition is gradual as …
This paper reports that ~10-nm-thick FeSe thin films exhibit insulator-like behavior in terms of the temperature dependence of their electrical resistivity even though bulk FeSe has a metallic electronic structure that has been confirmed by photoemission spectroscopy and first-principles calculations. This apparent contradiction is explained by potential barriers formed in the conduction band. Very thin FeSe epitaxial films with various [Fe]/[Se] were fabricated by molecular beam epitaxy and classified into two groups with respect to lattice strain and electrical properties. Lattice parameter …
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…
One of central issues in iron-based superconductors is the role of structural change to the superconducting transition temperature (T_c). It was found in FeSe that the lattice strain leads to a drastic increase in T_c, accompanied by suppression of nematic order. By angle-resolved photoemission spectroscopy on tensile- or compressive-strained and strain-free FeSe, we experimentally show that the in-plane strain causes a marked change in the energy overlap (DeltaE_{h-e}) between the hole and electron pockets in the normal state. The change in DeltaE_{h-e} modifies the Fermi-surface volume, lead…
We investigate the electronic and magnetic properties of FeSe monolayer on the anisotropic SrTiO$_3$ (110) surface. With compressive strain along $[1\bar{1}0]$ direction from the substrate, the monolayer FeSe possesses a staggered bipartite iron lattice with an height difference around 0.06Å~ along the out-plane direction. The staggering causes stronger magnetic frustration between the collinear and checkerboard antiferromagnetic orders, and the strain elongates one electron and two hole pockets along the strain direction and the remaining hole pocket along the orthogonal direction. The strain…
Motivated by the success of experimental manipulation of the band structure through biaxial strain in Sr$_2$RuO$_4$ thin film grown on a mismatched substrate, we investigate theoretically the effects of biaxial strain on the electronic instabilities, such as superconductivity (SC) and spin density wave (SDW), by functional renormalization group. According to the experiment, the positive strain (from lattice expansion) causes charge transfer to the $γ$-band and consequently Lifshitz reconstruction of the Fermi surface. Our theoretical calculations show that within a limited range of positive st…
The successful fabrication of one monolayer FeSe on SrTiO$_{3}$ represented a real breakthrough in searching for high-T$_{c}$ Fe-based superconductors (Ref. 1). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in plane tensile strain. FeTe, in both bulk and m…
Competing Constraints on Superconductivity in Thick FeSe films
Materials Genome Institute, Shanghai University（上海大学材料基因组研究院）；College of Ocean Science and Engineering, Shanghai Maritime University（上海海事大学海洋科学与工程学院）；Department of Physics and Shanghai Key Laboratory for High Temperature Superconductors, Shanghai University（上海大学物理系及高温超导上海市重点实验室）；Institute for Quantum Science and Technology, Shanghai University（上海大学量子科学与技术研究院）
The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the nematic/structural transition at $T_{s}\approx$90~K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strain-free samples. The anisotropy peaks slightly below $T_{s}$ and decreases to nearly zero on cooling down t…
The mechanism behind the nematicity of FeSe is not known. Through elastoresitivity measurements it has been shown to be an electronic instability. However, so far measurements have extended only to small strains, where the response is linear. Here, we apply large elastic strains to FeSe, and perform two types of measurements. (1) Using applied strain to control twinning, the nematic resistive anisotropy at temperatures below the nematic transition temperature Ts is determined. (2) Resistive anisotropy is measured as nematicity is induced through applied strain at fixed temperature above Ts. In…
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）
The electron-doped silicene under the influence of the biaxial tensile strain is predicted to be the phonon-mediated superconductor. By using the Eliashberg formalism, we investigate the thermodynamic properties of the superconducting silicene in the case when the tension is $5\%$ and the electron doping equals $3.5\times10^{14}~{\rm cm^{-2}}$. Under such conditions, silicene monolayer is expected to exhibit the highest superconducting transition temperature ($T_C$). In particular, based on the electron-phonon spectral function and assuming wide range of the Coulomb pseudopotential values ($μ^…
The recent study of $^{77}$Se nuclear magnetic resonance (NMR) in a $β$-FeSe single crystal proposed that ferro-orbital order breaks the $90^\circ$ $C_4$ rotational symmetry, driving nematic ordering. Here, we report an NMR study of the impact of small strains generated by gluing on nematic state and spin fluctuations. We observe that the local strains strongly affect the nematic transition, considerably enhancing its onset temperature. On the contrary, no effect on low-energy spin fluctuations was found. Furthermore we investigate the interplay of the nematic phase and superconductivity. Our …
We have carried out Raman spectroscopy experiments to investigate two-magnon excitations in epitaxial thin films of the quasi-two-dimensional antiferromagnetic Mott insulator Sr$_2$IrO$_4$ under in-plane misfit strain. With in-plane biaxial compression, the energy of the two-magnon peak increases, and the peak remains observable over a wider temperature range above the Néel temperature, indicating strain-induced enhancement of the superexchange interactions between $\it{J}_{eff}$ = 1/2 pseudospins. From density functional theory calculations, we have found an increase of the nearest-neighbor h…
Strain induced superconductivity in the parent compound BaFe2As2
在SrTiO_3衬底上生长的单层FeSe（体相Tc =8K）薄膜中发现了转变温度高达65K的超导电性（SC），引起了人们对铁基薄膜的特别关注。高Tc是从氧空位衬底到FeSe薄膜的电子转移和晶格拉伸应变的组合效应的结果。在这里，我们证明了SC的实现在母体化合物BaFe2As2（没有散装Tc）只是通过拉伸晶格应变没有电荷掺杂。研究了在Fe缓冲MgAl 2 O 4单晶衬底上外延BaFe 2 As 2薄膜中应变与SC之间的相互作用。Fe和FeAs亚晶格之间的强界面键合增加了由于晶格失配导致的反铁磁自旋密度波的抑制，并诱导SC与体Tc？一万这些结果突出了结构变化在控制铁基超导体相图中的作用。
We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beamspot (nano-ARPES), we identify clear stripe-like orthorhombic domains in FeSe with a length scale of approximately 1-5~$μ$m. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron pocket electronic structure of FeSe, observed without the application of uniaxial…
Dual Enhancement of Superconductivity in FeSe/SrTiO3 via Orbital and Correlation Synergy
State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University（清华大学物理系低维量子物理国家重点实验室）；Frontier Science Center for Quantum Information（量子信息前沿科学中心）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Southern University of Science and Technology（南方科技大学）；Hefei National Laboratory（合肥国家实验室）
Fluctuation-enhanced electron-phonon coupling in FeSe
Center for Solid State Physics and New Materials, Institute of Physics Belgrade, University of Belgrade（贝尔格莱德大学物理研究所固态物理与新材料中心）；Department of Applied Physics and Applied Mathematics, Columbia University（哥伦比亚大学应用物理与应用数学系）；COMMIT, Department of Physics, University of Antwerp（安特卫普大学物理系COMMIT）；Stavropoulos Center for Complex Quantum Matter, Department of Physics and Astronomy, University of Notre Dame（圣母大学物理与天文学系斯塔夫罗普洛斯复杂量子物质中心）
