Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors

Abstract

The superconducting critical temperature Tc of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high-Tc monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high-Tc iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer in both superconducting 28 K and 42 K samples. Using atomic multiplet calculations, we assign the low-energy ~0.3 and 0.7 eV Raman peaks as local eg − eg and eg − t2g orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a dominating contribution of the orbital fluctuations. Our results highlight the importance of the orbital degree of freedom for high-Tc iron selenides.

Magnetic excitations in iron-based superconductors as revealed by resonant inelastic X-ray scattering

“…由于插层 的引入, 该材料中FeSe层之间的间距很大, 耦合很弱, 可以近似认为是二维体系. Xiao等人 [42] 在ADRESS光 束线上采用RIXS 研究了(Li,Fe)OHFeSe薄膜中的激发 谱, 发现体系中存在多个本征局域激发模式(E ∼0.1,…”

“…∼ 140 meV的磁激发(强度明显低于FeSe体材料中的磁 激发) [43] , 该磁激发在RIXS实验中未被探测到. Xiao等 人 [42] 认为可能是掺杂效应导致该体系中的磁激发较 弱以至于在RIXS研究中未能观测到. 该研究表明轨道 自由度对FeSe类高温超导电性十分重要.…”

“…铁基超导体的RIXS谱的谱重通常被E 0.5 eV 表 表 表 1 铁基超导材料中元激发的RIXS研究列表(部分). ∆E表示RIXS谱仪分辨率, SE (Spin Excitations)表示磁激发 [22] ADRESS 70 N Y Ba 1−x K x Fe 2 As 2 (x = 0, 0.4) [19] ADRESS ADRESS 90 Y Y NaFe 1−x Co x As (x = 0, 0.03, 0.08) [26] ADRESS 110 Y Y NaFe 1−x Cu x As (x = 0, 0.47) [34] ADRESS 86 Y Y PrFeAsO 0.7 [44] BL07SU@Spring-8 230 N N SmFeAsO [27] ADRESS 110 Y N FeSe [36] ID32@ESRF 55 Y Y Detwinned FeSe [21] ADRESS 80 Y N Uniaxial-strained FeSe 1−x S x ADRESS 80 Y N Monolayer FeSe/SrTiO 3 [40] I21@Diamond 40 Y N/A (Li,Fe)OHFeSe [42] ADRESS 80 N bulk (Li 0.8 Fe 0.2 )ODFeSe [43] 的荧光信号主导. 精确描述荧光谱对提取磁散射信 号十分重要.…”

Enhancement of electron–phonon coupling due to increased magnetism and applied hydrostatic pressure in FeSe

“…Moreover, many experiments on various iron-based superconductors have revealed remarkable isotopic effects, suggesting the importance of EPC in superconductivity [10][11][12]. The significance of EPC becomes even more puzzling in FeSe, an iron-based superconductor with simplest lattice structure and without toxic elements, which shows a high superconducting T c of 65 ~109 K in monolayer FeSe grown on SrTiO 3 substrate [13,14] and 40 ~48 K in molecular-intercalated [15,16] or potassium-coated multilayer FeSe [17]. Investigations of bulk FeSe under hydrostatic pressure reveal a complicated phase diagram, including the emergence of anti-ferromagnetic states, non-monotonic behavior of superconducting T c , and structural phase transition as a function of pressure, implying an interplay between magnetism, EPC and superconductivity [18][19][20].…”