Multi-gap superconductivity in a BaFe1.84Co0.16As2 film from optical measurements at terahertz frequencies
A. Perucchi, L. Baldassarre, C. Marini, S. Lupi, J. Jiang, J. D. Weiss, E. E. Hellstrom, S. Lee, C. W. Bark, C. B. Eom, M. Putti, I. Pallecchi, P. Dore
测量了高质量铁基超导体BaFe $_{1.84}$ Co $_{0.16}$ As $_2$外延薄膜的THz反射特性，T_（$_c$）=22.5 K.通过脉冲激光沉积在具有外延SrTiO $_3$中间层的DyScO $_3$衬底上生长膜。测量的$R_S/R_N$谱，即超导和正常状态反射率之间的反射率比，提供了接近15 cm $^{-1}$的超导间隙$Δ_A$的明确证据。详细的数据分析表明，一个两带，两个间隙模型是绝对必要的，以获得一个很好的描述所测量的$R_S/R_N$频谱。低能$Δ_A$能隙结果被很好地确定（$Δ_A$ =15.5 $\pm$ 0.5 cm $^{-1}$），而高能能隙$Δ_B$的值更不确定（$Δ_B$ =55 $\pm$ 7 cm $^{-1}$）.我们的研究结果提供了一个无节点的各向同性双间隙的情况下，与存在的两个光学间隙对应的2 $Δ/kT_c$值接近2和7的证据。
英文摘要：
We measured the THz reflectance properties of a high quality epitaxial thin film of the Fe-based superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ with T$_c$=22.5 K. The film was grown by pulsed laser deposition on a DyScO$_3$ substrate with an epitaxial SrTiO$_3$ intermediate layer. The measured $R_S/R_N$ spectrum, i.e. the reflectivity ratio between the superconducting and normal state reflectance, provides clear evidence of a superconducting gap $Δ_A$ close to 15 cm$^{-1}$. A detailed data analysis shows that a two-band, two-gap model is absolutely necessary to obtain a good description of the measured $R_S/R_N$ spectrum. The low-energy $Δ_A$ gap results to be well determined ($Δ_A$=15.5$\pm$0.5 cm$^{-1}$), while the value of the high-energy gap $Δ_B$ is more uncertain ($Δ_B$=55$\pm$7 cm$^{-1}$). Our results provide evidence of a nodeless isotropic double-gap scenario, with the presence of two optical gaps corresponding to 2$Δ/kT_c$ values close to 2 and 7.
Transmittance and reflectance measurements at terahertz frequencies on a superconducting BaFe_{1.84}Co_{0.16}As_2 ultrathin film: an analysis of the optical gaps in the Co-doped BaFe_2As_2 pnictide
本文报道了40 nm的BaFe $_{1.84}$ Co $_{0.16}$ As $_2$超导薄膜在太赫兹波段的光学研究，超导转变温度T $_c$ = 17.5 K.结合反射率和透射率测量的详细分析表明，超导系统的光学性质可以用两带，两个间隙模型来描述。大间隙$Δ_B$的零温度值似乎遵循BCS类行为，结果为$Δ_B$（0）= 17 cm $^{-1}$。对于$Δ_A$（0）= 8 cm $^{-1}$的小间隙，不能清楚地建立温度依赖性。这些间隙值和文献中报道的BaFe $_{2-x}$ Co $_{x}$ As $_2$系统通过使用红外光谱，当放在一起作为T $_c$的函数时，显示出沿着两条主曲线聚集的趋势，提供了测量的光学间隙的统一观点。温度低于20 K时，禁带宽度与T_PDMATHABMP的关系呈现BCS型线性关系，但斜率不同。在此温度以上，两个间隙显示出不同的超线性行为。
Ultrastrong magnon-photon coupling in superconductor/antiferromagnet/superconductor heterostructures at terahertz frequencies
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院）；National Research University Higher School of Economics（国立研究型高等经济大学）
Observation of Local Pairs through Nonequilibrium Terahertz Spectroscopy in YBa2Cu3O7-δ Below Tc
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University（固体表面物理化学国家重点实验室，化学化工学院，厦门大学）；Department of Physics, Tsinghua University（物理系，清华大学）
Finite-momentum Cooper plasmons in superconducting terahertz microcavities
Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构与动力学研究所）；Center for Free-Electron Laser Science (CFEL)（自由电子激光科学中心）；Department of Physics, Columbia University（哥伦比亚大学物理系）；Department of Physics, University of California Santa Barbara（加州大学圣塔芭芭拉分校物理系）
