Line nodes in the energy gap of high-temperature superconducting BaFe_2(As_{1-x}P_x)_2 from penetration depth and thermal conductivity measurements
K. Hashimoto, M. Yamashita, S. Kasahara, Y. Senshu, N. Nakata, S. Tonegawa, K. Ikada, A. Serafin, A. Carrington, T. Terashima, H. Ikeda, T. Shibauchi, Y. Matsuda
备注：
4 pages, 3 figures, revised version to be published in Phys. Rev. B Rapid Communication
本文报道了高质量BaFe $_2$（As $_{1-x}$ P $_{x}$）$_2$（$T_c=30$ \，K）单晶的磁穿透深度和热导率数据，为该材料的能隙中存在线节点提供了有力的证据。这与（Ba，K）Fe $_2$ As $_2$中发现的无节能隙明显不同，后者具有相似的$T_c$和相图。我们的研究结果表明，排斥电子相互作用的Fe基高$T_c$超导性发挥了至关重要的作用，但独特的是有明显不同的配对状态，有和没有节点，具有可比的$T_c$。
英文摘要：
We report magnetic penetration depth and thermal conductivity data for high-quality single crystals of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ ($T_c=30$\,K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba,K)Fe$_2$As$_2$ which has similar $T_c$ and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-$T_c$ superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable $T_c$.
Ultralow-dissipative conductivity by Dirac fermions in BaFe$_2$As$_2$
报道了BaFe $_{2}$ As $_{2}$在太赫兹（THz）频率范围内与狄拉克锥有关的复电导率的反常行为.在自旋密度波（SDW）转变温度以上，电导率谱遵循Drude模型。在SDW状态下，与根据Drude模型预期的情况相比，复电导率的虚部$σ_2$被抑制。真正的部分，$σ_1$，表现出几乎像德鲁德一样的行为。这种行为（即，$σ_1$几乎没有变化和$σ_2$的降低）可以被认为是额外电导率的增加，而在Drude型电导率中没有任何耗散。这种超低耗散电导率的起源被认为是由于狄拉克锥内的准粒子的电导率贡献。也就是说，我们可以通过BaFe $_2$ As $_2$的电导谱，清晰而直接地观察到狄拉克费米子的动力学过程。
The issue of whether the quantum critical point (QCP) is hidden inside unconventional superconductors is a matter of hot debate. Although a prominent experiment on London penetration depth has demonstrated the existence of the QCP in the isovalent-doped iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$, with the observation of a sharp peak in the penetration depth in the vicinity of the disappearance of magnetic order at zero temperature, the nature of such an emerging QCP remains unclear. Here, we provide a unique picture to understand well the phenomena of the QCP based on the framewor…
Observation of a coherence peak and pair-breaking effects in THz conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$
本文报道了高质量磷属元素化物超导体BaFe $_{1.84}$ Co $_{0.16}$ As $_2$薄膜的时域THz光谱研究。近T $_c$，我们发现的相干峰和定性协议的证据与弱耦合的马蒂斯-Barbarian形式的导电性。在低温下，我们发现THz电导率的实部没有被完全抑制，并且$σ_2$明显小于Matthis-Barthanson期望。穿透深度$λ$的温度依赖性遵循具有异常高的指数3.1的幂律。我们解释这些结果与杂质散射诱导的对断裂一致。总之，我们的结果是一个扩展的s $\pm$对称序参量的强有力的证据。
Thermal conductivity of overdoped BaFe$_{1.73}$Co$_{0.27}$As$_2$ single crystal: Evidence for nodeless multiple superconducting gaps and interband interactions
测量了过掺杂的FeAs基超导体BaFe $_{1.73}$ Co $_{0.27}$ As $_2$（$T_c$ = 8.1 K）单晶的面内热导率$κ$，其热导率低至80 mK。在零磁场下，剩余的线性项$κ_0/T$可以忽略不计，表明在$ab$平面上存在无结超导能隙.在磁场中，$κ_0/T$迅速增加，与传统的s波超导体有很大不同。这种反常的$κ_0/T(H)$可能揭示了在过掺杂BaFe $_{1.73}$ Co $_{0.27}$ As $_2$中的一种奇异的超导能隙结构：消失空穴（$β$）的能隙比包含大部分载流子的电子（$γ$和$δ$）的能隙大得多。这种奇异的带隙结构是由带间相互作用诱导的超导态的证据，其中具有{ \it较小}态密度的带具有{ \it较大}带隙。
We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of BaFe2As2 single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band $s_{\pm}$-wave Eliashberg approach, able to account for all the main …
The optical properties of a large, detwinned single crystal of BaFe$_2$As$_2$ have been examined over a wide frequency range above and below the structural and magnetic transition at $T_{\rm N}\simeq 138$ K. Above $T_{\rm N}$ the real part of the optical conductivity and the two infrared-active lattice modes are almost completely isotropic; the lattice modes show a weak polarization dependence just above $T_{\rm N}$. For $T<T_{\rm N}$, the optical conductivity due to the free-carrier response is anisotropic, being larger along the $a$ axis than the $b$ axis below $\simeq 30$ meV; above this en…
Possible superconductivity above 25 K in single crystalline Co-doped BaFe$_{2}$As$_{2}$
通过测量Ba（Fe $_{0.9}$ Co $_{0.1}$）$_{2}$ As $_{2}$的磁化强度、电阻率、上临界场、霍尔系数和磁光图像，研究了其超导特性。磁化强度的测量揭示了鱼尾磁滞回线在高温和相对较高的临界电流密度在$J_{c}=10^{5}$ A/cm $^{2}$在低温。由电阻转变确定的上临界场是各向异性的，各向异性参数$\sim$为3.5。霍尔效应测量表明Ba（Fe $_{0.9}$ Co $_{0.1}$）$_{2}$ As $_{2}$是多带体系，电子迁移率占主导地位.磁光成像揭示了突出的豆状渗透的漩涡，虽然有轻微的不均匀性的样品。此外，我们发现了一个明显的超导电性高于25 K，这使我们推测，更高的转变温度可以通过微调Co掺杂水平实现。
Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case of BaFe$_{2-x}$Ni$_{x}$As$_{2}$
铁基超导体的特点表现在其电子、磁性和库珀对的配对对称性上，但后者仍有待探索。通常在这些材料中，超导性与磁序共存并竞争，从而产生非常规的配对机制。本文报道了BaFe $_{2-x}$ Ni $_{x}$ As $_{2}$单晶体在超导状态下的体磁化强度和低温比热的测量结果。{电子}比热在超导转变温度处显示出明显的异常，并且{在超导状态的低温下}显示出小的剩余部分。当$x$ = 0.092、0.096和0.10时，正常态索末菲系数随Ni掺杂而增加，这说明磁性和超导性之间存在竞争。我们的超导态比热和伦敦穿透深度的温度依赖性的分析提供了强有力的证据，一个两带$s$波序参数。此外，从低临界场计算的伦敦穿透深度的数据遵循指数温度依赖性，完全带隙超导体的特征。这些观察清楚地表明，在接近最佳掺杂的化合物中的超导能隙是无节点的。
Are there nodes in LaFePO, BaFe$_2$(AsP)$_2$, and KFe$_2$As$_2$ ?
我们重新研究了三种最可疑的铁磷族化合物LaFePO、BaFe $_2$（As $_{0.67}$ P $_{0.33}$）$_2$和KFe $_2$ As $_2$中可能存在超导能隙节点的实验证据。我们表明，虽然这三种化合物的渗透深度$T$ -线性温度依赖性表明非常干净的节隙超导体，热导率数据$\lim_{T,H \rightarrow 0} κ_S (H,T)/T$明确表明，LaFePO和BaFe $_2$（As $_{0.67}$ P $_{0.33}$）$_2$是非常脏的，而KFe $_2$ As $_2$可以是干净的。这种明显矛盾的实验数据使人们对LaFePO和BaFe $_2$（As $_{0.67}$ P $_{0.33}$）$_2$上结隙的可能性产生了严重怀疑。
我们提出了详细的温度依赖的光学数据BaFe $_{2-x}$ Co $_{x}$作为$_{2}$（BCFA），与x = 0.14，在4 meV和6.5 eV之间。我们分析我们的光谱，以确定主要的光学参数，并表明，在这种材料的带间电导率已经开始约10毫电子伏。我们确定超流密度为2.2 × 10^{7} $ cm^{-2}, which places optimally doped BFCA close to the Uemura line. Our experimental data shows clear signs of a superconducting gap with 2$ Δ_{1} $ = 6.2 $ \pm $ 0.8 meV. In addition we show that the optical spectra are consistent with the presence of an additional band of strongly scattered carriers with a larger gap, 2$ Δ_{2} $ = 14 $ \pm $2 meV.
The temperature dependences of the plasma frequency, superfluid density and London penetration depth were determined from terahertz spectra of conductivity and dielectric permittivity of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film with critical temperature $T_c=19.6$~K. Part of experimental data were analyzed within a simple three-band Eliashberg model where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fluctuations, whose characteristic energy $Ω_{0}$ scales with $T_{c}$ according to the empirical law $Ω_{0} = 4.65k_{B}T_{c}$, and with a total electron-boson coupli…
Non-exponential London penetration depth in RFeAsO$_{0.9}$F$_{0.1}$ (R=La,Nd) single crystals
测量了RFeAsO $_{0.9}$ F $_{0.1}$（R=La，Nd）单晶（R-1111）的超导穿透深度$λ(T)$。在Nd-1111中，我们发现$λ(T)$在冷却时出现了一个上升，并将其归因于Nd离子的顺磁性，类似于电子掺杂的铜酸盐Nd-Ce-Cu-O的情况。经过顺磁性校正后，发现伦敦穿透深度变化遵循幂律行为，在低温下为$Δλ_L(T)\propto T^{2}$。在非磁性La-1111晶体中发现了与$λ(T)$相同的$T^2$变化。在整个温度范围内的超流密度和渗透深度各向异性的分析是一致的双间隙超导。在此基础上，结合我们以前的工作，我们得出结论：RFeAsO（1111）和BaFe $_2$ As $_2$（122）族磷属元素化物超导体都具有非常规的双能隙超导性。
The influence of spin fluctuations on the thermal conductivity in superconducting Ba(Fe_{1-x}Co_x)_2As_2
本文研究了电子掺杂Ba（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$单晶在200 K以下的热导率，着重研究了在磁性和超导转变温度（T_c）附近的行为。对于所有样品，在低于T_c时观察到面内热导率$κ_{ab}$的增强，在最佳掺杂附近观察到最大的增强。所观察到的趋势是一致的散射的热载体的低能磁激发。在进入超导状态时，自旋能隙的形成导致散射减少和$κ(T)$的增强。类似地，$κ$的增强被观察到多晶BaFe 2 As 2的磁性转变以下，并利用单晶和多晶BaFe 2 As 2之间的$κ(T)$的定性差异来讨论各向异性散射。这项研究强调了如何测量$κ$附近的$T_c$在新的超导体可以作为一种手段来探测自旋波动的潜在作用。
Magnetism and Disorder Effects on muSR Measurements of the Magnetic Penetration Depth in Iron-Based Superconductors
结果表明，利用横场μ子自旋旋转（TF-muSR）方法精确推导过掺杂BaFe_{1.82} Co_{0.18} As 2单晶的磁穿透深度的尝试，由于磁场有序性和强的涡旋晶格无序性而受阻.我们解释了如何从磁场分布的一个或两个这些因素几乎完美的涡旋晶格的实质性偏差也是显着的其他铁基超导体，这引入了相当大的不确定性TF-muSR获得的磁穿透深度的值。
Optical conductivity and penetration depth in MgB2
在4 cm^{-1}< nu < 30 cm^{-1}和2.7 K < T <300 K的频率范围内研究了MgB_2薄膜的复电导率。复合电导率的两个组分的整体温度依赖性让人想起BCS型行为，尽管详细的分析揭示了一些差异。在超导态的电导率谱中没有观察到各向同性BCS间隙温度演化的特征。在低于9 cm^{-1}的频率下，检测到电导率实部的温度依赖性的峰值。超导穿透深度在低温下遵循T^2行为。
Optical Study of BaFe$_2$As$_2$ under pressure: Coexistence of spin-density-wave gap and superconductivity
在高达$\sim$ 5 GPa的压力下，在85 - 7000 cm $^{-1}$的频率范围内对BaFe $_2$ As $_2$单晶进行了温度相关的反射率测量。用Drude-Lorentz模型分析了相应的压力和温度依赖的光学电导率，以提取相干和非相干贡献。在3.6GPa下，随着压力的增加，自旋密度波（SDW）态逐渐被抑制，并出现了与SDW相共存的超导相。在3.6 GPa时，反射率在$\sim$ 95 cm $^{-1}$以下达到1，表明超导能隙打开，在6 K时表现为全能隙趋势。
Measurements of the superconducting fluctuations in optimally-doped BaFe$_{2-x}$Ni$_x$As$_2$ under high magnetic fields: Probing the 3D anisotropic Ginzburg-Landau approach
通过对几种高质量优化掺杂的BaFe $_{2-x}$ Ni $_x$ As $_2$晶体的面内介导率的测量，研究了铁基超导体正常态内的超导涨落。这些测量是在振幅高达14 T的磁场中进行的，并且相对于晶体c轴具有不同的取向（0、53和90度）。结果允许严格检查的适用性最近提出的Ginzburg-Landau方法的波动电导率的三维各向异性材料中存在的有限施加磁场。
Investigation of the Superconducting Gap Structure in SrFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ by Magnetic Penetration Depth and Flux Flow Resistivity Analysis
测量了掺磷铁基超导体SrFe $_2$（As $_{1-x}$ P $_{x}$）$_2$单晶（$x=0.30$，$T_c=25 \mathrm{K}$）的微波表面阻抗，获得了单晶中的超流密度和磁通流电阻率。在低温下，超流密度$n_s (T)/n_s(0)$服从幂律$n_s (T)/n_s (0)=1-C(T/T_c)^n$，分数指数为$n=1.5$-1.6。在低磁场下，磁通流电阻率显著增强。这些功能是一致的存在与线节点的间隙和无节点的间隙与深最小。实验中观察到的SrFe $_2$（As $_{1-x}$ P $_{x}$）$_2$和BaFe $_2$（As $_{1-x}$ P $_{x}$）$_2$超导能隙结构的显著差异，对阐明铁基超导机制具有重要意义。
Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy
利用超流密度和宽带准粒子电导率的高分辨测量，研究了欠掺杂YBCO中节状准粒子的低能激发谱。穿透深度的测量温度低至0.05 K。微波电导率是从0.1到20 GHz测量的，是零能量准粒子的直接探针。的数据进行了比较与预测的一些理论方案，竞争或以其他方式修改纯d波超导性，特别是相称和不相称的自旋和电荷密度波; d + i s和d + i d超导性;循环电流相;和BCS-BEC交叉。我们的结论是，在存在少量强散射无序的情况下，数据与纯d波态一致，并且能够完全排除大多数候选竞争态，或者排除到由无序的能量尺度设置的水平，~ 4 K。然而，相称的自旋和电荷密度顺序，预计不会改变节点的频谱，因此不能排除。
Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a- and b-axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasi-linear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis up…
We measured the complex conductivity, $σ$, of FeSe$_{1-x}$Te$_x$ ($x=0-0.5$) films in the superconducting state which show a drastic increase of the superconducting transition temperature, $T_\textrm{c}$, when the nematic order disappears. Since the magnetic penetration depth, $λ$ $(>$ 400 nm), of Fe(Se,Te) is longer than the typical thickness of the film ($\sim$100 nm), we combined the coplanar waveguide resonator and cavity perturbation techniques to evaluate both the real and imaginary parts of $σ$. Films with a nematic order showed a qualitatively different temperature dependence in penetr…
