测量了BaFe $_2$ As $_2$单晶、电子掺杂Ba $_{0.7}$ La $_{0.3}$ Fe $_2$ As $_2$和空穴掺杂Ba $_{0.5}$ K $_{0.5}$ Fe $_2$ As $_2$多晶的比热$C(T)$。对于未掺杂的BaFe $_2$ As $_2$单晶，在136 K处观察到非常尖锐的比热峰。这是由于在相同的温度下发生的结构和反铁磁转变。电子掺杂的非超导Ba $_{0.7}$ La $_{0.3}$ Fe $_2$ As $_2$的$C(T)$也在120 K处显示出小峰，表明类似但较弱的结构/反铁磁转变。对于空穴掺杂的超导Ba $_{0.5}$ K $_{0.5}$ Fe $_2$ As $_2$，在$T_c$ = 36 K处观察到$C/T$的明显峰，这是迄今为止在铁基高$T_c$超导体的超导转变处观察到的最高峰。由低温数据得到了这些化合物的电子比热系数$γ$和德拜温度$Θ_D$。
英文摘要：
The specific heat $C(T)$ of BaFe$_2$As$_2$ single crystal, electron-doped Ba$_{0.7}$La$_{0.3}$Fe$_2$As$_2$ and hole-doped Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ polycrystals were measured. For undoped BaFe$_2$As$_2$ single crystal, a very sharp specific heat peak was observed at 136 K. This is attributed to the structural and antiferromagnetic transitions occurring at the same temperature. $C(T)$ of the electron-doped non-superconducting Ba$_{0.7}$La$_{0.3}$Fe$_2$As$_2$ also shows a small peak at 120 K, indicating a similar but weaker structural/antiferromagnetic transition. For the hole-doped superconducting Ba$_{0.5}$K$_{0.5}$Fe$_2$As$_2$, a clear peak of $C/T$ was observed at $T_c$ = 36 K, which is the highest peak seen at superconducting transition for iron-based high-$T_c$ superconductors so far. The electronic specific heat coefficient $γ$ and Debye temperature $Θ_D$ of these compounds were obtained from the low temperature data.
Doping evolution of antiferromagnetism and transport properties in the non-superconducting BaFe2-2xNixCrxAs2
本文报道了Ni和Cr等量掺杂的铁磷属元素化物BaFe $_{2-2x}$ Ni $_{x}$ Cr $_{x}$ As $_{2}$的弹性中子散射和输运测量结果。与电子掺杂的BaFe $_{2-x}$ Ni $_{x}$ As $_{2}$相比，BaFe $_{2-2x}$ Ni $_{x}$ Cr $_{x}$ As $_{2}$中的长程反铁磁（AF）有序逐渐被抑制，在$x= 0.20$附近的有序矩和Néel温度逐渐消失，没有出现超导电性.对BaFe $_{2-x}$ Ni $_{x}$ As和BaFe $_{2-2x}$ Ni $_{x}$ Cr $_{x}$ As $_{2}$的输运性质的详细分析表明，与作为温度的函数的线性电阻率相关的非费米液体行为可能不对应于静态AF序的消失。从温度依赖性的电阻率在过掺杂的化合物没有静态AF秩序，我们发现，实际上是由Cr杂质散射，这可能会导致一个金属到绝缘体的交叉在高掺杂BaFe $_{1.7-y}$ Ni $_{0.3}$ Cr $_{y}$ As $_{2}$的影响。
Anisotropic Superconducting Properties of Optimally Doped BaFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ under Pressure
在不同晶轴方向施加磁场的情况下，对BaFe $_2$（As $_{1-x}$ P $_{x}$）$_2$（$x\approx0.35$）的最佳掺杂单晶进行了压力下的磁性测量，从而跟踪了相干长度和各向异性因子的压力演化。尽管超导临界温度降低，我们的研究表明，压力下的超导性能变得更加各向异性。通过适当的缩放，我们直接将这些性质与BaFe $_2$（As $_{1-x}$ P $_{x}$）$_2$作为磷含量的函数获得的值进行比较。
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 $2 meV.
The optical properties of a large, detwinned single crystal of BaFe$_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…
We investigated the out-of-plane transport properties of parent and chemically substituted BaFe$_{2}$As$_{2}$ for various types of substitution. Based on the studies of Hall coefficient and chemical-substitution effect, we have clarified the origin for the unusual temperature dependence of out-of-plane resistivity $ρ_c(T)$ in the high-temperature paramagnetic-tetragonal phase. Electron (hole) carriers have an incoherent (coherent) character, which is responsible for non-metallic (metallic) $ρ_c(T)$. Although both of electron and hole contributions are almost comparable, a slightly larger contr…
A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe$_2$As$_2$, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature $T_s$. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature $T_N$ ($<T_s$) and create in-plane resistivity anisotropy above $T_s$. Here we use neutron polarization analysis to …
The influence of the buffer layer architecture on transport properties for BaFe1.8Co0.2As2 films on technical substrates
在IBAD-MgO/Y $_2$ O $_3$缓冲技术衬底上制备的BaFe $_{1.8}$ Co $_{0.2}$ As $_2$ /Fe双层膜中，观察到正常态下的低电阻和几乎与温度无关的电阻，以及正常-超导转变中的反常峰效应。电阻器网络阵列充分再现了这种效果，假设随着缓冲层厚度的减小，带和膜之间的电导增加。基于此模型，我们评估了这种效应对临界电流密度的影响，并成功地重建了双层膜的超导转变。
The BaFe2(As1-xPx)2 compounds with x = 0 (parent), x = 0.10 (under-doped), x = 0.31, 0.33, 0.53 (superconductors with Tc = 27.3 K, 27.6 K, 13.9 K, respectively) and x = 0.70, 0.77 (over-doped) have been investigated versus temperature using 57Fe Mössbauer spectroscopy. Special attention was paid to regions of the spin-density-wave (SDW) antiferromagnetic order, spin-nematic phase, and superconducting transition. The BaFe2(As0.90P0.10)2 compound exhibits a reduced amplitude of SDW as compared to the parent compound and preserved universality class of two-dimensional magnetic planes with one-dim…
Electronic structure and exotic exchange splitting in spin-density-wave states of BaFe$_2$As$_2$
母体化合物的磁性通常与超导电性的微观机制密切相关。本文报道了对新发现的铁基超导体BaFe $_2$ As $_2$的电子结构的首次直接测量，为进一步的研究提供了基础。我们发现BaFe $_2$ As $_2$中的自旋密度波（SDW）的能量是通过能带结构的奇异交换劈裂而降低的，而不是通过费米表面的巡游电子嵌套。这清楚地表明，金属SDW状态可以单独由局部磁矩的相互作用引起，类似于铜酸盐母体化合物中的反铁磁序的性质。
Strange Metal Hall Effect in Underdoped BaFe$_2$(As$_{1-x}$P$_x$)$_2$
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质与结构动态研究所）；National High Magnetic Field Laboratory（国家强磁场实验室）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Boston University（波士顿大学）
Magnetotransport properties in the magnetic phase of BaFe$_{2-x}$T$_x$As$_2$ (T = Co,Ni): A magnetic excitations approach
由于铁基超导体复杂的费米面，导致电子散射的物理现象的识别是一项困难的任务。本文报道了两个系列的BaFe $_{2-x}$ T $_x$ As $_2$（T = Co，Ni）晶体在不同的$x$值下的电阻率、磁电阻和霍尔效应。T含量的选择，使大多数的研究样品呈现一个中间的磁有序状态和超导基态。我们解释所获得的结果在散射的载流子的磁激发，而不是描述它们作为唯一的多带传导相关的影响。我们的样品是单晶从结构的角度来看，其整体的磁输运性质是由一个单一的磁状态。
Electronic Phase Propagation Speed in BaFe$_2$As$_2$ Revealed by Dilatometry
International Center for Quantum Materials, Peking University（北京大学量子材料中心）；Hefei National Laboratory（合肥国家实验室）；Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家研究中心）
The scaling of $H$-linear magnetoresistance in field and temperature was measured in under-doped (x = 0.19) and optimally-doped (x=0.31)~BaFe$_2$(As$_{1-x}$P$_x$)$_2$. We analyze the data based on an orbital model in the presence of strongly anisotropic quasiparticle spectra and scattering time due to antiferromagnetism. The magnetoresistance is dominated by the properties of small regions of the Fermi surface called `hot spots' where antiferromagnetic excitations induce a large quasiparticle scattering rate. Approximate temperature-magnetic field scaling relations are derived and shown to be …
Uniaxial strain detwinning of CaFe2As2 and BaFe2As2: optical and transport study
砷化铁超导体的母体化合物$A$ Fe $_{2}$ As $_{2}$（$A$ =Ca，Sr，Ba）在温度$T_{\mathrm{TO}}$为135至205 K（取决于碱土元素）时经历四方晶系到正交晶系的结构转变。在$T_{\mathrm{TO}}$下方，独立式晶体分裂成相等分布的结构域，其掩盖了材料的固有的、面内的、各向异性的性质。在这里，我们演示了一种方法，机械解孪晶的CaFe $_{2}$ As $_{2}$和BaFe $_{2}$ As $_{2}$。偏振光成像和同步辐射$X$ -射线测量表明，脱孪晶几乎完全，并且是可逆的，应变释放后孪晶图案恢复。在孪晶和去孪晶状态的电阻率测量表明，电阻率，$ρ$，沿正交$a_{o}$ -轴减小，但沿正交$b_{o}$ -轴在两种化合物中增加。紧接在$T_{\mathrm{TO}}$之下，对于Ca和Ba化合物，比率$ρ_{bo}/ ρ_{ao}$分别= 1.2和1.5。与CaFe $_{2}$ As $_{2}$相反，BaFe $_{2}$ As $_{2}$在名义上的四元相中显示出各向异性，这表明在BaFe $_{2}$ As $_{2}$中波动在$T_{\mathrm{TO}}$以上起的作用比在CaFe $_{2}$ As $_{2}$中更大，或者存在更高的温度交叉或相变。
Optical spectroscopy was used to study the electrodynamics and hidden transport properties of a BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ thin superconducting film. We analyzed the normal state data using a Drude-Lorentz model with two Drude components: one narrow ($D_1$) and another broad one ($D_2$). In the superconducting state, two gaps with $2Δ_{0}^{(2)}/k_BT_c=1.9$--2.0 and $2Δ_{0}^{(1)}/k_BT_c=4.0$--4.3 are formed from the narrow component $D_1$ while the broad component $D_2$ remains ungapped. The calculated total DC resistivity of the film and the low-temperature scattering rate for the narrow…
Calculation of the specific heat of optimally K-doped BaFe$_2$As$_2$
用密度泛函理论计算的最佳K掺杂BaFe $_2$ As $_2$的比热比实验值小约5倍。我们报告说，通过调整铁原子上的电位，使其对电子的排斥性稍强，从而提高了Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$的计算热容以及结构、磁性和电子性质。对反铁磁态作同样的修正，我们发现电子-声子耦合得到了强烈的增强。
BaFe$_2$S$_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature, $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5%mol excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether an Fe off-stoichiometry is actually present in the samples, and how it affects the str…
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。
Alkaline-earth iron arsenide (122) is one of the most studied families of iron-based superconductors, especially for angle-resolved photoemission spectroscopy. While extensive photoemission results have been obtained, the surface complexity of 122 caused by its charge-non-neutral surface is rarely considered. Here, we show that the surface of 122 can be neutralized by potassium deposition. In potassium-coated BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$, the surface-induced spectral broadening is strongly suppressed, and hence the coherent spectra that reflect the intrinsic bulk electronic state recover.…
Topological properties of spin block magnetic ladders in proximity of a superconductor: application to BaFe$_{2}$S$_{3}$
Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Université Paris-Saclay, Centre National de la Recherche Scientifique, Laboratoire de Physique des Solides（巴黎萨克雷大学，法国国家科学研究中心，固体物理实验室）
Pressure tuning of Fermi surface topology of optimally doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$
采用高压下的电阻测量和同步辐射X射线衍射测量相结合的方法，研究了电子掺杂BaFe $_{1.9}$ Ni $_{0.1}$ As $_{2}$的超导、输运和结构性质.发现该体系的超导转变温度与$c/a$的轴比随压力的增加而降低的方式相似，但在7.5GPa的临界压力下，$c/a$的超导转变温度为零，此时发生了从四方相到塌缩四方相的同构转变。电阻服从线性温度依赖关系，证明了反铁磁自旋涨落输运机制。压力效应的压力诱导费米表面拓扑结构的修改，其中压力抑制准粒子的有效质量和强度的反铁磁自旋涨落导致超导性的降低，相应的框架内解释。在塌缩的四方相中不存在的超导电性被认为是由于反铁磁自旋涨落的完全抑制。
Sequential structural and antiferromagnetic transitions in BaFe$_2$Se$_3$ under pressure
在两条腿的梯形化合物BaFe $_2$ S $_3$中发现了超导电性，确立了123型铁硫族化合物作为铁基超导体家族中一个新的和有趣的亚群。然而，在这个123系列中，BaFe $_2$ Se $_3$是例外的成员，具有不同于所有其他的磁性顺序和晶体结构。最近，一个令人兴奋的实验报道了BaFe $_2$ Se $_3$在高压下出现超导性[J. - J. Ying等人，Rev. B 95，241109（R）（2017）]。在这篇文章中，我们报告了BaFe $_2$ Se $_3$的第一性原理研究。我们的分析揭示了BaFe $_2$ S $_3$和BaFe $_2$ Se $_3$之间的各种定性差异，包括在后者中，随着压力的增加，一个意想不到的转变链。首先，通过逐渐减小铁梯的倾斜角度，晶体结构在~6 GPa下从Pnma平滑地转变为Cmcm。第二，系统在10.4 GPa下变成金属。第三，其独特的环境压力Block反铁磁基态被~12 GPa下更常见的CX反铁磁序取代，与123-S阶梯的磁态相同。这种转变是在与实验超导转变非常相似的压力下发现的。最后，所有的磁矩在30 GPa时消失。这份完整的相演化理论图非常重要，因为在高压实验中捕获许多物理性质的技术挑战。计算结果表明BaFe $_2$ Se $_3$和BaFe $_2$ S $_3$的超导性具有不同的特征：在123-S中，当磁矩消失时，出现配…
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。
Disorder Induced Effects on the Critical Current Density of Iron Pnictide BaFe_1.8 Co_0.2 As_2 single crystals
研究无序在超导体中的作用是表征基本超导性质以及评估材料潜在应用的重要组成部分。在大多数情况下，缺陷矩阵上的信息是穷人，使这样的研究困难，但这种情况可以通过引入缺陷，在一个可控的方式，如中子辐照提供改善。在这项工作中，我们分析了中子辐照对Ba（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$单晶的影响。我们主要集中在磁性能，这是由磁强计测定。通过中子辐照引入无序导致对可逆和不可逆磁性的显著影响，例如转变温度、上临界场、各向异性和临界电流密度。通过与未辐照状态下的性质进行比较，对结果进行了详细讨论。
Magneto-elastically coupled structural, magnetic and superconducting order parameters in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$
我们测量了机械应变BaFe $_{2}$（As $_{1-x}$ P $_{x}$）$_{2}$单晶在宽的$x$范围内的输运性质。Néel转变对应力极其敏感，并且这种敏感性随着接近最佳掺杂而增加，即使转变本身被强烈抑制。此外，我们观察到显着的变化，超导转变温度与施加的应变，这反映了变化的组合物$x$的函数。这些实验直接说明了铁基超导体中不同自由度之间的紧密耦合，揭示了磁弹性耦合对磁性和超导转变温度的重要性。
Atomically-flat, chemically-stable, superconducting epitaxial thin film of iron-based superconductor, cobalt-doped BaFe$_2$As$_2$
最近报道了诸如Co掺杂的SrFe $_2$ As $_2$（SrFe $_2$ As $_2$：Co）的Fe基超导体的外延生长，但是由于它们具有粗糙的表面并且通过与环境大气中的水蒸气的反应而分解，因此对于器件应用而言仍然具有不足的性能。本文报道了在700 ℃下生长的Co掺杂BaFe $_2$ As $_2$外延膜的超导转变起始温度为20 K。该转变比在SrFe $_2$ As $_2$：Co膜上观察到的转变更尖锐，这将源于它们改进的结晶度。这些薄膜还具有原子级平坦的表面，具有台阶和露台结构，并表现出抗水蒸气暴露的化学稳定性。
Anomalous sharp peak in the London penetration depth induced by the nodeless-to-nodal superconducting transition in BaFe$_2$(As$_{1-x}$P$_x$)$_2$
量子临界点（QCP）是否隐藏在非常规超导体中的问题是一个激烈的争论。虽然一个关于伦敦穿透深度的著名实验已经证明了在等价掺杂的铁基超导体BaFe $_2$（As $_{1-x}$ P $_x$）$_2$中存在QCP，但在零温度下磁序消失附近的穿透深度中观察到一个尖峰，这种新兴的QCP的性质仍然不清楚。在这里，我们提供了一个独特的图片来理解的QCP现象的基础上的线性响应理论的框架。从态密度和超流密度计算的证据表明，无结到结的配对转变伴随着在BaFe $_2$（As $_{1-x}$ P $_x$）$_2$的穿透深度的尖峰的出现。这种配对转变起源于具有强层间超导配对的三维电子性质。这一发现为理解BaFe $_2$（As $_{1-x}$ P $_x$）$_2$中实验观察到的QCP提供了重要的见解。
Study of the hyperfine fields in the BaFe$_2$As$_2$ family and its relation to the magnetic moment
利用完全相对论Dirac方法研究了BaFe $_2$ As $_2$族的超精细场$B_\text{hf}$和磁性.该研究涵盖电子掺杂的Ba（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$和Ba（Fe $_{1-x}$ Ni $_x$）$_2$ As $_2$、空穴掺杂的（Ba $_{1-x}$ K $_x$）Fe $_2$ As $_2$以及同位素掺杂的Ba（Fe $_{1-x}$ Ru $_x$）$_2$ As $_2$和BaFe $_2$（As $_{1-x}$ P $_x$）$_2$，用于宽范围的浓度$x$。对于取代的化合物的超精细场表现出非常强的依赖于掺杂剂的类型和它的浓度$x$。相对论的贡献被发现有一个显着更强的影响，为铁pnictides相比，散装铁。作为一个重要的发现，我们表明，这是不明智的，涉及到的超精细场$B_\text{hf}$的平均磁矩$μ$的化合物，因为它是在早期的文献。
Avoided quantum criticality and magnetoelastic coupling in BaFe$_{2-x}$Ni$_{x}$As$_{2}$
利用高分辨同步辐射X射线和中子散射研究了电子掺杂BaFe_（2-x）Ni_xAs_2的结构和磁性有序。当Ni掺杂x时，BaFe_2As_2中几乎同时发生的四方晶到正交晶的结构T_s和反铁磁（T_N）相变被逐渐抑制和分离，导致T_s>T_N.当x> $, tending towards a quantum bi-critical point near optimal superconductivity at x=0.1. The zero-temperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088< x < 0.104, resulting in a finite value of T_N \approx T_c+10$ K时，T_s和T_N之间的温度间隔随x的增大而减小。我们的结果意味着避免量子临界点，这是预计将强烈影响正常和超导状态的属性。
