Possible Multiple Gap Superconductivity with Line Nodes in Heavily Hole-Doped Superconductor KFe2As2 Studied by 75As-NQR and Specific Heat
H. Fukazawa, Y. Yamada, K. Kondo, T. Saito, Y. Kohori, K. Kuga, Y. Matsumoto, S. Nakatsuji, H. Kito, P. M. Shirage, K. Kihou, N. Takeshita, C. -H. Lee, A. Iyo, H. Eisaki
备注：
5 pages, 5 figures, to be published in J. Phys. Soc. Jpn. No.8 issue (2009)
报道了重空穴掺杂超导体KFe_2As_2（Tc = 3.5K）的~（75）As核四极共振（NQR）和比热测量结果。超导态的自旋-晶格弛豫速率1/T1表现出相当平缓的温度依赖性，在Tc以下没有相干峰。准粒子比热C_QP/T在低于Tc时有很小的比热跃变，约为电子比热系数的30%。此外，在最低测量温度T = 0.4 K \simeq Tc/10时，还存在低能准粒子激发.这些1/T1和C_QP/T的温度依赖性可以用多节超导能隙方案来解释，而不是简单能隙分析中的多个全能隙s_ \pm波方案。
英文摘要：
We report the 75As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe2As2 (Tc = 3.5 K). The spin-lattice relaxation rate 1/T1 in the superconducting state exhibits quite gradual temperature dependence with no coherence peak below Tc. The quasi-particle specific heat C_QP/T shows small specific heat jump which is about 30% of electronic specific heat coefficient just below Tc. In addition, it suggests the existence of low-energy quasi-particle excitation at the lowest measurement temperature T = 0.4 K \simeq Tc/10. These temperature dependence of 1/T1 and C_QP/T can be explained by multiple nodal superconducting gap scenario rather than multiple fully-gapped s_\pm-wave one within simple gap analysis.
Zn-Neighbor Cu NQR in Zn-Substituted YBa2Cu3O7-d and YBa2Cu4O8
利用Cu（2）核四极共振（NQR）谱的卫星信号研究了最佳掺杂$_2$（Cu $_{1-x}$ Zn_x）$_3$ O $_{7-δ}$和欠掺杂$_2$（Cu $_{1-x}$ Zn_x）$_4$ O $_8$中Zn附近的局域电子态.从Cu NQR谱的相对强度，卫星信号被分配到Zn邻居Cu NQR线。卫星信号的Cu核自旋-晶格弛豫时间比主信号的短，表明无论是欠掺杂还是最佳掺杂体系，Zn附近的磁关联都局部增强。纯YBa $_2$ Cu $_4$ O $_8$是化学计量的、均匀的、欠掺杂的电子系统;然而，在CuO $_2$平面中的Zn诱导的不均匀磁响应比最佳掺杂的YBa $_2$ Cu $_3$ O $_{7-δ}$更显著。
A unified phase diagram of F-doped LaFeAsO by means of NMR and NQR parameters
我们目前$^{75}$作为核磁共振和四极共振结果（NMR，NQR）的一组新的LaFeAsO $_{1-x}$ F $_x$多晶样品。改进的合成条件导致更均匀的样品，更好地控制氟含量。结构$\equiv$的磁，超导转变温度已被确定的NMR自旋晶格弛豫率和交流磁化率测量。如此确定的相图偏离公布的一个，特别是低F掺杂浓度。然而，如果从NQR谱确定掺杂水平，则两个相图可以被调和。磁性和超导性的本体共存和纳米级分离成低掺杂样和高掺杂样区域的情况下，已被证实。在磁性和超导性的边界处对欠掺杂样品的附加频率依赖的强度、自旋-自旋和自旋-晶格弛豫率测量表明，正交性和磁性起源于低掺杂区，超导性首先在高掺杂区发展。
Cu NQR Wipeout Effect versus Charge Pseudogap in Zn/Ni Doped NdBa$_{2}$Cu$_{3}$O$_{6+y}$
本文报道了用$^{63,65}$ Cu NQR方法测量了重掺Ni和Zn杂质的轻度欠掺杂NdBa $_{2}$ Cu $_{3}$ O $_{6+y}$单晶。由于杂质掺杂，在这两种情况下，超导性都被完全抑制。镍强烈增强磁相关性，并诱导wipeout的NQR信号相比，发现在条纹有序铜酸镧。相反，在Zn掺杂的样品中，磁性被抑制，其中没有观察到擦除效应，并且核自旋弛豫速率降低。我们的研究结果是在一个惊人的对应关系的不同影响的Ni和Zn杂质的电荷pseudogap证明最近的光学数据，从而揭示了磁相关性和pseudogap现象之间的密切关系。
Doping dependence of Normal-state Property in the Iron-based Oxypnictide Superconductors LaFeAsO1-y probed by 57Fe-NMR and 75As-NMR/NQR
本文报道了贫氧铁基氧氮族化合物超导体LaFeAsO_{1-y} $. A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, 1/T_1T in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe and As sites regardless of the electron-doping level in LaFeAsO_{1-y}. On the basis of this result, we claim that $ 1/T_1 T $ is not always enhanced by antiferromagnetic fluctuations close to an antiferromagn…
Zn-induced wipeout effect on Cu NQR spectra in La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$
本文系统地研究了Zn取代对高$T_c$超导体La $_{2-x}$ Sr $_x$ Cu $_{1-y}$ Zn $_y$ O $_4$从载流子欠掺杂到过掺杂的NQR谱的影响（多晶样品，$x$ =0.10、0.15和0.20）。我们没有观察到明显的wipeout效应的过掺杂的样品，一个渐进的和部分的wipeout效应低于约80 K的最佳掺杂的，和非常突然的和完全的wipeout效应低于约40 K的欠掺杂的。擦除效应表明在低频率下的Cu自旋波动的高度增强的光谱权重。我们将擦除效应与远高于40 K的Zn诱导局部磁性和低于40 K的局部化效应相关联。
Pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by $^{75}$As NQR
我们报告了单晶Ca（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$（$0\leq x \leq 0.09$）的$^{75}$ As NQR测量。核自旋晶格弛豫速率$T_1^{-1}$作为温度$T$和Co掺杂剂浓度$x$的函数揭示了低于交叉温度$T^*$的下和最佳掺杂区域中的正常状态赝隙状相。所得的$x$-$T$相图显示，在抑制自旋密度波阶之后，$T^*$与$T_c$相交，在最佳掺杂制度附近迅速下降到零。赝能隙行为的可能起源进行了讨论。
Uniform mixing of antiferromagnetism and high-T_c superconductivity in multilayered copper oxides Ba_2Ca_{n-1}Cu_nO_{2n}F_2 with apical fluorines (n=2,3,4): Cu-NMR/NQR and F-NMR
Nematicity in the heavily hole-doped iron pnictide superconductors remains controversial. Sizeable nematic fluctuations and even nematic orders far from a magnetic instability were declared in RbFe$_2$As$_2$ and its sister compounds. Here we report a systematic elastoresistance study of series of isovalent- and electron-doped KFe$_2$As$_2$ crystals. We found divergent elastoresistance upon cooling for all the crystals along their [110] direction. The amplitude of elastoresistivity diverges if K is substituted with larger ions or if the system is driven towards a Lifshitz transition. However, w…
Cu Nuclear Quadrupole Resonance Study of La_{2-x}Sr_xCu_{1-y}Zn_yO_4 (x=0.10, 0.15 and 0.20): Zn-induced Wipeout Effect near the Magnetic and Electric Instability
Electronic structure of heavily electron-doped BaFe$_{1.7}$Co$_{0.3}$As$_2$ studied by angle-resolved photoemission
我们对重电子掺杂非超导BaFe $_{1.7}$ Co $_{0.3}$ As $_2$进行了高分辨角分辨光电子能谱研究。我们发现在空穴掺杂的超导体Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$中观察到的两个空穴费米面口袋在该化合物中不存在或非常小，而在M点处的两个电子口袋由于Co替代的电子掺杂而显著膨胀。非SC和SC样品的费米面的比较表明，通过反铁磁波矢连接的空穴和电子口袋的共存是实现铁基超导体的超导性机制的必要条件。
Novel Superconducting Characteristics and Unusual Normal-State Properties in Iron-based Pnictide Superconductors: 57Fe-NMR and 75As-NQR/NMR studies in REFeAsO_1-y (RE=La,Pr,Nd) and Ba0.6K0.4Fe2As2
利用$^{57}$ Fe-NMR和$^{75}$ As-NQR/NMR研究了铁基磷属元素化物超导体REFeAsO $_{1-y}$（RE=La，Pr，Nd）和Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$（$T_{c}=$ 38 K）的新颖超导特性和异常正常态性质.在$_{0.7}$的超导态（$T_{c}=$ 28 K），$^{57}$的Fe-Knight位移的自旋分量在低温下几乎为零，这为自旋-单重态Cooper配对形成超导态提供了有力的证据. LaFeAsO $_{0.7}$和Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$中的核自旋-晶格弛豫速率$(1/T_{1})$表现出类似$T^{3}$的依赖性，在$T_{c}$以下没有相干峰，表明在这些Fe基磷属元素化合物中通常实现非常规的超导态。所有这些事件$T_c$以下一直认为在扩展的s $_{\pm}$波配对与费米面之间的顺序参数的符号反转。在正常状态下，$1/T_1T$显着减少后冷却的Fe和As位的LaFeAsO $_{0.7}$。相反，在Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$中，它在冷却时逐渐增加。尽管这些化合物的超导性质之间的相似性，观察到一个关键的差异，在其正常状态下的属性取决于电子或空穴是否掺杂到FeAs层。这些结果可能为铁基…
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
Evolution of band structure from optimally doped to heavily overdoped Co-substituted NaFeAs
用角分辨光电子能谱研究了NaFe $_{1-x}$ Co $_x$ As从最佳掺杂超导化合物（$x=0.028$）到重过掺杂非超导化合物（$x=0.109$）的电子结构。与“122”型铁磷属元素化合物的情况类似，我们的数据表明，在NaFe $_{1-x}$ Co $_x$ As中的Co掺杂提供了额外的载流子，并相应地移动了费米能级。在$x=0.109$化合物中，围绕带中心$Γ$的空穴状带移动到更深的结合能，而出现电子口袋。在整个掺杂范围内，我们研究的整体能带重整化基本上保持不变，这表明局部磁/电子相关性不受载流子掺杂的影响。我们推测，流动载流子的巡游特性和局域相互作用之间的平衡可能对超导性起着重要的作用。
Angle-resolved photoemission spectroscopy of Na-doped Ca2CuO2Cl2 single crystals: Fingerprints of a magnetic insulator in a heavily underdoped superconductor
Superconducting gap with sign reversal between hole pockets in heavily hole-doped Ba1-xKxFe2As2
为了深入了解铁磷族元素化物的非常规超导性，没有电子口袋，我们测量了重空穴掺杂制度的Ba $_{1-x}$ K $_x$ Fe $_2$ As $_2$的热导率$κ$和穿透深度$λ$。剩余热导率$(κ/T)_{T \rightarrow 0\,{\rm K}}$和$λ$的$T$依赖性一致地表明在$x=0.76$处的完全带隙超导性和在较高空穴浓度处的（线）节超导性。在低温下的$\fracκ{T}\cdot T_c|_{T \rightarrow 0\,{\rm K}}$和$\frac{dλ}{d(T/T_c)}$的大小，这两者都是由低能量激发的性质决定的，表现出非常不寻常的非单调x依赖性。这些结果表明，在一个狭窄的掺杂范围内的节点特性的显着变化，表明掺杂交叉的s波状态之间的间隙函数与$Γ$中心的空穴袋之间的符号反转和没有。
Motivated by recent discovery of correlated insulating and superconducting behavior in twisted bilayer graphene, we revisit graphene's honeycomb lattice doped close to the van Hove singularity, using the truncated unity functional renormalization group approach. We consider an extended Hubbard model on the honeycomb lattice including on-site and nearest-neighbor Coulomb repulsions, and nearest-neighbor ferromagnetic exchange and pair hopping interactions. By varying the strength of the nearest-neighbor exchange coupling and Coulomb repulsion as free parameters, we present rich ground-state pha…
Crystalline anisotropy induces a second antiferromagnetic phase in the absence of SDW in the heavily hydrogen-doped LaFeAsO$_{1-x}$H$_x$ $(x\sim0.5 )$
采用密度泛函理论结合动力学平均场方法（DFT+DMFT）研究了重掺氢LaFeAsO $_{1-x}$ H $_x$ $(x\sim0.5 )$的电子和磁性.我们发现了氢原子和氧原子的条纹状有序结构，作为基态，具有与反铁磁（AF）顺序相同的配置。新的结构可以解释现有的实验结果有关的重H掺杂的LaFeAsO $_{1-x}$ H $_x$，如在平面内的电子各向异性和非均匀的磁性行为。在Fe- 3d $_{xz}$（xz）和Fe-3d $_{yz}$（yz）基态轨道之间观察到显著的各向异性，其中不存在由自旋密度波相产生的赝能隙，这被发现源于结晶各向异性。磁矩在空间上是不均匀的，并且对晶体构型很敏感。我们发现，非均匀的磁性行为与结构中的As-Fe-As键角有关。我们的研究结果将阐明铁基超导体中晶体细节和轨道自由度的重要性。
Hidden robust presence of a hole Fermi surface in a heavily electron doped iron based superconductor LaFe$_2$As$_2$
本文研究了最近发现的重电子掺杂铁基超导体LaFe $_2$ As $_2$的电子结构。尽管第一性原理计算表明在$Γ$点周围明显不存在空穴费米表面，但我们通过假设去除La $d$轨道贡献揭示了在$Γ$点周围的空穴费米表面基本上存在。在LaFe $_2$ As $_2$的塌缩阶段，这是非超导的，空穴费米面被发现是不存在的，和从非塌缩超导阶段的差异可以自然地理解内的自旋波动介导的配对方案。
Recently it was reported experimentally that the monolayer graphene can be doped to beyond the Van Hove singularity. We study theoretically the possible superconductivity and the corresponding spin excitations of the monolayer graphene in this doping region. A static spin-density-wave state is favorable due to the nested Fermi surface as the Fermi level is doped to the Van Hove singularity point. Superconductivity may be realized upon further doping. The spin excitations in the superconducting state are studied theoretically based on the random phase approximation. The overall features are qua…
Quantitative Determination of Eliashberg Function and Evidence of Strong Electron Coupling with Multiple Phonon Modes in Heavily Over doped (Bi,Pb)2Sr2CuO6
To elucidate the origin of re-enhanced high-$T_c$ phase in the heavily electron-doped Fe-pnictides, systematic $^{75}$As NMR studies are performed on heavily electron-doped LaFe$Pn$O$_{0.75}$H$_{0.25}$ by controlling the pnictogen height ($h_{Pn}$) from the Fe plane through the substitution at $Pn$(=As) site with Sb or P. The measurements of nuclear spin relaxation rate (1/$T_1$) and Knight shift ($K$) reveal that the moderate spin fluctuations at high temperatures are suppressed toward low temperatures. Such characteristic spin fluctuations with gap like feature at low energies are more enlar…
We theoretically study ferromagnetic (FM) fluctuations that are experimentally observed in the heavily overdoped region of cuprate superconductors. To explore the origin of FM fluctuations, we evaluate the spin susceptibilities of a single-band Hubbard model within the fluctuation exchange approximation. Model parameters are derived using the Wannierization technique and the constrained random phase approximation method based on the maximally localized Wannier functions. The constrained random phase approximation calculations reveal that the on-site Coulomb interaction decreases with an increa…
The pairing glue of high-$T_{\rm c}$ superconductivity in heavily electron-doped (e-doped) FeSe, in which hole-pockets are absent, has been an important unsolved problem. Here, we focus on a heavily e-doped bulk superconductor Li$_{1-x}$Fe$_x$OHFeSe ($T_{\rm c} \sim 40$K). We construct a multiorbital model beyond the rigid band approximation and analyze the spin and orbital fluctuations by taking both vertex corrections (VCs) and self-energy into consideration. Without e-doping ($x=0$), the ferro-orbital order without magnetism in FeSe is reproduced by the VCs.The orbital order quickly disappe…
