Within a two-band model for the recently discovered ferropnictide materials, we calculate the thermal conductivity assuming general superconducting states of $A_{1g}$ ("s-wave") symmetry, considering both currently popular isotropic "sign-changing" $s$ states and states with strong anisotropy, including those which manifest nodes or deep minima of the order parameter. We consider both intra- and interband disorder scattering effects, and show that in situations where a low-temperature linear-$T$ exists in the thermal conductivity, it is not always "universal" as in d-wave superconductors. We discuss the conditions under which such a term can disappear, as well as how it can be induced by a magnetic field. We compare our results to several recent experiments.
While the beginning decade of the high-Tc cuprates era passed under domination of local theories, Abrikosov was one of the few who took seriously the electronic band structure of cuprates, stressing the importance of an extended Van Hove singularity near the Fermi level. These ideas have not been widely accepted that time mainly because of a lack of experimental evidence for correlation between saddle point position and superconductivity. In this short contribution, based on the detailed comparison of the electronic band structures of different families of cuprates and iron based superconducto…
Microwave conductivity in the ferropnictides with specific application to Ba$_{1-x}$K$_x$Fe$_2$As$_2$
计算了具有$s^\pm$带隙对称性的两带超导体的微波电导率。非弹性散射近似包括在BCS模型增强的温度相关的准粒子散射率假设，然而，是频率无关的。的可能性，S波间隙的一个或另一个的电子或空穴口袋是各向异性的探索，包括费米表面上的间隙节点和没有间隙的情况下。我们的BCS结果与在两种流体模型（TFM）中得到的结果的比较，以及与铜酸盐的情况下，间隙具有d-波对称性和Ba $_{1-x}$ K $_x$ Fe $_2$ As $_2$的实验结果。目前可用的微波电导率数据在这种材料中提供了强有力的证据，大的各向异性的电子袋s波间隙。虽然最佳拟合有利于费米表面上具有节点的间隙，但这与一些但不是所有的穿透深度测量结果不一致，这些测量结果有利于无节点间隙，热导率和核磁共振数据也是如此。
Symmetry of spin excitation spectra in the tetragonal paramagnetic and superconducting phases of 122-ferropnictides
通过比较第一性原理计算和非弹性中子散射（INS）测量结果，研究了122-铁磷族化合物超导体中既没有静态磁相也没有结构相变的BaFe 1.85 Co 0.15 As 2和BaFe 1.91 Ni 0.09 As 2样品的自旋激发谱的对称性.在正常和超导（SC）状态下，光谱缺乏围绕（1/2 1/2 L）轴的42/m螺旋对称性，这是由I4/mmm空间群所暗示的。这在正常和SC状态自旋动力学的面内各向异性和自旋共振模式的面外色散中都是明显的。我们发现，这种效果源于较高的对称性的磁性Fe子晶格相对于晶体本身，因此INS信号继承了对称性的展开布里渊区（BZ）的Fe子晶格。面内各向异性是温度无关的，并且可以在正常状态密度泛函理论计算中定性地再现，而不调用先前提出的作为对这种效应的解释的破缺（“破缺”）基态。在SC转变之下，磁共振模式Er的能量以及其强度和SC自旋间隙继承了沿着面外方向L的正常态强度调制，其周期比体心四面体BZ对称性所预期的大两倍。这种调制的幅度在更高的掺杂降低，提供了一个类比，在双层铜酸盐的偶和奇谐振模式之间的分裂。结合我们和以前的数据，我们表明，在奇数L的一个普遍的线性关系Er=4.3*kB*Tc持有所有研究的铁基超导体，独立于他们的载体类型。它的有效性下降到最低的掺杂水平是一致的，与较弱的电子相关性在铁磷族化合物相比，欠掺杂的铜酸盐。
Using a second-order perturbative Green's functions approach we determined the normal state single-particle spectral function $A(\vec{k},ω)$ employing a minimal effective model for iron-based superconductors. The microscopic model, used before to study magnetic fluctuations and superconducting properties, includes the two effective tight-binding bands proposed by S.Raghu et al. [Phys. Rev. B 77, 220503 (R) (2008)], and intra- and inter-orbital local electronic correlations, related to the Fe-3d orbitals. Here, we focus on the study of normal state electronic properties, in particular the tempe…
Phase-incoherent superconducting pairs in the normal state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$
最近发现的铁磷族化合物超导体的正常态性质可能是理解其奇异超导性的关键。使用点接触光谱，我们表明，Andreev反射之间的外延薄膜的Ba（Fe $_{0.92}$ Co $_{0.08}$）$_2$作为$_2$和银尖可以看到在正常状态下的膜的温度$T\sim1.3T_c$，其中$T_c$是临界温度的超导体。只有当复超导序参量相位的强涨落所产生的超导对处于正常状态时，才能理解远高于$T_c$的Andreev反射。我们的结果提供了铁磷族化合物超导体正常态下存在相位非相干超导对的光谱证据。
Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, allows one to distinguish between closely related superconducting order parameters, e.g., d$_{x^2-y^2}$ and d$_{xy}$ superconductivity. We benchmark this method on YBa$_2$Cu$_3$O$_{7-δ}$ and, unsurprisingly, confirm that this is a d$_{x^2-y^2}$ superconductor. We then apply our method to $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which we discover is a d$_{xy}$ superconductor.
Absence of nodes in the energy gap of the high-temperature electron-doped Pr1.85Ce0.15CuO4-y superconductor using thermal conductivity and specific heat measurements
To shed light on the mechanism responsible for the in-plane resistivity anisotropy in the nematic phase of the iron-based superconductors, we investigate the impact of spin fluctuations on the anisotropic AC conductivity. On the one hand, the scattering of electrons off magnetic fluctuations causes an anisotropic scattering rate. On the other hand, the accompanying Fermi velocity renormalization contributes to both the plasma frequency and the scattering rate in antagonistic ways, giving rise to anisotropies in these quantities that exactly cancel each other in the DC limit. As a result, the e…
FeSe is an iron-based superconductor of immense current interest due to the large enhancements of Tc that occur when it is pressurized or grown as a single layer on an insulating substrate. Here we report precision measurements of its superconducting electrodynamics, at frequencies of 202 and 658 MHz and at temperatures down to 0.1 K. The quasiparticle conductivity reveals a rapid collapse in scattering on entering the superconducting state that is strongly reminiscent of unconventional superconductors such as cuprates, organics and the heavy fermion material CeCoIn5. At the lowest temperature…
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
我们计算了零温下洁净金属和超导体的低频光学电导率σ（w），忽略了杂质和声子的影响。一般来说，sigma的频率和温度依赖性几乎没有共同之处。例如，对于三维的小费米面（但不是二维的），我们发现对于低w，Re σ（w>0）=常数，这对应于散射率γ与w^2成比例，即使在没有Umklapp散射的情况下，当T^2对γ没有贡献时。本文的主要部分详细讨论了二维d波超导体的光学电导率，其中Re sigma（w>0）\propto w ^4对应于最小频率，Umklapp过程通常在小于最大能隙Δ的两倍的有限阈值w_0之上平滑地设置。在节点位于（pi/2，pi/2）的情况下，使得它们之间的直接Umklapp散射是可能的，得到Re sigma（w）\propto w ^2。
The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic field, the electronic residual linear term in the T = 0 limit, κ_0/T, is vanishingly small. Application of a magnetic field H causes no increase in κ_0/T initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of κ_0/T has the classic shape of a two-band superconductor, such as MgB2: it rises exponentially at very l…
In-plane superfluid density and microwave conductivity of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: evidence for d-wave pairing and resilient quasiparticles
Anisotropic microwave conductivity of cuprate superconductors in the presence of CuO chain induced impurities
在动能驱动的超导机制下，研究了邻位Ⅱ型$_2$的微波电导各向异性.邻位-Ⅱ $_2$ $_3$ O $_{6.50}$的特征在于填充和空$\hat{b}$轴CuO链的周期性交替。考虑CuO链诱导的扩展各向异性杂质散射，基于准粒子激发和散射过程的节块近似，再现了邻位Ⅱ型$_2$微波电导各向异性的主要特征，包括$\hat{a}$轴和$\hat{b}$轴微波电导的能量和温度依赖性的强度和线形.我们的研究结果也证实了$\hat{b}$轴CuO链诱导杂质是各向异性的主要来源。
Analysis of the resonance peak and magnetic coherence seen in inelastic neutron scattering of cuprate superconductors: a consistent picture with tunneling and conductivity data
Motivated by Sr2RuO4, we consider a chiral superconductor where the gap is strongly suppressed along certain momentum directions. We evaluate the thermal Hall conductivity in the gapless regime, i.e., at temperature small compared with the impurity band width γ, taking the simplest model of isotropic impurity scattering. We find that, under favorable circumstances, this thermal Hall conductivity can be quite significant, and is smaller than the diagonal component (the universal thermal conductivity) only by a factor of 1/ ln(2 Δ_M/γ), where Δ_M is the maximum gap.
