In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual $s_\pm$ state where there the gap changes sign between the electron and hole Fermi surface sheets. Here we report measurements of the penetration depth $λ(T)$ in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to $\sim$ 100 mK. We find that $λ(T)$ varies almost perfectly linearly with $T$ strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function may not be generic to all pnictide superconductors.
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。
Spin fluctuations are widely believed to play an important role in the superconducting mechanisms of unconventional high-temperature superconductors. Spin fluctuations have been observed in iron-based superconductors as well. However, in some iron-based superconductors such as LaFePO$_{0.9}$, they have not been observed by inelastic neutron scattering (INS). LaFePO$_{0.9}$ is an iron-based superconductor with a low superconducting transition temperature ($T_{\rm c}$= 5 K), where line nodes are observed in the superconducting gap function. The line-node symmetry typically originates from sign r…
We investigate the impact of s-wave spin-singlet pairing on antiferromagnetic semimetals with Dirac points or nodal loops at the Fermi level. The electron pairing is generally shown to convert the semimetal into a tunable nodal superconductor. The changeover from fully gapped to gapless phases is dictated by symmetry properties of the antiferromagnetic-superconducting state that set the occurrence of a large variety of electronic topological transitions. We provide a general criterion for predicting a series of transitions between nodal and fully gapped superconducting phases. Different types …
The three-band Emery model is applied to study the selected principal features of the $d$-$wave$ superconducting phase in the copper-based compounds. The electron-electron correlations are taken into account by the use of the diagrammatic expansion of the Guztwiller wave function (DE-GWF) method. The nodal Fermi velocity, Fermi momentum and effective mass are all determined in the paired state and show relatively good agreement with the available experimental data, as well as with the corresponding single-band calculations. Additionally, the influence of the next-nearest neighbor oxygen-oxygen…
We present a detailed analysis on the hopping between monolayer CuO$_2$ and bulk CuO$_2$ plane in the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ substrate. With a two-band model, we demonstrate that the nodeless gap can only exist when the hole concentration in monolayer CuO$_2$ plane is very large or very small. We argued that the possible phase separation may play important role in the recent experimental observation of nodeless gap.
Unlike the hole-doped cuprates, both nodal and nodeless superconductivity (SC) are observed in the electron-doped cuprates. To understand these two types of SC states, we propose a unified theory by considering the two-dimensional t-J model in proximity to an antiferromagnetic (AF) long-range ordering state. Within the slave-boson mean-field approximation, the d-wave pairing symmetry is still the most energetically favorable even in the presence of the external AF field. In the nodal phase, it is found that the nodes carry vorticity and are protected by the adjoint symmetry of time-reversal an…
Robust nodal superconductivity induced by isovalent doping in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and BaFe$_2$(As$_{1-x}$P$_x$)$_2$
研究了铁基超导体Ba（Fe $_{1-x}$ Ru $_x$）$_2$ As $_2$和BaFe $_2$（As $_{1-x}$ P $_x$）$_2$的超低温热输运性质.对于最佳掺杂的Ba（Fe $_{0.64}$ Ru $_{0.36}$）$_2$ As $_2$，观察到零场下的大的剩余线性项$κ_0/T$和$κ_0(H)/T$对$\sqrt{H}$的依赖性，这为超导能隙中存在节点提供了有力的证据.该结果表明，等价Ru掺杂也可以诱导节点超导性，如P在BaFe $_2$（As $_{0.67}$ P $_{0.33}$）$_2$中所做的那样。此外，在欠掺杂Ba（Fe $_{0.77}$ Ru $_{0.23}$）$_2$ As $_2$和重欠掺杂BaFe $_2$（As $_{0.82}$ P $_{0.18}$）$_2$中，$κ_0/T$表现出类似的节点行为，这表明节点超导性在欠掺杂区域的鲁棒性，并对理论模型施加约束。
Emergent loop-nodal s$_\pm$-wave superconductivity in CeCu$_2$Si$_2$: similarities to the iron-based superconductors
重费米子超导体由于具有自旋轨道耦合自由度和电子关联，是新电子配对态的主要候选者。1979年发现的CeCu $_2$ Si $_2$中的超导电性是强关联电子系统中非常规（非BCS）超导体的原型，至今仍没有解决。在这里，我们提供了基于先进的第一性原理理论方法的超导性的第一份报告。我们发现，有希望的候选者是一个$s_\pm$波态的费米面上的环形节点，不同于广泛预期的线节点$d$波态。主要的配对胶是磁性的，但高秩八极波动。该系统与铁基超导体共享多轨道自由度的重要性。我们的发现不仅揭示了这种材料中长期存在的谜团，而且促使我们重新考虑所有重费米子超导体中的配对状态和机制。
Unconventional superconductivity in the layered iron germanide YFe$_2$Ge$_2$
自从2006年在LaFePO中发现超导性以来，已经在不同的结构家族中鉴定出许多铁基超导体，所有这些都将铁与V族（磷属元素）或VI族（硫属元素）元素结合。非常规的超导性在这些层状铁系统和铜酸盐之外的过渡金属化合物中是极其罕见的，并且它几乎普遍与高度各向异性的电子性质和接近2D的费米表面几何形状相关联。铁基金属间化合物YFe $_2$ Ge $_2$具有3D费米面和强烈增强的低温热容，这表明强的电子相关性。我们目前的数据从新一代的高品质样品的YFe $_2$锗$_2$，这表明超导转变异常低于1.8 K的热力学以及运输测量，建立超导性是内在的，在这个层状铁化合物以外的已知的超导铁磷属元素化物或硫属元素化物家庭。YFe $_2$ Ge $_2$的费米面几何形状类似于KFe $_2$ As $_2$在高压下坍缩的四方相的费米面几何形状，其中最近报道了在高达10 K的温度下的超导性，这表明两个系统之间的潜在联系。
Nodal quasiparticle meltdown in ultra-high resolution pump-probe angle-resolved photoemission
高$T_c$铜氧化物超导体的特征在于沿布里渊区对角线（节方向）和沿布里渊区面（反节方向）的低能激发之间的强动量依赖的各向异性。最明显的是d波超导能隙，在波腹方向上发现最大的幅度，而在波节方向上没有能隙。此外，虽然波节准粒子激发只出现在$T_c$以下，但超导性被认为与波节激发无关，因为它们被认为是鲁棒的，并且对$T_c$不敏感。在这里，我们揭示了一个意想不到的结准粒子和超导电性之间的联系，使用高分辨率的时间和角度分辨光电发射最佳掺杂的Bi $_2$ Sr $_2$ CaCu $_2$ O $_{8+δ}$。我们观察到抑制的节准粒子光谱重量泵浦激光激发和测量其恢复动力学。这种抑制在超导状态下显著增强。这些结果减少了节点-反节点二分法，并挑战了超导中节点激发中性的传统观点。
We show the emergence of a new class of superconductivity in multiorbital systems, focusing on non-Kramers f$^2$ states. The Cooper pairs in this class of superconductivity mainly consist of local pairs with the same symmetry as the local f$^2$ ground states. When the local ground state is an anisotropic representation, the superconducting gap has nodes on the Fermi surface. This nodal superconductivity is mediated by the strong onsite inter-orbital attractions as a consequence of negative-$U$ physics generalized to multiorbital systems. We show that this is indeed realized in a simple two-orb…
We find evidence that the newly discovered Fe-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ ($T_c~=~33.36(7)$~K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation ($μ$SR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully-gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gap…
We study the intrinsic fully-gapped odd-parity superconducting order in doped nodal-loop materials with a torus-shaped Fermi surface. We show that the mirror symmetry, which protects the nodal loop in the normal state, also protects the superconducting state as a topological crystalline superconductor. As a result, the surfaces preserving the mirror symmetry host gapless Majorana cones. Moreover, for a Weyl loop system (two-fold degenerate at the nodal loop), the surfaces that break the mirror symmetry (those parallel to the bulk nodal loop) contribute a Chern (winding) number to the quasi-two…
Recently superconductivity was discovered in the Kagome metal AV3Sb5 (A = K, Rb, and Cs), which has an ideal Kagome lattice of vanadium. These V-based superconductors also host charge density wave (CDW) and topological nontrivial band structure. Here we report the ultralow-temperature thermal conductivity and high pressure resistance measurements on CsV3Sb5 with Tc = 2.5 K, the highest among AV3Sb5. A finite residual linear term of thermal conductivity at zero magnetic field and its rapid increase in fields suggest nodal superconductivity. By applying pressure, the Tc of CsV3Sb5 increases firs…
A plaquette-centered rotation symmetry $C_{4}^{p}$ is identified to play a significant role in determining and stabilizing the Fermi-surface structure of Fe-based superconductors. Together with the $S_{4}$ symmetry previously found, we are able to sort out the tangling orbitals and solve the puzzle of pairing symmetry of superconductivity in $\text{KFe}_{2}\text{As}_{2}$ in a simple but comprehensive way. By modeling the material with a strong coupling $t-J_{1}-J_{2}$ model, we find phase transitions of pairing symmetry driven by the competition between the local spin antiferromagnetic couplin…
Owing to the specific topological states in nodal-line semimetals, novel topological superconductivity is expected to emerge in these systems. In this letter, by combination of the first-principles calculations and resistivity, susceptibility and specific heat measurements, we demonstrate that La$_3$Pt$_3$Bi$_4$ is a topologically nontrivial nodal-ring semimetal protected by the gliding-mirror symmetry even in the presence of spin-orbit coupling. Meanwhile, we discover bulk superconductivity with a transition temperature of $\sim$1.1 K, and an upper critical field of $\sim$0.41 T. These findin…
By employing a series of experimental techniques, we provide clear evidence that CaPtAs represents a rare example of a noncentrosymmetric superconductor which simultaneously exhibits nodes in the superconducting gap and broken time-reversal symmetry (TRS) in its superconducting state (below $T_c$ $\approx$ 1.5 K). Unlike in fully-gapped superconductors, the magnetic penetration depth $λ(T)$ does not saturate at low temperatures, but instead it shows a $T^2$-dependence, characteristic of gap nodes. Both the superfluid density and the electronic specific heat are best described by a two-gap mode…
We report low-temperature heat transport measurements on superconducting iron sulfide FeS with $T_c \approx$ 5 K, which has the same crystal structure and similar electronic band structure to the superconducting iron selenide FeSe. In zero magnetic field, a significant residual linear term $κ_0/T$ is observed. At low field, $κ_0/T$ increases rapidly with the increase of field. These results provide strong evidence for nodal superconducting gap in FeS. We compare it with the sister compound FeSe, and other iron-based superconductors with nodal gap.
The combination with superconductivity and topological nontrivial band structure provides a promising route towards novel quantum states such as topological superconductivity. Here, we report the first observation of superconductivity (4.1 K) in Ba5In4Bi5 single crystal, a noncentrosymmetric topological semimetal featuring nodal chain loops at the high-symmetry points R and X. The magnetization, resistivity, and specific heat capacity measurements reveal that Ba5In4Bi5 is a moderately coupled type-II Bardeen-Cooper-Schrieffer superconductor. Bulk superconductivity is suggested from the magneti…
The extension of the topological classification of band insulators to topological semimetals gave way to the topology classes of Dirac, Weyl, and nodal line semimetals with their unique Fermi arc and drum head boundary modes. Similarly, there are several suggestions to employ the classification of topological superconductors for topological nodal superconductors with Majorana boundary modes. Here, we show that the surface 1H termination of the transition metal dichalcogenide compound 4Hb-TaS$_2$, in which 1T-TaS$_2$ and 1H-TaS$_2$ layers are interleaved, has the phenomenology of a topological …
We have determined the superconducting and magnetic properties of four samples of Lu$_x$Zr$_{1-x}$B$_{12}$ ($x=0.04$, $0.07$, $0.17$, and $0.8$) using muon spin rotation ($μ$SR) and magnetometry measurements. We observed a strong magnetic signal in both the $μ$SR and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $\approx 1\,μ_{\rm B}$ due to a clustering effect of the Lu$^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $μ$SR data thought to originate from boron nuclei in the B$_{12}$ cages. The superconductivi…
Topological semimetals with superconducting properties provide an emergent platform to explore the properties of topological superconductors. We report magnetization, and magneto-transport measurements on high quality single crystals of transition metal dichalcogenide SnTaS2. It is a nodal line semimetal with superconducting transition below Tc = 2.9 K. Moderate anisotropy (3.1) is observed in upper critical fields along H||c and H||ab plane. In the normal state we observe large magneto-resistance and weak anti-localization effect that provide unambiguous confirmation of topological features i…
We found superconductivity in CaSb$_2$ with the transition temperature of 1.7 K by means of electrical-resistivity, magnetic-susceptibility, and specific-heat measurements. This material crystallizes in a nonsymmorphic structure and is predicted to have multiple Dirac nodal lines in the bulk electronic band structure protected by symmetry even in the presence of spin-orbit coupling. We discuss a possible topological superconductivity for the quasi-2-dimensional band originating mainly from one of the antimony sites.
The pairing mechanism and gap structure in Ba122 pnictides have been hotly discussed for long time as one of the central issues in Fe-based superconductors. Here, we attack this problem by taking account of the vertex corrections (VCs) for the Coulomb interaction $U$ ($U$-VCs), which are totally dropped in conventional Migdal-Eliashberg formalism. The $U$-VC in the charge susceptibility induces strong orbital fluctuations, and the $U$-VC also enlarges the orbital-fluctuation-mediated attractive interaction. By analyzing the effective multiorbital Hubbard model for Ba122 pnictides, we find that…
