The vanadium doping effects on superconductivity and magnetism of iron pnictides are investigated in Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ by transport, susceptibility and neutron scattering measurements. The doping of magnetic impurity V causes a fast suppression of superconductivity with T$_c$ reduced at a rate of 7.4~K/1\%V. On the other hand, the long-range commensurate $C$-type antiferromagnetic order is recovered upon the V doping. The value of ordered magnetic moments of Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ follows a dome-like evolution versus doping concentration x. A possible Griffiths-type antiferromagnetic region of multiple coexisting phases in the phase diagram of Ba(Fe$_{0.92-x}$Co$_{0.08}$V$_x$)$_2$As$_2$ is identified, in accordance with previous theoretical predictions based on a cooperative behavior of the magnetic impurities and the conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida interactions between them.
The heavy fermion CeMIn5 family with M = Co, Rh, Ir provide a prototypical example of strange superconductors with unconventional d-wave pairing and strange metal normal state, emerged near an antiferromagnetic quantum critical point. The microscopic origin of strange superconductor and its link to antiferromagnetic quantum criticality and strange metal state are still open issues. We propose a microscopic mechanism for strange superconductor, based on the coexistence and competition between the Kondo correlation and the quasi-2d short-ranged antiferromagnetic resonating-valence-bond spin-liqu…
Superconductivity was recently observed in CrAs as the helimagnetic order is suppressed by applying pressure, suggesting possible unconventional superconductivity. To reveal the nature of the superconducting order parameter of CrAs, here we report the angular dependence of the upper critical field under pressure. Upon rotating the field by 360 degrees in the $bc$-plane, six maxima are observed in the upper critical field, where the oscillations have both six-fold and two-fold symmetric components. Our analysis suggests the presence of an unconventional odd-parity spin triplet state.
The advances in the field of unconventional superconductivity are largely driven by the discovery of novel superconducting systems. Here we report on the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional RbMn6Bi5 via measurements of resistivity and magnetic susceptibility under high pressures. With increasing pressure, its antiferromagnetic transition with TN = 83 K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and exhibits a dome…
Temperature dependence of resistivity under high pressures with magnetic fields parallel and perpendicular to the FeSe planes are measured in FeSe single crystals. It is found that the structural transition (nematic) temperature is suppressed by pressure and ends at around $P$ = 1.18 GPa. Below around 0.85 GPa, the superconducting transition shows a narrow width with no indication of antiferromagnetic order. While above this pressure, the superconducting transition temperature drops slightly forming a small dome of superconducting region with the maximum $T_c$ at around 0.825 GPa. Furthermore,…
We report high pressure studies on the C-type antiferromagnetic semiconductor EuTe2 up to 36.0 GPa. A structural transition from the I4/mcm to C2/m space group is identified at ~16 GPa. Superconductivity is discovered above ~5 GPa in both the I4/mcm and C2/m space groups. In the low-pressure phase (< 16 GPa), the antiferromagnetic transition temperature is enhanced with increasing pressure due to the enhanced magnetic exchange interactions. Magnetoresistance measurements indicate an interplay between the local moments of Eu2+ and the conduction electrons of Te 5p orbits. The upper critical fie…
We study superconductivity in a normal metal, arising from effective electron-electron interactions mediated by spin-fluctuations in a neighboring antiferromagnetic insulator. Introducing a frustrating next-nearest neighbor interaction in a Néel antiferromagnet with an uncompensated interface, the superconducting critical temperature is found to be enhanced as the frustration is increased. Further, for sufficiently large next-nearest neighbor interaction, the antiferromagnet is driven into a stripe phase, which can also give rise to attractive electron-electron interactions. For the stripe pha…
Two-magnon Raman scattering in A$_{0.8}$Fe$_{1.6}$Se$_2$ systems: competition between superconductivity and antiferromagnetic order
我们对几种组分的高质量A $_{0.8}$ Fe $_{1.6}$ Se $_2$超导体单晶进行了拉曼散射测量。我们发现一个广泛的，不对称的峰周围1600厘米$^{-1}$（200 meV），我们确定为一个两个磁振子过程涉及光学磁振子。双磁振子峰的强度在进入超导相时急剧下降。这种效应在非超导系统KFe $_{1.5}$ Se $_2$中完全不存在，它需要在反铁磁性和超导电性之间有很强的相互排斥，这是由微尺度相分离区域内的邻近效应引起的。
High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near $T=0K$ by slightly modifying pressure or doping. CaKFe$_4$As$_4$ is a superconducting, stoichiometric iron pnictide compound showing optimal superconducting critical temperature with $T_c$ as large as $38$ K. Doping with Ni induces a decrease in $T_c$ and the onset of spin-vortex antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of CaK(Fe$…
We report the $^{11}$B nuclear magnetic resonance (NMR) measurements on non-centrosymmetric superconductors NbRh$_2$B$_2$ (superconducting transition temperature $T_c$ = 7.8 K) and TaRh$_2$B$_2$ ($T_c$ = 5.9 K) with a chiral crystal structure. The nuclear spin-lattice relaxation rate $1/T_1$ shows no coherence peak below $T_{\rm c}$, which suggests unconventional nature of the superconductivity. In the normal state, $1/T_1T$ increases with decreasing temperature $T$ at low temperatures below $T$ = 200 K for TaRh$_2$B$_2$ and $T$ = 15 K for NbRh$_2$B$_2$, while the Knight shift remains constant…
The pressure dependence of the structural ($T_s$), antiferromagnetic ($T_m$), and superconducting ($T_c$) transition temperatures in FeSe is investigated on the basis of the 16-band $d$-$p$ model. At ambient pressure, a shallow hole pocket disappears due to the correlation effect, as observed in the angular-resolved photoemission spectroscopy (ARPES) and quantum oscillation (QO) experiments, resulting in the suppression of the antiferromagnetic order, in contrast to the other iron pnictides. The orbital-polarization interaction between the Fe $d$ orbital and Se $p$ orbital is found to drive th…
The magnetic order in CaK(Fe[1-x]Ni[x])4As4 (1144) single crystals (x = 0.051 and 0.033) has been studied by neutron diffraction. We observe magnetic Bragg peaks associated to the same propagation vectors as found for the collinear stripe antiferromagnetic (AFM) order in the related BaFe2As2 (122) compound. The AFM state in 1144 preserves tetragonal symmetry and only a commensurate, non-collinear structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe plane and simple AFM stacking along the c direction is consistent with our observations. The SVC order is promoted by the redu…
The discovery of high-temperature superconductivity in Fe-based compounds [1,2] has triggered numerous investigations on the interplay between superconductivity and magnetism [3] and, more recently, on the enhancement of transition temperatures through interface effects [4]. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture of this relationship remains elusive [1] due to the lack of data with atomic spatial resolution [5-7]. Here, we present a spin-pol…
Superconductivity in iron pnictides is unconventional and pairing may be mediated by magnetic fluctuations in the Fe-sublattice. Pressure is a clean method to explore superconductivity in iron based superconductors by tuning the ground state continuously without introducing disorder. Here we present a systematic high pressure transport study in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals with $x=$ 0.057, which is near the antiferromagnetic instability. Resistivity $ρ=ρ_0+AT^{n}$ was studied under applied pressure up to 7.90 GPa. The parameter $n$ approaches a minimum value of $n\approx 1$…
Nanoscale phase separation of antiferromagnetic order and superconductivity in K$_{0.75}$Fe$_{1.75}$Se$_2$
本文报道了铁硒化物超导体K $_{0.75}$ Fe $_{1.75}$ Se $_2$的面内光谱研究。测量结果表明，在T $_c$以下，在相对非相干的电子背景下，在比超导能隙小得多的频率下，出现了尖锐的反射边，这是迄今为止在任何其他铁基超导体中都没有看到的现象。此外，该特征可以被显著抑制并通过适度的磁场向较低频率偏移。我们的分析表明，这种边缘结构产生的约瑟夫森耦合等离子体激元的超导冷凝的发展。结合透射电子显微镜分析，我们的研究为超导性和磁性之间存在纳米级相分离提供了令人信服的证据。这些结果也使我们能够理解从不同技术探测到的各种看似有争议的实验数据。
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 …
Emergent Phases of Nodeless and Nodal Superconductivity Separated by Antiferromagnetic Order in Iron-based Superconductor (Ca4Al2O6)Fe2(As1-xPx)2: 75As- and 31P-NMR Studies
我们报告$^{31}$ P-和$^{75}$ As-NMR研究（Ca $_4$ Al $_2$ O $_{6}$）Fe $_2$（As $_{1-x}$ P $_x$）$_2$与P等价取代As。我们提出了新的演化的紧急阶段，节点无超导（SC）在0 $\le x \le$ 0.4和节点的$x$ =1附近是密切分离的开始相称的条纹型反铁磁（AFM）的顺序在0.5 $\le x \le$ 0.95，作为一个等价的替代P为As降低了氮族元素的高度$h_{Pn}$从Fe平面测量。结果表明，原子力显微镜的有序化发生在1.32 π $\le h_{Pn} \le$ 1.42 π的条件下，这也适用于（Fe $Pn$）$^{-}$层中具有Fe $^{2+}$态的其他铁磷族化合物.这种新的相位演化与$h_{Pn}$的变化点的重要性，电子相关的出现SC以及AFM秩序。
We show that magnetic fluctuations can destroy the Hebel-Slichter peak in conventional superconductors. The Hebel-Slichter peak has previously been expected to survive even in the presence of strong electronic interactions. However, we show that antiferromagnetic fluctuations suppress the peak at $\bf{q}=0$ in the imaginary part of the magnetic susceptibility, $χ_{+-}''\left(\bf{q},ω\right)$, which causes the Hebel-Slichter peak. This is of general interest as in many materials superconductivity is found near a magnetically ordered phase, and the absence of a Hebel-Slichter peak is taken as ev…
Antiferromagnetic Spin Fluctuations and Unconventional Nodeless Superconductivity in an Iron-based New Superconductor (Ca_4Al_2O_{6-y})(Fe_2As_2):75As-NQR Study
We report 75As-NMR/NQR results on new iron-arsenide compounds (La0.5-xNa0.5+x)Fe2As2. The parent compound x=0 exhibits a stripe-type antiferromagnetic (AFM) order below T_N=130 K. The measurement of nuclear spin relaxation rate at hole-doped x=+0.3 and heavily electron-doped x=-0.5 revealed that the normal-state properties are dominated by AFM spin fluctuations (AFMSFs), which are more significant at x=+0.3 than at x=-0.5. Their superconducting (SC) phases are characterized by unconventional multi-gap SC state, where the smaller SC gaps are particularly weaken in common. The experimental resul…
High-T_c Superconductivity with T_c = 52 K under Antiferromagnetic Order in Five-layered Cuprate Ba_2Ca_4Cu_5O_10(F,O)_2 with T_N = 175 K: 19F- and Cu-NMR Studies
The influence of a partial substitution with sulphur into Te sites on the crystal, electronic and magnetic structures of FeTe is investigated by DFT calculations. The results reveal a phase transition from the antiferromagnetic double-stripe order for pure FeTe to the single-stripe order for S-substituted samples, which coincides with the previously observed appearance of the superconducting state. The magnetic transition is caused by the variations of the average chalcogen position in the unit cell. The analyzed normal-state properties of Fe(Te,S) and Fe(Se;S) compounds allow a detection of t…
Unconventional superconductivity and antiferromagnetic quantum critical behavior in the isovalent-doped BaFe2(As1-xPx)2
用$^{31}$ P NMR研究了BaFe $_2$（As $_{1-x}$ P $_x$）$_2$的自旋动力学随P浓度的变化.我们的核磁共振研究表明，二维反铁磁（AF）的波动显着增强与静态磁化率的变化不大接近AF阶段从超导圆顶。此外，磁有序温度$θ$推导出的弛豫速率在最佳掺杂消失。这些结果为量子临界点（QCP）提供了明确的证据，表明与QCP相关的AF涨落在高- $T_c$超导性中发挥着核心作用。
We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions ca…
We propose as key of superconductivity in (hole-doped) cuprates a novel excitation of magnetic origin, characteristic of two-dimensions and of purely quantum nature: the antiferromagnetic spin vortices. In this formalism the charge pairing arises from a Kosterlitz-Thouless-like attraction between such vortices centered on opposite Néel sublattices. This charge pairing induces also the spin pairing through the action of a gauge force generated by the no-double occupation constraint imposed in the t-J model of the CuO planes of the cuprates. Superconductivity arises from coherence of pairs of ex…
Local antiferromagnetic correlations in the iron pnictide superconductors LaFeAsO{1-x}F{x} and Ca(Fe,Co)2As2 as seen via normal-state susceptibility
本文研究了LaFeAsO $_{1-x}$ F $_x$和Ca（Fe $_{1-x}$ Co $_x$）$_2$ As $_2$中磁性和超导电性的相互作用。虽然反铁磁自旋密度波的形成被抑制和超导性的发展，所有的样品显示出掺杂独立的正常状态下的磁化率的加热后，出现了铁磷属元素的一般功能的强烈增加。这些数据提供了证据，强大的本地反铁磁相关坚持即使在超导制度的相图。
We perform Eliashberg calculations for magnon-mediated superconductivity in a normal metal, where the electron-magnon interaction arises from interfacial coupling to antiferromagnetic insulators. In agreement with previous studies, we find $p$-wave pairing for large doping when the antiferromagnetic interfaces are uncompensated, and $d$-wave pairing close to half-filling when the antiferromagnetic interfaces are compensated. However, for the $p$-wave phase, we find a considerable reduction in the critical temperature compared to previous weak-coupling results, as the effective frequency cutoff…
We investigate theoretically magnon-mediated superconductivity in a heterostructure consisting of a normal metal and a two-sublattice antiferromagnetic insulator. The attractive electron-electron pairing interaction is caused by an interfacial exchange coupling with magnons residing in the antiferromagnet, resulting in p-wave, spin-triplet superconductivity in the normal metal. Our main finding is that one may significantly enhance the superconducting critical temperature by coupling the normal metal to only one of the two antiferromagnetic sublattices employing, for example, an uncompensated …
We study the contribution of the electron-spin fluctuation coupling to the superconducting state of the two dimensional Hubbard model within dynamical cluster approximation (DCA) using a numerical exact continuous time Monte Carlo solver. By analyzing the frequency dependence of the self energy, we show that only about half of the superconductivity can be attributed to a "pairing glue" arising from treating spin fluctuations as a pairing boson in the standard one-loop theory.
