Bulk superconductivity and disorder in single crystals of LaFePO
James G. Analytis, Jiun-Haw Chu, Ann S. Erickson, Chris Kucharczyk, Alessandro Serafin, Antony Carrington, Catherine Cox, Susan M. Kauzlarich, Hakon Hope, I. R. Fisher
We have studied the intrinsic normal and superconducting properties of the oxypnictide LaFePO. These samples exhibit bulk superconductivity and the evidence suggests that stoichiometric LaFePO is indeed superconducting, in contrast to other reports. We find that superconductivity is independent of the interplane residual resistivity $ρ_0$ and discuss the implications of this on the nature of the superconducting order parameter. Finally we find that, unlike $T_c$, other properties in single-crystal LaFePO including the resistivity and magnetoresistance, can be very sensitive to disorder.
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…
The influence of weak disorder on the superconductivity in ordinary metals can be formally described by the Abrikosov-Gorkov diagrammatic approach. The vertex correction is ignored in this approach because an inequality $k_F l \gg 1$, where $k_F$ is the Fermi momentum and $l$ mean free path, is satisfied in ordinary metals with a large Fermi surface. In a Dirac semimetal that has discrete Fermi points, this inequality may break down even for arbitrarily weak disorder since $k_F \rightarrow 0$, and thus the vertex correction could be important. We incorporate the vertex correction into the self…
Disorder-sensitive superconductivity in the iron silicide Lu$_2$Fe$_3$Si$_5$ studied by the Lu-site substitutions
通过少量取代Lu位，研究了非磁性杂质和磁性杂质对Lu $_2$ Fe $_3$ Si $_5$超导电性的影响。（Lu $_{1-x}$ Sc $_x$）$_2$ Fe $_3$ Si $_5$，（Lu $_{1-x}$ Y $_x$）$_2$ Fe $_3$ Si $_5$和磁性（Lu $_{1-x}$ Dy $_x$）$_2$ Fe $_3$ Si $_5$。非磁性杂质对$T_c$的快速抑制与剩余电阻率的增加一致，揭示了以无序为主的强对破缺。我们为Lu $_2$ Fe $_3$ Si $_5$中超导序参量的符号反转提供了令人信服的证据。
Bulk Superconductivity driven by Disorder-Induced Delocalization in 4Hb-Ta(S$_{1-x}$Se$_x$)$_2$
Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；University of California, Berkeley（加州大学伯克利分校）；University of Illinois Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；Technion（以色列理工学院）
Disorder Effects on Competition between Antiferromagnetism and Superconductivity in Cuprate Superconductors through the Enhancement in Charge Susceptibility
Full-gap superconductivity robust against disorder in heavy-fermion CeCu2Si2
反铁磁自旋涨落机制的非常规配对的一个关键方面是超导能隙在费米表面的不同部分必须具有相反的符号。最近在重费米子超导体CeCu $_2$ Si $_2$中观察到的非节带隙结构非常令人惊讶，因为这种材料长期以来一直被认为是库珀对是磁性介导的超导体的原型。在这里，我们提出了一个控制点缺陷的影响，引入电子辐照，对温度依赖的磁穿透深度$λ(T)$在CeCu $_2$ Si $_2$的研究。我们发现，完全带隙的状态是强大的，对无序，表明低能量的束缚态，预期的符号变化的间隙结构，不诱导的掺杂杂质。这为$s_{++}$波超导性提供了大量证据。
Single crystals of pristine and 6% Pd-intercalated 2H-TaSe$_2$ have been studied by means of $^{77}$Se nuclear magnetic resonance (NMR). The temperature dependence of the $^{77}$Se spectrum, with an unexpected line narrowing upon Pd intercalation, unravels the presence of correlated local lattice distortions far above the transition temperature of the charge density wave (CDW) order, thereby supporting a strong-coupling CDW mechanism in 2H-TaSe$_2$. While, the Knight shift data suggest that the incommensurate CDW transition involves a partial Fermi surface gap opening. As for spin dynamics, th…
Pressure tuning of superconductivity independent of disorder in Tl$_{2}$Ba$_{2}$CaCu$_{2}$O$_{8+δ}$
在大范围内改变铜氧化物超导体的超导转变温度是确定超导机制的必要步骤。化学掺杂或元素取代已被证明是有效的，但也带来晶格无序。即使在固定的掺杂水平下，这种无序也可以完全破坏超导性。压力被认为是调整超导性的最干净的方法。然而，从最近的实验中认识到压力引起的无序增加。通过选择一个无序的Tl $_{2}$ Ba $_{2}$ CaCu $_{2}$ O $_{8+δ}$在最佳掺杂，我们进行单晶X射线衍射和磁化率测量在高压下。所获得的结构数据提供了证据的强大功能，这种材料的无序在研究的压力范围内。这一特征确保了压力对超导性的影响与无序区分开来。导出的抛物线状行为的转变温度与压力高达近30 GPa，具有最大值约7 GPa，提供了一个平台，用于测试任何现实的理论模型在一个几乎恒定的无序环境。当考虑载流子浓度和配对相互作用强度作为两个压力本征变量时，可以理解这种行为。
Connections between crystal chemistry and critical temperature $T_c$ have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that $T_c$ correlates with the average anion height above the Fe plane, i. e. with the geometry of the FeAs4 or FeCh4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe$_{1-y}$Se$_{1-x}$S$_x$ (0$\leq$$x$$\leq$1, $y$$\leq$0.1), we find that in alloyed crystals $T_c$ is not correlated with the anion height like it i…
Layered three-dimensional centrosymmetric crystals can exhibit characteristics of noncentrosymmetric materials. This happens when each individual layer alone lacks inversion, but, when combined, inversion symmetry is restored; hence the designation: locally noncentrosymmertic superconductors (LNCSs). In LNCSs, the effects of impurities and subdominant magnetic field induced pairing channels remain unexplored. Using a minimal model, we examine all pairing channels and show that there is always a subdominant superconducting instability that is favored at high magnetic fields, which can substanti…
We investigate the superconducting gap function of topological superconductor PbTaSe$_2$. Temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe$_2$ is fully-gapped, with two isotropic $s$-wave gaps. The pair-breaking effect is probed by systematically increasing non-magnetic disorders through H$^+$-irradiations. The superconducting transition temperature, $T_{\rm{c}}$, is found to be robust against disorders, which suggests that the pairing should be sign-preserved rather than sign-reversed.
We study the electronic densities of states (DOS) of strongly disordered superconducting thin films of TiN. We find, using Scanning Tunneling Microscopy (STM) that the DOS decreases towards the Fermi level in the normal phase obtained by applying magnetic fields. The DOS shows spatial fluctuations whose length scale is related to the energy dependent DOS and is similar in normal and superconducting phases. This suggests that Coulomb interactions lead to a spatially varying DOS in the normal phase of a disordered superconductor.
Real-space modulated Charge Density Waves (CDW) are an ubiquituous feature in many families of superconductors. In particular, how CDW relates to superconductivity is an active and open question that has recently gathered much interest since CDWs have been discovered in many cuprates superconductors. Here we show that disorder induced by proton irradiation is a full-fledged tuning parameter that can bring essential information to answer this question as it affects CDW and superconductivity with different and unequivocal mechanisms. Specifically, in the model CDW superconductor Lu$_5$Ir$_4$Si$_…
Unconventional superconductivity has been discovered in a variety of doped materials, including topological insulators, semimetals and twisted bilayers. A unifying property of these systems is strong orbital hybridization, which involves pairing of states with non-trivial Bloch wave functions. In contrast to naive expectation, many of these superconductors are relatively resilient to disorder. Here we study the effects of a generic disorder on superconductivity in doped 3D Dirac systems, which serve as a paradigmatic example for the dispersion near a band crossing point. We argue that due to s…
In the layered iron-based superconductor YFe2Ge2, a high Sommerfeld ratio of ~100 mJ/molK^2 and a T^(3/2) temperature dependence of the electrical resistivity at low temperature T indicate strong electronic correlations and point towards an unconventional pairing state. We have investigated the role of composition and annealing conditions in optimizing the growth of high-quality YFe2Ge2. Our findings confirm that bulk superconductivity is observed in samples with disorder scattering rates less than 2 k_B T_c/hbar. Fe deficiency on the Fe site is identified as the dominant source of disorder, w…
Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic $s$-wave superconductivity in epitaxial Rb$_3$C$_{60}$ films against merohedral …
Enhancement of superconductivity by disorder in Remeika-type quasiskutterudites
Institute of Low Temperature and Structure Research, Polish Academy of Sciences（波兰科学院低温与结构研究所）；Institute of Theoretical Physics, Wrocław University of Science and Technology（弗罗茨瓦夫科技大学理论物理研究所）
The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many body physics. The conventional superconductors are insensitive to dilute nonmagnetic impurities, known as the Anderson's theorem. Destruction of superconductivity and even superconductor-insulator transitions occur in the regime of strong disorder. Hence disorder-enhanced superconductivity is rare and has only been observed in some alloys or granular states. Because of the entanglement of various effects, the mechanism of enhancement is still under debate. Here we report well-controlled …
The Dependence of the Superconducting Transition Temperature of Organic Molecular Crystals on Intrinsically Non-Magnetic Disorder: a Signature of either Unconventional Superconductivity or Novel Local Magnetic Moment Formation
