Strongly anisotropic s-wave gaps in exotic superconductors

The exotic superconductors, defined as those that follow the phenomenological trend reported by Uemura (T _c approximately proportional to λ_L ^?2), at present constitute the most broad and general class of superconductors which can reasonably be considered 'similar to the high-T _c cuprates'. It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have an even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed.     

Evidence for two superconducting gaps in the unconventional superconductor PrPt4Ge12

We report the results of magnetization measurements on the unconventional superconductor PrPt₄Ge₁₂ in its superconducting state. A distinct positive curvature is observed in the temperature dependence of the lower critical field H _C1(T) and the upper critical field H _C2(T) around 5.5?K. The zero field critical current density J _C(T, H?=?0) is found to vary nearly linearly with temperature. We argue that these observed features are due to the existence of two superconducting gaps in the material. The normalized superfluid density is estimated from the temperature dependence of H _C1. Our detailed analysis of the normalized superfluid density using a two-gap model indicates the presence of line nodes in the larger gap. The H _C2(T) line can be fitted with the Usadel equations, and the positive curvature in H _C2(T) is shown to be consistent with the two-gap picture of superconductivity.     

Electronic structure of new LiFeAs high-T c superconductor

We present results of ab initio LDA calculations of electronic structure of “next generation” layered iron-pnictide high-T _c superconductor LiFeAs (T _c = 18 K). Obtained electronic structure of LiFeAs very similar to recently studied ReOFeAs (Re = La, Ce, Pr, Nd, Sm) and AFe₂As₂ (A = Ba, Sr) compounds. Namely close to the Fermi level its electronic properties are also determined mainly by Fe 3d-orbilats of FeAs₄ two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe₂As₂ systems as well as the shape of the Fe-3d density of states and Fermi surface. The article is published in the original.     

Vortex lattice in LiFeAs superconductor in the two-band Ginzburg-Landau model

Time-dependent equations of the two-band Ginzburg-Landau model are simulated numerically. Calculations are performed for square thin films of the LiFeAs two-band superconductor in the vicinity of the superconducting transition temperature in the transverse magnetic field; the quasi-hexagonal nature of the vortex lattice in the intermediate state is demonstrated.     

Probing the unconventional superconducting state of LiFeAs by quasiparticle interference

A crucial step in revealing the nature of unconventional superconductivity is to investigate the symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which sensitively depends on the superconducting pairing mechanism. A particularly well-suited material to apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral cleaved surfaces without surface states and a relatively high T(c)～18 K. Our data reveal that in LiFeAs the quasiparticle scattering is governed by a van Hove singularity at the center of the Brillouin zone which is in stark contrast to other pnictide superconductors where nesting is crucial for both scattering and s(±) superconductivity. Indeed, within a minimal model and using the most elementary order parameters, calculations of the QPI suggest a dominating role of the holelike bands for the quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing symmetries s(++), s(±), and d wave. This brings to mind that the superconducting pairing mechanism in LiFeAs is based on an unusual pairing symmetry such as an elementary p wave (which provides optimal agreement between the experimental data and QPI simulations) or a more complex order parameter (e.g., s+id wave symmetry).     

Properties of anisotropic s-wave superconductivity in MgB2

Earlier this year a new type of high-temperature superconductivity has been reported in MgB₂ with a transition temperature of 39 K. In a recent paper we proposed a simple BCS model with an anisotropic s-wave order parameter which accounts for the available thermodynamic and scanning tunneling microscopy data obtained from polycrystalline palets of MgB₂. In this work, we further investigate the physical consequences of this theory by calculating the temperature dependence of the thermal conductivity, the nuclear spin relaxation rate, and the angular dependence of the upper critical field.     

铁基高温超导体的超导能隙和自旋涨落：核磁共振研究

文章介绍了利用核磁共振法对铁基高温超导体的研究结果.笔者研究组断定铁基超导是自旋单态配对,并首先发现超导多能隙的存在.正常态的反铁磁自旋涨落在自旋空间是各向异性的,它与超导的关系,是人们关心的热点之一.     

铁基高温超导体的超导能隙和自旋涨落:核磁共振研究

文章介绍了利用核磁共振法对铁基高温超导体的研究结果.笔者研究组断定铁基超导是自旋单态配对,并首先发现超导多能隙的存在.正常态的反铁磁自旋涨落在自旋空间是各向异性的,它与超导的关系,是人们关心的热点之一.     

de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs

We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T(c) ≈ 18 K and LiFeP with T(c) ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.     

Validation of the Wiedemann–Franz law in a granular s-wave superconductor in the nanometer scale

The present study tries to evaluate the validity of the Wiedemann–Franz law in a granular s-wave superconductor in the presence of concentrated impurities. By using Green's function method and the Kubo formula technique, three distinct contributions of the Aslamazov–Larkin, the Maki–Thompson and, the density of states are calculated for both the electrical conductivity and the thermal conductivity in a granular s-wave superconductor. It is demonstrated that these different contributions to the fluctuation conductivity depend differently on the tunneling because of their different natures. This study examines the transport in a granular superconductor system in three dimensions in the limit of large tunneling conductance,which makes it possible to ignore all localization effects and the Coulomb interaction. We find that the tunneling is efficient near the critical temperature and that there is a crossover to the characteristic behavior of a homogeneous system.When it is far from the critical temperature, the tunneling is not effective and the system behaves as an ensemble of real zero-dimensional grains. The results show that the Wiedemann–Franz law is violated in both temperature regions.     

Effect of harmonic confinement on correlation studies of a spin-polarized s-wave superconductor

We study different correlation functions for a spin imbalanced and harmonically trapped Fermi gas in two dimensions described by an attractive Hubbard model. Eigensolutions obtained via numerically solving Bogoliubov de Gennes equations are used to compute the local pairing amplitudes which show significantly different behaviour for the trapped case where the profile is radially modulated in contrast with a spatial modulation extending throughout the lattice geometry when trap effects are switched off. Further, different experimentally accessible quantities, such as pair–pair, density–density correlations and local density fluctuations show characteristic fluctuations for the spin polarized phase, which however wash away as the trapping effects are turned on. A contrasting scenario is presented corresponding to the case when the spin polarization effects are turned off.     

Islands of stability of the d-wave order parameter in s-wave anisotropic superconductors

In this paper we find and present on diagrams in the coordinates of η=2t₁/t₀ (the ratio of the second and the first nearest neighbor hopping integrals) and n (the carrier concentration) the areas of stability for the superconducting spin-singlet s- and d-wave and the spin-triplet p-wave order parameters hatching out during the phase transition from the normal to the superconducting phase. The diagrams are obtained for an anisotropic two-dimensional superconducting system with a relatively wide partially-filled conduction band. We study a tight-binding model with an attractive nearest neighbor interaction with the amplitude V₁, and the on-site interaction (with the amplitude V₀) taken either as repulsive or attractive. The problem of the coexistence of the s-, p- and d-wave order parameters is addressed and solved for chosen values of the ratio V₀/V₁. A possible island of stability of the d-wave order parameter in the s-wave order parameter environment for a relatively strong on-site interaction is revealed. The triple points, around which the s-, d-, and p-wave order parameters coexist, are localized on diagrams. It is shown that results of the calculations performed for the two-dimensional tight-binding band model are dissimilar with some obtained within the BCS-type approximation.     

Inelastic neutron-scattering measurements of incommensurate magnetic excitations on superconducting LiFeAs single crystals

Magnetic correlations in superconducting LiFeAs were studied by elastic and by inelastic neutron-scattering experiments. There is no indication for static magnetic ordering, but inelastic correlations appear at the incommensurate wave vector (0.5±δ,0.5-/+δ,0) with δ~0.07 slightly shifted from the commensurate ordering observed in other FeAs-based compounds. The incommensurate magnetic excitations respond to the opening of the superconducting gap by a transfer of spectral weight.     

Phase fluctuations in a strongly disordered s-wave NbN superconductor close to the metal-insulator transition

We explore the role of phase fluctuations in a three-dimensional s-wave superconductor, NbN, as we approach the critical disorder for destruction of the superconducting state. Close to critical disorder, we observe a finite gap in the electronic spectrum which persists at temperatures well above T(c). The superfluid density is strongly suppressed at low temperatures and evolves towards a linear-T variation at higher temperatures. These observations provide strong evidence that phase fluctuations play a central role in the formation of a pseudogap state in a disordered s-wave superconductor.     

Evidence for the coexistence of an anisotropic superconducting gap and nonlocal effects in the nonmagnetic superconductor LuNi2B2C

A study of the dependence of the heat capacity C(p)(alpha) on the field angle in LuNi2B2C reveals an anomalous disorder effect. For pure samples, C(p)(alpha) exhibits a fourfold variation as the field H (alpha=0). A slightly disordered sample, however, develops anomalous secondary minima along <110> for mu(0)H>1 T, leading to an eightfold pattern at 2 K and 1.5 T. The anomalous pattern is discussed in terms of coexisting superconducting gap anisotropy and nonlocal effects.     

Lifshitz时空s波超导模型的关联长度和穿透深度

在D=d+2维各向异性的Lifshitz黑洞时空背景中, 在探子极限下, 用解析方法研究了临界温度附近引力系统的微扰, 计算出超导的关联长度ξα(1/T_c)(1-(T/T_c)^-1/2, 这与平均场论的结果一致. 进一步, 考虑在该系统中加一个均匀外磁场, 计算出穿透深度λα(T_c-T)^-1/2, 该结果与Ginzburg-Landau理论相符.     

Bulk evidence for single-Gap s-wave superconductivity in the intercalated graphite superconductor C6Yb

We report measurements of the in-plane electrical resistivity rho and thermal conductivity kappa of the intercalated graphite superconductor C6Yb down to temperatures as low as Tc/100. When a field is applied along the c axis, the residual electronic linear term kappa0/T evolves in an exponential manner for Hc1相似文献