Looking at the superconducting gap of iron pnictides

THz and infrared spectroscopies are widely utilized to investigate the electrodynamic properties of the novel iron-based superconductors in the normal and superconducting states. Besides electronic excitations and correlations, electron-phonon coupling and the influence of magnetism, the experiments yield important information on low-lying excitations and help to clarify the number and symmetry of superconducting gaps. While the experimental data of different groups converge, the interpretation is still under debate. Here we review the status of optical investigations on the superconducting state for the 122 and 11 family of iron pnictides.     

Anion height as a controlling parameter for the superconductivity in iron pnictides and cuprates

Both families of high T_c superconductors, iron pnictides and cuprates, exhibit material dependence of superconductivity. Here, we study its origin within the spin fluctuation pairing theory based on multiorbital models that take into account realistic band structures. For pnictides, we show that the presence and absence of Fermi surface pockets is sensitive to the pnictogen height measured from the iron plane due to the multiorbital nature of the system, which is reflected to the nodeless/nodal form of the superconducting gap and T_c. Surprisingly, even for the cuprates, which is conventionally modeled by a single orbital model, the multiorbital band structure is shown to play a crucial role in the material dependence of superconductivity. In fact, by adopting a two orbital model that considers the d_z² orbital on top of the d_x²−y² orbital, we can resolve a long standing puzzle of why the single layered Hg cuprate have much higher T_c than the La cuprate. Interestingly, here again the apical oxygen height measured from the CuO₂ plane plays an important role in determining the relative energy difference between d_x²−y² and d_z² orbitals, thereby strongly affecting the superconductivity.     

Vortex analysis of the periodic Ginzburg–Landau model

We study the vortices of energy minimizers in the London limit for the Ginzburg–Landau model with periodic boundary conditions. For applied fields well below the second critical field we are able to describe the location and number of vortices. Many of the results presented appeared in [H. Aydi, Doctoral Dissertation, Université Paris-XII, 2004], others are new.     

Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa1−xBa1.75−xLa0.25+xCu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting Ca_xLa_1−xBa_1.75−xLa_0.25+xCu₃O_y (0≤x≤0.4 and 6.4≤y≤7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO₂ plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (T_c^max) in different cuprate families at optimum hole density.     

What drives pseudogap physics in high-Tc cuprates? A view from the (resistance) bridge

Taking into account the evolution of the in-plane resistivity with temperature and doping, a candidate proposal for the (hole-doped) cuprate phase diagram is constructed. Many features of the phase diagram are viewed as a consequence of an anisotropic interaction that intensifies with decreasing doping from the heavily overdoped side. At a critical doping p_c, that coincides with the development of the normal state pseudogap, the interaction becomes sufficiently strong that all electronic states near the zone boundary are effectively incoherent even at T = 0.     

Optical properties of the iron-chalcogenide superconductor FeTe0.55Se0.45

The complex optical properties of the iron-chalcogenide superconductor FeTe_0.55Se_0.45 with T_c=14 K have been examined over a wide frequency range for light polarized in the Fe-Te(Se) planes above and below T_c. At room temperature the optical response may be described by a weakly interacting Fermi liquid; however, just above T_c this picture breaks down and the scattering rate takes on a linear frequency dependence. Below T_c there is evidence for two gap features in the optical conductivity at and . Less than 20% of the free carriers collapse into the condensate for T?T_c, and this material is observed to fall on the universal scaling line for a BCS dirty-limit superconductor in the weak-coupling limit.     

As-NQR study of superconductivity in LaRAsO1−xFx (R=Fe and Ni)

We report ⁷⁵As nuclear quadrupole resonance (NQR) studies on oxypnictide superconductors LaFeAsO_1−xF_x (x=0.08, 0.15) and LaNiAsO_1−xF_x (x=0, 0.06, 0.10, 0.12). In LaFeAsO_0.92F_0.08 (T_c=23 K), nuclear spin-lattice relaxation rate 1/T₁ shows no coherence peak just below T_c and decreases with decreasing temperature accompanied by a hump structure at T∼0.4T_c, which is a characteristic of the multigap superconductivity. In the normal state, the quantity 1/T₁T increases with decreasing temperature to T_c, indicating that the existence of antiferromagnetic correlation originating from its multiple electronic band structure. On the other hand, LaNiAsO_1−xF_x shows a clear Hebel-Slichter (coherence) peak just below T_c, evidencing that the LaNiAsO_1−xF_x is a BCS superconductor. In the normal state, 1/T₁T is constant in the temperature range for all LaNiAsO_1−xF_x, which indicates electron correlations are weak. We suggest that the contrasting behavior of both superconductivity and electron correlations in LaFeAsO_0.92F_0.08 and LaNiAsO_1−xF_x between them relate to the difference of electronic band structure configuration. We also provide a possible interpretation for the pseudogap-like behavior in the normal state observed in both compounds.     

Can electrochemistry help to understand superconductivity – β-Fe1+xSe as a case study

Superconductivity is a riddling phenomenon. Even after more than a century of research, the mechanism that drives the so-called ‘unconventional’ superconductors has not been fully understood. Although the phenomenon is closely linked to low-temperature research which is usually not the preferred condition for an electrochemist, the structures itself determining the superconducting behaviour are synthesised, studied and modified at much higher temperatures. Electrochemistry provides an unmatched lever to change the thermodynamic ground state and to modify or synthesise compounds. Concomitantly, it affords a precision for the investigation of compositions superior to other chemical methods. In this review, the example of the iron-based superconductor β-Fe_1+xSe is chosen to evaluate achievements and prospects of this research at the cross section of (electro)chemistry and (solid state) physics.