Superconducting Gap and Pseudogap in Iron-Based Layered Superconductor La(O_1-xF_x)FeAs

Abstract

We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O0.93F0.07)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s-or axial p-wave case, while it is 4.1 meV in the polar p-or d-wave case. We also found a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing temperature and closes at temperature far above Tc similarly to copper-oxide high-temperature superconductors.KEYWORDS: superconducting gap, pseudogap, iron-based superconductor, photoemission spectroscopy Superconductivity observed at as high as 26 K (onset temperature = 32 K) in La(O 1−x F x )FeAs 1 provided a deep impact in condensed-matter physics, since this new superconductor does not belong to any known categories of "high-temperature superconductors" such as copper-oxides (cuprates), 2 fullerenes, 3 and MgB 2 . 4La(O 1−x F x )FeAs consists of alternatingly stacked insulating lanthanum oxide (LaO) and conductive iron arsenide (FeAs) layers. While undoped LaOFeAs is a metal or a degenerate semiconductor at room temperature with no sign of superconductivity, substitution of oxygen with fluorine (F) atoms gives rise to the superconductivity at the F doping of x = 0.03. Further doping causes a gradual increase of superconducting transition temperature (T c ) up to the maximum T c of 26 K with the onset temperature over 30 K at x = 0.11. 1 Although the crystal structure is substantially different from that of cuprate superconductors -it is not a perovskite structure and does not contain CuO 2 planes -, both compounds share intriguing similarities such as the two-dimensional crystal/electronic structure, 1, 5-8 presence of a superconducting dome in the electronic phase diagram where the T c is controlled by a systematic aliovalent ion doping into the insulating block layers, and a characteristic anomaly in the transport property in the under-doped region.1 It has been suggested that La(O 1−x F x )FeAs is situated at the boundary to the ferromagnetic phase, since the F doping increases the number of Fe 3d electrons from six to that (seven) of Co 3d electrons in LaOCoAs which is known to undergo a ferromagnetic transition at 66 K.1, 9 Such a situation may invoke an exotic pairing mechanism related to ferromagnetic spin fluctuations. It is thus particularly important to elucidate the mechanism and origin of superconductivity in this new category of high-T c superconductor in relation to cuprates and other layered novel superconductors like ruthenium-10 and cobalt-oxides.11 * E-mail address: t-sato@arpes.phys.tohoku.ac.jpIn this letter, we report ultrahigh-resolution (∆E = 1.7 meV) photoemission spectroscopy (PES) on La(O 0.93 F 0.07 )FeAs to study the electronic structure and the superconducting gap. We found that the PES spectrum at 5 K shows a suppression of spectral intensity within 4 meV indicative of opening of a superconducting gap. We also observed a pseudogap ...

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

“…An evidence for the pseudogap has been reported from photoemission experiments on polycrystalline samples (e.g., see [142,143]) and in some ARPES experiments on single crystals [144], but this is supported neither by other numerous ARPES studies [41,42,57,59,64,145,146] nor by STM measurements [147,148]. The absence of the pseudogap in ARPES spectra may be just a consequence of low spectral weight modulation by the magnetic ordering that may question its importance for superconductivity, discussed in previous section.…”

Magnetic and metallic state at intermediate HubbardUcoupling in multiorbital models for undoped iron pnictides

“…19,20 The "antiferromagnetic metallic" nature of LaOFeAs is clearly different from an antiferromagnetic insulator or a non-magnetic metal. Further confirming the need to focus on the intermediate coupling regime for the Fe pnictides parent compounds, a pseudogap has been observed 21,22,23,24 in their density of states (DOS), which is different from the featureless DOS of a good metal or the gapped DOS of an insulator. Much of the current theoretical effort 17,18,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46 has focused thus far on two well defined limits.…”

High-temperature superconductivity in the iron pnictides

“…And although much has been made recently of the differences these materials have from the cuprates, other studies show some similarities. Various photoemission [43], tunneling [44], and optics [45] studies are quite reminiscent of the cuprates, including the possible presence of a precursor gap (a "pseudogap") existing above T c . Pursuing the various similarities and differences between these two classes of materials may help unravel the physics of both of them.…”