Unusual Pseudogap Features Observed in Iron Oxypnictide Superconductors

Abstract

We present angle-integrated photoemission spectroscopy (PES) study of LaOFeAs:F (Tc = 26 K) and LaOFeP:F (Tc = 5 K) using He I line (hν = 21.2 eV) and laser (hν = 6.994 eV) as excitation light sources. We find quantitatively consistent results between the He I and the laser PES measurements: LaFeAsO:F exhibits a temperature-dependent pseudogap as large as ∼0.1 eV at 300 K, whereas LaFePO:F exhibits a ∼20-meV pseudogap. We discuss possible origin of the T -dependent pseudogap specifically observed in LaFeAsO:F through comparison with other materials that exhibit unusual T -dependent pseudogaps. KEYWORDS: Iron oxypnicite superconductors, pseudogap, photoemission spectroscopyThe discovery of T c = 26 K in F-doped LaFeAsO 1) and subsequent reports on T c 's as high as 55 K in the layered FeAs superconductors 2-5) have generated great interests both in the search for new high-T c materials and in the elucidation of the mechanism of the high T c . The superconductivity in high-T c FeAs compounds appears with aliovalent substitution 1,[3][4][5] or with applying pressure 6)to the non-superconducting parent compounds that exhibit stripe-type antiferromagnetism 7) and orthorhombic distortion.8) On the other hand, there are series of transition-metal oxypnictides that do not show high T c 's: LaFePO:F 1) and LaNiAsO 10) exhibit T c 's of 5 K at most, whereas LaCoAsO and LaCoPO are itinerant ferromagnets.11) Interestingly, low-T c LaFePO 9) and LaNiPO 10)superconduct without F doping in contrast to the parent compounds of the high-T c FeAs superconductors being antiferromagnets. In order to investigate the common/different features between the high-T c and the low-T c iron-oxypnictide superconductors, we have previously reported a comparative laser photoemission spectroscopy (PES) study of LaFeAsO:F (As26) and LaFePO:F (P05) showing T c 's of 26 and 5 K, respectively.12) Through investigating the spectra near the Fermi level (E F ) over a wide temperature and wide energy range, we showed that there is a As26-specific T -dependent pseudogap that becomes as large as ∼0.1 eV at 250 K. We also showed ∼20-meV pseudogaps and signatures of superconducting gaps both in high-T c As26 and in low-T c P05. Temperature-dependent angle-integrated PES studies by other groups have also shown evidence for pseudogaps in the iron-oxypnictide compounds.13-17) Most of the re- * E-mail: ishida@spring8.or.jp ports showed a pseudogap having an energy scale of ∼20 meV, [13][14][15][16][17] indicating that this feature occurs commonly in the layered iron oxypnictides and is insensitive to T c or to doping. Through a wide-temperature-range study, Ou et al. also showed T -dependent pseudogap features in SmFeAsO:F 15) similar to that in As26, 12) although there are rooms for speculation concerning its origin. 15, 16)In this proceedings, we present He I (hν = 21.2 eV) PES spectra of As26 and P05 and compare them with the previous results obtained by laser PES.12) He I PES provides information complementary to laser PES since the former covers the entire...

Pseudogap formation above the superconducting dome in iron pnictides

“…Synchrotron-based ARPES thus show that SC gap in AsP122 is smaller than PG, consistent with laser-ARPES. PG with an energy scale higher than SC gap has been also reported for a variety of iron-based superconductors by several experimental probes [32,33,35,36,39,40].…”

“…Although several studies have been reported on the PG formation [32][33][34][35][36][37][38][39][40], the PG phase has not been completely established in the phase-diagrams of the iron-pnictides. In particular, only one angle-resolved photoemission spectroscopy (ARPES) study has shown the presence of a normal state energy gap in momentum space for Ba 0.75 K 0.25 Fe 2 As 2 [41].…”

High-Temperature Majorana Fermions in Magnet-Superconductor Hybrid Systems

“…For instance, FeSe on SrTiO 3 (001) has a gap as large as 18meV 32,33 . Gap sizes up to 20 meV have also been reported in LaFeAsO and LaFePO 34 .…”

“…Thus it would be most desirable to reproduce similar experiments on superconducting substrates with large gap sizes as realized in some of the high-temperature superconductors. For example, the Re(0001)-O(2×1) substrate has a gap size of 280-330 µeV 24 whereas superconducting gaps of up to 20 meV have been reported in monolayer FeSe 32,33 and also up to 20 meV in LaFeAsO and LaFePO 34 . Obvious candidates are the high T c cuprate and iron-based superconductors as substrates for Shiba systems.…”

A review on angle resolved photoemission spectroscopy studies of Fe-based superconductors

“…In addition, other issues to be resolved are confirmation of pseudo-gaps [90][91][92][93][94][95][96][97][98][99][100][101], finding the relationship between pseudo-gaps and superconductivity, finding the origin of the T c enhancement in FeSe monolayer grown on SrTiO 3 substrate [34,[107][108][109][110][111], confirmations of quantum critical points [101][102][103][104][105][106], material and doping dependence of orbital order, electronic structure studies of 1111 systems [35]. Information on these issues could provide clues to the microscopic mechanism.…”