Band Structure and Fermi Surface of an Extremely Overdoped Iron-Based SuperconductorKFe2As2

Abstract

We have performed high-resolution angle-resolved photoemission spectroscopy on heavily overdoped KFe2As2 (transition temperature Tc = 3 K). We observed several renormalized bands near the Fermi level with a renormalization factor of 2-4. While the Fermi surface (FS) around the Brillouin-zone center is qualitatively similar to that of optimally-doped Ba1−xKxFe2As2 (x = 0.4; Tc = 37 K), the FS topology around the zone corner (M point) is markedly different: the two electron FS pockets are completely absent due to excess of hole doping. This result indicates that the electronic states around the M point play an important role in the high-Tc superconductivity of Ba1−xKxFe2As2 and suggests that the interband scattering via the antiferromagnetic wave vector essentially controls the Tc value in the overdoped region.PACS numbers: 74.70. Dd, 71.18.+y, 74.25.Jb, 79.60.Bm The discovery of superconductivity at 26 K [1] (43 K under high pressure [2]) in LaFeAsO 1−x F x , has triggered intensive researches on the high-temperature (T c ) superconductivity of iron (Fe) pnictides. The T c value has already exceeded 55 K by replacing La atom with other rare-earth atoms or by introducing oxygen vacancies [3,4], opening a new avenue for high-T c material research beside cuprates. Remarkable aspects of the FeAs-based superconductors are (i) electrons in the Fe orbitals, generally believed to be the foe, indeed play an essential role in superconductivity [1,5,6], (ii) non-doped parent compounds commonly exhibit a collinear antiferomagnetic (AF) spin density wave (SDW) [7,8], and (iii) the superconductivity emerges either by the hole or electron doping into the parent compounds [1,9]. To elucidate the mechanism of high-T c superconductivity in terms of the electronic structure, angle-resolved photoemission spectroscopy (ARPES) has been performed on both holeand electron-doped compounds in the optimally-and non(under)-doped region [6,10,11,12,13,14,15,16,17] and it clarified key features on the band structure, the FS topology, and the superconducting gap. On the other hand, little is known about the electronic states in the overdoped region. As demonstrated by electrical resistivity measurements, the T c value of the holedoped Ba 1−x K x Fe 2 As 2 monotonically decreases from the optimally-doped region (T c = 37 K) upon hole doping but does not completely disappear even at the highest doping level (x = 1.0; T c ∼3 K) [18,19], unlike the overdoped cuprates. The resistivity does not show SDW-related anomalies in the overdoped region [19]. Clarifying the microscopic origin of this T c reduction would be a key to find an essential ingredient to achieve high-T c values in the iron-based superconductors. It is thus of particular importance to gain insight into the band structure and the FS by performing ARPES measurements on overdoped samples and directly compare the electronic states with the optimally-doped ones for a comprehensive understanding of the high-T c mechanism.In this Letter, we report high-resolution ARPES results on KFe 2 A...

s+isstate with broken time-reversal symmetry in Fe-based superconductors

“…32) indicate that superconducting T c most likely remains non-zero from x = 0.4 → 1. (iii) For the x = 1 material KFe 2 As 2 , ARPES measurements 33,34 show that only hole pockets are present. According to theory, in this situation, both d−wave and s−wave pairing amplitudes are attractive 5,[9][10][11]35 , and which state wins depends on delicate interplay between system parameters.…”

“…(ii) Recent measurements on Ba 1−x K x Fe 2 As 2 with x = 1 (Refs. 33,34) and x = 0.93 and x = 0.88 (Ref. 32) indicate that superconducting T c most likely remains non-zero from x = 0.4 → 1.…”

Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case ofBaFe2−xNixAs2

“…It has been well characterized that both cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetries with nodal and nodeless gap distributions, respectively. There is no general consensus on the nature of pairing in iron-based superconductors leaving the perspectives * xjchen@hpstar.ac.cn ranging from S ++ wave, to S ± , and to d wave [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In addition, from 59 Co and 75 As nuclear magnetic resonance measurements, the spin-triplet order parameter was ruled out in BaFe 1.8 Co 0.2 As 2 [25].…”

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

“…BFA goes into magnetically ordered phase below 140 K [30] and never superconducts. An extremely overdoped BKFA is a stoichiometric KFe 2 As 2 (KFA) [31], which is non-magnetic, with = c T 3 K. There is also an interesting case of isovalent dopping, BaFe 2 (As 1-x P x ) 2 (BFAP) ( = c T 30 K) [22] with similar phase diagram (see Fig. 3).…”