Measurement of a Sign-Changing Two-Gap Superconducting Phase in Electron-DopedBa(Fe1−xCox)2As2

Abstract

Scanning tunneling spectroscopic studies of BaðFe 1Àx Co x Þ 2 As 2 (x ¼ 0:06, 0.12) single crystals reveal direct evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish above the superconducting transition T c . The two-gap nature and the slightly doping-and energy-dependent quasiparticle scattering interferences near the wave vectors ( AE , 0) and (0, AE) are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T c ratios suggest dominant unitary impurity scattering. DOI: 10.1103/PhysRevLett.106.087004 PACS numbers: 74.20.Rp, 74.25.Jb, 74.55.+v, 74.70.Xa The recent discovery of iron-based superconductors [1-10] has renewed intense research activities in superconductivity. Comparison of the similarities and contrasts between the iron-based compounds and the cuprates can provide useful insights into the microscopic mechanism for high-temperature superconductivity [11]. In particular, antiferromagnetic spin fluctuations appear to influence the physical properties of both cuprate and iron-based superconductors [11][12][13][14][15][16]. On the other hand, theoretical calculations [8,[15][16][17] and angle resolved photoemission spectroscopy (ARPES) [18][19][20] suggest that multibands and inter-Fermi surface interactions are crucial to superconductivity in the iron-based compounds.While substantial experimental results from ARPES studies [18][19][20] and phase sensitive measurements of various iron-based compounds [21,22] are supportive of the scenario of two-gap superconductivity with signchanging s-wave (s AE ) order parameters for the hole and electron Fermi pockets, tunneling and point-contact spectroscopic studies of the iron pnictides appear to be inconclusive [23][24][25][26][27][28]. For instance, reports of point-contact spectroscopy and scanning tunneling spectroscopy (STS) have suggested either BCS-like s-wave superconductivity [23] or nodes in the superconducting order parameter [24,28] of the ''1111'' iron-arsenides LnFeAsO 1Àx F x (Ln: trivalent rare-earth elements). On the other hand, STS studies of the electron and hole-doped ''122'' iron arsenides BaðFe 1Àx Co x Þ 2 As 2 and ðBa 1Àx K x ÞFe 2 As 2 have revealed findings ranging from strong spatial variations in the tunneling spectra with occasional observation of a large superconducting gap [25] to moderate spatial variations with predominantly a small superconducting gap [26]. In the nonsuperconducting limit, nematic surface reconstructions have been observed [27].In this Letter we report direct STS evidence for twogap superconductivity in the electron-doped 122 system BaðFe 1Àx Co x Þ 2 As 2 of two different doping levels. For each doping level, two different energy gaps can be clearly resolved at T ( T c . Both gaps decrease monotonically with increasing temperature and then completely vanish above T c . The gap values agree favorably with those obtained from ARPES so that the larger gap Á À may be associated with the holelike F...

Calculation of the effect of random superfluid density on the temperature dependence of the penetration depth

“…9 Similar gap maps have been observed in the iron pnictide family [10][11][12] and in disordered titanium nitride films close to the superconductor to insulator transition.…”

Interrogating the superconductor Ca10(Pt4As8)(Fe2−xPtxAs2)5 Layer-by-layer

“…5b) exhibits a finite zero bias conductance (ZBC). Such a feature has been observed previously in other Fe-based superconductors20212223242526, even when probed at much lower temperatures than their T c , indicating the ZBC is not due to the thermal broadening effect. It was also shown that spectral features can vary on different reconstructed surfaces in BaFe 2−x Co x As 2 26 suggesting reconstructions may be closely related to the observed ZBC in Fe-based superconductors.…”

Conventional superconductivity in SrPd2Ge2

“…As temperature is raised, the dip appearing at approximately ±1.3 mV between the two above-mentioned peaks reaches even negative conductance values. These two observations indicate that SrPd 2 Ge 2 is indeed an s-wave single-gap superconductor, and s ± pairing proposed for the isostructural 122 iron pnictides, 29 which has been associated with unconventional pairing mediated by magnetic fluctuations, is probably absent here.…”