Momentum dependence of the superconducting gap inBa1−xKxFe2As2

Abstract

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor Ba 1−x K x Fe 2 As 2 ͑BKFA͒ with T c = 32 K was determined from angle-resolved photoemission spectroscopy ͑ARPES͒ via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner ⌫ barrel, X pocket, and "blade" pocket, and equals 9 meV, while the gap on the outer ⌫ barrel is estimated to be less than 4 meV, resulting in 2⌬ / k B T c = 6.8 for the large gap and 2⌬ / k B T c Ͻ 3 for the small gap. A large ͑77Ϯ 3 %͒ nonsuperconducting component in the photoemission signal is observed below T c . Details of gap extraction from ARPES data are discussed in Appendixes A and B.

Penetration depth, lower critical fields, and quasiparticle conductivity in Fe-arsenide superconductors

“…6, the overall temperature dependence of n s in crystal #3 cannot be fully reproduced by the single gap calculations. Considering the multiband electronic structure in this system [22,23,24], we fit the data again to the two-gap model Eq. (3).…”

“…In the electron-doped LnFeAs(O,F) or '1111' systems (where Ln is Lanthanoide ions), while several experiments [17,18] suggest nodes in the gap, the tunnelling measurements [19] and angle resolved photoemission (ARPES) [20] suggest fully gapped superconductivity. In the hole-doped Ba 1−x K x Fe 2 As 2 or '122' system [21], experimental situation is controversial as well: ARPES [22,23,24] and lower critical field measurements [25] support nodeless gaps while µSR measurements [26] imply the presence of line nodes in the gap function. One of the effective ways to judge the presence or absence of nodes in the gap is to investigate the properties of thermally excited quasiparticles at low temperatures.…”

Specific Heat of Ba0.59K0.41Fe2As2, an Fe-Pnictide Superconductor with Tc = 36.9 K, and a New Method for Identifying the Electron Contribution

“…ARPES measurements give more detailed information but the results are often summarized by two averages over narrow ranges of gap magnitude. For a sample with T c = 32 K, Evtushinsky et al 19 report Δ 1 (0) = 9.2 ± 1 meV for an inner hole-like barrel at the Г point, and smaller gaps on all other elements of the Fermi surface. However, the feature that showed the opening of the larger gap was not observed for the smaller gaps, and they conclude only that Δ 2 (0) < 4 meV.…”

Probing the pairing symmetry of the iron pnictides with electronic Raman scattering

“…While ARPES experiments [16][17][18][19][20][21] have reported weakly anisotropic gaps, this may be due to momentum resolution issues and the difficulty of preparing good surfaces in these systems at present. In particular, the samples used in these experiments may be sufficiently dirty at the surface that considerable momentum averaging, with concomitant gap averaging, could be taking place 23 .…”

“…However, penetration depth measurements [9][10][11][12][13][14][15] have been fit both to exponential activated T -dependence, indicative of a fully gapped state, and low-T power laws. ARPES measurements on single crystals of 122-type materials [16][17][18][19][20][21] measured the spectral gap reporting isotropic or nearly isotropic gaps on all Fermi surface sheets. It is possible that these differences reflect genuinely different ground states in different materials.…”