Momentum dependence of superconducting gap, strong-coupling dispersion kink, and tightly bound Cooper pairs in the high-Tc(Sr,Ba)

Abstract

We present a systematic angle-resolved photoemission spectroscopic study of the high-Tc superconductor class ͑Sr/ Ba͒ 1−x K x Fe 2 As 2 . By utilizing a photon-energy-modulation contrast and scattering geometry we report the Fermi surface and the momentum dependence of the superconducting gap, ⌬͑k ជ ͒. A prominent quasiparticle dispersion kink reflecting strong scattering processes is observed in a binding-energy range of 25-55 meV in the superconducting state, and the coherence length or the extent of the Cooper pair wave function is found to be about 20 Å, which is uncharacteristic of a superconducting phase realized by the BCS-phonon-retardation mechanism. The observed 40Ϯ 15 meV kink likely reflects contributions from the frustrated spin excitations in a J 1 -J 2 magnetic background and scattering from the soft phonons. Results taken collectively provide direct clues to the nature of the pairing potential including an internal phase-shift factor in the superconducting order parameter which leads to a Brillouin zone node in a strong-coupling setting. DOI: 10.1103/PhysRevB.78.184508 PACS number͑s͒: 74.20.Mn, 74.25.Jb, 74.70.Ϫb, 74.72.Ϫh The recent discovery of superconductivity ͑T c up to 55 K͒ in iron-based layered compounds promises a new route to high-temperature superconductivity. 1-3 This is quite remarkable in the view that T c in the pnictides is already larger than that observed in the single-layer cuprates. Preliminary studies suggest that the superconducting state in these materials competes with a magnetically ordered state, and the proper description of the ordered state lies somewhere in between a strong correlation-mediated local-moment magnetism and quasi-itineracy with stripelike frustration. [3][4][5][6][7][8][9][10][11][12][13][14][15] This calls for a microscopic investigation of pair formation and related electron dynamics in these superconductors. Angle-resolved photoemission spectroscopy ͑ARPES͒ is a powerful tool for investigating the microscopic electronic behavior of layered superconductors. 16 In this work we report electronic structure results focusing on the details of the low-lying quasiparticle dynamics on very high-quality ͑␦T c Շ 1 K and surface flatness root mean square ϳ1 Å͒ single domain single-crystal samples, which allow us to gain insight into connections between the superconductivity and magnetism. We observe that the electrons are strongly scattered by collective processes around the 15-50 meV binding-energy range depending on the Fermi-surface ͑FS͒ sheet while a magnitude-oscillating gap structure persists nearly along the spin density wave ͑SDW͒ vector of the parent compound. We also show that a Cooper pair in this superconductor is strongly bound ͑ Շ4a o ͒. Our overall results can be self-consistently interpreted in a phase-shifting order-parameter scenario.ARPES measurements were performed using 18-60 eV photons with better than 8-15 meV energy resolution, respectively, and overall angular resolution better than 1% of the Brillouin zone ͑BZ͒. Most of the data wer...

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.…”

Observation of ordered vortices with Andreev bound states in Ba0.6K0.4Fe2As2

“…In their article, the authors demonstrated the expected magnetic field dependence of the interference pattern due to quasi-particle scattering off magnetic vortices. Since there are many families in the iron-based superconductors, the available results of STM measurements 1,[23][24][25][26][27][28][29] are still limited to reach a consensus. An intriguing question appears to us is how the hole-doped pnictide superconductors will behave in a magnetic field.…”

The 3-Dimensional Fermi Liquid Description for the Iron-Based Superconductors

“…The ε 3 ln |ε| dependence of has been really observed in Ba 1−x K x Fe 2 As 2 [6]; thus, the 3-dimensionality of the system has been confirmed.…”

“…The quasiparticle behaviours, energies and states around Fermi surfaces of (Ba, K)Fe 2 As 2 and (Sr, K)Fe 2 As 2 were thoroughly examined by Wray et al [6] through the angle-resolved-photoemission spectroscopic (ARPES) experiments. They measured the self-energy of quasiparticles in the superconducting and normal states and found characteristic kinks in Re (ε) around ε = 40 meV.…”