Raman scattering investigation of the electron-phonon coupling in superconducting Nd(O,F)BiS2

Abstract

We have performed polarized Raman scattering measurements on the newly discovered superconductor Nd(O,F)BiS2 (Tc = 4 K). We observe 2 Raman active modes, with frequencies in accordance with first-principles calculations. One A1g phonon mode at 112.4 cm −1 exhibits a Fano line shape due to electron-phonon coupling. We find a resonance for this mode at 2.45 eV excitation energy. We estimate a 0.68 contribution of this mode to the electron-phonon coupling constant λ. Our Raman results suggest that the BiS2-based superconductors are possibly phonon-mediated BCS superconductors. 74.25.nd, 74.25.Kc, Despite low superconducting (SC) critical temperatures (T c 's) of 3 K -10 K in Bi 4 O 4 S 3 and (La/Nd/Ce/Pr)O 0.5 F 0.5 BiS 2 [1-11], the BiS 2 -based superconductors are often regarded as possible hightemperature superconductor candidates. Indeed, their square BiS 2 layers are quite similar to the CuO 2 layers in high-T c cuprate superconductors [12] and the FeAs planes in the iron-based superconductors [13]. First-principles calculations [14,15] indicate that LaO 0.5 F 0.5 BiS 2 evolves from a band insulator to a bad metal upon doping. Interestingly, a large electron-phonon coupling constant λ = 0.85 is predicted, leading to a T c value of 10.6 K from the Allen-Dynes formula [16,17] that is compatible with experiments, suggesting a phonon-mediated pairing mechanism. Recent angle-resolved photoemission spectroscopy (ARPES) data were also interpreted in terms of the presence of polarons [18]. However, a neutron scattering work [19] suggests that the electronphonon coupling in LaO 0.5 F 0.5 BiS 2 is weaker than expected and a strong SC paring exceeding the limit of simple phonon mediated scenarios was proposed from upper critical field and magnetoresistance measurements [20]. Consequently, whether phonons are the paring glue for the BiS 2 -based superconductors is still under debate.In this paper we use Raman scattering to investigate directly the crystallographic structure and the electronphonon coupling in Nd(O,F)BiS 2 single-crystals. We report 2 Raman active modes, with frequencies in accordance with first-principles calculations. A photon energy resonance at about 2.45 eV for the 112.4 cm −1 phonon mode is observed and assigned to an electronic transition. The asymmetric phonon spectral profile of this mode indicates a strong electron-phonon coupling and suggests that phonons are possibly the paring glue in the BiS 2based superconductors. * p.richard@iphy.ac.cn † dingh@iphy.ac.cnThe Nd(O,F)BiS 2 single-crystals used in our Raman scattering measurements were grown by a flux method with KCl/LiCl as the flux [21]. Energy-dispersive X-ray spectroscopy (EDX) measurements performed on several pieces of samples with T c = 4 K give an averaged compo-The crystals were cleaved in air to obtain flat surfaces and then quickly transferred into a low-temperature cryostat ST500 (Janis) for the Raman measurements between 5 and 300 K under a working vacuum better than 2 × 10 −6 mbar. Raman scattering measurements were performed using...

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