Superconducting gap and nematic resonance at the quantum critical point observed by Raman scattering in BaFe2(As1−xPx)2<

Abstract

We report comprehensive temperature and doping-dependences of the Raman scattering spectra for BaFe 2 (As 1−x P x ) 2 (x = 0, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing x, the bare nematic transition temperature estimated from the Raman spectra reaches T = 0 K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair breaking peaks observed in the A 1g and B 1g spectra, another strong B 1g peak appears below the superconducting transition temperature which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.

Lattice-Shifted Nematic Quantum Critical Point in FeSe$_{1-x}$S$_x$

“…The QEP by contrast, is strongly affected by T s and displays a significant doping and temperature dependence : its intensity is maximum close to T s for lower x compositions, and continuously increases down to the lowest measured temperature (17K) for x=0.22. The enhancement and collapse of the QEP across the nematic/structural transition temperature is consistent with previous studies in various Fe SC [50][51][52]54], and is ascribed to critical nematic fluctuations near T s . The QEP intensity is significantly reduced for x=0.27 (see fig.…”

“…Here we use the ability of Raman scattering to probe symmetry resolved electronic fluctuations in the long wavelength limit (q=0) [48][49][50][51][52][53][54] to investigate the evolution of nematic fluctuations in FeSe 1−x S x as a function of doping and temperature. Our study spans a significant portion of the phase diagram of FeSe 1−x S x , from x=0 to x=0.27.…”

Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

“…At 9 K, below T c , the unstrained Raman response shows a relatively broad superconductivity-induced peak centered around 75 cm −1 . This was observed before [37,38], and interpreted as a nematic resonance mode where the usual Raman pair-breaking peak at twice the SC gap energy, 2∆, is replaced by a collective mode below 2∆ because of significant nematic correlations in the SC state [33,39,40]. Under compressive and tensile strains, the Raman response in the B 2g channel is strongly reduced at 9 K and 26 K, indicating a suppression of the nematic fluctuations below and just above T c .…”

Quasiparticle Scattering in a Superconductor near a Nematic Critical Point: Resonance Mode and Multiple Attractive Channels

“…We demonstrate that the nematic propagator possesses a charge exciton at a frequency Ω res (q), which near an instability is far below the threshold for the continuum at twice the gap value, 2∆. A nematic resonance at q = 0 has been observed in the B 1g Raman spectra of several iron-based superconductors [12][13][14][15][16]; here we show that Ω res (q) and the spectral weight of the resonance are strongly q-dependent, which is tied to the (ω/|q|)-form of the Landau damping of a nematic propagator in the normal state. We analyze how the dispersing charge exciton affects the fermionic spectral function at energies above the threshold at ∆ + Ω res (q = 0).…”