The origin of spontaneous electronic nematic ordering provides important information for understanding iron-based superconductors. Here, we analyze a scenario where the dxy orbital strongly contributes to nematic ordering in FeSe. We show that the addition of dxy nematicity to a pure dxz/dyz order provides a natural explanation for the unusual Fermi surface and correctly reproduces the strongly anisotropic momentum dependence of the superconducting gap. We predict a Lifshitz transition of an electron pocket mediated by temperature and sulfur doping, whose signatures we discuss by analysing available experimental data. We present the variation of momentum dependence of the superconducting gap upon suppression of nematicity. Our quantitatively accurate model yields the transition from tetragonal to nematic FeSe and the FeSe1−xSx series, and puts strong constraints on possible nematic mechanisms.

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Motivated by the recent development of terahertz pump-probe experiments, we investigate the short-time dynamics in superconductors with multiple attractive pairing channels. Studying a single-band square lattice model with spin-spin interaction as an example, we find the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse can be used to control the symmetry of the non-equilibrium state as well as frequencies and relative intensities of the contributions of different collective modes. We find particularly strong signatures of the Bardasis-Schrieffer mode in the dynamics of the quasiparticle distribution function. Our work shows the potential of modern ultrafast experiments to address the collective excitations in unconventional superconductors and highlights the importance of sub-dominant interactions for the nonequilibrium dynamics in these systems.Ultrafast pump-probe techniques became recently a powerful tool to probe the temporal evolution of symmetry broken states and relaxation in conventional and unconventional superconductors. 1-12 An intense pulse couples non-linearly to the Cooper pairs of the superconductor and, as was argued theoretically, should lead to a coherent excitation of the Higgs amplitude mode, i.e. |∆(t)| performs a damped oscillation with frequency ω H = 2|∆(∞)|. [13][14][15][16][17][18][19][20][21][22][23][24] Nonlinear terahertz spectroscopy has enabled the observation of the Higgs mode in conventional superconductors in the form of a free or forced oscillation and the resulting third-harmonic generation 4,11,25 . Interestingly, this technique has been also recently applied to the unconventional superconductors such as high-T c cuprates with the d-wave symmetry of the superconducting gap 12,26 where some additional oscillations have been reported. 26 In contrast to conventional superconductors, where the pairing is driven by the attractive electron-phonon interaction, the pairing interaction in unconventional superconductors is most likely of repulsive nature. To overcome the net repulsion among the quasiparticles, the superconducting gap has to change its sign across different parts of the Fermi surface, which typically yields the superconducting gap of a lower symmetry than an isotropic s-wave. For example, it is generally known that the antiferromagnetic spin fluctuations peaked near wave vector Q AF = (π, π) within a single-band model on a square lattice give rise to a d x 2 −y 2 -wave symmetry of the superconducting gap, yet states having other symmetries, such as strongly anisotropic sign-changing (extended) s-wave symmetry and the d xy -symmetry, are closely competing. As a result, the temporal dynamics of single-band unconventional superconductors might be significantly richer than that of the conventional ones....

a These authors contributed equally to this work 1 arXiv:1807.10752v1 [physics.comp-ph] a These authors contributed equally to this work

Motivated by the recent observation of the time-reversal symmetry broken state in K-doped BaFe2As2 superconducting alloys, we theoretically study the collective modes and the short time dynamics of the superconducting state with s + is-wave order parameter using an effective four-band model with two hole and two electron pockets. The superconducting s + is state emerges for incipient electron bands as a result of hole doping and appears as an intermediate state between s ± (high number of holes) and s ++ (low number of holes). The amplitude and phase modes are coupled giving rise to a variety of collective modes. In the s ± state, we find the Higgs mode at frequencies similar to a two-band model with an absent Leggett mode, while in the s + is and s ++ state, we uncover a new coupled collective soft mode. Finally we compare our results with the s + id solution and find similar behaviour of the collective modes.