Spectral analysis for the iron-based superconductors: Anisotropic spin fluctuations and fully gapped s^{\pm}-wave superconductivity

Spin fluctuations are considered to be one of the candidates that drive a sign-reversed s^{\pm} superconducting state in the iron pnictides. In the magnetic scenario, whether the spin fluctuation spectrum exhibits certain unique fine structures is an interesting aspect for theoretical study in order to understand experimental observations. We investigate the detailed momentum dependence of the short-range spin fluctuations using a 2-orbital model in the self-consistent fluctuation exchange approximation and find that a common feature of those fluctuations that are capable of inducing a fully gapped s^{\pm} state is the momentum anisotropy with lengthened span along the direction transverse to the antiferromagnetic momentum transfer. Performing a qualitative analysis based on the orbital character and the deviation from perfect nesting of the electronic structure for the 2-orbital and a more complete 5-orbital model, we gain the insight that this type of anisotropic spin fluctuations favor superconductivity due to their enhancement of intra-orbital, but inter-band, pair scattering processes. The momentum anisotropy leads to elliptically shaped magnetic responses which have been observed in inelastic neutron scattering measurements. Meanwhile, our detailed study on the magnetic and the electronic spectrum shows that the dispersion of the magnetic resonance mode in the nearly isotropic s^{\pm} superconducting state exhibits anisotropic propagating behavior in an upward pattern and the coupling of the resonance mode to fermions leads to a dip feature in the spectral function.Comment: 9 pages, 8 figure

Theory of spin-fluctuation induced superconductivity in iron-based superconductors

In this dissertation we focus on the investigation of the pairing mechanism in the recently discovered high-temperature superconductor, iron pnictides. Due to the proximity to magnetic instability of the system, we considered short-range spin fluctuations as the major mediating source to induce superconductivity. Our calculation supports the magnetic fluctuations as a strong candidate that drives Cooper-pair formation in this material. We find the corresponding order parameter to be of the so-called sign-reversed s wave type and show its evolution with temperature as well as the capability of supporting high transition temperature up to several tens of Kelvin. On the other hand, our itinerant model calculation shows pronounced spin correlation at the observed antiferromagnetic ordering wave vector, indicating the underlying electronic structure in favor of antiferromagnetic state. Therefore, the electronic degrees of freedom could participate both in the magnetic and in the superconducting properties. Our work shows that the interplay between magnetism and superconductivity plays an important role to the understanding of the rich physics in this material. The magnetic-excitation spectrum carries important information on the nature of magnetism and the characteristics of superconductivity. We analyze the spin excitation spectrum in the normal and superconducting states of iron pnictides in the magnetic scenario. As a consequence of the sign-reversed gap structure obtained in the above, a spin resonance mode appears below the superconducting transition temperature. The calculated resonance energy, scaled with the gap magnitude and the magnetic correlation length, agrees well with the inelastic neutron scattering (INS) measurements. More interestingly, we find a common feature of those short-range spin fluctuations that are capable of inducing a fully gapped sy state is the momentum anisotropy with elongated span along the direction transverse to the antiferromagnetic momentum transfer. This calculated intrinsic anisotropy exists both in the normal and in the superconducting state, which naturally explains the elliptically shaped magnetic responses observed in INS experiments. Our detailed calculation further shows that the magnetic resonance mode exhibits an upward dispersion-relation pattern but anisotropic along the transverse and longitudinal directions. We also perform a qualitative analysis on the relationship between the anisotropic momentum structure of the magnetic fluctuations and the stability of superconducting phase by intraorbital but interband pair scattering to show the consistency of the magnetic mechanism for superconductivity. As discussed for cuprates, an important identification of the mediating boson is from the fermionic spectrum. We study the spectral function in the normal and superconducting state. Not only do we extract the gap magnitude on the electron- and hole-pockets to show the momentum structure of the gap, but also find a peak-dip-hump feature in the electron spectrum, which reflects the feedback from the spin excitations on fermions. This serves as an interpretation of the kink structure observed in ARPES measurements

Post-transient relaxation in graphene after an intense laser pulse

High intensity laser pulses were recently shown to induce a population inverted transient state in graphene [T. Li et al. Phys. Rev. Lett. 108, 167401 (2012)]. Using a combination of hydrodynamic arguments and a kinetic theory we determine the post-transient state relaxation of hot, dense, population inverted electrons towards equilibrium. The cooling rate and charge-imbalance relaxation rate are determined from the Boltzmann-equation including electron-phonon scattering. We show that the relaxation of the population inversion, driven by inter-band scattering processes, is much slower than the relaxation of the electron temperature, which is determined by intra-band scattering processes. This insight may be of relevance for the application of graphene as an optical gain medium.Comment: 10 pages, 4 figures, submitted as contribution of the IMPACT Special Topics series of the EP