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Register a new userNon-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides
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Angle-resolved photoemission spectroscopy (ARPES) is used to study the band dispersion and the quasiparticle scattering rates in two ferropnictides systems. Our ARPES results show linear-in-energy dependent scattering rates which are constant in a wide range of control parameter and which depend on the orbital character of the bands. We demonstrate that the linear energy dependence gives rise to weakly dispersing band with a strong mass enhancement when the band maximum crosses the chemical potential. In the superconducting phase the related small effective Fermi energy favors a Bardeen-Cooper-Schrieffer (BCS),cite{Bardeen1957}-Bose-Einstein (BE),cite{Bose1924} crossover state.
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We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.
While multiband systems are usually considered for flat-band physics, here we study one-band models that have flat portions in the dispersion to explore correlation effects in the 2D repulsive Hubbard model in an intermediate coupling regime. The FLEX+DMFT~(the dynamical mean-field theory combined with the fluctuation exchange approximation) is used to show that we have a crossover from ferromagnetic to antiferromagnetic spin fluctuations as the band filling is varied, which triggers a crossover from triplet to singlet pairings with a peculiar filling dependence that is dominated by the size of the flat region in the dispersion. A curious manifestation of the flat part appears as larger numbers of nodal lines associated with pairs extended in real space. We further detect non-Fermi liquid behavior in the momentum distribution function, frequency dependence of the self-energy and spectral function. These indicate correlation physics peculiar to flat-band systems.
While the beginning decade of the high-Tc cuprates era passed under domination of local theories, Abrikosov was one of the few who took seriously the electronic band structure of cuprates, stressing the importance of an extended Van Hove singularity near the Fermi level. These ideas have not been widely accepted that time mainly because of a lack of experimental evidence for correlation between saddle point position and superconductivity. In this short contribution, based on the detailed comparison of the electronic band structures of different families of cuprates and iron based superconductors I argue that a general mechanism of the Tc enhancement in all known high-Tc superconductors is likely related with the proximity of certain Van Hove singularities to the Fermi level. While this mechanism remains to be fully understood, one may conclude that it is not related with the electron density of states but likely with some kind of resonances caused by a proximity of the Fermi surface to topological Lifshitz transition. One may also notice that the electronic correlations often shifts the electronic bands to optimal for superconductivity positions.
The electronic structure of single crystals Na$_{0.6}$CoO$_2$, which are closely related to the superconducting Na$_{0.3}$CoO$_2$.$y$H$_2$O ($T_c sim 5K$), is studied by angle-resolved photoelectron spectroscopy. While the measured Fermi surface is found to be consistent with the prediction of a local density band theory, the energy dispersion is highly renormalized, with an anisotropy along the two principle axes ($Gamma$-$K$, $Gamma$-$M$). Our ARPES result also indicates that an extended flat band is formed slightly above $E_F$ along $Gamma$-$K$. In addition, an unusual band splitting is observed in the vicinity of the Fermi surface along the $Gamma$-$M$ direction, which differs from the predicted bilayer splitting.
Nuclear quadrupole resonance measurements were performed on the heavy fermion superconductor Ce2PdIn8. Above the Kondo coherence temperature T_coh simeq 30K, the spin-lattice relaxation rate 1/T_1 is temperature independent, whereas at lower temperatures, down to the onset of superconductivity at T_c = 0.64K, it is nearly proportional to T^{1/2}. Below T_c, 1/T_1 shows no coherence peak and decreases as T^3 down to 75mK. All these findings indicate that Ce2PdIn8 is close to the antiferromagnetic quantum critical point, and the superconducting state has an unconventional character with line nodes in the superconducting gap.
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