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Linkage between scattering rates and superconductivity in doped ferropnictides
2021, Fink, J., Rienks, E.D.L., Yao, M., Kurleto, R., Bannies, J., Aswartham, S., Morozov, I., Wurmehl, S., Wolf, T., Hardy, F., Meingast, C., Jeevan, H.S., Maiwald, J., Gegenwart, P., Felser, C., Buechner, B.
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 inner hole pocket, which is the hot spot in these compounds. More specifically, we determine the energy (E)-dependent scattering rate Γ(E) as a function of the 3d count. Moreover, for the compounds K0.4Ba0.6Fe2As2 and BaCr2As2, we derive the energy dependence of the renormalization function Z(E) and the imaginary part of the self-energy function ImΣ(E). We obtain a non-Fermi liquidlike linear in energy scattering rate Γ(E≫kBT), independent of the dopant concentration. The main result is that the slope β=Γ(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 Γ(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.
Electron–phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission
2013, Rettig, L., Cortés, R., Jeevan, H.S., Gegenwart, P., Wolf, T., Fink, J., Bovensiepen, U.
Based on the results from femtosecond time-resolved photoemission, we compare three different methods for the determination of the electron–phonon coupling constant λ in Eu- and Ba-based 122 FeAs compounds. We find good agreement between all three methods, which reveal a small λ < 0.2. This makes simple electron–phonon-mediated superconductivity unlikely in these compounds.
High-field phase diagram of the heavy-fermion metal YbRh2Si2
2006, Gegenwart, P., Tokiwa, Y., Westerkamp, T., Weickert, F., Custers, J., Ferstl, J., Krellner, C., Geibel, C., Kerschl, P., Müller, K.-H., Steglich, F.
The tetragonal heavy-fermion (HF) metal YbRh2Si2 (Kondo temperature TK≈ 25 K) exhibits a magnetic field-induced quantum critical point related to the suppression of very weak antiferromagnetic (AF) ordering (TN = 70 mK) at a critical field of Bc = 0.06 T (B⊥ c). To understand the influence of magnetic fields on quantum criticality and the Kondo effect, we study the evolution of various thermodynamic and magnetic properties upon tuning the system by magnetic field. At B > Bc, the AF component of the quantum critical fluctuations becomes suppressed, and FM fluctuations dominate. Their polarization with magnetic field gives rise to a large increase of the magnetization. At B* = 10 T, the Zeeman energy becomes comparable to kB TK, and a steplike decrease of the quasi-particle mass deduced from the specific-heat coefficient indicates the suppression of HF behaviour. The magnetization M(B) shows a pronounced decrease in slope at B* without any signature of metamagnetism. The field dependence of the linear magnetostriction coefficient suggests an increase of the Yb-valency with field, reaching 3+ at high fields. A negative hydrostatic pressure dependence of B* is found, similar to that of the Kondo temperature. We also compare the magnetization behaviour in pulsed fields up to 50 T with that of the isoelectronic HF system YbIr2Si2, which, due to a larger unit-cell volume, has an enhanced TK of about 40 K.