2021, Bannies, J., Razzoli, E., Michiardi, M., Kung, H.-H., Elfimov, I.S., Yao, M., Fedorov, A., Fink, J., Jozwiak, C., Bostwick, A., Rotenberg, E., Damascelli, A., Felser, C.

Several early transition metal dipnictides (TMDPs) have been found to host topological semimetal states and exhibit large magnetoresistance (MR). In this paper, we use angle-resolved photoemission spectroscopy (ARPES) and magnetotransport to study the electronic properties of a TMDP ZrP2. We find that ZrP2 exhibits an extremely large and unsaturated MR of up to 40 000% at 2 K, which originates from an almost perfect electron-hole (e-h) compensation. Our band structure calculations further show that ZrP2 hosts a topological nodal loop in proximity to the Fermi level. Based on the ARPES measurements, we confirm the results of our calculations and determine the surface band structure. This paper establishes ZrP2 as a platform to investigate near-perfect e-h compensation and its interplay with topological band structures.

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.

2003, Pichler, T., Liu, X., Knupfer, M., Fink, J.

This paper reviews recent investigations of the electronic structure and the optical properties of intercalated single-wall carbon nanotubes (SWCNTs) and C60 filled SWCNTs (peapods) using electron energy-loss spectroscopy (EELS) in transmission as a probe. The results from these one-dimensional nanostructures are compared to C60 fullerides and intercalated graphite, which are well understood prototypes of carbon-based intercalation compounds. In detail, the structural changes were analysed by electron diffraction and the doping level and the matrix element weighted unoccupied density of states (DOS) by an analysis of the C 1s core-level excitations. Regarding the optical properties, the intercalation gives rise to a charge transfer to the peapods (SWCNTs) which leads to the formation of a free charge carrier plasmon in the loss function which is analysed within the framework of an effective Drude–Lorentz model.

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.

2016, Avigo, I., Thirupathaiah, S., Ligges, M., Wolf, T., Fink, J., Bovensiepen, U.

Using femtosecond time- and angle-resolved photoemission spectroscopy we investigate the effect of electron doping on the electron dynamics in $\mathrm{Ba}{({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ in a range of $0\leqslant x\lt 0.15$ at temperatures slightly above the Néel temperature. By analyzing the time-dependent photoemission intensity of the pump laser excited population as a function of energy, we found that the relaxation times at $0\lt E-{E}_{{\rm{F}}}\lt 0.2\,\mathrm{eV}$ are doping dependent and about 100 fs shorter at optimal doping than for overdoped and parent compounds. Analysis of the relaxation rates also reveals the presence of a pump fluence dependent step in the relaxation time at $E-{E}_{{\rm{F}}}=200\,\mathrm{meV}$ which we explain by coupling of the excited electronic system to a boson of this energy. We compare our results with static ARPES and transport measurements and find disagreement and agreement concerning the doping-dependence, respectively. We discuss the effect of the electron–boson coupling on the energy-dependent relaxation and assign the origin of the boson to a magnetic excitation.

2020, Yao, M., Manna, K., Yang, Q., Fedorov, A., Voroshnin, V., Valentin Schwarze, B., Hornung, J., Chattopadhyay, S., Sun, Z., Guin, S.N., Wosnitza, J., Borrmann, H., Shekhar, C., Kumar, N., Fink, J., Sun, Y., Felser, C.

Non-symmorphic chiral topological crystals host exotic multifold fermions, and their associated Fermi arcs helically wrap around and expand throughout the Brillouin zone between the high-symmetry center and surface-corner momenta. However, Fermi-arc splitting and realization of the theoretically proposed maximal Chern number rely heavily on the spin-orbit coupling (SOC) strength. In the present work, we investigate the topological states of a new chiral crystal, PtGa, which has the strongest SOC among all chiral crystals reported to date. With a comprehensive investigation using high-resolution angle-resolved photoemission spectroscopy, quantum-oscillation measurements, and state-of-the-art ab initio calculations, we report a giant SOC-induced splitting of both Fermi arcs and bulk states. Consequently, this study experimentally confirms the realization of a maximal Chern number equal to ±4 in multifold fermionic systems, thereby providing a platform to observe large-quantized photogalvanic currents in optical experiments.

2013, Allan, M.P., Tamai, A., Rozbicki, E., Fischer, M.H., Voss, J., King, P.D.C., Meevasana, W., Thirupathaiah, S., Rienks, E., Fink, J., Tennant, D.A ., Perry, R.S., Mercure, J.F., Wang, M.A., Lee, Jinho, Fennie, C.J., Kim, E.A., Lawler, M.J., Shen, K.M., Mackenzie, A.P., Shen, Z.X., Baumberger, F.

The phase diagram of Sr3Ru2O7 shows hallmarks of strong electron correlations despite the modest Coulomb interaction in the Ru 4d shell. We use angle-resolved photoelectron spectroscopy measurements to provide microscopic insight into the formation of the strongly renormalized heavy d-electron liquid that controls the physics of Sr3Ru2O7. Our data reveal itinerant Ru 4d-states confined over large parts of the Brillouin zone to an energy range of <6 meV, nearly three orders of magnitude lower than the bare band width. We show that this energy scale agrees quantitatively with a characteristic thermodynamic energy scale associated with quantum criticality and illustrate how it arises from a combination of back-folding due to a structural distortion and the hybridization of light and strongly renormalized, heavy quasiparticle bands. The resulting heavy Fermi liquid has a marked k-dependence of the renormalization which we relate to orbital mixing along individual Fermi surface sheets.