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End date: 2023 × Start date: 2020 × Publisher: [London] : IOP × WGL Institute: IFWD ×

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    Interplay of charge density waves, disorder, and superconductivity in 2H-TaSe2 elucidated by NMR
    ([London] : IOP, 2022) Baek, Seung-Ho; Sur, Yeahan; Kim, Kee Hoon; Vojta, Matthias; Büchner, Bernd
    Single crystals of pristine and 6% Pd-intercalated 2H‐TaSe2 have been studied by means of 77Se nuclear magnetic resonance. The temperature dependence of the 77Se spectrum, with an unexpected line narrowing upon Pd intercalation, unravels the presence of correlated local lattice distortions far above the transition temperature of the charge density wave (CDW) order, thereby supporting a strong-coupling CDW mechanism in 2H‐TaSe2. While, the Knight shift data suggest that the incommensurate CDW transition involves a partial Fermi surface gap opening. As for spin dynamics, the 77Se spin-lattice relaxation rate T1-1 as a function of temperature shows that a pseudogap behavior dominates the low-energy spin excitations even within the CDW phase, and gets stronger along with superconductivity in the Pd-6% sample. We discuss that CDW fluctuations may be responsible for the pseudogap as well as superconductivity, although the two phenomena are unlikely to be directly linked each other.
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    Strong surface termination dependence of the electronic structure of polar superconductor LaFeAsO revealed by nano-ARPES
    ([London] : IOP, 2022) Jung, Sung Won; Rhodes, Luke C; Watson, Matthew D; Evtushinsky, Daniil V; Cacho, Cephise; Aswartham, Saicharan; Kappenberger, Rhea; Wurmehl, Sabine; Büchner, Bernd; Kim, Timur K
    The electronic structures of the iron-based superconductors have been intensively studied by using angle-resolved photoemission spectroscopy (ARPES). A considerable amount of research has been focused on the LaFeAsO family, showing the highest transition temperatures, where previous ARPES studies have found much larger Fermi surfaces than bulk theoretical calculations would predict. The discrepancy has been attributed to the presence of termination-dependent surface states. Here, using photoemission spectroscopy with a sub-micron focused beam spot (nano-ARPES) we have successfully measured the electronic structures of both the LaO and FeAs terminations in LaFeAsO. Our data reveal very different band dispersions and core-level spectra for different surface terminations, showing that previous macro-focus ARPES measurements were incomplete. Our results give direct evidence for the surface-driven electronic structure reconstruction in LaFeAsO, including formation of the termination-dependent surface states at the Fermi level. This experimental technique, which we have shown to be very powerful when applied to this prototypical compound, can now be used to study various materials with different surface terminations.