Evidence for Suppressed Screening on the Surface of High TemperatureLa2−xSrxCuO4andNd2<

Abstract

Hard X-ray Photoemission spectroscopy (PES) of copper core electronic states, with a probing depth of ∼60Å, is used to show that the Zhang-Rice singlet feature is present in La2CuO4 but is absent in Nd2CuO4. Hole-and electron doping in La2−xSrxCuO4 (LSCO) and Nd2−xCexCuO4 (NCCO) result in new well-screened features which are missing in soft X-ray PES. Impurity Anderson model calculations establish metallic screening as its origin, which is strongly suppressed within 15Å of the surface. Complemented with X-ray absorption spectroscopy, the small chemical-potential shift in core levels (∼ 0.2 eV) are shown to be consistent with modifications of valence and conduction band states spanning the band gap (∼ 1 eV) upon hole-and electron-doping in LSCO and NCCO. [2]. Soft X-ray (SX, hν∼1000-1500 eV) core level photoemission spectroscopy (PES) with a probing depth of ∼10-15Å is valuable in studying valence change, chemical-potential-shift and screening effects in solids [3]. Combination of core level PES with model calculations have been used to describe the parent insulating cuprates La 2 CuO 4 (LCO) and Nd 2 CuO 4 (NCO) as charge-transfer insulators in the Zaanen-SawatzkyAllen classification scheme [4], with the on-site Coulomb energy (≈8 eV), being much larger than the charge transfer energy (≈2 eV) between the O 2p and Cu 3d states [5,6,7,8].La 1.85 Sr 0.15 CuO 4 (LSCO) and Nd 1.85 Ce 0.15 CuO 4 (NCCO) are prototypical of hole-and electron-doped cuprates and exhibit a d x 2 −y 2 superconducting gap. The normal phase resistivity (ρ∝T 2 ) is like a Fermi-liquid for NCCO[9] but non-Fermi-liquid-like (ρ∝T ) for LSCO [10]. The strong correlations lead to special spectral behaviour such as non-local screening effects [11], and anomalous spectral weight transfer upon doping [12]. While valency and chemical potential changes in the high-Tc cuprates can be probed with SX-PES, in spite of several core level and valence band PES studies, there remains a seemingly simple and yet unresolved puzzle about the doping dependent electronic structure of the superconducting cuprates [5,6,7,8,11,12,13,14,15]. The puzzle involves distinguishing between 'mid-gap pinning' or 'crossing the gap' scenario to simultaneously explain changes in core levels and valence bands. The mid-gap pinning scenario [5,6,14,15] involves formation of new states within the band gap on hole-and electron-doping. It explains the small chemical potential shift of -0.2 eV (or +0.2 eV) in O 1s core levels PES of LSCO (or NCCO) compared to undoped LCO (or NCO), but is inconsistent with the large optical gap onset (∼1.0 eV) of the insulating parents [16]. In an alternative picture, the chemical potential moves to the top of the valence band by hole-doping and bottom of the conduction band on electron-doping. Using resonant PES [13], it was shown that electron-and hole-doping leads to a crossing of the gap (∼1.0 eV) from NCCO to LSCO. However, the small chemical potential shift in O 1s core levels cannot be explained by this scenario.While many SX-PES of the Cu 2p core ...

X-ray spectroscopic study ofBaFeO3thin films: AnFe4+ferromagnetic insulator

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High resolution, low hν photoelectron spectroscopy with the use of a microwave excited rare gas lamp and ionic crystal filters

“…Then, higher E K hard x-ray PES named HAXPES is understood to be more favorable for probing bulk electronic structures of SCES. 5,6 However, the photoionization cross sections become orders of magnitude decreased with increasing h from 1 to 8-10 keV. 7 In spite of the higher possible h resolution, the resulting total resolution of HAXPES is often spoiled by the single nucleus recoil effects.…”

Bulk electronic structures and strong electron–phonon interactions in an electron-doped high-temperature superconductor

“…It is obvious that the spectral feature in the 'Bulk' is drastically different from that at the 'Surface'. The structure at 931-932 eV in figure 1(b) corresponds to the feature induced by the non-local screening effect reported by hard x-ray PES [21] for the core-hole excitation. Thus, the 'Bulk' components are confirmed to be dominant in our high-hν near-normal emission PES spectra with a kinetic energy of around 500 eV.…”

“…For quasi-two-dimensional electron systems such as HTSCs, it has long been thought that the electronic structures of the top (or surface) conducting layer are equivalent to those of the bulk layers, which are responsible for the superconductivity. High-hν core-level PES studies [21]- [26], however, recently suggested that the electronic structures of the bulk CuO 2 layers, in particular for optimally doped Nd 1.85 Ce 0.15 CuO 4 , are noticeably different from those of the surface CuO 2 layer, which are mainly detected by low-hν ARPES [6,8,22]. High-hν ARPES [23,24] is therefore essential for directly revealing the bulk QD.…”