We have studied the chemical potential shift in the electron-doped superconductor Nd2−xCexCuO4 by precise measurements of core-level photoemission spectra. The result shows that the chemical potential monotonously increases with electron doping, quite differently from La2−xSrxCuO4, where the shift is suppressed in the underdoped region. If the suppression of the shift in La2−xSrxCuO4 is attributed to strong stripe fluctuations, the monotonous increase of the chemical potential is consistent with the absence of stripe fluctuations in Nd2−xCexCuO4. The chemical potential jump between Nd2CuO4 and La2CuO4 is found to be much smaller than the optical band gaps.PACS numbers: 74.72.Jt, 71.30.+h, 75.50.Ee High-T c superconductivity occurs when the parent antiferromagnetic (AF) insulator with the CuO 2 plane is doped with holes or electrons. In the p-type materials, the long-range AF order vanishes for a slight amount of hole doping whereas in the n-type materials, the AF order persists up to a high doping concentration of ∼0.14 electrons per Cu and the superconducting (SC) doping range is much narrower [1]. The p-type materials show T -linear in-plane electrical resistivity [2] and split neutron peaks around q = (π, π) indicating incommensurate spin fluctuations [3] whereas the n-type materials show T 2 dependence of the in-plane resistivity [4] and (π, π) commensurate spin fluctuations [5]. In order to elucidate the mechanism of high-T c superconductivity, it is very important to clarify the origin of the similarities and the differences between the p-type and the n-type materials.In this Letter, we report on a study of the chemical potential shift in Nd 2−x Ce x CuO 4 (NCCO) as a function of doped electron concentration. The shift can be deduced from the core-level shifts in photoemission spectra because the binding energy of each core level is measured relative to the chemical potential µ. In a previous study [6], we found that in La 2−x Sr x CuO 4 (LSCO) the chemical potential shift is unusually suppressed in the underdoped region and attributed this observation to the strong stripe fluctuations which exist in this system. As for the chemical potential jump between La 2 CuO 4 and Nd 2 CuO 4 , which would represent the band gap of the parent insulator, it was estimated to be at most 300 meV in previous valence-band photoemission studies [7,8], which is much smaller than the 1.5-2.0 eV charge-transfer (CT) gap of the parent insulator estimated from optical studies [9].High-quality single crystals of NCCO (x = 0, 0.05, 0.125 and 0.15) were grown by the traveling-solvent floating-zone method as described elsewhere [10]. Uncertainties in the Ce concentration were ±0.01. For x = 0.15, both as-grown and reduced samples were measured while for the other compositions only as-grown samples were measured. The as-grown samples were all antiferromagnetic and did not show superconductivity. Only the x = 0.15 sample showed superconductivity after reduction in an Ar atmosphere and its T c was ∼25 K.X-ray photoemission spectrosc...
We have studied the chemical potential shift in the high-temperature superconductor Bi 2 Sr 2 Ca 1Ϫx R x Cu 2 O 8ϩy (RϭPr, Er͒, where the hole concentration is varied from 0.025 to 0.17 per Cu, by precise measurements of core-level photoemission spectra. The result shows that the shift is depressed in the underdoped region as in the case of La 2Ϫx Sr x CuO 4 ͑LSCO͒ but the depression is much weaker than in LSCO. The observed shift in the present system can be relatively well explained by numerical results on the doped two-dimensional Hubbard model and suggests that the change of the electronic structure induced by hole doping is less influenced by stripe fluctuations than in LSCO.
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