Chemical potential shift inNd2−xCexCuO

Abstract

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...

Doping-dependent evolution of low-energy excitations and quantum phase transitions within an effective model for high-Tc copper oxides

“…The dependence of the chemical potential shift ∆µ on x shows pinning at x < x opt , and evolves smoothly at higher doping concentrations 10 . The measured nodal Fermi velocity v F is almost dopingindependent within experimental error of 20% 11,12 .…”

Disorder Effects on Competition between Antiferromagnetism and Superconductivity in Cuprate Superconductors through the Enhancement in Charge Susceptibility

“…[38][39][40][41][42] The theoretical prediction of the chemical potential shift ∆µ ∝ δ 2 , which yields κ ≡ −∂δ/∂∆µ ∝ 1/δ, is satisfied in LSCO, Bi-2212, and YBCO within the range of experimental error.…”

At the interface between electron and hole-doped cuprates

“…From the core level shifts, the chemical potential of LSCO stays nearly constant up to optimal doping [85], but in the heterojunctions a shift of around 0.1 eV is observed for optimal doping [79]. For NCCO, there is a continuous shift, increasing up to 0.2 eV for a doping level of x = 0.15 [85]. [84]).…”

“…Here, we again assume that most of the band bending takes place in the NCCO layer. The addition of an overdoped Nd 2-x Ce x CuO 4 layer creates a larger Fermi level mismatch [85]. This will lead to a higher barrier, but with a smaller width, due to the higher carrier density associated with the overdoped phase.…”

“…The chemical potential shift as a function of doping for both materials has been determined from core level shifts by Harima et al [85]. Li et al [79] have measured the chemical potential shift in LSCO using LSCO/Nb:STO junctions.…”