The origin of superconductivity in nominally ‘undoped’ T′-La_2−xY_xCuO₄ films

Abstract

We have systematically studied the transport properties of the La2−xYxCuO4(LYCO) films of T'-phase (0.05 ≤ x ≤ 0.30). In this nominally "undoped" system, superconductivity was acquired in certain Y doping range (0.10 ≤ x ≤ 0.20). Measurements of resistivity, Hall coefficients in normal states and resistive critical field (H ρ c2 )in superconducting states of the T'-LYCO films show the similar behavior as the known Ce-doped n-type cuprate superconductors, indicating the intrinsic electron-doping nature. The charge carriers are induced by oxygen deficiency. Non-superconducting Y-doped Pr-or Nd-based T'-phase cuprate films were also investigated for comparison, suggesting the crucial role of the radii of A-site cations in the origin of superconductivity in the nominally "undoped" cuptates. Based on a reasonable scenario in the microscopic reduction process, we put forward a self-consistent interpretation of these experimental observations.

Electron Doping of the Parent CuprateLa2CuO4without Cation Substitution

“…Over this entire range, the electron doping level showed no significant dependence on oxygen pressure and was consistently measured to be within the range of x = 0.10 ± 0.03 e − /Cu. If oxygen vacancies are indeed responsible for electron doping in our samples, this result suggests that their formation is robust to growth conditions and that a fixed concentration of vacancies may be energetically favored in the growth of T -La 2 CuO 4 thin films, also suggested in work on TLa 2−x Y x CuO 4 thin films [50,51].…”

Metastable T′-phase in bulk La2−xLnxCuO4 (Ln=Sm and Y)

“…1 of Ref. [13]) for low-doped La 2Àx Ln x CuO 4 system. For the lower synthesis temperature, the finely mixed precursor is crucial.…”

“…However, no corresponding bulk material is reported so far. The results of nominally ''undoped" superconducting T 0 -La 2Àx Ln x CuO 4 films have shown that the nominally ''undoped" superconducting films exhibit the similar transport properties to the electron-doped cuprate superconductors with Ce-doping [11][12][13]. The substitution of isovalent Ln +3 for La +3 does not bring net charge; the charge carrier in CuO 2 layer arises only from the oxygen deficiency due to the reduction procession.…”

“…The T 0 -phase films are prepared by molecular beam epitaxy (MBE), superconducting transition temperature (T c ) above 20 K can be achieved. Subsequently, other groups also grew superconducting T 0 -La 2Àx Y x CuO 4 (T 0 -LYCO) and T 0 -La 2Àx Sm x CuO 4 (T 0 -LSmCO) films by different growth methods, such as pulsed laser deposition (PLD) [11] and magnetic sputtering [12,13]. However, no corresponding bulk material is reported so far.…”

At the interface between electron and hole-doped cuprates

“…This explains why NCCO with the small ionic radius for Nd naturally crystallizes into the T ′ -phase whereas LSCO prefers the T-phase [27]. A material like La 2-x Ce x CuO 4 is on the border of the T ′ to T transition and its crystal structure can be tuned by choice of substrate and by doping; higher Ce doping will favor the T ′ -phase and isovalent substitution of La by Y can also stabilize the T ′ -phase [27,28].…”