Spin Gap Effects on Bulk R1+xba2−x Cu3O7−δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R1+xBa2−xCu3O7−δ (R = Eu or Nd and 0 ≤ x ≤ 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R3+ for Ba2+ ions in the system rather than adjusting the oxygen deficiency. The excess R3+ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above Tc, and come across with superconducting transition at Tc. The increasing feature observed for spin gap temperature and the decreasing one for Tc, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.