X-ray absorption near edge structure study of -type revived superconducting compounds, (NACCO), A = Ca, Sr or Ba,

Abstract

Effects of the substitution of divalent A (Ca, Sr or Ba) ion in Ce excess non-superconducting compound, leading to revival of superconductivity, are studied by measuring absorption edges of all the cations with synchrotron radiation. It is found that in these NACCO samples, Cu exists mainly in the 2+ state; the weak pre-edge feature near the position is marginally affected due to ions. Except for the case of Ba doping, a fall in the intensity of the white line at the Cu K-edge is observed. This is attributed to a decrease in lattice oxygen content (oxygen vacancy creation) leading to an increase in electron carriers. It is noted that when the effective charge on Cu is , the -type samples are superconducting.

The interrelationship of Cu effective charge and superconductivity in the T´-type Gd1.85-xPrxCe0.15CuOysystem

“…This, in turn, induces additional electron carriers in the system and makes Gd(Pr)214 superconducting whenever n 1.87 ± 0.01 (table 1). Though the superconductivity is induced around n ∼ 1.87, the optimum T c is found in the vicinity of n ∼ 1.79 as has also been seen earlier in NCCO and NACCO T -type compounds [22]. n = 1.79 (±0.01) for optimum T c is, remarkably, in good agreement with the values of n (1.77 n 1.84) calculated from the values of y, reported by others [11,[36][37][38][39][40], for the optimum T c in various T -type cuprates.…”

“…A reduction annealing step is mandatory to generate sufficient additional electron carriers through oxygen vacancy creation [17][18][19]. Further, experiments have shown that in T -type Ln 2−x Ce x CuO y (Ln = Pr, Nd, Sm, Eu, Gd), the higher Ce 4+ content (x 0.18) and/or the smaller radius Gd 3+ ion have led to enough lattice compression [20,21] to make reduction process (Ar, N 2 or He annealing) somewhat ineffective to generate adequate additional carriers and, therefore, Gd 1.85 Ce 0.15 CuO y and Nd 2−x Ce x CuO y (x 0.18) compounds have failed to exhibit superconductivity [5,20,22]. Hall effect data on the above T -compositions are very scanty and mutually at too large variance to bring out any definitive view on the sign and the density of majority carriers [17,[23][24][25][26][27][28][29].…”

“…Recently, it has been reported [22] that T -type Nd 2−x Ce x CuO y , x < 0.18 (NCCO) and Nd 1.82−x A x Ce 0.18 CuO y , A = Ca, Sr or Ba, 0.6 x 0.18 (NACCO) compounds exhibit superconductivity whenever the effective charge (n) on Cu is 1.88 (±0.01). This is an important observation and merits further validation.…”