Oxygen non-stoichiometry of Nd1.85Ce0.15Cu1+δOy as a function of δ and its relation with the superconducting properties

Abstract

Thermogravimetric measurements of the equilibrium oxygen partial pressure (Po) as a function of the oxygen content y were performed on Ndl.ssCe0.15Cul,~Oy samples with cdntrolled Cu contents (1 +6= 1.00, 1.01 and 1.02) at 1173 and 1237 K. The data obtained showed a dependence of the oxygen non-stoichiometry on the nominal Cu content. These measurements are discussed in terms of the presence of Cu defects in the T' structure. The superconducting response of this material in samples with controlled oxygen and Cu contents was also studied. The superconducting response was found to be very sensitive to the presence of small amounts of cationic and anionic defects.

Microscopic annealing process and its impact on superconductivity in T′-structure electron-doped copper oxides

“…This microscopic oxygen-reduction mechanism was based on the resistivity values from undoped Nd 2 CuO 4 samples, which were found to decrease dramatically upon quenching from 1150 °C to room temperature. While experiments using a thermo-gravimetric apparatus (TGA) confirm that the reduction process removes some of the oxygen [4][5][6] , this simple model is inconsistent with observations that as-grown samples have excess oxygen per Cu atom [7][8][9] and that superconductivity is not achieved at any Ce (i.e., electron) doping levels without subsequent oxygen reduction.…”

“…This microscopic oxygen-reduction mechanism was based on the resistivity values from undoped Nd 2 CuO 4 samples, which were found to decrease dramatically upon quenching from 1150 °C to room temperature. While experiments using a thermo-gravimetric apparatus (TGA) confirm that the reduction process removes some of the oxygen 4-6 , this simple model is inconsistent with observations that as-grown samples have excess oxygen per Cu atom [7][8][9] and that superconductivity is not achieved at any Ce (i.e., electron) doping levels without subsequent oxygen reduction.Using neutron diffraction, resistivity and Hall-effect measurements, Greene and co-workers argue that the annealing process increases the number of mobile charge carriers 7 or decreases impurity scattering by removing apical oxygens (proposed oxygen just above and below the Cu atoms of the CuO 2 planes) which should be absent in the ideal T' structure of electron-doped materials 8,10,11,23 . In this latter scenario, the presence of a small amount of randomly doped apical oxygen in the as-grown materials induces localization of doped electrons and thus prohibits superconductivity.…”

“…5%) entirely suppress AF order and superconductivity appears over a wide range of hole concentrations (from 6% to 30%). In the case of T' structured electron-doped superconductors, doping alone by substituting the trivalent ions R 3+ in R 2 CuO 4 with tetravalent Ce 4+ is insufficient to induce superconductivity and the as-grown materials such as Nd 2-x Ce x CuO 4±δ (NCCO), Pr 2-x Ce x CuO 4±δ (PCCO), and Pr 0.88 LaCe 0.12 CuO 4 (PLCCO) are semiconducting with static long-range AF order [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] .In these materials, superconductivity (maximum T c ~25 K) is achieved only when the samples within a relatively narrow dopant range (0.1≤x≤0.18) are annealed in a reducing atmosphere . The reduction process suppresses the AF order most severely at higher Ce-doping levels above x≥0.1 (ref.…”

“…31) have recently determined the Fermi surface topology and the superconducting gap function in electron-doped materials. However, the quality of the sample surfaces studied has not been conclusively established, and one must now consider the possible effect of an exposed NSC R 2 O 3 surface in future ARPES and scanning tunneling microscopy experiments.Furthermore, it should also be possible to grow higher-quality single-crystal T' structured copper oxides by adding excess Cu in the starting materials to eliminate Cu deficiencies and subsequent R 2 O 3 contamination.Because of the availability of single crystals of the T' structured copper oxides, the vast majority of transport, structural, magnetic, and electronic-property measurements of electron-doped superconductors have been obtained from these materials [9][10][11][12][13][14][15][16][17][18][19][20][21][22]25,[27][28][29][30][31] . For other electron-doped high-T c superconductors with different crystal structure types, such as the infinite-layer Sr 1-x La x CuO 2 family 32-34 , the microscopic process of inducing superconductivity is still unclear 33,34 .…”

“…), electron-doping alone is insufficient, and annealing the as-grown sample in a low oxygen environment to remove a tiny amount of oxygen is necessary to induce superconductivity 2,3 . Previous work [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] suggests that oxygen reduction may influence mobile carrier concentrations 7 , decrease disorder/impurity scattering 8,10,11,23 , or suppress the long-range AF order 16,17,22 . However, the microscopic process of oxygen reduction, its effect on the large electron-hole phase diagram asymmetry and mechanism of superconductivity 2,3 are still unknown.…”