M. V. Subbarao

Correlation between Tc and hole concentration in superconducting NdBa2(Cu1−xGax)3Oz system

Abstract The structural and superconducting properties of Ga substituted for Cu in NdBa 2 Cu 3 O 7− δ samples have been studied by X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Ga concentration in NdBa 2 (Cu 1− x Ga x ) 3 O z reduces orthorhombicity very fast displaying the destruction of oxygen vacancy order (OVO) for x ⩾0.03 accompanied by an increase in the normal state resistivity and decreases both carrier concentration and T c , which is attributed to hole filling by Ga. The calculated hole concentration from oxygen content data and T c display a strong correlation between the two.

Structural Studies of the Superconducting La2.5Y0.5CaBa3(Cu1−xFex)7Oz (0 ≤ x ≤ 0.1) Perovskite System by Neutron Diffraction

We have investigated the effect of Fe substitution on the structural and superconducting properties of La2.5Y0.5CaBa3(Cu1−xFex)7Oz system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B1), x = 0.06 (B2), and x = 0.10 (B3) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B1, B2, and B3 are superconducting with TcR=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in Tc to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured Tc values of three samples.

Superconductivity in (La2−xYx)Ba2(CayCu4+y)Oz system

The structural and superconducting properties of single phase (La2−xYx)Ba2(CayCu4+y)Oz (0.0≤x≤0.5, 0.0≤y≤1.0) compounds with tetragonal triple perovskite structure are investigated using X-ray diffraction, resistivity and oxygen content measurements. Samples with 0≤x≤0.5 and 0≤y≤1.0 are superconducting. Increasing y=2x, both the hole concentration in the Cu2O layers and TcR=0 increases to a maximum value of 0.349 and 78 K at x=0.4 and y=0.8, respectively. A correlation between the Tc and hole concentration seems to hold.

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La2.5−yY0.5Ca1+y Ba3 (Cu0.88Fe0.12)7Oz (LYCaBCuFe) (y= 0.0−1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. TcR=0 shows a marginal increase from 31 K to 37 K for y = 0.0−0.21 and thereafter it decreases with increasing y leading to zero TcR=0 at y ≥ 0.84. This shows that the suppression of Tc from 80 K to 31 K by Fe doping at x = 0.12 La2.5Y0.5CaBa3(Cu1−xFex)7Oz cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.

Effect of M and M-Ca Substitution on the Structure and Superconductivity of GdBa2Cu3O7−δ [M = Mo, Hf ]

The structural and superconducting properties of (Gd1−x−yCayMx)Ba2Cu3Oz with M = Mo, Hf are investigated using X-ray diffraction, electrical resistivity, and oxygen content measurements. The effect of increasing the Mo concentration in (Gd1−xMox)Ba2Cu3Oz changes the structure from orthorhombic to tetragonal accompanied by a large increase in resistivity and a fast decrease in Tc at the rate of 1.9 K per at.% of Mo, unlike that of Hf substitution in (Gd1−xHfx)Ba2Cu3Oz, which maintains the orthorhombic structure and decreases Tc very slowly at the rate of 0.6 K per atm.% of Hf with nearly no change in resistivity. The suppression of Tc by M = Mo, Hf can be counterbalanced by hole doping by Ca which increases Tc with increasing Ca content showing maximum compensation for Mo. A comparative study of M = Mo, Hf doped samples in (Gd1−x−yCayMx)Ba2Cu3Oz indicates that the valence of the dopant M = Mo4+,6+, Hf4+ and its ionic radius play an important role in controlling the structural and superconducting properties of the systems.