Amish G. Joshi

Influence of hole-filling by La and hole-doping by Ca on the superconductivity of NdBa2Cu3O7-δ

Abstract The structural and superconducting properties of La- and Ca-substituted (Nd 1− x Ca x )(Ba 2− y La y )Cu 3 O z samples prepared under identical conditions have been investigated by X-ray diffraction (XRD), resistivity, ac susceptibility and oxygen content measurements. The effect of increasing La-concentration in Nd(Ba 2− y La y )Cu 3 O z is in lowering the carrier concentration and decreasing T c , which is attributed to hole-filling by La. This suppression of T c has been compensated by appropriate hole-doping with Ca in (Nd 1− x Ca x )(Ba 2− y La y )Cu 3 O z , which shows that for a fixed La content at Ba-site, T c increases dramatically as the Ca content increases. For instance, in the oxide (Nd 1− x Ca x )(Ba 1.6 La 0.4 )Cu 3 O z , the T c increases from 0.0 K ( x =0.0, non-superconductor) to 84.0 K ( x =0.4, compensated oxide) as the hole-filling by La is completely balanced by hole-doping from Ca.

Effect of Mo and MoCa substitution on the superconductivity of GaBa2Cu3O7−δ

Abstract The structural and superconducting properties of (Gd 1− x − y Ca y Mo x )Ba 2 Cu 3 O z samples are investigated using X-ray diffraction, ac susceptibility, electrical resistivity and oxygen content measurements. The effect of increasing the Mo concentration in (Gd 1− x Mo x )Ba 2 Cu 3 O z changes the structure from orthorhombic to tetragonal accompanied by an increase in the normal state resistivity and decreases both the oxygen content and T c which is attributed to hole filling by Mo. The substitution of Ca for Gd in (Gd 0.85− y Ca y Mo 0.15 )Ba 2 Cu 3 O z provides proper matching between the ionic radius and valence of Gd 3+ (0.94 A) and the average ionic radius and valence of Mo 4+ (0.70 A) and Ca 2+ (0.99 A). As the Ca content increases, the T c increases from 58.5 K ( y = 0) to 86 K ( y = 0.15, compensated oxide), closer to the value of 91 K for pure GdBa 2 Cu 3 O 7−δ due to the balance between the hole filling by Mo and hole doping by Ca.

Effect of Cu-Ga and coupled Nd-Ca/Cu-Ga substitution on the superconductivity of NdBa2Cu3O7-δ

The structural and superconducting properties of (Nd 1-y Ca y )Ba 2 (Cu 1-x Ga x ) 3 O z samples have been studied by x-ray diffraction, resistivity, ac susceptibility and oxygen content measurements. The effect of increasing the Ga concentration in NdBa 2 (Cu 1-x Ga x ) 3 O z reduces the orthorhombicity without much disturbing the oxygen-vacancy order. This is accompanied by an increase in the normal state resistivity and decreases in both the carrier concentration and T c , which is attributed to hole filling by Ga. This suppression of T c has been compensated by appropriate hole doping with Ca in (Nd 1-y Ca y )Ba 2 (Cu 1-x Ga x ) 3 O z , which shows that for a fixed Ga content T c increases dramatically as the Ca content increases. For instance, in the oxide (Nd 1-y Ca y )Ba 2 (Cu 0.94 Ga 0.06 ) 3 O z , the T c increases from 0 K (y = 0.0, semiconductor) to 76 K (y = 0.36, compensated oxide) as the hole filling by Ga is nearly balanced by hole doping from Ca.

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.

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

Abstract The structural and superconducting properties of (Gd 1− x − y Ca y Hf x )Ba 2 Cu 3 O z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd 1− x Hf x )Ba 2 Cu 3 O z lowers the oxygen content and decreases T c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd 0.85− y Ca y Hf 0.15 )Ba 2 Cu 3 O z provides proper matching between the ionic radius and valence of Gd 3+ (0.94 A) and the average ionic radius and valence of Hf 4+ (0.78 A) and Ca 2+ (0.99 A). As the Ca content increases, the T c increases from 81 K ( y = 0.05) to 86.5 K ( y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa 2 Cu 3 O 7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R 1− x Hf x )Ba 2 Cu 3 O z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T c .

Neutron Structural Studies of the Superconducting (Nd1−yCay)Ba2(Cu0.94Ga0.06)3Oz System

We investigate the influence of Ca ions on the structural and superconducting properties of (Nd1−yCay)Ba2(Cu0.94Ga0.06)3Oz system by Rietveld refinement of the neutron diffraction patterns of three samples with y = 0.0 (A1), y = 0.18 (A2), and y = 0.36 (A3) along with X-ray diffraction, resistivity, and ac susceptibility measurements. Samples A2 and A3 are superconducting with Tc values of 37 K and 76 K, respectively, whereas sample A1 is semiconducting to 20 K. Neutron diffraction data confirms (a) the occurrence of a single-phase tetragonal structure (space group Pmmm) for all three samples; (b) Ca ions substitute at the Nd site; and (c) increasing y from 0.0 to 0.36 decreases Cu(1)-O(1), Cu(2)-O(2), and Nd-Cu(2) bond lengths, whereas Cu(2)-O(4) bond length increases with corresponding increase in Tc to 76 K due to increasing occupancy of Ca ions at Nd site. A correlation exists between the Cu(1)-O(1), Cu(2)-O(2), Cu(2)-O(4), and Nd-Cu(2) bond lengths 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.

Effect of Cd and Cd-Ca Substitution on the Superconductivity in Y1−xPrxBa2Cu3O7−δ Superconductor

The structural and superconducting properties of Y1−v−ηPrNCd{η}¨zCazBa2Cu3O7−δsystem are investigated using X-ray diffraction, ac susceptibility, dc resistivity, and oxygen content measurements. The effect of increasing Cd concentration substituting the Y-site in Y0.8Pr0.2Ba2Cu3O7-δ suppresses the superconducting transition temperature and lowers the hole concentration, unlike that of Ca substitution in Y0.8Pr0.2Ba2Cu3O7-δ which increasesTc due to hole doping by Ca. The suppression ofTc due to Cd substitution can be counterbalanced by simultaneous hole doping by Ca which increases the Tc with increasing Ca concentration. In spite of similarity in the ionic radii and valency, the role played by Cd and Ca substitution at the Y-site in the Y0.8Pr0.2Ba2Cu3O7-δ system is opposite in nature as Cd doping helps in Tc suppression due to the Pr effect, suggesting that Cd does not provide the necessary holes like Ca substitution which helps to increase the Tc by the hole doping mechanism.