Systematic studies on the (Formula presented) ((Formula presented) system were carried out in order to determine the effect of the rare-earth ionic size and magnetic moment on the (Formula presented) suppression rate. The phases were characterized by powder x-ray diffraction (XRD), resistivity, and ac susceptibility measurements. XRD studies indicate a higher solubility limit of (Formula presented) ions in the (Formula presented) system as compared to the (Formula presented) system. Resistivity and ac susceptibility studies indicate that the (Formula presented) suppression rate for a given (Formula presented) ion depends on the ionic radius of the rare earth ((Formula presented)) and is higher for larger rare earths. The trend in (Formula presented) suppression as a function of concentration ((Formula presented)) shows deviation from Abrikosov-Gorșkov behavior. A metal-insulator transition is observed at higher dopant concentrations, and the semiconducting phases are found to obey the Mottșs variable range hopping mechanism of conduction. The parameters related to hopping conduction; viz., the characteristic temperature ((Formula presented)), localization length ((Formula presented)), hopping range ((Formula presented)), and hop energy ((Formula presented)) have been calculated, and a comparative study of the variation of these parameters in the two systems has been made. © 1996 The American Physical Society.

Upadhyayula V Varadaraju

Department Of Chemistry

P. Sumana Prabhu

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Physical Review B - Condensed Matter and Materials Physics

Compounds in the system L1-xPrxBa1.7Sr0.3Cu3O7 (L=Yb and Lu) were synthesized to study the effect of Pr substitution on superconductivity. The nature of variation of Tc with Pr concentration is significantly different from that in other Pr-doped L-123 systems and deviates from Abrikosov-Gor'kov-like behavior. The results are explained on the basis of hole localization and magnetic pair-breaking mechanisms. The ion size effect on Tc is attributed to the dependence of the extent of orthorhombicity and oxygen ordering on the L ion leading to retention of mobile holes in the CuO2 planes. © 1995 The American Physical Society.

Physical Review B

Suppression of superconductivity in the L1-xPrxBa1.7Sr0.3Cu3O7 (L=Yb and Lu) system: Observation of the hole localization effect

Semiconducting phases in the system YLaxBa2-xCu3O7- (x=1.0-0.0) exhibit three-dimensional variable-range hopping conduction over a wide temperature (T) range (100-900 K). Thermopower (S) measurements support this observation. S increases with T for x=1.0 and 0.8 indicative of transport at the chemical potential E. The decrease in S with T for x=0.4 and 0.3 is indicative of gap at E. A schematic band picture is used to explain the observed behaviors. © 1995 The American Physical Society.

Observation of variable-range hopping up to 900 K in the YLaxBa2-xCu3O7- system

Systematic substitutional studies in the Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R=Pr,Gd,Er; 0≤x≤0.3 in steps of 0.05 and 0.4≤x≤0.97 in steps of 0.1) system were carried out in order to determine the effect of the magnetic moment and ionic radius of the rare-earth ion on the Tc suppression rate. X-ray-diffraction studies indicate that the solid solubility of Gd and Er exists up to x=0.97 whereas that of Pr is limited to x=0.6 under the preparative conditions employed. Resistivity and ac susceptibility studies have shown that superconductivity persists up to x=0.4 and a metal-semiconductor transition occurs for x>0.4. The most interesting observation is that the rate of Tc suppression for the superconducting phases is found to be identical for all rare earths. We have explained that hole filling rather than Abrikosov-Gor'kov pair breaking is responsible for the decrease in Tc. The insulating phases with 0.5≤x≤0.97 exhibit the phenomenon of Mott's variable-range-hopping mechanism (VRH). The physical parameters related to VRH such as localization length (α-1), hopping range (R), and activation energy (W) for conduction, have been evaluated and discussed in detail. © 1994 The American Physical Society.

Tc suppression and conduction mechanisms in Bi2.1Sr1.93Ca0.97-xRxCu2O8+y (R=Pr, Gd, and Er) systems

The effect of substitution at the Cu site by the transition elements Fe, Co, Ni, and Zn on the structure and superconducting properties of NdBa2Cu3O7 has been investigated. Compared to the YBa2Cu3-xMxO7+δ (M=Fe, Co, Ni, Zn) system, considerably high solid solubility and drastic Tc suppression is achieved in the Nd 1:2:3 system. The maximum solid solubilities achieved for the dopants are x=1.0 for M=Fe and Co and x=0.8 for M=Ni and Zn. An orthorhombic-to-tetragonal (O→T) transition is observed at x=0.08 for M=Fe, Co similar to the case of the M-doped Y 1:2:3 system. However, a second T→O transition is observed in Nd 1:2:3 at x=0.5 for M=Co and x=0.8 for M=Fe. For the Ni- and Zn-doped phases, an O→T transition is observed at x=0.4 for M=Ni and x=0.5 for M=Zn. The doped phases are superconducting up to x=0.2 (M=Fe, Co, Ni) and x=0.08 (M=Zn). The initial rate of Tc suppression (up to 3.33 at. %) is ∼14 K/at. % for M=Fe, Co, and Ni and ∼28 K/at. % for M=Zn and is much higher than that reported for the Y 1:2:3 system. The Fe- and Zn-doped phases, heated in flowing N2 gas at 800 °C followed by reoxygenation at 450 °C show higher Tc and sharper superconducting transition. Hole doping by Ca substitution (y) at the Nd site in the Zn-doped (x=0.1) semimetallic-semiconducting Nd 1:2:3 phase induces metallicity and superconductivity for y0.1 and a Tc of 32 K is obtained for y=0.2. © 1993 The American Physical Society.

Influence of Cu-site substitution on the structure and superconducting properties of the NdBa2Cu3-xMxO7+δ (M=Fe,Co) and NdBa2Cu3-xMxO7-δ (M=Ni,Zn) systems

Journal	Physical Review B - Condensed Matter and Materials Physics
ISSN	10980121
Open Access	No