Crystal structure and resistivity studies on PrBa2 −x Pr x Cu3O7 −y oxide system

The compound PrBa₂Cu₃O_{7 −y} is not superconducting while most other RBa₂Cu₃O_{7 −y} (R=rare earth) compounds exhibit superconductivity in the 90K range. The system PrBa_{2 −x} Pr _x Cu₃O_{7 −x} has been prepared to study the effect of excess Pr at the Ba site on the structure, resistivity and magnetic behaviour of this system. It is observed that single-phase compounds in the above series form forx=0·8—that is up to the composition Pr_1·8Ba_1·2Cu₃O_{7 −y} . While stoichiometric PrBa₂Cu₃O_{7 −y} is orthorhombic, the compounds with excess Pr show tetragonal structure. Four-probe dc resistivity measurements show that all the single-phase compounds in the above series do not exhibit superconductivity and are semiconducting down to 12 K. Magnetic susceptibility measurements reveal deviation from Curie-Weiss behaviour starting at a characteristic temperature, which is taken to be the ordering temperature (T _N ) of the Pr moments. BothT _N and overall resistivity decrease with increasingx and may have a common origin.     

Thermal conductivity of Zn, Pr and Tb doped YBCO single crystals: Theory and experiment

The thermal conductivity K(T) vs temperature has been measured for (Y,Pr)Ba₂Cu₃O₇, (Y,Tb)Ba₂Cu₃O₇ and YBa₂(Cu,Zn)₃O₇ single crystals from 10T280K. For YBa₂Cu₃O₇(YBCO), a strong enhancement in K(T) is observed for TT_cwith a peak in K(T) at approximately TT_c/2 with a sharp change in slope of K(T) at T_c. These results are similar to those reported previously. For the selectively doped YBCO, the peak in K(T) is shifted to lower temperatures and occurs at T 20K independent of T_c and the impurity concentration. In addition, the sharp change in slope of K(T) at T_c is not apparent for the doped crystals. These results are discussed along with a theoretical model for electronic conduction including both impurity and inelastic scattering from spin fluctuations.     

Y1?xPrxBa2Cu3O7: Charge redistribution between planes and chains?

A qualitative discussion is given of the electronic structure and properties of the system Y_1–x Pr _x Ba₂Cu₃O₇ in which there is a transition from a high-T _c superconductor to a magnetic insulator. To reconcile the apparently contradicting experimental data, it is suggested that the progressive hole redistribution between CuO₂ planes and chains occurs with increasingx so that for large Pr content all the holes are in chains and none remain in planes. In this picture one can naturally explain the main properties of this system; notably, one gets an insulating state for pure PrBa₂Cu₃O₇, the valence of Pr being still Pr³⁺. Many other properties of Y_1–x Pr _x Ba₂Cu₃O_7– are naturally explained as well, and several predictions are made.     

Superconductor-Insulator Transition in Composite Granular (1?n)Gd123-nPr123 and (1?m)Gd123-mPrBa123

Granular composite samples of GdBa₂Cu₃O_7– (Gd123), PrBa₂Cu₃O_7– (Pr123), and Pr_0.5Ba_0.5Ba₂Cu₃O_7– (PrBa123) have been prepared by the solid state reaction technique. The characterization of samples has been done by SEM and XRD measurements. We have investigated the effect of Pr123 and PrBa123 insulating grains on the superconductor-insulator transition and the normal state resistivity of the (1–n)Gd123-nPr123 and (1–m)Gd123-mPrBa123 systems. The dominant diffusion of Pr ions onto the neighboring Gd123 grains in (1–n)Gd123-nPr123 causes high rate of suppression of superconductivity similar to the chemical-doped GdPr123 system. For (1–m)Gd123-mPrBa123 system, the suppression rate is slower and we have superconducting sample with T_c^mid = 41 K for the second phase of m = 0.6 sample. Comparison of the superconductor-insulator and metal-insulator transitions in the granular samples and the chemical substituted GdPr123 indicates optimization of the superconducting state in (1–n)Gd123-mPrBa123 relative to (1–n)Gd123-nPr123 and GdPr123 systems. The more stable characteristic of PrBa123 relative to Pr123, when they are mixed with Gd123 grains, makes PrBa123 more suitable for any superconductor-insulator application. The mechanism of suppression of superconducting state for Pr123 has been discussed by comparison of experimental results against the hole filling and hole localization models presented for the insulating Pr123.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

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

The structural and superconducting properties of Y_1−v−η Pr_NCd{η}¨zCa _z Ba₂Cu₃O_7−δ system 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 Y_0.8Pr_0.2Ba₂Cu₃O_7-δ suppresses the superconducting transition temperature and lowers the hole concentration, unlike that of Ca substitution in Y_0.8Pr_0.2Ba₂Cu₃O_7-δ which increasesT _c due to hole doping by Ca. The suppression ofT _c due to Cd substitution can be counterbalanced by simultaneous hole doping by Ca which increases the T_c 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 Y_0.8Pr_0.2Ba₂Cu₃O_7-δ system is opposite in nature as Cd doping helps in T_c suppression due to the Pr effect, suggesting that Cd does not provide the necessary holes like Ca substitution which helps to increase the T_c by the hole doping mechanism.     

Possible Superconductivity in Ba-Rich Pr123

Polycrystalline (Pr_1–xBa_x)Ba₂Cu₃O_7– samples with 0.0 x 0.6 are prepared by the standard solid state reaction technique. X-ray diffraction and resistivity measurements are performed. For x 0.3, the 123 phase is formed, but for x > 0.3, due to the solubility limit, the 123 phase is not the dominant phase. The normalized resistivity increases with the increase of Ba doping to about x = 0.08, and then decreases in the 123 phase up to x = 0.3. The existence of an extremum in the resistivity is explained as the competition between scattering by impurities and increase of the number of carriers by Ba doping. For x > 0.3, due to a large increase in the non-123 phase formation, the resistivity increases drastically. The normalized resistivity results suggest possible observation of superconductivity in Pr123 system by Ba doping. It also indicates that the reported superconductivity in Pr123 could be as the result of extra Ba concentration positioning at Pr site.     

Superconductor-Insulator Transition in Composite Granular (1−<Emphasis Type="Italic">n</Emphasis>)Gd123-<Emphasis Type="Italic">n</Emphasis>Pr123 and (1−<Emphasis Type="Italic">m</Emphasis>)Gd123-<Emphasis Type="Italic">m</Emphasis>PrBa123

Granular composite samples of GdBa₂Cu₃O_7– (Gd123), PrBa₂Cu₃O_7– (Pr123), and Pr_0.5Ba_0.5Ba₂Cu₃O_7– (PrBa123) have been prepared by the solid state reaction technique. The characterization of samples has been done by SEM and XRD measurements. We have investigated the effect of Pr123 and PrBa123 insulating grains on the superconductor-insulator transition and the normal state resistivity of the (1–n)Gd123-nPr123 and (1–m)Gd123-mPrBa123 systems. The dominant diffusion of Pr ions onto the neighboring Gd123 grains in (1–n)Gd123-nPr123 causes high rate of suppression of superconductivity similar to the chemical-doped GdPr123 system. For (1–m)Gd123-mPrBa123 system, the suppression rate is slower and we have superconducting sample with T_c^mid = 41 K for the second phase of m = 0.6 sample. Comparison of the superconductor-insulator and metal-insulator transitions in the granular samples and the chemical substituted GdPr123 indicates optimization of the superconducting state in (1–n)Gd123-mPrBa123 relative to (1–n)Gd123-nPr123 and GdPr123 systems. The more stable characteristic of PrBa123 relative to Pr123, when they are mixed with Gd123 grains, makes PrBa123 more suitable for any superconductor-insulator application. The mechanism of suppression of superconducting state for Pr123 has been discussed by comparison of experimental results against the hole filling and hole localization models presented for the insulating Pr123.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

Upper critical and irreversibility fields in selectively doped YBa2Cu3O7

The electrical resistivity of (Y_1–xPr_x)Ba₂Cu₃O_7–gd and YBa₂(Cu_1–xZn_x)₃O_7– thin films and (Y_1–x. Tb_x)Ba₂Cu₃O_7– thin films and single crystals has been measured as a function of 0xx_crit, 2KT300K and OTH20T. The samples were oriented withc-axis parallel to applied fields. Upper critical field H_c2 and irreversibility field H_irr values have been determined from these measurements. Increased Tb doping appears to shift H_irr to higher temperatures. This coupled with observed twin peaks in magnetization measurements reflects an enhancement of flux pinning. Unlike Tb which does not appear to alterT _c , Pr and Zn doping of this system tends to depress bothT _c and the slope of the mean field normal phase-mixed phase boundary line (dH _c2/dT).     

Fe57-Mössbauer Study in Y1-x PrxBa2(Cu1-yFey)3O7-z

In order to investigate the effect of Pr-substitution for Y element in Y123 high-Tc superconductors, Fe⁵⁷-Mossbauer effect is measured for the samples of Y_1-xPr_xBa₂(Cu_1-yFe_y)₃O_7-z. The Mossbauer spectrum can be decomposed into 4 sets of doublet spectrum, D1,D2,D3 and D4, and isomer shift () and quadruple splitting (Eq) for each component are obtained. We find that on CuO₂plane increases with x inY_1-xPr_xBa₂(Cu_0.96Fe_0.04)₃O_7-z as well as increase with oxygen reduction in YBa₂(Cu_0.96Fe_0.04)₃O_7-z while in the samples containing Pr decreases by oxygen reduction above z=0.25. These variation of can be interpreted by the change of 4p electrons in virtual Fe4p orbital which is induced by hybridization with O2p of the nearest oxygen ions.     

Extraordinary behaviour of the Y1?xPrxBa2Cu3O7?δ system

Investigations of Y_1–x M _x Ba₂Cu₃O_7– (M=Ce, Th)c-axis oriented thin film specimens show that the rate of depression ofT _c withx is larger for M=Th, than for M=Ce and Pr, and suggest that Ce, like Th, is tetravalent in this compound. Hall effect measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals reveal aT ² dependence of the cotangent of the Hall angle in the normal state and a negative Hall anomaly belowT _c in the superconducting state, in agreement with recent reports. Our research shows that the depth, , of the negative Hall signal scales withT/T _c and that the maximum value of decreases linearly withx and vanishes atx0.24. Magnetoresistance measurements on Y_1–x Pr _x Ba₂Cu₃O_7– single crystals indicate that the irreversibility lineH(T ^*) obeys a universal scaling relation characterized by anm=3/2 power law nearT _c, with a crossover to a more rapid temperature dependence of belowT/T _c 0.6, similar to that observed for polycrystalline specimens.     

Y1−xPrxBa2Cu3O7: Charge redistribution between planes and chains?

A qualitative discussion is given of the electronic structure and properties of the system Y_1?x Pr _x Ba₂Cu₃O₇ in which there is a transition from a high-T _c superconductor to a magnetic insulator. To reconcile the apparently contradicting experimental data, it is suggested that the progressive hole redistribution between CuO₂ planes and chains occurs with increasingx so that for large Pr content all the holes are in chains and none remain in planes. In this picture one can naturally explain the main properties of this system; notably, one gets an insulating state for pure PrBa₂Cu₃O₇, the valence of Pr being still Pr³⁺. Many other properties of Y_1?x Pr _x Ba₂Cu₃O_7?δ are naturally explained as well, and several predictions are made.     

Pressure Effect on Hall Coefficient in Multilayered High-T c Cuprates

Temperature dependence of the Hall coefficient and the resistivity in Tl₂Ba₂CaCu₂O _y (TL-2212), Tl₂Ba₂Ca₂Cu₃O _y (Tl-2223) and Cu_0.7C_0.3Ba₂Ca₃Cu₄O _y (Cu-1234) have been measured under high pressure up to 6 GPa. The values of the intrinsic T _c enhancement not related to charge transfer by applied pressure were estimated to be 0.8K/GPa for the Tl system and 1.1K/GPa for Cu-1234.     

Pseudoelastic behavior in Y1−xPrxBa2Cu3Oy compounds

Tensile tests show that ferroelastic loops always occur at 100–150 K, 200–250 K and room temperature in the Y_1–xPr_xBa₂Cu₃O_y samples withx=0 and 0.1 for whichT _c is 92 K and 82.5 K respectively, and the shape memory effect is always observed in the compact sample near 130 K, just similar to that of thermal elastic martensitic alloys. The loss fraction (W/W) which is proportional to the area of ferroelastic loop as a function of temperature shows that there always exist static hysteresis (W/W) peaks at 130 K and 110 K which are attributed to the phaselike transition (PLT) characterized by the jump of lattice parameters. No ferroelastic loops and shape memory effect are observed in the range of 100 K to 150 K for the lowerT _c samples withx=0.3, 0.4 and 0.6.     

Superconducting gap in PrxY1?xBa2Cu4O8 and YBa2?ySryCu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1–x Ba₂Cu₄O₈ and YBa_2–y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration at300 cm^–1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.On leave from the Institute for Semiconductor Physics, Ukrainian Academy of Sciences, 252650 Kiev-28, Ukraine.     

The Superconducting and Normal-State Transport Properties in Pure, Pr-Doped, and Ca-Doped NdBa2Cu3O7?y

We report detailed studies of the superconducting and normal-state transport properties in NdBa₂Cu₃O_7–y , Nd_0.9Ca_0.1Ba₂Cu₃O_7–y , and Nd_0.8Pr_0.2Ba₂Cu₃O_7–y , systems. We found a linear temperature dependence of resistivity and the Hall number n _H above T _c. For a given sample n _H, which is inconsistent with any simple transport model in science (1/n). However, and n _H at a fixed temperature for these three samples follow a relation 1/n _H. This is evidence that n _H could scale with actual mobile carrier concentration. The unit cell volume V is a linear function of n _H. The T _c –V–n _H diagram indicates that there exist an optimal concentration n _H and an optimal V for T _c in these systems.