The Material-Dependent Parameter Controlling the Universal Phase Diagram of Cuprates

The critical temperature T _c in the universal phase diagram of cuprate superconductors is a function of two variables: the hole-doping δ and a material dependent parameter. Here we focus on the behavior of T _c,max as a function of the material dependent parameter (MDP) at the optimum hole doping. We have discussed the correlation between (1) the average Cu—O (planar) distance, or the strain of the Cu—O bond, (2) the nearest-neighbor hopping t′ and (3) the Lifshitz parameter z. These Lifshitz parameter z = μ_{δ = 0.16}−E _vHs which are all material dependent parameters, where μ_{δ = 0.16} is the chemical potential at optimum doping and E_vHs is the energy of the Van Hove singularity, defines the proximity to the Fermi surface topological transition from electron-like to hole-like. The results show that the striped phases occur for z < 0, the highest T _c,max for and the drop of T _c,max for z > 75 meV.     

Cu0.5Tl0.5Ba2Ca3Cu4−y Zn y O12−δ (y=0, 1.0, 2.0, 3.0, 3.5): Superconductor with Four ZnO2 Planes

A new Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) superconductor with four ZnO₂ planes is reported. The structure of the material remains tetragonal for all Zn doping concentration. The substitution of Zn at CuO₂ planar site was carried out following Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ (y=0, 1.0, 2.0, 3.0, 3.5) formula. Contrary to all previous studies of Zn doping in all copper oxide high temperature superconductors, the zero resistivity critical temperature T _c(R=0), critical current density and quantity of diamagnetism increase with increased Zn concentration. The onset temperature of superconductivity in these samples was observed at 128 K and T _c(R=0) at 122 K for y=3.5. The volume of the unit cell observed through X-ray diffraction scan is found to decrease with increase Zn doping; promoting an increase in Fermi vector K _F and effective density of states which results in enhanced superconductivity parameters. The synthesis of Cu_0.5Tl_0.5Ba₂Ca₃Cu_4−y Zn _y O_12−δ material by this method is highly reproducible.      

Effect of non-stoichiometry and Ce-doping on the tetragonal superconducting phase CaBaLaCu3O7−δ

We have synthesized CaBaLaCu₃O_7−δ and related compounds Ca_0.9BaLaCu₃O_7−δ , Ca₁₁ BaLaCu₃O_7−δ and Ca_0.9Ce_0.1 BaLaCu₃O_7−δ in order to investigate the effect of doping and non-stoichiometry at Ca-sites on the superconducting behaviour of tetragonal superconductor CaBaLaCu₃O_7−δ . The superconducting transition temperature (T _c) of Ca_0.9 BaLaCu₃O_7−δ is the same as that of pure CaBaLaCu₃O_7−δ (67 ± 1 K). Calcium excess compound Ca_1.1 BaLaCu₃O_7−δ showed a drop inT _c to 72 K while the cerium-substituted compound Ca_0.9 Ce_0.1 BaLaCu₃ O_7−δ showed a drop inT _c to 43 K. The results are explained on the basis of difference in valency between the host and the dopant ion.     

Synthesis Conditions and Superconductivity in 123-Type Superconductors

We have synthesized REBa₂Cu₃O_7–, where RE = Y, Sm, Gd, and SM(123):Ag under the identical conditions and characterized R – T and SEM–EDAX measurements by XRD. It is observed for sample SmBa₂Cu₃O_7– with = 0.3 that substitution of 10% Ag in place of Cu reduces T _c by 13 K along with the enhancement of normal-state resistivity by almost 200 times. When the T _c of the samples were optimized, it was found that the normal-state resistivity decreases by the substitution of Ag along with a decrease in T _c.     

Microwave Response of Perfect YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films Deposited on CeO<Subscript>2</Subscript>-Buffered Saphire: A Probe for Pairing Symmetry

Microwave surface impedance, Z _s(T), of epitaxial YBCO thin films deposited on CeO₂-buffered sapphire substrates, was measured at several discrete frequencies within the range 5–134 GHz by use of coplanar resonator and end-plate cavity resonator techniques. The main features of obtained experimental results are as follows: (i) surface resistance R _s(T) at low temperatures obeys the exponential law: R _s(T) = R _res+R ₀⋅exp [−δ/T] with a small gap δ value (δ≈ 0.7 T _c); (ii) the most perfect quasi-single-crystalline films reveal a distinct two-peak structure of R _s(T) dependence, which is not observable in films with a less ordered crystal structure. These features are believed to reveal some intrinsic electron properties of such films, namely: (i) mixed (d+is) type symmetry of electron pairing, and (ii) dominant role of extended c-oriented defects (e.g., edge dislocation arrays or twin planes) in quasiparticles scattering for the most perfect films, which demonstrate the two-peak anomalous R _s(T) behavior.     

Effect of orthorhombic distortion and second-nearest neighbor hopping on gap-to-T cratio

Within the framework of the BCS theory, we perform a numerical study of the effect of orthorhombic distortion and second nearest neighbor hopping on the ratioR = 2δ(0)/T _c based on the van Hove singularity scenario. We find that the gap-to-T _c ratio depends on the distortion and hopping parameters, in contradiction to the recent conclusion of Sarkaret al. [Phys. Rev. B 51, 12854 (1995)]. In particular, the introduction of orthorhombic distortion increases the value ofR, and with decreasing second neighbor hopping,R decreases.     

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu_1−x Tl _x Ba₂Ca₃Cu₄O_12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J _c) has also been investigated. From the power law dependence of H _ac∼(1−T _p/T _c) ⁿ , it was observed that n=1 gives a best fit for the J _c of thallium rich samples and n=2 provides a best fit for the J _c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J _c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.      

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.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

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.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedT _c in calcium-free Tl compounds of the series Tl _m Ba₂Ca _n−1Cu _n O _x (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestT _c(onset) ranges from 103 K to ∼ 115 K andT _c (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentT _cs.     

Effect of substitution at the barium site in YBa2Cu3O7-δ

The oxygen content in YBa₂Cu₃O_7-δ has been varied by substitution at Ba site with Sr or K and its influence on the structural and superconducting properties has been studied. These properties are compared with that of unsubstituted YBa₂Cu₃O_7-δ prepared by quenching. The increase inδ value of YBa₂Cu₃O_7-δ decreases theT _c drastically, whereas in YBa_2-χK_χCu₃O_7-δ system increase in theδ value (as high as 0·92) does not affect the superconducting transition temperature, which remains above 77 K. In the YBa_2-χSr_χCu₃O_7-δ system, as Sr content increases there is marginal decrease in the oxygen content as well as in the superconducting transition temperature.     

Interplay of low and highT c phases in Bi-Sr-Ca-Cu-O superconductor

In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highT _c phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi₂Sr₂Ca _n−1Cu _n O_{4 + 2n} . X-ray diffraction studies show that the lowerT _c material is a relatively pure phase while the higherT _c phase only co-exists with the lowerT _c phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.     

Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Prediction ofT c for YBa2Cu3Oz doped with Ca using neural network

In order to predict the superconducting transition temperatureT _c of YBa₂Cu₃O_z doped with Ca, we have constructed a neural network NN which is composed of three layers; we considered 28 constituent elements and their concentration in the input layer to provideT _c in the output layer. The NN was trained by the error-back-propagation method using data from the database “SUPERCON” for high-T_c superconducting materials. The effect of Ca doping onT _c of (Y_1-x-Ca_x)Ba₂Cu₃O_z and Y(Ba_2-xCa_x)Cu₃O_z was investigated using this method. It turned out that a higherT _c is expected for the sample withx = 0.3 andz = 6.5 in the former system and that the oxygen concentrationz can be varied for different Ca contents in the latter system.     

Correlation Between Superconducting Transition Temperature and Bond Length Ba–O in High-Tc Cuprate Superconductors

A well-known correlation of the maximum superconducting transition temperature (T _c,max) with ionic radius of rare earths and Y (R) in the series RBa₂Cu₃O_{7 –} is converted to the intrinsic correlation of T _c,max with bond length between Ba and oxygen in CuO₂ plane, being valid not only for RBa₂Cu₃O_{7 –} but also for other cuprate superconductors containing BaO plane such as HgBa₂CaCu₂O_{6 +} . It is pointed out that this correlation places a constraint on possible mechanisms inducing the high-T _c superconductivity.     

Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors

The nature of pairing mechanism as well as transition temperature of yttrium cuprates is discussed using the strong coupling theory. An interaction potential has been developed for the layered structure with two conducting CuO₂(a–b) layers in a unit cell. The interaction potential properly takes care of electron-electron, electron-phonon and electron-plasmon interactions. Furthermore, the electron-phonon coupling parameter (λ), the modified Coulomb repulsive parameter (μ*) and the 2D acoustic phonon (plasmon) energy as a function of oxygen deficiency is worked out. Finally, the superconducting transition temperature (T _c) is then evaluated by using these coupling parameters and obtainedT _c = 95(92)K for Y(Yb)Ba₂Cu₃O_7−δ superconductors withδ = 0·0. The model parameters estimated from the layered structure approach are consistent with the strong coupling theory. The result deduced on the variation ofT _c withδ are in fair agreement with the earlier reported data on yttrium cuprates. The analysis of the above results are discussed.