Comparison of Superconductivity and Structure for YBa2Cu3Oy with Potassium and Sodium Doping

The nominal composition of YBa_2–x M _x Cu₃O _y (M = K, Na) cuprates with x 0.30 were synthesized by the standard solid-state reaction technique. X-ray diffraction (XRD) and the resistivity measurements were used to characterize the structure and the superconductivity of the doped cuprates. There was no impurity phase detected within the whole doping range. The structure of the main phase (123) has the orthorhombic with P _mmm symmetry. With increasing the content of dopants, the lattice constants and some other structural parameters are almost unchanged for M = K, whereas they changed for M = Na. The refined contents of dopants are consistent with that of the nominal ones. The zero resistance temperature T _c0 decreases sharply with the increase of the content of potassium in potassium-doped samples as x 0.20. For sodium-doped YBa_2–z Na _x Cu₃O _y cuprates, T _c0 varies very little. The difference in superconductivity depression may result from the shift oxygen, which transfers conducting carriers from Cu-O chains to Cu-O₂ sheets or the structural stress effect.     

Comparison of Superconductivity and Structure for Lanthanum Replacing for Barium and Copper in YBa2Cu3Od

X-ray diffraction studies and the resistivity measurements are used to characterize the structure and the superconductivity of the nominal composition of YBa₂Cu_{3 – x} La _x O _d (YBCLO) cuprates with x 0.30. There was a BaCuO₂ impurity phase detected with x 0.10. The structure of the main phase (123) has the orthorhombic form with Pmmm symmetry in the whole doping range. With increasing content of lanthanum, x, the lattice constants increase for x < 0.04, and decrease for x 0.04. Rietveld refinements for X-ray diffraction show that the dopant of lanthanum substitutes for copper in the lower doping level, and replaces for both barium and copper in the high doping level. The zero-resistance temperature T _c0 first increases with the increase of the content of lanthanum in YBCLO as x 0.04 and then decreases with x as x 0.04. We compared the results with those of La-doped YBa_{2 – z} La _z Cu₃O _y cuprates. The different relationship in superconductivity dependence of lanthanum content may result from the strains due to the different occupancy of lanthanum in the unit cell of YBa₂Cu₃O _d .     

Electronic Raman scattering of superconducting YBa2Cu3Oy single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^–1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^–1 fory7.0 to 550 cm^–1 fory6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

X-Ray Diffraction Studies on LaBa2Cu3Ox Cuprates with Iron Substitution

LaBa₂Cu_3–y Fe _y O _x ceramic samples with y = 0.00–1.50 are synthesized by the solid-state reaction technique. Rietveld analysis for X-ray diffraction is performed on these iron-doped samples. A BaCuO₂ impurity phase and a ceramic cuprate phase coexist in each sample. An orthorhombic-to-tetragonal (OT) phase transition occurs in the doping range of 0.03y0.06, and a tetragonal-to-orthorhombic (TO) one occurs in the doping level of 0.10y0.25. There is a jump in the structural parameters due to the iron doping. The occupancy of oxygen at the O(4) site, which is in the La plane at z = 1/2, increases with increase in iron content. These results may relate to the iron preferential occupancy for the Cu(1) site at the lower doping level, and for Cu(2) sites at the higher doping level.     

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

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (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. T _c ^R=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 T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.     

Eliashberg Spectral Functions for Some High-Tc Superconductors

We used a simplified but more realistic form of a model spectral functions for some high-T _c superconductors (e.g., LSCO, YBCO, BSCCO), avoiding the elaborate process of solving the Eliashberg equations numerically. The resulting spectral functions, constructed using the phonon density of states (PDOS), obtained from INS experiments with the easy use of available expressions, reproduce the observed critical temperature, gap ratio, and several other parameters. T _c is found to have a stronger increase with the electron–phonon coupling constant compared to those obtained from several equations, constructed from numerical solution of the Eliashberg equations for lower values. The obtained functions show widely spreading spectra that inhibit lattice instability by yielding not too large values of (< 2.95).     

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.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7– (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (31) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Joint interval reliability for Markov systems with an application in transmission line reliability

We consider Markov reliability models whose finite state space is partitioned into the set of up states and the set of down states . Given a collection of k disjoint time intervals I_ℓ=[t_ℓ,t_ℓ+x_ℓ], ℓ=1,…,k, the joint interval reliability is defined as the probability of the system being in for all time instances in I₁I_k. A closed form expression is derived here for the joint interval reliability for this class of models. The result is applied to power transmission lines in a two-state fluctuating environment. We use the Linux versions of the free packages Maxima and Scilab in our implementation for symbolic and numerical work, respectively.     

Theory of superconductivity. 3. 2D conduction bands for high Tc. Bose-Einstein condensation transition of the third order

A general theory of superconductivity is developed, starting with a BCS Hamiltonian in which the interaction strengths (V ₁₁,V ₂₂,V ₁₂) among and between electron (1) and hole (2) Cooper pairs are differentiated, and identifying electrons (holes) with positive (negative) masses as those Bloch electrons moving on the empty (filled) side of the Fermi surface. The supercondensate is shown to be composed of equal numbers of electron and hole ground (zero-momentum) Cooper pairs with charges ±2e and different masses. This picture of a neutral supercondensate naturally explains the London rigidity and the meta-stability of the supercurrent ring. It is proposed that for a compound conductor the supercondensate is formed between electron and hole Fermi energy sheets with the aid of optical phonons having momenta greater than the minimum distance (momentum) between the two sheets. The proposed model can account for the relatively short coherence lengths observed for the compound superconductors including intermetallic compound, organic, and cuprous superconductors. In particular, the model can explain why these compounds are type II superconductors in contrast with type I elemental superconductors whose condensate is mediated by acoustic phonons. A cuprous superconductor has 2D conduction bands due to its layered perovskite lattice structure. Excited (nonzero momentum) Cooper pairs (bound by the exchange of optical phonons) aboveT _c are shown to move like free bosons with the energy-momentum relation=1/2v_Fq. They undergo a Bose-Einstein condensation atT _c = 0.977v _F k _b ^–1 n ^1/2, wheren is the number density of the Cooper pairs. The relatively high value ofT _c (100 K) arises from the fact that the densityn is high:n ^1/2–1 10⁷ cm^–1. The phase transition is of the third order, and the heat capacity has a reversed lambda ()-like peak atT _c .     

Minimal cut sets of s–t networks with k-out-of-n nodes

In this paper, we extend traditional directed s–t network by letting nodes have k-out-of-n property: To generate output flows, a node must receive at least k flows from its n input links, where k is an integer assigned for the node and its value can be any number from 1 to n. To evaluate the system reliability, minimal cut sets for the extended network are defined for nodes. Under this definition, an extended network and its sink node have the same minimal cut sets. A new algorithm is designed to generate minimal cut sets for all nodes, starting with the source node and ending with the sink node. With different initializations, the algorithm can be applied for extended s–t networks with or without node failures.     

Investigation on the Structure and Superconducting Properties of Mg1 ? xMxB2 (M = deficiency or Ca)

We report the preparation of Mg_{1 – x} M _x B₂ (M = deficiency or Ca) compounds and their structure and superconducting properties. For Mg_{1 – x} B₂, although nearly single-phase samples can be obtained for x = 0, MgB₄ coexists with the MgB₂ phase and some minor impurity phases, and the amount of MgB₄ increases with x for 0 < x 0.5. The lattice parameters a and c of MgB₂ decreases and increases, respectively, with the increase of x, and T _c also decreases. While for Mg_{1 – x} Ca _x B₂, the superconducting transition temperature remains unchanged for x 0.3 and loss of superconductivity occurs for x > 0.3. X-ray diffraction patterns for x 0.3 samples show that MgB₂ phase coexists with CaB₆, Mg, and MgO. With increasing x, the amount of CaB₆, Mg, and MgO increases, while the amount of MgB₂ decreases. The lattice parameters of MgB₂ phase do not show any obvious change in contrast to Mg_{1 – x} B₂. The results were discussed by considering some possible contributions.     

Upper limit ofTc for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c.A new parameter, intrinsicT _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Polarons, Bipolarons, and Possible High-Tc Superconductivity in Metal-Ammonia Solutions

We examine the nature of fluid metal-ammonia solutions with a special emphasis on the electronic structure and dynamics of polaronic and bipolaronic charge carriers. Importantly, we find that close to the compositionally-induced Nonmetal-to-Metal Transition in the fluid at low temperatures (ca. 240K), the vast majority (ca. 85% or above) of current carriers are highly mobile, diagmagnetic (S = 0) bipolarons. This raises the intriguing possibility, first proposed by R. A. Ogg in 1946, of a Bose–Einstein Condensation (BEC) of trapped electron pairs in vitreous, quenched metal-ammonia solutions. From a modern (2000) perspective we believe that there are important similarities to the situation in the crystalline layered cuprates, where we have argued elsewhere that High-T _c superconductivity derives from the BEC of bipolarons in the electronically active CuO₂ planes [A. S. Alexandrov and P. P. Edwards, Physica C 331, 97 (2000)]. We now propose that the search begins for high temperature superconductivity in quenched metal-ammonia and related solutions.     

500–600 K Dilatation Anomaly and Tc Increase in YBa2Cu3Ox

YBa₂Cu₃O _x bulk ceramics have been simultaneously investigated by the thermal expansion and the acoustic emission methods in the 500–600 K temperature range. An error of the differential dilatometer does not exceed 10^–7. At T = 555 K, the dilatation anomaly accompanied by the acoustic emission has been observed and measured. Based on this measured anomaly, a value of c = –0.003 m has been determined. And based on this c size the stoichiometric coefficient x = 7.01 has been calculated too. The possible mechanism for T _c increase in YBa₂Cu₃O _x as a result of closing b axis by the absorbed oxygen atoms during the samples' heating through 500–600 K has been discussed.     

Far- and mid-infrared spectrum of YBa2Cu3O6.0 in high magnetic fields

The far- and mid-infrared spectrum of antiferromagnetic YBa₂Cu₃O_6.0 was investigated by infrared transmission measurements (kc-axis) in high magnetic fields up to 16.5Tesla at T=1K. A peak at 1436cm ^–1 which previously was assigned to the excitation of single optical magnons did not show a measurable shift with magnetic field. In the far-infrared, no signature of acoustic magnon absorption has been observed in the magneto transmittance. However, at 83 cm^–1 the sixth phonon of symmetry E_u in YBa₂ Cu₃ O_6.0 has been found.