Charge Reservoir Research Articles

On the role of apical oxygen in the charge transfer of YBCO superconductors

We have observed the effects of cation substitution on the charge distribution and superconductivity of Y 1− x Ca x Ba 2Cu 3O y and YBa 2− z La z Cu 3O y . As Ca concentration increases, hole concentration on the CuO 2 plane increases beyond the optimum region due to the hole transfer from the Cu–O chain to the CuO 2 plane. However, it decreases as the holes move from the CuO 2 plane to the Cu–O chain with increasing La concentration. Apical oxygen moves in the opposite direction to the hole transfer for both cases. The hole distribution between the superconducting plane and the charge reservoir is considered to be determined mainly by the environmental ionic configuration including the apical oxygen but not by the movement of the apical oxygen alone.

Non-linear temperature dependence of Hg–O and Cu–O distances in Hg-1212 superconductor

The crystal structure of HgBa 2CaCu 2O 6+ δ (Hg-1212) has been carefully examined as a function of temperature by the single-crystal X-ray diffraction method. A significant anisotropy above the T c (=120 K) appeared on out-of-plane interatomic distances, such as Cu–O(2) and Hg–O(2), although the structural environment at room temperature is generally considered to maintain in the temperature vicinity of superconducting transition. On decreasing the temperature, the oxygen atom in the apical O(2) site of CuO 5 pyramid approaches Hg atom and then leaves it. The anisotropic behavior would be precursor with superconductive charge-carriers, where the O(2) site plays the role of a “channel” between Hg charge reservoir and Cu–O superconducting plane.

Four predicted high-temperature superconductors

The charge-reservoir oxygen model has successfully predicted that four materials will superconduct. In all cases the successful predictions were based on the idea that the superconductivity inhabits the charge reservoirs, not the cuprate-planes.

New tailored cuprates grown by pulsed laser deposition

This paper reports on the use of pulsed laser deposition to grow tailored infinite-layer-derived structures. The goal is to modify the carrier by changing the nature of the charge reservoir intergrown with the infinite-layer structure. The challenges associated with the development of superlattices containing CaCuO 2 are focussed upon. Both structural and electrical properties of materials with various charge reservoirs are presented.

Suppression of thermal fluctuation and improvement of the irreversibility line by Sr substitution in Hg 0.7Pb 0.3Ba 2Ca 2Cu 3O y superconductor

We have investigated the effects of Sr substitution on Ba site in Hg 0.7Pb 0.3Ba 2Ca 2Cu 3O y superconductor. The substitution leads to the suppression of thermal fluctuation and the improvement of the irreversibility line position. This feature results from the shrinkage of charge reservoir blocks and the increase of interlayer coupling.

(Hg,Pb)(Ba,Sr) 2Ca 2Cu 3O 8+δ single crystals: Influence of Sr substitutions on structure, hole concentration in CuO 2 planes and irreversibility line

The hole concentration in CuO 2 planes, structure and irreversibility field (H irr) as a function of Sr content in (Hg,Pb)(Ba,Sr) 2Ca 2Cu 3O 8+δ single crystals were investigated. The main results are: (1) H irr increases significantly with Sr substitution, (2) in nonsubstituted crystals almost all holes are concentrated in the inner CuO 2 planes, (3) with increasing Sr content the distribution of holes between outer and inner CuO 2 planes becomes more homogeneous, (4) the thickness of the charge reservoir decreases with Sr substitution by 0.7 Å.

DEPOSITION OF (Ba, La)CuO2/CaCuO2 SUPERCONDUCTING MULTILAYERS BY PULSED LASER ABLATION

Superconducting multilayers, whose growth and structure are determined by the kinetic deposition instead by thermodynamics, are a powerful tool for investigating high T c superconductors (HTSC) properties. By means of Pulsed Laser Deposition (PLD), we grew such materials alternating infinite layer phases both pure and doped, to supply CuO 2 planes and charge reservoir (CR) blocks respectively. We found out that using chemically doped CR leads to a semiconducting behavior, whereas oxygen doping seems to be much more effective, since the BaCuO 2 infinite phase, grown in high oxygen partial pressure, does actually bring about a superconducting behavior. We deposited BaCuO 2/ CaCuO 2 superlattices with various periodicities and studied their transport and structural properties. By varying the deposition parameters, we reached both kinetic and thermodynamic growth and we observed semiconducting and superconducting behaviors.

XAS STUDY OF (BaCuO2)2/(CaCuO2)n SUPERLATTICES

EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-Ray Absorption Near Edge Structure) spectroscopies have been used to probe the local bonding of the artificially layered superconducting (BaCuO2)2/(CaCuO2)n (n = 2, 3, 4) superlattices. Fluorescence-yield measurements have been performed above the Cu and Ba K-edges. This study shows that the charge reservoir (CR) block of the Ba/Ca superlattice contains oxygen vacancies randomly distributed in the CuO2 planes and that the two apical oxygen atoms are displaced out of the Ba plane giving rise to buckled BaO planes. These oxygen atoms have a distorted pyramidal and octahedral coordination around Cu at Ba/Ca interface and in the (BaCuO2) block, respectively.

Magnetic-field irreversibility in superconductiveCu2Ba2(Y, Ca)Cu2O8±δ phase with Cu2O2 double-chaincharge reservoir

The magnetic irreversibility-field lines (Hirr(T)) for bothnon-substituted (x = 0) and Ca-substituted (x = 0.1) Cu2Ba2(Y1-xCax)Cu2O8±δ superconductorsof the Cu-2212 structure (so-called `124') with a Cu2O2±δ double chain as the charge-reservoir constituent have beenestablished. The obtained Hirr(T) lines are compared to data measured forCuBa2(Y1-xCax)Cu2O7-δ samples ofthe Cu-1212 structure (so-called `123') with a CuO1-δsingle-chain charge reservoir. The Ca-free Cu-2212 sample with Tc = 80 K is assumed to be underdoped in terms of the hole-doping level. Increasingthe hole-doping level through the Ca(II)-for-Y(III) substitution not onlyincreases the value of Tc, but also shifts the Hirr line towards thehigher magnetic field regime. Nevertheless, the Hirr(T) line for theCa-substituted Cu-2212 sample (Tc = 87 K) is clearly located lower thanthat for the Ca-free sample, which is of the optimally-doped Cu-1212(Tc = 91 K) phase. This is attributed to the yet higher overallhole-doping level of the Cu-1212-phase sample. It is interesting to note thatfor the Cu-1212 phase, the Hirr lines are nearly identical for both theCa-free and the Ca-substituted samples, even though the Tc decreases withCa substitution. This is believed to be due to a slightly lower oxygen contentin the Ca-substituted Cu-1212 sample.

Oxygen non-stoichiometry and hole distribution in multi-layered copper oxides: understanding of the magnetic-irreversibility characteristics

The dependence of the magnetic-irreversibility characteristics on the amount of excess oxygen and the hole distribution in superconductive multi-layered copper oxides, M m A 2 Q n−1 Cu n O m+2+2 n± δ or M– m ( A) 2 ( Q) ( n−1) n:P/RS, is discussed with relevant examples of such phases, i.e. Cu−1 (Ba,Sr)2 (Yb)12:P, Bi-2 (Sr)2 (Ca,Y)12:RS, Hg-1 (Ba)2 (Ca)23:RS, Cu-1 (Ba)2 (Ca)23:P and Cu-1 (Ba)2 (Ca)34:P. At a fixed cation stoichiometry, the irreversibility-field line, H irr( T), is shifted in a continuous manner to higher magnetic fields with increasing oxygen content. This trend applies not only to the M– m ( A) 2 ( Q) ( n−1) n:P/RS phases with a perovskite-type (P) charge reservoir but also to those with a rock-salt-type (RS) charge reservoir, and furthermore, in both cases to both under- and overdoped regions. However, when the increase in the oxygen content is accompanied with aliovalent cation substitution, e.g. trivalent-Y-for-divalent-Ca substitution in the Bi-2 (Sr)2 (Ca,Y)12:RS system, the H irr characteristics are not necessarily enhanced with increasing oxygen content but may rather be depressed. This provides us with one of the experimental evidences supporting the general conclusion that it is not the oxygen content but the concentration and the distribution of holes which control the H irr characteristics.

Irreversibility-field enhancement by optimizing hole density distribution in the Hg-1212 superconductor

In this paper, enhancement of the irreversibility field characteristics for the n=2 member of the Hg-12( n−1) n homologous series by optimizing hole density distribution is reported. The hole density distribution in the high- T c layered copper oxides can be regarded as a source of anisotropy and also has been found to govern superconductivity properties such as transition temperature [M. Karppinen, H. Yamauchi, Philos. Mag. B, 79 (1999) 343–366], irreversibility field [H. Yamauchi, M. Karppinen, K. Fujinami, T. Ito, H. Suematsu, K. Matsuura, K. Isawa, Supercond. Sci. Technol., 11 (1998) 1006–1010], etc. In order to control the hole density distribution, solid solutions of HgBa 2(Ca 1− x Y x )Cu 2O 6+ δ (0≤ x≤0.5) have been synthesized by an encapsulation technique and then post-annealed by a high-pressure oxidization (HPO) technique. The total amount of doped holes depends on both the amount of Y substitution for Ca ( x) and that of excess oxygen in the HgO δ layer ( δ). The former would reduce the hole density in the neighborhood of the CuO 2 planes and the latter would increase the amount of holes especially in the charge reservoir (CR) block. The experimental data have demonstrated that the best H irr-vs.-(1− T/ T c) characteristics is obtained by the excess oxygen doping solely, i.e. upon over-doping the samples. In such over-doped states, the T c's tend to decrease. On the other hand, the samples simultaneously doped with small amounts of both yttrium and oxygen showed remarkably improved H irr characteristics without large suppression of the T c value.

New superconducting lead cuprates prepared by molecular beam epitaxy

We report the synthesis of a new superconducting cuprate PbSr 2CuO 5+ δ (Pb-1201; T c∼40 K) with a pure PbO charge reservoir layer by a low-temperature technique using molecular beam epitaxy (MBE). This new superconductor is the first member of the Pb-12( n−1) n homologous series. Low-temperature synthesis by MBE and an appropriate choice of substrates were essential to our discovery of Pb-1201 superconductor. This work demonstrates that MBE provides a novel synthetic route for new cuprates.

Selective reduction for hole-doping in Cu 1− xTl x-1223 (Cu 1− xTl xBa 2Ca 2Cu 3O 10− y) system with [formula omitted

A new hole-doping process due to the selective reduction of Tl in the charge reservoir layer is discovered in Cu 1−x Tl x Ba 2 Ca 2 Cu 3 O 10−y ( Cu 1−x Tl x -1223) system. The process is very effective for the enhancement of T c up to 132 K. The selective reduction effect is related to the reduction of Tl ions from 3+ valence to 1+ and the energy shift of Tl 6s level from above to below the Fermi level. The mechanism of the hole-doping is explained by electron transfer from Cu 3d ( x 2– y 2) orbital to Tl 6s orbital.

Pairing mechanism for electron-doped cuprates in a layered two-dimensional electron gas model

An effective two-dimensional (2D) interaction potential for super-lattice of electron-doped cuprates (Nd 2− x Ce x CuO 4) treated as a layered electron gas is developed which incorporates screening of electrons as carriers by phonons and by plasmons. The layer stacking sequence is modelled with single superconducting CuO 2 layer, which is an isolated free electron layer well, separated by the charge reservoir layers. A model dielectric function in the random phase approximation is set up for two oscillators fulfilling the appropriate sum rules on polarisabilities. Attention is focussed on the description of the inter-layer electron pairing and superconducting transition temperature, T c. Deduced values of the Coulomb repulsive parameter and of the electron–phonon coupling strength from the 2D effective interaction potential indicate that the Nd-doped cuprate superconductor is in the strong coupling regime with effective screening of the electrons. The superconducting transition temperature of Nd 1.85Ce 0.15CuO 4 is estimated as 28 K, which is consistent with the reported data. The implications of the proposed approach and its analysis are discussed.

Geodynamic and hydrodynamic evolution of the Broad Fourteens Basin and the development of its petroleum systems: an integrated 2D basin modelling approach

Results of integrated 2D basin modelling indicate that both sedimentary loading and gas generation play an important role in the overpressure history of the Broad Fourteens Basin. We analysed the influence of time-dependent permeability of faults on the overpressure history and evolution of petroleum systems. We favour time-dependent permeability of faults as the most likely option to explain the present-day near-hydrostatic conditions in the basin, the known location of oil fields, and the location and characteristics of the gas occurrences. The model results further show that the difference in timing of oil charging of the P9 and Q1 Lower Cretaceous oil reservoirs, explains the observed differences in geochemical composition of the accumulated oils. The biodegraded and water-washed nature of the pre-inversion charged Q1 oil reservoir is consistent with the concentrated topography-induced groundwater flow through the Lower Cretaceous reservoir units during the Late Cretaceous inversion period. Late charging of the oil reservoir in the P9 area explains the non-biodegraded character of the accumulated oils. Present-day conditions in the Broad Fourteens Basin are near-hydrostatic. In general, this paper shows that paleohydrodynamic conditions and time-dependent permeability of the hydrogeological framework have influenced significantly the petroleum systems in the basin.

High-pressure synthesis and Jc-related properties of Cr- and V-based high Tc superconductors

Homologous series of cuprate superconductors (Cu,Cr)Sr 2Ca n−1 Cu n O 2 n+3 [(Cu,Cr)-12( n−1) n, n=1–9] and (Cu,V)Sr 2Ca n−1 Cu n O 2 n+3 [(Cu,V)-12( n−1) n, n=3–7] prepared under high pressure were discussed. They have M-12( n−1) n-type structures with the M site occupied by the (Cu,Cr) [or (Cu,V)] mixed atom. The highest T c in each series is 103 K for (Cu,Cr)-1223 and 107 K for (Cu,V)-1234. The n=3 phases of both series show extremely high critical current densities and irreversibility fields which are comparable to or better than those of the YBa 2Cu 3O 7. This seems to be related to the partial occupation of the Cu atom in the charge reservoir layer and the random arrangement of Cu and Cr (or V) there.

EXAFS study of the [BaCuO 2] 2/[(Ca,Sr)CuO 2] n artificial superconducting superlattices

Polarised extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies have been used to probe the local bonding of the infinite layer compounds CaCuO 2 and SrCuO 2 and of artificially layered superconducting (BaCuO 2) 2/(CaCuO 2) n and nonsuperconducting (BaCuO 2) 2/(SrCuO 2) 2 superlattices. Fluorescence-yield measurements have been performed at T=77 K above the Cu and Ba K-edges. The XANES spectrum of CaCuO 2 is described in terms of mixing of electronic configurations in the final state. EXAFS results show that the charge reservoir (CR) block of the Ba/Ca superlattice contains 0.6 oxygen vacancies per unit cell randomly distributed in the CuO 2 plane sandwiched between the BaO planes and that the two apical oxygens are not in the Ba plane but are shifted toward Cu by 0.21 Å. These oxygens have a pyramidal and distorted octahedral coordination with Cu at the Ba/Ca interface and in the (BaCuO 2) 2 block, respectively. The results clarify the origin of the oxygen doping mechanism in these artificially layered compounds.

Coherence effects above Tc in YBa 2Cu 4O 8

High temperature superconductors (HTSC) consist of CuO 2 layers which are separated by charge reservoirs (CuO-chains). They show strongly anisotropic physical parameters. Lower doped samples reveal metallic conductivity inside the layers but insulating temperature dependence for the out-of-plane direction. The resistivities of highly doped samples are metallic for all three directions. Evidence is presented that the temperature dependence for the resistivity perpendicular to the CuO 2 layers is related to the charge reservoirs in between the planes. The metallic out-of-plane resistivity might be traced back to a metallic conduction channel due to the charge reservoirs.

Luminescence spectra and crystal structure of high-temperature superconductors

A study is reported of the effect of temperature (4.2 K≤T≤150 K), the content of weakly bonded oxygen [6.1≤(7−δ)≤6.9], illumination [Eexc=3.4 eV, (Φt)max∼1.5 ×1020 photon cm−2, and substitution (Cu → Ag, Cr, Fe, Mn) on the luminescence spectra of the high-T c superconductor YBa2Cu3O7−δ. Only two bands with Elum∼2.4 and ∼2.8 eV were observed in the luminescence spectra in all these cases. A clearly pronounced correlation between the electronic and structural changes in YBa2Cu3O7−δ, caused by the influence of temperature, illumination, doping with oxygen or metal ions, and the spectral parameters (peak position Elum, width Δλ lum, and intensity Ilum of the luminescence bands), has been established. It is shown that luminescence spectra of HTSCs can be employed as a fairly reliable optical probe to study the electronic processes occurring in these substances, in particular, the electron (hole) transfer between the CuO2 plane and the CuO1−δ chain plane serving as a charge reservoir.

Theoretical study of activation of oxirane by bidentate acids

The reaction of oxirane with amines in the presence of some bidentate and monodentate Bronsted acids has been studied theoretically by taking some simplified reaction models. Acids have been shown to serve as temporary reservoirs of electronic charge during the reaction to strengthen the nucleophilicity of the substrate. The shift of electronic charges and enhancement of the nucleophilicity of oxirane have been represented by means of paired interaction orbitals and projected reactive orbitals. The reason the transition state appears at an earlier stage in the reaction catalyzed by 1,8-biphenylenediol relative to the reactions catalyzed by monodentate phenols and alcohols has been clarified.