Layered Cuprate Superconductors Research Articles

Collective Josephson vortex dynamics and Josephson plasma excitation in layered cuprate superconductors

We perform numerical simulations for the coupled sine-Gordon equation in order to investigate collective Josephson vortex dynamics under the presence of the c-axis parallel transport current and the ab-plane parallel magnetic field. The simulations reveal that Josephson vortices change their own flowing lattice structures with increasing the transport current. When the vortex density is high enough the structural transition voltages almost coincide with resonant points with transverse Josephson plasma modes. On the other hand, the coincidences fail with decreasing the vortex density and chaotic and more complicated flow patterns appear in low current regions. In both high and low vortex densities, we confirm that useful rectangular lattice flow states exciting the in-phase electromagnetic field oscillations can appear in high current regions.

Transport properties of separated pairs ofCuO2planes in(Pb2Cu)Sr2DyxCen−x−δCu2O2n+6(n=3,5)epitaxial films

The in-plane resistivity and thermopower for $\mathrm{Pb}\ensuremath{-}32n2$ phase, ${(\mathrm{P}\mathrm{b}}_{2}{\mathrm{C}\mathrm{u})\mathrm{S}\mathrm{r}}_{2}{\mathrm{Ln}}_{x}{\mathrm{Ce}}_{n\ensuremath{-}x\ensuremath{-}\ensuremath{\delta}}{\mathrm{Cu}}_{2}{\mathrm{O}}_{2n+6}$ $(\mathrm{L}\mathrm{n}=\mathrm{D}\mathrm{y}$ or Eu, $n=3$ or 5), have been studied. The distance, d, between a pair of ${\mathrm{CuO}}_{2}$ planes can be arbitrarily chosen in the $\mathrm{Pb}\ensuremath{-}32n2$ family. So, one can expect the control of electronic anisotropy using this family. The samples were grown by sequential deposition using the molecular beam epitaxy technique. The hole concentration per [CuO], p, was estimated from the thermopower at room temperature. The highest value of p for the Pb-3252 film sample is 0.089, which is within the range for the occurrence of superconductivity in layered cuprates, although resistivity of the Pb-3252 sample shows weak localization behavior. The increase in hole concentration by the substitution of ${\mathrm{Dy}}^{3+}$ for ${\mathrm{Ce}}^{4+}$ is lower than expected from the simple calculation of the valence. We discuss possible reasons for these findings and for the absence of superconductivity. The cation deficiency in the fluorite block is supposed to be one of the reasons for the low efficiency of carrier doping and localization behavior. Sheet conductance per ${\mathrm{CuO}}_{2}$ plane is similar for the Pb-3232 phase $(d=0.87\mathrm{nm})$ and Pb-3252 phase $(d=1.41\mathrm{nm}).$

Role of interlayer coupling in the superconducting state of layered cuprate superconductors

The role of interlayer coupling in the superconducting state of high- T c layered cuprate systems has been studied using one-band two-layer tight binding Hamiltonian with intra- and interlayer attractive interactions. The expressions of charge carrier (hole) density and momentum dependent energy-gap function are derived on the basis of BCS-type pairing in narrow band within Hubbard-III like decoupling approximations. The hole density and generalized gap-parameter are found to be dependent on the interlayer interactions. It is found that for a two layer per unit cell cuprate systems there is a contribution in the hole density due to interlayer interactions and numerical analysis shows that change in the holes between the cuprate planes depends on the magnitude of interlayer hopping element and superconducting transition temperature.

Structure and Properties of 1256 and 1267 Type Hg1−xRexBa2Can−1CunO2n+2+4x+δSingle Crystals

Single crystals of Hg0.75Re0.22Ba2Ca5Cu6O15and of Hg1−xRexBa2Ca6Cu7O16+4x+δhave been grown with a gas-phase high pressure technique at 10 kbar. They crystallize in space groupP4/mmmwith lattice parametersa=3.8542(3) Å,c=25.162(3) Å (HgRe-1256) anda=3.8514(5) Å,c=28.388(7) Å (HgRe-1267). They represent then=6, 7 members of the layered cuprate superconductors of the type HgBa2Can−1CunO2n+2+δ. X-ray single crystal structure analysis showed that Re doping decreases the size of the rock-salt block. Rhenium substitutes mercury up to 25% and forms short bonds of 1.86 Å to oxygen atoms. They coordinate Re octahedrally and are shifted off their ideal positions to (0.34, 0.34, 0). The crystals contain stacking faults which are included in the refinements. In the HgRe-1267 crystal they are so numerous that they influence the refinement severely. A simulation of the Fourier maps proved that observable maxima in the difference electron density map are in fact due to stacking faults. The transition temperature onset of HgRe-1256 is 100 K and that of HgRe-1267 is assumed to be 84 K.

Dependence of the superconducting transition temperature on the type and number ofCuO2layers inTl2Ba2Can−1CunO2n+4−y

The ${\mathrm{CuO}}_{2}$ layer dependence of the superconducting transition temperature ${T}_{c}$ at ambient pressure on the intrinsic transition temperature ${T}_{c}(i)$ of the type-I ${\mathrm{CuO}}_{2}$ plane in which the copper atom has fivefold pyramid coordination of oxygen and the fourfold square coordinated type-II plane is studied in terms of the generalized Lawrence-Doniach theory. Calculations show that the increase of ${T}_{c}$ with the number of ${\mathrm{CuO}}_{2}$ layers benefits from the difference of the intrinsic ${T}_{c}(i)$ of the two types of ${\mathrm{CuO}}_{2}$ layers and that interlayer coupling between the neighboring ${\mathrm{CuO}}_{2}$ layers can enhance ${T}_{c}$ for the multilayer cuprates. The upper limit of ${T}_{c}$ is predicted to be 146 K for the bilayer thallium-based series. We present an extended pressure-induced charge transfer model for layered cuprate superconductors, assuming that the charge distribution among the crystallographically inequivalent ${\mathrm{CuO}}_{2}$ layers is nonhomogeneous, which enables us to investigate the pressure effect on the intrinsic ${T}_{c}(i).$ The intrinsic ${T}_{c}(i)$ of the two types of ${\mathrm{CuO}}_{2}$ layers is predicted to behave with pressure in a paraboliclike manner. For the optimally doped single, double, and triple ${\mathrm{CuO}}_{2}$ sheets compounds, the saturation values of ${T}_{c}(I)$ of the type-I ${\mathrm{CuO}}_{2}$ plane of 91.1, 119.6, and 133.9 K are obtained when $P=2.3,$ 2.9, and 6.0 GPa, respectively. For the underdoped Tl-2234 compound with ${T}_{c}=113$ K, the calculated ${T}_{c}(I)$ of 118.5 K is obtained at $P=6.0$ GPa. Under the application of pressure, the intrinsic ${T}_{c}(\mathrm{II})$ of the type-II ${\mathrm{CuO}}_{2}$ plane in Tl-2234 increases strongly compared with a modest increase of ${T}_{c}(\mathrm{II})$ in Tl-2223, possibly resulting from its underdoped nature. We suggest that at low pressure the ${T}_{c}$ is the intrinsic ${T}_{c}(i)$ of the type-I ${\mathrm{CuO}}_{2}$ plane, and at relatively high pressures the intrinsic effect of the type-II plane dominates. Our theoretical results are in agreement with experiments.

C-Axis Optical Reflectivity of Layered Cuprate Superconductors

Using a conventional BCS -- Fermi liquid model we calculate the c-axis optical reflectivity of the layered high temperature cuprate superconductors by obtaining the finite temperature dynamical dielectric function in a microscopic self-consistent gauge invariant formalism. We get good semi-quantitative agreement with all the existing experimental data by using the measured normal state $dc$ resistivities as the input parameters in obtaining the c-axis hopping amplitude and the normal state level broadening in our microscopic calculation.

On the relation between anisotropic 3D-XY/Josephson-junction array model and layered high Tc cuprate superconductors

We modeled the ceramic high Tc materials as a quantum-capacitive Josephson-junction array, with weakly coupled superconducting grains on a 3D lattice and Angstrom-scale parameters. We studied that Josephson-junction coupling energy via Cooper pair tunneling within and between CuO2 layers enhances the critical temperature Tc for layered cuprate superconductors, while the charge coupling energy fo rthe capacitance between CuO2 layers depresses the critical temperature Tc. We argue the high Tc superconductive glass behavior from Monte Carlo simulation of highly anisotropic 3D-XY/JJA model with frustration (vector potential) and bonding disorder (probability).

Normal-State c-Axis Resistivity of the High-Tc Cuprate Superconductors

It is shown that a strong intraplanar incoherent scattering can effectively block the interplanar coherent tunneling between the weakly coupled planes of the highly anisotropic but clean (intrinsic) materials such as the optimally doped high-T c layered cuprate superconductors. The calculated normal-state c-axis resistivity ρ c (T) then follows the metal-like temperature dependence of the ab-plane resistivity ρab(T) at high temperatures. At low enough temperatures, however, ρ c (T) exhibits a non-metal like upturn even as ρab(T) remains metallic. Moreover, in the metallic regime, ρ c (T) is not limited by the maximum metallic resistivity of Mott–Ioffe–Regel. This correlation between the intrinsic ρ c (T) and ρab(T) is observed in the normal state of the high-T c stoichiometric cuprates.

Angular dependence of the in-plane energy gap of Bi2Sr2CaCu2O8 by tunneling spectroscopy.

A technique for tunneling into the a-b plane of the layered cuprate superconductors is described and applied to study the in-plane gap anisotropy of the high-${\mathit{T}}_{\mathit{c}}$ superconductor ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$. The results show unambiguously that the in-plane energy gap is highly anisotropic with a gap minimum along the Cu-${\mathrm{O}}_{2}$ bond direction. Using these results, we present an angular mapping of the in-plane gap anisotropy and compare our results to those from other experiments. Effects of the gap anisotropy on the tunneling spectra will also be discussed. \textcopyright{} 1996 The American Physical Society.

Critical and noncritical behavior of the Kosterlitz-Thouless-Berezinskii transition.

The standard treatment of the Kosterlitz-Thouless-Berezinskii (KTB) transition is supplemented here by the inclusion of free-vortex screening effects. We obtain the high-density corrections to the KTB critical behavior. The size of the KTB critical region is extracted, and is found in most cases to be quite small. The tricritical point, separating second-order (KTB) phase transitions from first-order transitions, is also determined. We make predictions concerning similar KTB-like transitions, such as in the layered cuprate superconductors.

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T c (10 ± 1 K for the organic metal (BEDT-TTF) 3Ta 2F 11 and 92 to 12 K for YBa 2Cu 3O 7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T c for the organic metal. Here the exchange field between the aligned AF domains: J AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ AF < 10 -10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ 1 for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ 1 = C H /α H , C H and α H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T c are correlated to electron-phonon interactions.

The intermediate phase during (Bi,Pb) 2Sr 2CaCu 2O 8+δ to (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+δ phase transformation

We report our observations on and identification of an intermediate phase, (Bi,Pb) 4Sr 4Ca 3Cu 5O 18+δ (denoted as Bi/4435), during the phase transformation of (BiPb) 2Sr 2CaCu 2O 8+δ (Bi/2212) to (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+δ (Bi/2223) in powder-in-tube processed tapes. The until cell of the phase consists of one half each of the Bi/2212 and the Bi/2223 cell, with lattice parameters of a ≈ b = 5.4 A ̊ and c = 33.9 A ̊ . The space group of the new phase was determined to be Pbmn using the extinction rule of the selected-area electron diffraction, as well as convergent beam electron diffraction and high-resolution electron microscopy. The presence of the Bi/4435 phase and the abundance of its local structure during the early stage of the transformation suggests that the Bi/2223 phase is formed by the rapod insertion of two extra Ca/CuO bi-layers, one at a time, into the two identical CuO/Ca/CuO building blocks of the Bi/2212 matrix. This finding may shed light on the mechanism of structural phase transformation in layered cuprate superconductors.

Electromagnetic phenomena generated by the excitation of low frequency plasma in layered cuprate super-conductors

The nature of the low frequency plasma in cuprate superconductors and the electromagnetic phenomena related to the plasma excitation are theoretically studied. The interaction of electromagnetic wave with the transverse part of the low frequency plasma causes a peculiar frequency and magnetic field dependence of the reflectivity and transmissivity for a film of cuprate superconductor. It is predicted that the low frequency plasma is excited by the ac Josephson effect.

Block-by-Block Deposition of Complex Oxide Films

Since the discovery of superconductivity in layered cuprates there has been a strong desire to grow thin films of these materials in a so-called layer-by-layer manner, where each layer represents one atomic monolayer. There has been worldwide interest in this problem and numerous groups have invested time and equipment in an attempt, first, to grow the known materials using a sequential deposition technique so that the atomic monolayers are stacked artificially on top of one another in a sequence defined by the structure of the given unit cell, and second, to search for new stacking sequences of such atomic monolayers, thus making metastable compounds.Understanding the growth of thin films using any deposition technique requires information about two fundamental processes, the nucleation of a crystal and its growth. Both are strongly affected by the three thermodynamic parameters, temperature, pressure, and chemical potential (composition), at the growth front. The third parameter, the surface composition from which the desired compound can nucleate and propagate, probably provides the greatest degree of freedom in the growth process. The compositional phase diagrams of most cuprates show that several phases can coexist with the superconducting phase at a certain temperature and pressure. This is also the main reason why a widely changing surface composition, such as that present during layer-by-layer or block-by-block deposition, still produces high-quality thin films.

Defects and Superconductivity in Layered Cuprates

After the discovery of superconductivity at 40 K in cuprates by Georg Bednorz and Alex Müller in 1986, many researchers became involved in the synthesis of new higher-temperature superconductors. I believe that many of them failed in this quest simply because they did not realize that the crystal chemistry of such materials is extremely complicated. By the following spring, of course, groups all over the world were fabricating new superconducting cuprates of yttrium, bismuth and thallium that allowed critical temperatures to exceed the all-important 77-K boiling point of liquid nitrogen. But because of the complex crystal chemistry involved, it will take a long time to optimize the superconducting properties of these cuprates for the development of useful superconducting wires. Thin films of superconducting cuprates, by contrast, are already quite close to being commercially available for SQUIDS and microwave applications.

Substitution of calcium for strontium in the SrCuO 2 structure : The solid solution Sr 1−xCa xCuO 2

The study of the substitution of calcium for strontium in SrCuO 2 is of capital importance for the understanding of superconductivity in layered cuprates. We describe here a large solid solution Sr 1−xCa xCuO 2 for 0≤x≤0.7, which is single phase to x=0.6. The parameters of the orthorhombic cell (a=3.576 to 3.401Å, b=16.335 to 16.033Å, c=3.895 to 3.857Å) are strongly affected by the Ca-Sr substitution. The HREM study of this phase shows lamellar character and the existence of disoriented domains formed through coherent boundaries.

Smearing of the Hebel-Slichter Peak by 2D Fluctuationsin Layered Cuprate Superconductors

The absence of the Hebel-Slichter peak in the observed temperature dependence of the nuclear spin relaxation rate, 1/ T 1 , is investigated by evaluating the effect of fluctuations in the superconducting order parameter on the local electronic susceptibility. The interactions between fluctuations have been taken into account in order to smear out the peak in the immediate vicinity of the critical temperature T c . It is found that, below T c , 1/ T 1 rapidly decreases in agreement with the experimental data.

A microscopic scenario for s-channel superconductivity in layered cuprates

Two anionic complexes within a cationic lattice frame are the common structural element in all layered cuprate superconductors. One is the extensively investigated current carrying CuO 2 layer. It is assumed that the second one creates states near the Fermi level, too, and it is probably quasi-one-dimentional in nature not only for the 1-2-3 and 1-2-4 Y-Ba-Cu phases, but in all hole-doped cuprates, a fact not sufficiently appreciated up to now. The low dimensionality of the doping complex in connection with a strong electron deformation potential at the bridging oxygen may lead to a bipolaronic resonance which is capable of producing s-channel superconductivity as proposed by Friedberg and Lee [1] phenomenologically. Consequences for the temperature dependence of thermodynamic properties as the gap and the upper critical field resulting from a closely related Peierls transition in the chain subsystem are briefly discussed.

Perspectives in high-temperature superconductivity

Based on the known layered cuprate superconductors, the pertinent electronic properties of high-Tc materials are reviewed both above and below their transition temperatures. Recent evidence of anharmonic oxygen dynamics is summarized.

Atomic resolution STM/STS on oxide superconductors down to 4.2 K

Atomic resolution STM/STS measurement on the cleaved basal plane and on the edge of it has been performed for the first time on the layered cuprate superconductors at low temperatures down to 4.2 K. The topographic image of the a-b plane taken at 4.2 K clearly showed the atomic corrugation structure along the b-axis. The STS observations both on the a-b plane and the cross sectional edge surface lead to the electronic scheme that the superconductivity appears only in CuO 2 layer, and that the BiO layer is essentially semiconducting even below T c .