Showing papers in "Physica C-superconductivity and Its Applications in 1997"
TL;DR: In this paper, a quasi-two-dimensional organic system, (ET)2X, has been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation.
Abstract: A family of quasi-two-dimensional organic systems, (ET)2X, have been studied by NMR, susceptibility and specific heat measurements in the light of role of electron correlation A systematic variation of antiferromagnetic spin fluctuations toward metal(superconductor)-to-insulator transition and the ordered spin structure in the insulating phase are uncovered Based on the results, we propose a conceptual phase diagram for (ET)2X, where the λ-type of family span a critical region of Mott transition The superconductivity, which is enhanced near the Mott transition, is suggested to have line nodes in gap parameter by NMR relaxation and specific heat profiles
226 citations
TL;DR: In this article, two buffer layer architectures were developed on rolled-Ni substrates using an electron beam evaporation technique, one consisting of an epitaxial laminate of CeO 2 /Pd/Ni and the other consisting of a YSZ/CeO 2/Ni.
Abstract: This paper describes the development of two buffer layer architectures on rolled-Ni substrates using an electron beam evaporation technique. The first buffer layer architecture consists of an epitaxial laminate of CeO 2 /Pd/Ni. The second alternative buffer layer consistes of an epitaxial laminate of YSZ/CeO 2 /Ni. The cube (100) texture in the Ni was produced by cold-rolling followed by recrystallization. The CeO 2 films were grown epitaxially on both Pd-buffered and textured-Ni substrates. The YSZ films were grown epitaxially on CeO 2 -buffered Ni substrates. The crystallographic orientation of the Pd, CeO 2 , and YSZ films were all (100). We also studied the effect of CeO 2 layer thickness and crack formation on textured-Ni substrates. The layer thickness was found to be critical. For some thickness, cracks formed in the CeO 2 layer. The presence of YSZ layers on the CeO 2 layers seem alleviate the cracks that are formed underneath. Our SEM studies showed that both CeO 2 (3–10 nm thick underlayer) and YSZ layers were smooth and continuous.
190 citations
TL;DR: In this article, the authors reported that La 1.85 Sr 0.15 CuO 4 thin films on LaSrAlO 4 substrates exhibit T c (R = 0) as high as 44 K, which is significantly higher than those of bulk samples of the same material under ambient pressure.
Abstract: We report that (001) La 1.85 Sr 0.15 CuO 4 thin films on LaSrAlO 4 substrates exhibit T c ( R = 0) as high as 44 K, which is significantly higher than those of bulk samples of the same material under ambient pressure. The increase in T c seems to be due to the plane stress produced by the lattice mismatch between the films and the substrates, which compresses the a -axis of the films, but expands the c -axis. By comparing our results with the previous pressure-effect studies, we suggest that not only the compression of the a -axis but also the expansion of the c -axis may be important in increasing T c .
166 citations
TL;DR: In this paper, sputter deposition of biaxially oriented buffer-layers on textured Ni tapes is described, and the effects of Ni surface smoothness on buffer-layer texture are also investigated.
Abstract: Techniques are reported for sputter deposition of biaxially oriented buffer-layers on textured Ni tapes. These buffered tapes can be employed as long, flexible, or large area substrates for biaxially-aligned high-temperature superconductors (HTS) with high critical current density Jc. Using deposition techniques at temperatures as low as 25°C, epitaxial Pd or Pt films were first deposited as a base layer on the textured Ni tapes, followed by deposition of biaxially oriented Ag or CeO2 buffer layers. Using Ar/4%H2 sputter gas, biaxially oriented CeO2 films were also grown directly on the textured Ni tapes, followed by the epitaxial growth of YSZ films. All the films show both strong in-plane and out-of-plane orientations. The effects of Ni surface smoothness on buffer-layer texture were also investigated.
132 citations
TL;DR: Close to the border of antiferromagnetic order at high pressure, the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures as mentioned in this paper.
Abstract: Close to the border of antiferromagnetic order at high pressure the cubic stoichiometric compound CeIn 3 displays an unconventional normal-state resistivity followed by superconductivity at low temperatures. The superconducting transition temperature varies rapidly with pressure near the critical pressure where the Neel temperature collapses towards absolute zero.
131 citations
TL;DR: In this article, the universal correlations between Tc and n s /m ∗ (superconducting carrier density/effective mass) and the pseudo-gap behavior in the underdoped region were analyzed.
Abstract: Combining (1) the universal correlations between Tc and n s /m ∗ (superconducting carrier density/effective mass) and (2) the pseudo-gap behavior in the underdoped region, we obtain a picture to describe superconductivity in cuprate systems in evolution from Bose-Einstein to BCS condensation. Overdoped and Zn-substituted cuprate systems show signatures of reduced superfluid density in a microscopic phase separation. Scaling of Tc to the superfluid volume density ns in all these cases indicate importance of Bose condensation.
126 citations
TL;DR: In this article, the superconducting gap of Bi2Sr2CaCu2O8+σ has been studied for 0.1 ⪅p⪅ 0.21 by STM/STS.
Abstract: Doping effects on the superconducting gap of Bi2Sr2CaCu2O8+σ have been studied for 0.1 ⪅p⪅ 0.21 by STM/STS. The gap is consistent with dx2−y2 symmetry in the p-range examined. The gap magnitude 2 Δ0 increases with decreasing p even below p0 ∼ 0.17, at which Tc starts to decrease, and seems to correlate with the development of an antiferromagnetic correlation. The in-plane resistivity is reduced just like a superconducting precursor below T ∗ , at which pseudogaps open in the electronic and magnetic energy spectra. Furthermore, the reduced gap 2 Δ 0 /K B T ∗ is nearly constant for p ⪅ 0.21, 4 ∼ 5, comparable to the mean field value 2 Δ0/kBTc0 ∼ 4.3 for d-wave pairing states. These results suggest convincingly that incoherent pairs are preformed well above Tc at low doping levels where the pairing interaction is extremely strong but Tc is largely reduced.
99 citations
TL;DR: In this paper, the magnetic field and current distributions inside superconducting strips arranged in a z-stack and an x-array are calculated analytically using the transformation method proposed by Mawatari for the case where a transport current is passed through the strips.
Abstract: The magnetic field and current distributions inside superconducting strips arranged in a z-stack and an x-array are calculated analytically using the transformation method proposed by Mawatari for the case where a transport current is passed through the strips. The magnetic field distributions are used to calculate the self-field ac hysteresis losses of both the z-stack and the x-array. The self-field ac hysteresis loss of the z-stack is always larger than the loss of the x-array. q 1997 Elsevier Science B.V.
96 citations
TL;DR: In this paper, Y 1 Ba 2 Cu 3 O x films, ranging in thickness from 0.9 to 10 μm, were grown on MgO(100) substrates by liquid phase epitaxy.
Abstract: Y 1 Ba 2 Cu 3 O x films, ranging in thickness from 0.9 to 10 μm, were grown on MgO(100) substrates by liquid phase epitaxy. These films were characterized structurally and electrically. From θ−2σ and ƒ scan X-ray diffraction, and Rutherford backscattering spectrometry measurements, it was found that the films were c -axis oriented and had good in-plane alignment, showing that good epitaxial growth was achieved. The T c,end values in a 20 mm long and 15 mm wide Y 1 Ba 2 Cu 3 O x film exceeded 90 K. The T c and resistivity values did not become degraded after patterning into a strip. Although J c values decreased with increasing film thickness, even a 7 μm-thick film showed a J c value of 9.3 × 10 5 A/cm 2 at 77 K. These data are comparable to those reported for Y 1 Ba 2 Cu 3 O x films prepared on CeO 2 /YSZ(100) by pulse laser deposition, and thought to be due to the c -axis orientation and good in-plane alignment of the liquid phase epitaxially grown thick films.
93 citations
TL;DR: In this paper, the growth of (La,Sr) 2 CuO 4 and (Nd,Ce) 2CuO 4 thin films by reactive coevaporation is reviewed.
Abstract: The growth of (La,Sr) 2 CuO 4 and (Nd,Ce) 2 CuO 4 thin films by reactive coevaporation is reviewed. By (1) careful adjustment of the cation stoichiometry, (2) optimum oxidation, and (3) appropriate choice of substrates, we have successfully grown c -axis and non- c -axis oriented high-quality thin films of (La,Sr) 2 CuO 4 and (Nd,Ce) 2 CuO 4 . The key points for thin film growth and the characterization of grown films are discussed.
92 citations
TL;DR: Theoretical expressions for the induced DC electric field, E, are derived as a function of the applied current density, J, near the glass-liquid transition temperature of T = T GL as E(J) = E (J)[1 + ( δE(j) E (j) )], where E(E) is the scalable part and E(j), is the unscalable part given by a power series of |T GL − T | nv ( z + 2 − D ) wi as discussed by the authors.
Abstract: Theoretical expressions for the induced DC electric field, E , are derived as a function of the applied current density, J , near the glass-liquid transition temperature of T = T GL as E(J) = E (J)[1 + ( δE(J) E (J) )] , where E (J) is the scalable part and δE(j) E (j) is the unscalable part given by a power series of | T GL − T | nv ( z + 2 − D ) wi Then the condition of δE E ⪡1 gives a measure of the width of the critical regime, in which E = E ( J ) is well scalable. The general scaling characteristics of E= E (J) are the same as those predicted by Fisher and coworkers based on the conventional theories of phase transitions with the scaling hypothesis, while the present theory provides a concrete expression for E (J) . Furthermore, the expressions for the scaled master curves of the AC impedance are derived. The reason, why the observed E vs. J characteristics are scalable over wider ranges of the temperature and flux density than the critical regime, is also discussed.
TL;DR: In this paper, high quality Bi2Sr2CaCu2O8+δ single crystals have been grown successfully by the traveling solvent floationg zone (TSFZ) method with the necking technique.
Abstract: High-quality Bi2Sr2CaCu2O8+δ single crystals have been grown successfully by the traveling solvent floationg zone (TSFZ) method with the necking technique. The crystallinity was characterized by a double-crystal X-ray diffractometry. The full-width at half maximum (FWHM) of the rocking curve was improved from 120 arcsec to 75 arcsec by the necking technique using a single crystal seed.
TL;DR: In this paper, the substrate is held fixed and the oxygen pocket is set in linear reciprocation, which allows simultaneous deposition on a square of 20×20 cm2, and stable rate control by atomic absorption spectroscopy (AAS).
Abstract: In the last decade we have developed thermal reactive co-evaporation as a technique to produce high quality YBCO and other oxide films of very large size up to 9 inches in diameter. This was achieved by intermittent deposition and reaction with oxygen using a heater which rotates the substrate in and out of an oxygen pocket. Even larger substrates, e. g. coated conductors, cannot be rotated. Therefore we have recently developed a new setup where the substrate is held fixed, and the oxygen pocket is set in linear reciprocation. This technique allows simultaneous deposition on a square of 20×20 cm2. Moreover, we have developed an instant refill mechanism for the thermal boats, and stable rate control by atomic absorption spectroscopy (AAS), in order to obtain a continuous process suitable for small scale mass production.
TL;DR: Hardness and fracture toughness of VGF melt-textured Y1Ba2Cu3O7−x are investigated as well as the influence of thermal cycling on these properties as mentioned in this paper.
Abstract: Hardness and fracture toughness of VGF melt-textured Y1Ba2Cu3O7−x are investigated as well as the influence of thermal cycling on these properties. The hardness is found to be load dependent. Introducing a true hardness, Ho, concept, leads to a load independent isotropic hardness, Ho = 5.5–6.5 GPa. The fracture toughness, KIC is isotropic and amounts to 1.5–2.0 MPaVm. The absence of any anisotropy in KIC can be ascribed to the homogeneous distribution of fine Y2Ba1Cu1O5 inclusions. Thermal cycling changes the measured hardness but does not affect the true hardness. After thermal cycling the fracture toughness drops substantially and the threshold for fracture decreases.
TL;DR: In this paper, the guiding principles to reduce superconducting-anisotropy were derived for high-T c cuprate superconductors, and based on the principles, they have attained the least SE γ = 1.6 for a Cu-based CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) superconductor.
Abstract: The guiding principles to reduce superconducting-anisotropy were derived for high-T c cuprate superconductors. Based on the principles, we have attained the least superconducting-anisotropy γ=1.6 for a Cu-based CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) superconductor. Further lower superconducting-anisotropy has been proposed for Cu-1245 (γ=1.3) and Cu-1256 (γ≌1.0) Cu-based superconductors.
TL;DR: In this paper, the current-voltage characteristics are measured for two Bi-2223 tape specimens with different critical current densities at various temperatures under the magnetic field parallel to the c-axis.
Abstract: The current-voltage characteristics are measured for two Bi-2223 tape specimens with different critical current densities at various temperatures under the magnetic field parallel to the c -axis. It is found that the current-voltage curves are scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical indices increased, and the static critical indices decreased appreciably with increasing magnetic field. These experimental results are compared with the numerical calculation based on the flux creep and flow model, taking the distribution of the flux pinning strength into account. Agreement is obtained between the experimental and theoretical results not only on the scaling curve but also on the critical indices and the transition line. The static critical index showing a divergence of the correlation length near the transition temperature can also be explained from the temperature dependence of the pinning correlation length. These results suggest that the scaling behavior is not correlated to the phase transition of fluxoids assumed in the vortex glass-liquid transition theory but seems to be correlated to a kind of transition governed by the flux pinning.
TL;DR: In this paper, Pb-doped Bi2212 single crystals with carefully controlled oxygen compositions were prepared and their magnetization curves were studied as a function of carrier doping level from heavily overdoped (Tc = 67 K) to near optimally doped (tc = 96 K) states.
Abstract: Heavily Pb-doped Bi2212 single crystals with carefully controlled oxygen compositions were prepared and their magnetization curves were studied as a function of carrier doping level from heavily overdoped (Tc = 67 K) to near optimally-doped (Tc = 96 K) states. All the Pb-doped samples showed much improved Jc-H properties compared to the conventional Pb-free Bi2212 crystals. In particular, the moderately overdoped crystals maintained high Jc and Hirr up to high temperatures and these values surpassed those for the Pb-ion irradiated Bi2212 with an optimized fluence. Dramatically improved flux pinning properties in the present material are possibly due to generation of temperature and field-induced new pinning centers, as suggested by anomalous dependence of Jc both on temperature and magnetic field.
TL;DR: In this article, the authors have succeeded in fabricating Nd-Ba-Cu-O bulk superconductors, which exhibit high T c and a sharp superconducting transition, by the melt process in air without additional high temperature annealing in a reduced oxygen atmosphere.
Abstract: We have succeeded in fabricating Nd-Ba-Cu-O bulk superconductors, which exhibit high T c and a sharp superconducting transition, by the melt process in air without additional high temperature annealing in a reduced oxygen atmosphere The key to this achievement is the use of Ba-rich Nd 4−2 z Ba 2+2 z Cu 2− z O 10− δ as a precursor Fabricated bulk samples exhibit onset T c values of 94–95 K and the secondary peak effect in the M-H loop like the case of Nd-Ba-Cu-O superconductors produced by melt processing in a reduced oxygen atmosphere The peak position and the irreversibility field for the best sample were about 12 T and 5 T at 773 K, respectively, and the estimated J c at the peak field was about 23 × 10 4 A cm −2
TL;DR: In this article, the authors determined the δ-dependences of the condensation energy U(0) and the superconducting and normal state energy gaps Δ(0), and Γ.
Abstract: From the electronic specific heat of Y0.8Ca0.2Ba2Cu3O7−δ we determine the δ-dependences of the condensation energy U(0) and the superconducting and normal state energy gaps Δ(0) and Γ. The data supports d-symmetry for both gaps. Although Δ(0) increases with δ across the entire series, a rapid increase in Γ∼940(δ−0.2) (K) results in a peak in U(0) at δ∼0.25 and the suppression of U(0) and Tc in underdoped samples. We conclude that the normal state gap results from competing singlet correlations and not precursor superconducting pairing.
TL;DR: In this paper, the microstructures and superconducting properties of melt-processed (RE,RE′)Ba-consisting of Nd, Sm, Eu, Gd and Y are studied.
Abstract: We have studied the microstructures and superconducting properties of melt-processed (RE,RE′)BaCuO composite bulks (RE and RE′ are two different rare earth elements selected from a group of Nd, Sm, Eu, Gd and Y). It was found that the peritectic decomposition temperatures ( T m ) of the (RE,RE′)BaCuO composite increased linearly with increasing the average ionic radius between two different rare earth elements. Large grain growth was observed in almost all the (RE,RE′)BaCuO bulks except those containing Y, when they were melt-processed in a reduced oxygen atmosphere. Compositional analyses revealed that the ratio of RE to RE′ in the (RE,RE′)Ba 2 Cu 3 O 7− d matrix phase was almost the same as that of the nominal composition, showing that the mixture of the two different rare earth elements was very uniform. All the samples showed onset superconducting transition temperatures ( T c ) exceeding 93 K with a sharp transition, and exhibited a secondary peak effect in the M - H loops.
TL;DR: The phase evolution, critical current density (J c ), and irreversibility field (H irr ) for (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (2223) tapes have been studied through a two-step sintering process.
Abstract: The phase evolution, critical current density ( J c ) and irreversibility field ( H irr ) for (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (2223) tapes have been studied through a two-step sintering process. The tapes sintered at 840°C without second-step annealing at 820°C contain a small fraction of (Bi,Pb) 2 Sr 2 CuO 6 (2201). 2201 phase vanishes when the tapes are annealed at 820°C for 30 h after sintering at 840°C. (Bi,Pb) 2 Sr 2 CuO 6 (2201) phase was found to be the predominant factor for the depression of J c of the tapes. The J c vs. magnetic field shows that the depression of J c occurs only in the low field region, indicating that 2201 phase act as weak links in the tapes.
TL;DR: In this paper, superconducting (001) thin films of La 2 CuO 4+δ were synthesized by cooling in ozone gas, demonstrating that this simple process produces a strong oxidizing environment.
Abstract: Superconducting (001) thin films of La 2 CuO 4+δ were synthesized by cooling in ozone gas, demonstrating that this simple process produces a strong oxidizing environment. The films cooled in ozone gas until 150°C exhibit the superconducting onset temperature of 52 K and the zero-resistance temperature of 48 K on LaSrAlO 4 substrates. The room-temperature resistivity is as low as 500 μΩ cm for these films, indicating that they were homogeneously oxidized. The superconducting transition temperatures for the films on LaSrAlO 4 substrates are higher than those for the films on SrTiO 3 substrates, suggesting a strain effect caused by lattice mismatch with the substrates, which was investigated by X-ray diffraction. Surface analysis by reflection high energy electron diffraction and photo-emission spectroscopy indicate that degraded layers were formed on the surfaces of the films during the oxidation process. The thickness of these degraded layers seems to be much less than 50 A because they do not seriously affect the resistivity of the films with thickness of 50 A.
TL;DR: In this article, the effect of eddy current dampers on the vibrational properties in superconducting levitation has been investigated using melt-processed YBaCuO bulk superconductors.
Abstract: The effect of eddy current dampers on the vibrational properties in superconducting levitation has been investigated using melt-processed YBaCuO bulk superconductors. In vertical vibration, the damping was about 100 times improved by inserting eddy current dampers into the gap between a superconductor and a magnet. On the other hand, in horizontal vibration, eddy current dampers did not greatly affect the vibrationaal properties of superconducting levitation up to a given thickness of eddy current dampers.
TL;DR: In this article, a phase diagram of the normal state of cuprate superconductors is presented, which yields qualitative and quantitative agreement with the measured changes in the quasiparticle, transport, magnetotransport, and optical properties.
Abstract: I review progress in measurements of the dynamic spin susceptibility in the normal state which yield a new phase diagram and discuss microscopic calculations which yield qualitative, and in many cases, quantitative agreement with the measured changes in the quasiparticle, transport, magnetotransport, and optical properties of the cuprate superconductors as one varies doping and temperature provided one describes the systems as nearly anti-ferromagnetic Fermi liquids in which the effective magnetic interaction between planar quasiparticles mirrors the dynamic spin susceptibility measured in NMR and INS experiments. Together with the demonstration that the NAFL pairing potential leads inexorably to a d x 2 −y 2 , pairing state, this work provides a “proof of concept” for the NAFL description of high T c materials. I review Eliashberg calculations of the mean-field behavior found in overdoped systems and discuss the extent to which the crossovers to pseudoscaling and pseudogap behavior found in the effective magnetic interaction and quasiparticle behavior in the optimally doped and underdoped systems may be derived microscopically. I conclude with a tentative scenario for the dependence of T c on doping level and imperfections in different systems.
TL;DR: In this paper, the properties of Josephson junction including a ferromagnetic layer as the insulator are studied theoretically and the Josephson current formula is expressed by using the energy levels of bound states located near the interface.
Abstract: The properties of Josephson junction including a ferromagnetic layer as the insulator are studied theoretically The Josephson current formula is expressed by using the energy levels of bound states located near the interface The calculated current shows the anomalous dependence on the magnitude of the exchange interaction at the insulator As the interaction strength is enhanced, the Josephson junction changes from 0-junction to π-junction
TL;DR: In this article, the suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically.
Abstract: The suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically. Complete disappearance of the hysteresis of the magnetization curve in YBCO melt-textured samples was observed. Nonmonotonous behavior of M with the increase of the ac magnetic field amplitude was found. The dynamics of the collapse of the magnetic moment was studied in the pulse regime of the transverse field. The experimental results indicate homogeneity of the static magnetic induction in the sample regions where the ac field penetrates. The data obtained are interpreted within the framework of a two-liquid hydrodynamic model.
TL;DR: In this paper, the eddy current AC power loss in the metal superconductor of a metal-superconductor strip, such as the hastelloy-Y123 flexible tape, is calculated analytically in the low frequency limit.
Abstract: Eddy current AC power loss in the metal and hysteresis loss in the superconductor of a metal-superconductor strip, such as the hastelloy-Y123 flexible tape, are calculated analytically in the low frequency limit. Cases studied are (i) where an AC magnetic field is applied perpendicular to the tape, (ii) where an AC field is applied parallel to the tape and (iii) where an AC current is applied to the tape. The losses are smallest in case (ii) while the losses in case (i) and (iii) show an approximate scaling. In cases (i) and (iii) the eddy current loss becomes important in high AC fields or at high AC currents, respectively, while in case (ii) the eddy current loss becomes important at high fields and at low fields, but less important at moderately strong fields. Because of the large resistivity of the hastelloy, the eddy current loss of a typical hastelloy-Y123 flexible tape can be neglected at frequencies below 100 Hz for all practical AC applications and the hysteresis loss in the superconductor is the only loss.
TL;DR: In this paper, the relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated.
Abstract: The relationship between the solubility parameter, x, in the solid solutions Nd 1+x Ba 2−x Cu 3 O 6.5+x 2+δ (Nd123SS) and XRD patterns, powder surface area (SA), particle size, morphology and melting points was investigated. An efficient way to determine the value of x and residual BaCuO2 content during Nd123SS powder synthesis is presented. The method is based on calculation of the orthorhombic splitting (OS) factor from the unit cell parameters obtained from XRD data. The final phase in the Nd123SS system is formed through a diffusion controlled reaction between BaCuO2 and Nd123SS and a kinetic model is developed to describe the formation of the Nd123 (x=0) superconducting powder. Finally, high values of Tc (measured by DC magnetization) were found in the powders. The highest Tc of 98.7 K was measured for the x=0 case. This value is the highest ever reported for the NdBaCuO system. The Tc was found to be insensitive to the value of x, ranging between 98.7 and 94 K for x between 0 and 0.25, respectively. This data is contrast to published results on samples synthesized at higher temperatures, where Tc fell to 40 K at x = 0.25.
TL;DR: In this article, it was shown that a single crystal of 3 μm thickness should behave like a stack of 2000 Josephson junctions, i.e. every pair of superconducting and non-superconducting bilayers forms a Josephson junction.
Abstract: The large anisotropy and the extremely short coherence lengths of the high-Tc superconductors suggest that the layered crystal structure is mapped onto a periodic modulation of the superconducting order parameter. Even an ideal single crystal should consist of a stacked series of superconducting and non-superconducting layers. Three-dimensional phase coherence is provided by Josephson currents between the layers. As the typical interlayer distance is approximately 15 A, a single crystal of 3 μm thickness should behave like a stack of 2000 Josephson junctions. This hypothesis is proved in every detail by measurements of the DC as well as the AC Josephson effects on single crystals of Bi2Sr2CaCu2O8, (Bi1−y Pby)2Sr2CaCu2O8, Tl2Ba2Ca2Cu3O10 and Pr2−x CexCuO4. Microwave emission experiments at frequencies between 3.5 and 95 GHz reveal explicitly the number of junctions in the samples. This number is given by the crystal thickness divided by 15 A, i.e. every pair of CuO2 bilayers forms a Josephson junction.
TL;DR: In this paper, a microscopic model of the evolution from antiferromagnetic insulator to superconductor on oxidation of the parent-phase (CuO 2 ) 2− sheets of a cuprate superconductuctor is presented.
Abstract: A microscopic model of the evolution from antiferromagnetic insulator to superconductor on oxidation of the parent-phase (CuO 2 ) 2− sheets of a cuprate superconductor starts with the assumption that strong electron-lattice interactions are dominant and give a heterogeneous electronic distribution. Introduction of pseudo-Jahn-Teller vibronic coupling associated with the δ holes in the (CuO 2 ) (2−δ) − sheets is shown to stabilize, below a critical temperature T p ≈ 850 K, large non-adiabatic polarons containing 5 to 7 copper centers; cooperative low-symmetry in-plane vibrations also stabilize an elastic attractive force between polarons that can overcome the longer-range Coulomb repulsion between polarons. Utilizing established parameters for isolated CuO 6 complexes gives a calculated polaron size of 5 to 7 copper centers, which compares with a measured mean size of 5.3 copper centers in underdoped samples 0 δ ≤ 0.10. A large polaron is shown to move by a piece-wise tunneling of a fraction of itself across a peripheral CuO vibronic bond rather than by an activated hopping. This type of motion, which is not described by conventional transport theories, gives a linear increase of the resistivity with temperature above a temperature T ϱ due to scattering of the polaron at its own border, which separates regions inside and outside the polaron of slightly different mean CuO bond length. At lower temperatures, the polaron mobility becomes activated, but at higher concentrations this change is obscured because the elastic interpolaron attractive force causes the polarons to condense into a “polaron liquid,” and below some critical temperature T d ≥ T c the polarons undergo long-range ordering into one-dimensional 〈110〉 polaronic stripes separated by stripes of the parent phase, which support antiferromagnetic spin fluctuations. The zig-zag polaron stripes consist of polaron pairs oriented alternately along [100] and [010] axes of a CuO 2 sheet. Formation of the ordered superstructure permits conduction of hole pairs without scattering from lattice vibrations provided there is also coupling in the third dimension between CuO 2 sheets. The vibronic coupling introduces an anisotropic dispersion curve and superconductive gap for the states within the polarons, which lie within the energy gap between residual Hubbard-band states of the parent phase. The magnitude of the superconductive gap is determined by the elastic forces responsible for ordering the superstructure rather than by the energy of coupling of superconductive pairs.