Bi-Sr-Ca-Cu-O Research Articles

Thermodynamics and tunneling spectroscopy in the pseudogap regime of the boson–fermion model

Motivated by the STM experimental data on Bi 2Sr 2CaCu 2O 8+ x which indicate the tunneling conductance asymmetry σ(− V)≠ σ( V), we report that such a behavior can be explained in terms of the boson–fermion model. It has been shown in the recent studies, based on various selfconsistent techniques to capture the many-body effects, that the low energy spectrum of the boson–fermion model is featured by an appearance of the pseudogap at T ∗>T c . We argue that the pseudogap structure has to exhibit a particle–hole asymmetry. This asymmetry may eventually depend on the boson concentration.

Pseudogap state in overdoped Bi 2Sr 2CaCu 2O 8+ y

We use interlayer intrinsic tunneling resistivity to probe the pseudogap in Bi 2Sr 2CaCu 2O 8+ y in magnetic fields up to 60 T. A map of the field–temperature ( H– T) diagram of the pseudogap state reveals the pseudogap as a dominant feature up to very high levels of hole doping. The unconventionally large quantum fluctuation regime observed on the overdoped side is consistent with the pseudogap in the vortex cores. Unlike the characteristic fields of the superconducting state, the field that closes the pseudogap at low temperatures is temperature-independent. It is related to the pseudogap temperature T ★ through a simple Zeeman energy scale, pointing to the pseudogap controlled by the spin rather than the orbital degrees of freedom.

STM study of novel resonances in Bi 2Sr 2CaCu 2O 8+ δ

Low temperature scanning tunneling microscopy (STM) of various samples of the high temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ consistently reveals the presence of quasi-particle scattering resonances, similar both spectroscopically and spatially to those observed around Zn atoms in Zn-doped BSCCO. As the resonances appear at energies indicative of nearly unitary scattering ( ∼0.5 meV ) and are always accompanied by topographic depression of the surface Bi atom around which they are centered, we postulate that the source of scattering may be Cu vacancies in the CuO 2 plane. Such resonances should thus provide a simpler test case for theoretical models than those created by Zn or Ni substitution.

Electrical properties of superconducting Bi-2212 thick films

Bi 2Sr 2CaCu 2O 8+ x (Bi-2212) thick films were prepared by a partial melt technique on Al 2O 3 and Ag substrates. Well ground Bi-2212 powder with particle size of about 1 μm was used as starting material. The films have been prepared with melting temperatures T melt in the range from 860°C to 900°C. The critical temperature T c is determined from the temperature dependence of the film resistance, measured by a four-probe technique. The film structure and its critical current J c are significantly influenced by the melting temperature. The maximum J c≅10 3 A/cm 2 has been obtained on the sample prepared at T melt=880–885°C. The electrical properties of these materials are discussed by comparing the experimental data with results, calculated on the basis of the classical dirty limit model and the proximity effect model.

Extended-range short-pulse interlayer tunneling spectroscopy for Bi 2Sr 2CaCu 2O 8+ δ intrinsic Josephson junctions

The energy range for the short-pulse interlayer tunneling spectroscopy for Bi 2Sr 2CaCu 2O 8+ δ has been extended from less than 100 meV per single junction to greater than 200 meV by adopting composite current pulses. This improvement eliminates the breakage of specimens during the measurements almost completely, leading to the observation of a likely doping-dependent dip-and-hump structure in the tunneling spectrum of Bi 2Sr 2CaCu 2O 8+ δ .

Energy dissipation in Bi 2Sr 2CaCu 2O 8+ δ single crystal

The in-plane resistivity of Bi 2Sr 2CaCu 2O 8+ δ single crystal in different magnetic fields was measured for magnetic field H parallel and perpendicular to the CuO 2 plane. The irreversibility line H irr ⊥ c ( T) for H parallel to the CuO 2 plane locates in higher field and higher temperature region in the H– T phase diagram than H irr ∥ c ( T) for H perpendicular to the CuO 2 plane. H irr ∥ c ( T) is dominated by surface barrier pinning, while H irr ⊥ c ( T) shows different temperature dependencies above and below 2 T, which may indicate different energy dissipation above and below the field. The field dependence of activation energy above 2 T in H⊥ c configuration could be explained by vortex–antivortex expansion in CuO 2 plane.

Structure and performance of Bi-2212 hollow cylinders prepared by a new diffusion process

Bi 2Sr 2CaCu 2O 8+ X (Bi-2212) high- T c superconductors (HTS) bulk conductors have been prepared by a new diffusion process for current lead application. The Bi-2212 HTS diffusion layer is synthesized through the reaction between a Sr–Ca–Cu oxide substrate and a Bi–Cu oxide coating with Ag addition. The HTS diffusion layers about 150 μm in thickness are formed around both outside and inside of the hollow cylinders. The Ag added to the coating enhances the diffusion reaction, and precipitates on the surface of the specimen decreasing the contact resistance between the HTS ceramic layer and the current terminal. The Bi-2212 HTS layer consists of thin plate-like grains grown in the diffusion direction. They are mainly composed of grains with a-axis orientation. The transport current is expected to pass longitudinally through the cylinders on the ab planes. The transport current of the hollow cylinder 27/19 mm in outside/inside diameter and 200 mm in length reaches 6000 A at 4.2 K and self-field, which corresponds to the current density of 28,000 A/cm 2. The total heat load, including the heat leakage conducted through the cylinder between 4 and 40 K, is estimated to be less than 400 mW at 5000 A. Present Bi-2212 HTS conductors seem to be promising as large transport current leads for superconducting magnets.

AC transverse magnetic field dependence of magnetization in Bi 2Sr 2CaCu 2O 8 single crystal

We measured the role of small ac transverse magnetic field on the magnetization of Bi 2Sr 2CaCu 2O 8 single crystal with dc magnetic field along the crystalline c-axis. We observed that the irreversible magnetization diminishes monotonically with increasing ac transverse field, as the dithering of vortices induces easier flux entry and exit into and from the crystal. The frequency dependence showed non-monotonic dependence of the irreversible magnetization with the most reduction near 30 Hz of ac field. We find strong frequency dependence of the irreversible magnetization and the occurrence of flux jumps above the second magnetization peak, that are interpreted in terms of dimensional crossover of vortices from three to two dimension in the peak region with increasing dc magnetic field.

Origin of the irreversible microwave absorption versus the state of vortex matter in Bi 2Sr 2CaCu 2O x single crystals

We have investigated properties of hysteretic microwave absorption (MWA) by the Bi 2Sr 2CaCu 2O x single crystals in the temperature range from 10 K to T c. It was found that the shape of the MWA hysteresis loop and its dependence on the magnetic field modulation amplitude undergo essential changes with temperature. The analysis has shown that near 50 K, the origin of the signal detected by electron spin resonance spectrometer with magnetic field modulation changes. It is due to crossover to another pinning type. At T<50 K where the bulk pinning is the most effective pinning mechanism, the signal registered by a spectrometer is the magnetically modulated MWA. At high temperatures, 50 K< T< T c, when vortices are pinned by the surface or the edge barrier, the signal is the derivative of the absorption power with respect to field d P/d H. The shape of the MWA hysteresis loop and its modulation enable one to determine unambiguously the pinning type (bulk or surface) controlling the irreversible properties of a superconductor.

Collective Josephson vortex dynamics in long Josephson junction stacks

We investigate the collective phase dynamics in conventional long Josephson junction (LJJ) stacks and in layered superconductors, exhibiting intrinsic LJJ behaviors. Using a theoretical model which accounts for both the magnetic induction effect and the breakdown of local charge neutrality (i.e., charging effect), we show that the collective motion of Josephson vortices, including the dispersion of Josephson plasma mode and the Swihart-type velocity, in an intrinsic LJJ stack such as Bi 2Sr 2CaCu 2O 8+ y (BSCCO) is significantly modified from those in a conventional LJJ stack. In BSCCO, the strength of the charging effect α is small (i.e., α∼0.1–0.4), but it leads to notable changes in collective phase dynamics, including changes to the stability condition. Also, we show that splitting of the supercurrent branch in the resistive state is due to collective motion of Josephson vortices. The width of spread of these sub-branches in the linear current–voltage regime depends on α, suggesting another way to measure the charging effect in BSCCO.

Anomalous magnetization in a heavily Pb-doped Bi 2Sr 2CaCu 2O 8+ δ as-grown single crystal

Anomalous magnetization is investigated on a heavily Pb-doped Bi 2Sr 2CaCu 2O 8+ δ as-grown single crystal [(Bi,Pb)-2212] with T c=92 K. Both the anomalous magnetization peak H 2p and the onset of the anomalous magnetization peak H on are detected in temperature region from 0.3 T c to 0.9 T c based on the descending field branch of magnetic hysteresis loops. At low temperatures below 20 K, the magnetic hysteresis loop behaves as an obvious asymmetry. The double magnetic hysteresis behavior is observed at high temperatures close to T c. The temperature dependence of H 2p can be described according to the thermal disorder induced decoupling theory. The double magnetization behavior may be ascribed to the existence of intra-grain granularity due to the inhomogeneity of oxygen distribution induced by heavily Pb-doping.

Normal-state re-entrant peak in the vortex transformer measurement for Bi-2212/Bi-2223 intergrowth single crystals

A normal-state re-entrant behaviour in the Bi 2Sr 2CaCu 2O 8/Bi 2Sr 2Ca 2Cu 3O 10 (Bi-2212/Bi-2223) intergrowth single crystals is observed with the ‘flux-transformer’ measurements. After entering the superconducting state at T c1=107 K, the intergrowth system re-enters the normal state at a temperature slightly higher than the superconducting transition temperature of the Bi-2212 phase ( T c2=92 K). This re-entrant phenomenon is consistent with our previous observations in the measurements with the conventional four-probe technique. However, some new behaviours have been found in the flux-transformer measurements, namely a two-head peak near T c2 is observed. The position and the height of the peak are sensitive to driving currents and applied magnetic fields. The origin of the two-head peak has been attributed to the vortex motion and the superconducting transition of the Bi-2212 matrix.

The effect of the surface state and shape of Bi 2Sr 2CaCu 2O 8+ δ single crystal on anomalous magnetization

Magnetic hysteresis loops are measured on Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) single crystal with different surface states and shapes. The result obtained on Bi-2212 single crystals with dirty and clean surfaces demonstrates that variation of surface state does not result in the disappearance of anomalous magnetization peak H 2p and has only an influence on the width of magnetic hysteresis loops. The result obtained on Bi-2212 single crystals with elongated and circular shapes shows that the variation of shape also does not give rise to the disappearance of H 2p. The origin of H 2p of Bi-2212 single crystal is suggested to originate from other intrinsic reasons and should not be attributed to the surface state and surface shape.

Effect of thermal cycling on the strength and superconducting properties of laser floating zone textured Bi-2212 rods

The influence of the thermal cycling on the flexure strength and superconducting properties of textured Bi 2Sr 2CaCu 2O 8+ δ rods processed by the laser-heated floating zone method was studied. Two different temperature ranges (77–300 and 77–393 K) were used to separate the contributions of chemical reactions with liquid water from those due to the thermo-elastic stresses. The rods showed excellent resistance to thermal cycling in both cases and neither the superconducting nor the mechanical properties were significantly affected. In particular, the variation of the critical current at 77 K was below ±4% after 50 thermal cycles. The flexure strength of rods increased or decreased by ≈10% after a few tens of thermal cycles, and crack deflection along the rod axis was observed in the specimens which showed a reduction in strength. This decrease was transitory, and the flexure strength after 50 thermal cycles was similar to that measured on the as-received rods.

Vortex lattice in Bi 2Sr 2CaCu 2O 8+ δ well above the first-order phase-transition boundary

Measurements of non-local in-plane resistance originating from transverse vortex–vortex correlations have been performed on a Bi 2Sr 2CaCu 2O 8+ δ high- T c superconductor in a magnetic field up to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid vortex lattice does exist over a broad portion of the magnetic field–temperature ( H– T) phase diagram, well above the first-order transition (FOT) boundary H FOT( T). The results also provide evidence for the vortex lattice melting and vortex liquid decoupling phase transitions, occurring above the H FOT( T).

ARPES studies of c-axis intracell coupling in Bi 2Sr 2CaCu 2O 8+ δ

Using angle-resolved photoemission spectroscopy we have performed a detailed study of bilayer splitting in Bi 2Sr 2CaCu 2O 8+ δ as a function of doping level and temperature. In heavily overdoped samples where the splitting is the clearest, we extract an intracell coupling t ⊥∼55 meV. As a function of photon energy the intensity ratio of the bonding and antibonding bands varies, allowing us to detect the bilayer splitting effect in the optimal and underdoped regimes. Surprisingly, with reduced doping the intracell coupling is not measurably reduced. Upon cooling to the superconducting state, a gap Δ opens in both bands yet the magnitude of the splitting remains unchanged.

Characterisation of an optimised high current MgO/Bi 2Sr 2CaCu 2O 8.21 composite conductor using pulsed transport currents with pulsed magnetic fields

High temperature superconducting conductors are already used in hybrid magnets to produce fields that enhance the performance of conventional magnets made from A-15 type low temperature superconducting wires. For such applications it is vital that the interdependence of the critical parameters such as critical current versus magnetic field can be mapped under high field and high current conditions. However these superconductors have high critical currents even at fields over 20 T, making accurate measurements difficult due to the thermal and mechanical problems. In this paper, we compare measurements on the fully optimised Bi 2Sr 2CaCu 2O 8.21 flat rigid conductors using an innovative pulsed high transport current and pulsed high field technique. We show how analysis of the voltage signal from Bi 2Sr 2CaCu 2O 8.21 tape in pulsed conditions may be used to extract the critical current under quasi-stationary conditions.

BaZrO 3 insulation coatings for HTS coils

Barium zirconium oxide (BaZrO 3) thick films were grown on Ag and AgMg sheathed Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) superconducting tapes for electrical insulation purposes. A BaZrO 3 (BZO) precursor solution was prepared by sol–gel synthesis using Ba and Zr based alkoxide. Reel-to-reel continuous sol–gel dip coating method was used to grow BZO coatings on superconducting tapes. Processing, microstructure and electrical properties of high temperature BZO insulation coatings have been studied. The BZO coatings were grown in the temperature range of 450 and 600 °C in air. ESEM and X-ray diffraction were used to investigate microstructure and crystallinity of the coatings. Room temperature resistance, dielectric constant, and high voltage breakdown strength of the coatings were measured to be 13 M Ω , 22.5 and 1.0 kV at 1.5 mA, respectively.

Collective dynamics of Josephson fluxons in Bi 2Sr 2CaCu 2O 8+ δ intrinsic junctions

The dynamic characteristics of Josephson fluxons in naturally stacked intrinsic Josephson junctions in Bi 2Sr 2CaCu 2O 8+ δ single crystals were studied experimentally. The Josephson fluxons were generated either by irradiating microwaves or by applying dc magnetic fields in parallel with the junction planes. In the presence of a finite tunneling bias current the fluxons are driven along the junctions in resonance with the excitation modes of Josephson plasma and thus exhibit a variety of dynamic characteristics. Thus in the flux-flow state of fluxons in both cases above, for a relatively low vortex density, sub-branch splitting of the ‘supercurrent’ branch in the current–voltage ( I– V) characteristics was observed, which corresponded to different plasma excitation modes. For a higher vortex density, on the other hand, all the quasiparticle branches merged into a single non-hysteretic I– V curve which exhibited step-like kink features, very similar to the recently predicted ones corresponding to different moving fluxon patterns. The detailed agreement with the theoretical prediction, however, is yet to be confirmed.

The X-ray characterization of Bi 2Sr 2CaCu 2O 8+ x single crystals grown by different methods

We report on results of systematic X-ray investigations of the Bi 2Sr 2CaCu 2O 8+ x (2212) single crystals grown by slow cooling, top seeded solution growth and travelling solvent floating zone (TSFZ) methods. The typical concentrations of dislocations in the Bi-2212 crystals are estimated from width of peaks. According to the obtained X-ray topography and diffractometry data, these crystals demonstrate misorientation anisotropy around different crystallographic axes, which is a consequence of incommensurate modulation in this compound. The highly perfect crystals grown by TSFZ method are found to demonstrate the abnormal transmission X-ray diffraction effect (Borrman effect).