Mechanism of Bi2Sr2CaCu2Ox crystallization and superconducting properties for Bi2Sr2CaCu2Ox/Ag tapes prepared using isothermal partial melting method

The mechanism of Bi₂Sr₂CaCu₂O_x (Bi-2212) crystallization from a partially molten state has been studied by means of thermal analysis, X-ray diffraction (XRD) measurement, and microstructural observation in Bi-2212/Ag tapes. Tapes have been prepared using the isothermal partial melting (IPM) method, under which partial melting and solidification are carried out at the same temperature in an nitrogen atmosphere and 20% oxygen partial pressure (p(O₂)), respectively. The Bi-2212 phase crystallizes by peritectic reaction between (Sr, Ca)CuO₂ (1:1 phase) and the liquid phase at processing temperatures (T_p)≥855°C. At the beginning of Bi-2212 crystallization, only the 1:1 and the liquid phases are present in tapes. The Bi-2212 nucleation occurs in the vicinity of 1:1 grains (homogeneous nucleation). This peritectic reaction progresses by recovery of oxygen in the liquid phase. In contrast, the nucleation of the Bi-2212 phase occurs heterogeneously at T_p≤850°C. The Bi-2212 phase co-exists with four Bi-free phases ((Sr, Ca)₁₄Cu₂₄O₄₁ (14:24 phase), (Sr, Ca)₂CuO₃ (2:1 phase), (Sr, Ca)O (1:0 phase), and the 1:1 phase) and the liquid phase in the early stage of the Bi-2212 crystallization step. Since the precipitation temperature of the Bi-2212 phase is higher than 850°C, the Bi-2212 crystallization progresses by direct precipitation. The crystallization paths affect microstructure and transport critical current density (J_c) of Bi-2212 tapes.     

Josephson vortex lattice melting in Bi-2212

The B-T diagram of Josephson vortex lattice melting in Bi-2212 is analyzed (B is magnetic induction parallel to the layers, T is temperature). It is shown that the Josephson vortex lattice melting at B > B* = 0.6–0.7 T is associated with Berezinsky-Kosterlitz-Thouless transition in individual Bi-2212 superconducting layers and is a second-order phase transition.     

Josephson vortex lattice melting in Bi-2212 probed by commensurate oscillations of Josephson flux-flow

We studied the commensurate semifluxon oscillations of Josephson flux-flow in Bi-2212 stacked structures near T_c as a probe of melting of a Josephson vortex lattice. We found that oscillations exist above 0.5 T. The amplitude of the oscillations is found to decrease gradually with the temperature and to turn to zero without any jump at T = T₀ (3.5 K below the resistive transition temperature T_c), thus, indicating a phase transition of the second order. This characteristic temperature T₀ is identified as the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature, T_BKT, in the elementary superconducting layers of Bi-2212 at zero magnetic field. On the basis of these facts, we infer that melting of a triangular Josephson vortex lattice occurs via the BKT phase with formation of characteristic flux loops containing pancake vortices and antivortices. The B-T phase diagram of the BKT phase found from our experiment is consistent with theoretical predictions.     

Geometry dependent resistivity behavior in mesoscopic Bi2Sr2CaCu2O8+δ single crystals

In order to investigate the interaction between Josephson vortices and pancake vortices in a layered Bi₂Sr₂CaCu₂O_8+δ superconductor, c-axis resistivity measurements were carried out on fabricated mesoscopic single crystals for two experimental geometries, when an applied high magnetic field of 14 T is perpendicular (B_ex⊥L) and parallel (B_ex∥L) to the width of sample. In the first case, the angular dependence of the resistance obtained near the ab-plane at T = 30 K exhibited hysteretic non-monotonic behavior with local maxima at θ = 0.6° away from the ab-plane and dips and spikes at θ = 0.2° marking a vortex lock-in transition. In the second geometry (B_ex∥L), instead of a local maximum, peculiar resistance shoulders were observed. At higher temperatures, hysteresis is reduced, while the vortex lock-in transition becomes considerably broader indicating the interaction of pancake and Josephson vortices.     

Experimental evidence for Coulomb charging effects in submicron Bi-2212 stacks

Focused ion beam (FIB) and ion milling techniques are developed for the fabrication of Bi₂Sr₂CaCu₂O_8+1d (Bi-2212) stacked junctions with in-plane size L ab ranging from several microns down to the submicron scale without degradation of superconducting transition temperature T _c. It is found that the behavior of submicron junctions (L _ab<1 μm) is quite different from that of larger ones. The critical current density is considerably suppressed, the hysteresis and multibranched structure of the current-voltage (I-V) characteristics are eliminated, and a periodic structure of current peaks appears reproducibly on the I-V curves at low temperatures. The period ΔV of the structure is consistent with the Coulomb charging energy of a single pair, ΔV=e/C, where C is the effective capacitance of the stack. It is considered that this behavior originates from the Coulomb blockade of the intrinsic Josephson tunneling in submicron Bi-2212 stacks. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 1, 75–80 (10 January 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Intrinsic Josephson tunneling for basic studies of high-temperature superconductors

High-temperature superconductors (HTS) are highly anisotropic with layers of CuO₂ separated by layers of charge reservoirs. The superconducting CuO₂ planes are weakly coupled by Josephson tunneling of Cooper pairs in the c-axis direction. It may be possible to use the intrinsic tunneling to realize a stack of intrinsic Josephson junctions at the surface of a single crystal (or epitaxial film) of HTS. These will form excellent tools in the study of superconducting properties of HTS.     

Intrinsic tunneling spectroscopy for Bi2−xPbxSr2CaCu2O8+δ of nm-thickness mesa structure

We have investigated intrinsic tunneling spectroscopy (ITS) with short-pulse bias to mesa structures consisting of several layers of intrinsic Josephson junction superlattices of Bi_1.8Pb_0.2Sr₂CaCu₂O_8+δ(PbBi2212). Through ITS, the superconducting gap 2Δ = 75 meV (at 10 K) is obtained for a PbBi2212 crystal. The large 2Δ value corresponds to the underdoped property of Pb-free Bi2212, which is consistent with the ab-plane transport measurement results performed simultaneously. The normal tunneling resistance R_N derived from the high bias region of the I–V characteristics is significantly small in comparison with underdoped Bi2212. Moreover, J_c of PbBi2212 is less deviated from the Ambegaokar–Baratoff value than the case of underdoped Bi2212. It is interpreted that the Pb substitution makes the tunnel barrier lower, resulting in a reduced anisotropy and a high J_c even with a lower doping.     

Attraction-repulsion transition between abrikosov vortices and Josephson vortices in the strongly layered superconductor Bi2Sr2CaCu2O8

A change in the effect of a frozen magnetic field parallel to the c-axis on rf power absorption, which is associated with the motion of Josephson vortices, is observed in the layered superconductor Bi₂Sr₂CaCu₂O₈ at a low temperature (～15 K). The effect is interpreted as a change in the interaction between an Abrikosov vortex and a Josephson vortex from attraction (at high temperatures) to repulsion (at low temperatures). It is found that the dynamics of the magnetic flux parallel to the ab plane of the single crystal becomes irreversible upon a transition of the superconductor to the layered state. Possible reasons behind the observed effect are considered, one of them being a manifestation of the second superconducting transition in the elementary-excitation spectrum of a d-type superconductor near the core of Abrikosov vortices.     

Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La_1.87Sr_0.13CuO₄ are directly measured with the electric field polarized perpendicular to the CuO planes (E∥c) covering the frequency range 10-40 cm⁻¹ and temperatures 5-300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper-oxygen planes of La_1.87Sr_0.13CuO₄. A strongly enhanced low-frequency (below 15 cm⁻¹) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.     

Orientation-induced crossover in pinning force density and peak effect in Bi2Sr2CaCu2O8+x single crystal

Magnetization–hysteresis measurements on single crystal of Bi₂Sr₂CaCu₂O_8+x (Bi-2212) at various temperatures (20–40 K) are reported for different applied field orientations (θ) relative to the unit cell's c-axes. An orientation-induced crossover in the magnitude of the volume pinning force density F_p around θ≥20° is observed which does not conform with predictions of the existing 2D-pancake model. The results further indicate that the `peak-effect' in Bi-2212 is associated with the crossover in the vortex matter from a quasi-lattice to a disordered vortex solid.     

Vortex states in mesoscopic single crystals Bi2Sr2CaCu2O8+δ in high magnetic fields

The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi₂Sr₂CaCu₂O_8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.     

Magnetically dependent superconducting transport in oxide heterostructures with an antiferromagnetic layer

The superconducting current in hybrid superconducting structures Nb/Au/Ca_1?x Sr _x CuO₂/YBa₂Cu₃O_7?δ with an antiferromagnetic layer is experimentally shown to have a Josephson nature, and the deviation from the sinusoidal dependence of the superconducting current on the phase difference between superconducting electrodes is about 20% of the second harmonic. These heterostructures are found to have sensitivity to an applied magnetic field that is much higher than that of conventional Josephson junctions. The experimental shape of the magnetic-field dependence of the critical current in the heterostructures differs from the usual Fraunhofer shape by oscillation with a significantly smaller period along a magnetic field.     

Spin-polarized quasiparticle injection effects in YBCO thin films

We report detailed transport studies on ferromagnet-superconductor heterostructures. Epitaxial heterostructures of half-metal colossal magnetoresistive La_2/3Ca_1/3MnO₃ (HM-CMR) and high T_c superconducting YBa₂Cu₃O₇ (YBCO) are grown on SrTiO₃ (100) single crystal substrates by pulsed laser deposition. Using the HM-CMR layer as source for spin-polarized quasiparticles, we show the effect of injection of spin-polarized quasiparticles into the ab-plane and along the c-axis of YBCO. The results show a drop in the ab-plane resistance R_ab (T) in the case of injection along the c-axis that is discussed to be related to the opening of a pseudogap.     

First-principles calculations for anisotropy of iron-based superconductors

We perform first-principles calculations for anisotropy of various iron-based superconductors. The anisotropy ratio γ_λ of the c-axis penetration depth to the ab-plane one is relatively small in BaFe₂As₂, LiFeAs and FeSe, indicating that the transport applications are promising in these superconductors. On the other hand, in those having perovskite-type blocking layers such as Sr₂ScFePO₃ we find a very large value, γ_λ ? 200, comparable to that in strongly anisotropic high-T_c cuprate Bi₂Sr₂CaCu₂O_8−δ. Thus, the intrinsic Josephson junction stacks are expected to be formed along the c-axis, and novel Josephson effects due to the multi-gap nature are also suggested in these superconductors. We also examine the doping effect on the anisotropy of LaFeAsO.     

Effective activation energy U(J,H) in epitaxial YBa2Cu3O7−δ thin films

We have measured the resistivity of epitaxial YBa₂Cu₃O_7?δ thin film as a function of temperature, current density and the magnetic field up to 8 T. The current density dependence of the effective activation energy exhibits a slight increase in the low current density range below 10³ A/cm² and a logarithmic decline in higher current density with increasing current density. The magnetic field dependence of the effective activation energy showed a crossover of vortex state from a quasi-2D for H//ab-plane to a 3D line liquid state for H//c-axis. The possible dissipation mechanisms responsible for the ln H dependence of the effective activation energy were discussed.     

Anomalous electrical properties of linear chain mercury compounds

The electrical resistivity of Hg_2.86AsF₆ has been studied as a function of temperature. At room temperature, the resistivity along the chain direction is 10^?4 Ω-cm with an anisotropy of about 10². This incommensurate linear chain system remains metallic at low temperatures with resistance ratio ?_ab(300 K)/ ?_ab(1.4 K) ? 3000 and still increasing with no apparent sign of residual resistivity. A large anisotropic magnetic field dependence of the resistivity is observed below 30 K. Near 4 K, the c-axis resistance drops abruptly more than three orders of magnitude, apparently to zero, while ?_ab is continuous. The c-axis transition is suppressed in a small magnetic field.     

Anisotropic and large low-field magnetic entropy change in a La4/3Sr5/3Mn2O7 single crystal

We have studied the magnetocaloric effect (MCE) in a bilayered La_4/3Sr_5/3Mn₂O₇ single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the MCE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2 J/kg K. A large low-field magnetic entropy change, reaching 3.2 J/kg K for a magnetic field change of 15 kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane.     

Microstructure and flux distribution in both pure and carbon-nanotube-embedded Bi2Sr2CaCu2O8+δ superconductors

In order to improve pinning properties of bulk Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) materials, samples of both pure Bi-2212 and Bi-2212 with carbon nanotubes embedded (CNTE Bi-2212) have been prepared by partial-melting processing. The preparation conditions are chosen so that a significant fraction of carbon nanotubes can be successfully embedded in the material, as indicated by thermogravimetric analysis. The microstructure and composition of non-superconducting second phases are found to be different in these two types of samples. By means of magneto-optical (MO) imaging, flux distributions in both types of samples are investigated up to T=77 K. The MO investigation reveals the propagation of a flux front in both pure and CNTE Bi-2212, showing that there is a strong coupling between grains (clusters) which enables the flow of inter-granular currents. This grain coupling persists in our field range of ±180 mT. In bulk non-textured ceramic high-T_c superconductors, the flux fronts caused by currents flowing through the entire sample are observed for the first time. Intra-granular current densities are obtained from the images as a function of grain size. The MO investigations have revealed the differences in the current densities caused by the presence of carbon nanotubes, showing that the carbon nanotubes are indeed functioning like columnar defects produced by heavy-ion irradiation. The increase of the flux penetration field is also a manifestation of the increase of the transport current density in the CNTE Bi-2212. The superconducting properties in our samples are very well reproducible.     

Numerical research on RF induced zero-crossing steps of stacked Josephson junction in flux-flow state

RF responses of intrinsic Josephson junction stacks in the flux-flow state have been studied to explore vortex motion. We have reported various RF induced effects on flux-flow of Bi₂Sr₂CaCu₂O_8+y intrinsic Josephson junctions. In a pinning free state, a remarkable zero-crossing step appears at a certain voltage on I–V curve, which is closed to the voltage V = NΦ₀f, where N is the number of junction, Φ₀ is the flux quantum, f is the RF frequency, respectively. It is shown that vortex motion phase-locked to external microwaves play an important role in the responses. In this report, we have carried out numerical simulation by using the coupled sine-Gordon equations in order to compare with the experimental results. The numerical simulations reveal that the alternating magnetic field of microwaves drives vortices into the stack and generates the zero-crossing step.     

Study of the influence of hole concentration on high-Tc superconductivity of cuprates Bi-2212 by X-ray photoelectron spectroscopy

In this paper, we report the observation of X-ray photoelectron spectra (XPS), which were obtained from high-T_c superconducting compounds Bi-2212 with some percentage of 3d elements (Fe, Co, and Ni) substituted for copper. The changes in characteristics of XPS spectra Sr 3d and Cu 2p_3/2 with the doping content were explained in the relation to the hole concentration on CuO₂ planes, which resulted in high-T_c superconductivity.