Enhancement of Interlayer Phase Coherence in the Vortex Liquid State of Bi2Sr2CaCu2O8+y with Columnar Defects

We studied the phase coherence in the vortex liquid state of Bi ₂ Sr ₂ CaCu ₂ O _8+y with columnar defects (CDs) using Josephson plasma resonance. The phase coherence as a function of magnetic field is nonmonotonic in samples with higher density of CDs, and we observe multiple resonance peaks in a limited temperature range. The characteristic field H ^* for the enhancement of the phase coherence normalized by the matching field (B ) is constant between 1/5 and 1/3 for samples with larger B , while H ^* is constant in samples with smaller B . In samples with CDs introduced at angles from the c-axis, H ^* is determined only by the density of CDs in the CuO ₂ layers. In a sample with CDs at 80° from the c-axis, a large enhancement of the phase coherence was observed when the field is applied parallel to the direction of CDs and clear triple resonance peaks are observed.     

High-temperature X-ray Study of Melting and Solidification of the Bi2Sr2CaCu2O8+δ on Silver

The melting and solidification of a Bi ₂ Sr ₂ CaCu ₂ O ₈₊ compound on Ag were studied by in-situ high-temperature x-ray analysis. Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag samples were heated by the heating schedule known as partial melting and slow cooling under a 1-100%-O ₂/He gas flow. A strong c-axis alignment of the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ crystals was observed at the incongruent-melting temperature where the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and the molten phase coexist. Similarly, a strong c-axis alignment was observed at the initial stage of solidification. However, the subsequent solidification of the residual molten phase caused less aligned Bi ₂ Sr ₂ CaCu ₂ O ₈₊ on Ag. An improved heating schedule is proposed which increases the critical current of Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag superconducting tapes.     

Engineered flux pinning centres in Pb-doped high temperature superconducting “Bi2Sr2CaCu2O8” ceramics

The Pb solubility of Bi₂Sr₂CaCu₂O₈ is determined within the concentration range Bi_2.18-x Pb _x Sr₂CaCu_8+d with 0 x 0.6 and the temperature range from 800 to 900°C. From a consideration of the Pb solubility in Bi₂Sr₂CaCu₂O₈ a simple annealing procedure was developed to precipitate (Pb,Bi)₄(Sr,Ca)₅Cu0_10+d (451 phase) in high-temperature superconducting Pb-rich Bi₂Sr₂CaCu₂O₈ ceramics. The precipitation results in a two-fold increase of the critical current density which is believed to express improved pinning properties of the superconducting ceramics. The microstructures of the samples with increased critical current density were studied with respect to the grain size and volume content of the 451 phase.     

Magnetic field dependence of the Josephson coupling energy along the c-axis in Bi2Sr2CaCu2O8+y

The magnetic field dependent c-axis dissipation in Bi₂Sr₂CaCu₂O_8+y single crystals is shown to agree with that of a series stack of Josephson tunnel junctions. We extract the Josephson coupling energy E_j from the measured the c-axis resistance as a function of the angle between the c-axis and the magnetic field B using an extended Ambegaokar-Halperin model. For B > 0.5T, E_j increases continuously with both decreasing temperature and decreasing magnetic field. At T = 78K, E_j exhibits a B^–0.5 behavior.     

c-axis Superfluid Response and Quasiparticle Conductivity in Bi2Sr2CaCu2O8+δ and Bi2Sr2CuO6+δ

Josephson plasma resonance for underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and Bi ₂ Sr ₂ CuO ₆₊ have been measured by sweeping the microwave, frequency continuously. The resonance enables us to determine the superfluid density and quasiparticle conductivity in the c-axis accurately. We show that the. superfluid response and the low energy excitations out of the condensate in the c-axis of these materials are very different from those in the ordinary Josephson multilayer tunnel junctions. The Josephson coupling energy in single layer Bi ₂ Sr ₂ CuO ₆₊ is more than 5000 times smaller than is predicted in interlayer tunneling model.     

Magnetic-field-induced non-linear effects of Josephson coupled superconductor Bi2Sr2CaCu2O8+δ

The dc current-voltage (I-V) relation along the c-axis of single crystalline Bi₂Sr₂CaCu₂O_8+δ has been measured in magnetic fields parallel and perpendicular to the c-axis. In zero field a clear and sharp jump with large hysteresis in theI-V curve was observed, indicative of the dc-Josephson effect. In magnetic field below a characteristic fieldB ^*≈0.4 T (at T=0 K) parallel to the c-axis the magnetic field suppresses the hysteresis and reduces the critical currentI _c drastically, whereas aboveB ^* theI-V curve becomes broad and featureless behavior. The characteristic field scaleB ^* can be interpreted as an energy scale of the Josephson coupling between superconducting layers in Bi₂Sr₂CaCu₂O_8+δ and is argued with emphasis on the correlation length of pancake vortices in this system.     

Atomic force microscopy observation of Bi2Sr2CaCu2O8+δ thin films prepared by molecular beam epitaxy

We observed the surface structures of 1200 Å-thick c-axis oriented Bi₂Sr₂CaCu₂O₈₊ thin films, which we intend to use for the base electrode of a planar type Josephson junction. The films were prepared by molecular beam epitaxy on the Nd:YAlO₃(001)substrates at a growth rate of 20 Å/min.Atomic force microscopy observation revealed that the surface of the films prepared at a substrate temperature of 780 °Cand an ozone pressure of 2.3 × 10^–3 Pahas island-like structures. Their shape is like a stack of cylindrical plates and the diameter of the island is typically 250 nm.Most of the steps have a height of 15 16 Å,which corresponds to the primitive-cell thickness of Bi₂Sr₂CaCu₂O8+.We discuss the result in comparison with that observed in Bi₂Sr₂Ca₂Cu₃O₁₀₊ films grown by metal organic chemical vapor deposition and YBa₂Cu₃O_7– films by sputtering. The occurrence of the structure with several steps is attributed to the insufficient supply of elements. The roughness less than 100 Åis favorable for the fabrication of junctions with thin barriers.     

Studies of Both Intralayer and Interlayer Electrodynamics of Bi2Sr2CaCu2O y in the Superconducting State

We studied the intralayer and interlayer electrodynamics of the superconducting Bi ₂ Sr ₂ CaCu ₂ O _y single crystals in the microwave and millimeter wave regions. The c-axis superfluid density fraction _s ^c shows a more rapid growth than _s ^ab just below T _c and a weaker temperature dependence at low temperatures. These features suggest the anisotropic growth mechanism of _s . The temperature dependence of _ab ¹ shows a large increase below T _c with a broad peak around 20 K. However, to discuss the detailed behavior of ₁ ^c or the anisotropy of conductivity _ab ¹ / _c ¹ in the superconducting state, much more careful measurement is required in the c-axis direction.     

Magnetic Torque Measurements on Bi2201 Single Crystal in High Magnetic Fields

The magnetic field direction dependence of the magnetic torque in the superconducting state of Bi ₂ Sr _1.65 La _0.35 CuO ₆₊ (Bi2201) single crystal has been measured in several static magnetic fields up to 200 kOe. The applied field dependence of the anisotropy parameter, , defined by the square root of the effective mass ratio has been obtained by fitting the magnetic torque curves at 17 K by means of the effective mass model. For Bi ₂ Sr ₂ CaCu ₂ O ₈₊ (Bi2212), the values up to 210 kOe were reported previously. At each field, for Bi2201 is larger than that for Bi2212. Since the value decreases as the field increases both for Bi2201 and Bi2212, the reduction of by the application of the high magnetic field is considered to be common for Bi-based layered superconductors.     

Synthesis of high-T c superconducting Bi-Pb Sr-Ca-Cu-O ceramics prepared by an ultrastructure processing via the oxalate route

The oxalate precursor yielded by a sol-gel processing can be pyrolysed and oxidized at 840 ° C for 40h into the desired high-T _c superconductor in the lead-doped Bi-Sr-Ca-Cu-O system withT _c(onset) = 140 K andT _c(zero) = 104K. The mechanisms of the sol-gel reaction were monitored by analysis of Fourier transformation infrared (FTIR) spectroscopy. Moreover, identification of the various phases existing on the specimens after firing at selected temperatures, was made by analysis by FTIR spectroscopy and X-ray diffractometry. Based on the experiment results, the mechanisms of forming Bi₂Sr₂CaCu₂O₈₊ (phase 2212) might be identified as the reaction of Bi₂Sr₂CuO⁶⁺, Ca₂CuO₃ and CuO at 800 ° C. Furthermore, Bi₂Sr₂Ca₂Cu₃O₁₀₊ (phase 2223) might be synthesized by the reaction of phase 2212, Ca₂CuO₃ and CuO at 840 ° C. Ca₂PbO₄ found in lead-doped materials may catalyse the formation reaction of Ca₂CuO₃ at 650 ° C and play a role of a flux to fuse Bi₂Sr₂CaCu₂O₈₊, CuO and residual Ca₂CuO₃ and then improve the further synthesis of Bi₂Sr₂Ca₂Cu₃O₁₀₊ at 840 C.