Synthesis of Bi2Sr2CaCu2O8+δ Single Crystals by Alumina Crucible

Journal of Superconductivity and Novel Magnetism - The contamination by alumina crucible and the effect of Al impurity on the crystal growth of Bi2Sr2CaCu2O8+δ are always a thorny problem. In...     

Degradation of Superconductivity in Annealed Irradiated Bi2Sr2CaCu2O8+δ Single Crystals During Exposure to Air

As for irradiated Bi₂Sr₂CaCu₂O₈₊ single crystals annealed in flowing N₂ atmosphere, flowing O₂ atmosphere, and air, their superconductivity was degraded during exposure to air. The degradation of superconducting transition temperature T _c is attributed to the doping of oxygen. The small recovery of T _c may be related to the redistribution of oxygen. The superconductivity of samples annealed in N₂ atmosphere and air is more stable than that of the sample annealed in O₂ atmosphere during exposure to air.     

Study of Pinning Effects for Vortices in Bi2Sr2CaCu2O8+δ Single Crystals Using a Bitter Decoration Technique

Distribution of vortices has been analyzed to study on the pinning effects of the vortices in Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals, observed with a Bitter decoration technique. On the cleaved surfaces of the samples, vortices are pinned in the disordered configurations at lower magnetic fields, which change to a hexagonal lattice structure with increasing a magnetic field. Furthermore, a dense concentration of vortices has been observed in the voids and on the lower terrace of the steps. These distributions of the vortices are found to be very stable from the estimation of the pinning energy.     

Study of Current-Voltage Characteristics of the Mesa-Structured Bi2Sr2CaCu2O8+δ Single Crystals under Magnetic Fields

To study on the interlayer coupling between Cu-O superconducting layers in Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals, we have performed the current-voltage measurements along the c-axis in the magnetic fields perpendicular and parallel to the axis at 4.2K. A pronounced effect of magnetic fields to the current-voltage characteristics appears in the multiple-branch behaviors, when the magnitude of the parallel field exceeds the 2D-3D dimensional-crossover field B _2D . This is considered as a consequence of missing a long-range order of the correlation of the vortex lines along the c-axis.     

Oxygen diffusion in epitaxial Bi2Sr2CaCu2O8+δ thin films

Superconducting Bi ₂ Sr ₂ CaCu ₂ O ₈₊ thin films (T _c 85K) were prepared on off-c-axis (001) SrTiO ₃ substrates by pulsed laser deposition. High quality oriented film growth with a c-axis inclined with respect to the surface normal by a tilt angle is found up to 15° from SEM, XRD and resistivity measurements. Isothermal oxygen diffusion is monitored by in-situ electrical resistance measurements at temperatures ranging from 200°C...400°C. From the temporal evolution of the resistance the in-plane diffusion constant D _ab is determined.     

Systematic Magnetization Measurements on Single Crystalline Bi2Sr2CaCu2O8+δ with Columnar Defects

We have performed systematic magnetization measurements on single crystalline Bi ₂ Sr ₂ CaCu ₂ O ₈₊ with columnar defects of B = 0.005 to 1 T by using a SQUID magnetometer. Magnetization hysteresis curves of the pristine sample show a weak irreversible behavior in the vortex liquid state, suggesting the existence of the new vortex state in the vortex liquid state. This weak irreversible region persists systematically in the samples with columnar defects even up to B = 1 T. It is shown that the weak hysteresis of magnetization is sensitive to the disorder level of the sample and shifts systematically to higher temperature and field region with increasing the number of columnar defects. This behavior clearly indicates that effective pinning mechanism exists even in the vortex liquid state and generates a finite critical current.     

Anisotropic 3D fluctuations conductivity in Bi2Sr2CaCu2O8+δ films

In zero applied magnetic field the fluctuation conductivity is measured and analyzed for a high-quality thin film (100 nm) of Bi ₂ Sr ₂ CaCu ₂ O ₈₊ grown on a MgO substrate. The two-band model formulas derived by Konsin et al. for the temperature dependence of the anisotropic three-dimensional (3D) paraconductivity in the vicinity of a superconducting phase transition are used. One of the two correlation lengths is critical and along with the other parameters of the model it determines the fluctuation conductivity (T).The global result of two-band model mimics closely d-wave-type pairing. The theory is compared with the experiment, and we obtained reasonable values and the slope of (T), the superconducting critical temperature T _c and the coherence length (0).     

Josephson plasma resonance in Bi2Sr2CaCu2O8+δ under parallel magnetic field

Josephson plasma resonance in under-doped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals has been observed when magnetic field is applied parallel to the ab plane and its vicinity. The resonance mode splits into two branches at higher and lower temperature regions, and a definite gap appears in the temperature region between them. As the magnetic field is tilted from the ab plane, these branches come closer and finally merge to a single mode. This leads to a general interpretation that the resonance mode observed in a parallel field may be due to continuous extension of the c axis plasma mode. However, splitting of the mode as well as the peculiar temperature dependence of the resonance field, the line shape, and its intensity strongly imply that these behaviors may originate from the inherent resonance modes in parallel field. In particular, the high temperature mode fields goes even higher in temperature beyond the zero field resonance as the field increases, suggesting that a new explanation may be required for the case of parallel magnetic field.     

Substitution Effect of Pr for Ca in Bi2Sr2CaCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction, and photoemission experiments have been performed. Both X-ray diffraction and photoemission measurements show that Pr ion enters the lattice with a valence higher than 3⁺, which supports the hole-filling mechanism of the suppression of superconductivity.     

Necking Effect on the Growth of Bi2Sr2CaCu2O8+δ Single Crystal by TSFZ Method

High-quality Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals have been grown by a travelling solvent floating zone method with a necking technique using a single crystal seed. The full-width at half maximum of the X-ray rocking curves was improved from 120 arcsec to 70 arcsec by the necking technique. The topographic results show that the crystals grown using the necking technique have better crystallinity. Especially, this technique is very effective on the growth of the atomically substituted single crystals. It results not only in the improvement of crystallinity but also in the enhancement of the single crystal size, especially for the crystals substituted by other elements.     

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.     

Studies of the residual absorption of a Bi2Sr2CaCu2O8+δ single crystal at THz frequencies

We measured the residual absorption of a Bi₂Sr₂ CaCu₂ O₈₊ single crystal at low temperature in the frequency range between 0.6 THz and 4.2 THz using a new method, photothermal interference spectroscopy. By combining with reflectivity data obtained with a Fourier-transform IR spectrometer (frequency range: 3 THz to 100 THz) we have calculated the dynamical conductivity using a Kramers-Kronig-transformation. The results are compared with calculations of the far infrared conductivity and show qualitative features expected for a superconductor with nodes in the gap.     

Anomalous Scattering Rate and Microwave Absorption in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7?δ

We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two-fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as YBa₂Cu₃O_7–. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.     

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2−xSnxO8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi₂Sr₂CaCu₂O_8+δ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.     

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.     

Instability of the Critical State in NdBa2Cu3O7−δ Single Crystals

Instabilities of the critical state are observed in various NdBa ₂ Cu ₃ O _7– single crystals by means of direct magneto-optic (MO) imaging while warming up flux states containing vortices of opposite polarity. Using the same samples as in the MO investigations, we observe for the first time characteristic steps in the integral magnetization measured by means of a SQUID system, which allows a constant temperature sweep. The integral measurement technique enables the study of the instability phenomenon (turbulence) in a much wider range of temperatures and fields than the MO investigations, and also in the orientations H _a c and H _a c. As a result, we find turbulence not only in a narrow window of temperatures (65 T 80 K), but also in a window of applied negative fields ranging between 30 and 80 mT.     

Out-of-Plane Negative Magnetoresistance of Bi2Sr1.6La0.4CuO6+δ Single Crystals in the Underdoped Region

In order to investigate a relationship between the carrier concentration and negative magnetoresistance in the underdoped region, we have performed out-of-plane electrical resistivity measurements for underdoped Bi ₂ Sr _1.6 La _0.4 CuO ₆₊ single crystals. Giant negative magnetoresistance has been observed in the most abundant carrier sample with = 0.12 in our experiment. The negative magnetoresistance reduces with decreasing carrier concentration.     

XRD and PES Studies of the Bi2Sr2Ca1−xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

Upper Critical Field of Four-CuO2-Layer TlBa2Ca3Cu4O11+δ Single Crystals

Magnetic measurements have been performed on single crystals of TlBa ₂ Ca ₃ Cu ₄ O ₁₁₊ in the magnetic fields up to 5.5 T. No change in the onset temperature of superconducting transition (T _c ) was observed in the magnetic fields between 0.4 and 5.5 T, while the superconducting transitions were broadened significantly in the same field. The linear temperature-dependence of the magnetization was found in a large temperature range below T _c . The slope of the magnetization versus temperature decreased significantly with increasing magnetic field. The experimental findings indicated that the upper critical field (H _C2 ) in this compound was very large and traditional methods of deriving H _C2 value were not applicable. Using the measured magnetization in the reversible region, the H _C2 properties of TlBa ₂ Ca ₃ Cu ₄ O ₁₁₊ single crystals have been studied based on a new model developed by Hao et al.     

On-Demand Local Modification of High-Tc Superconductivity in Few Unit-Cell Thick Bi2Sr2CaCu2O8+δ

High-temperature superconductors (HTSs) are important for potential applications and for understanding the origin of strong correlations. Bi₂Sr₂CaCu₂O_8+δ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, is of immense interest for superconducting electronics. A route to modify superconductivity locally by depositing metal on the surface is reported here by transport studies on few unit-cell thick BSCCO. Deposition of chromium (Cr) on the surface over a selected area of BSCCO results in insulating behavior of the underlying region. Cr locally depletes oxygen in CuO₂ planes and disrupts the superconductivity in the layers below. This technique of modifying superconductivity is suitable for making sub-micrometer superconducting wires and more complex superconducting devices.