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).     

Studies Of Dynamic Critical Behavior in Bi2Sr2CaCu2O8+δ Single Crystals

Up to now, the value of the dynamic critical exponent z is still an open question. We have studied this exponent in the intermediate (XY) critical regime in zero field for a set of high quality Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals both by previously obtained critical exponents of the dc critical conductivity fluctuations [1] and in terms of Fisher-Fisher-Huse (FFH) dynamic scaling [2]. In this regime (i.e. the absence of Coulomb interactions), one would expect model-E dynamics (z = 1.5) to apply with the conserved charge-density conjugate to the phase fluctuations. However, we find the dynamic critical exponent to be z = 2, which is in agreement with the suggestion of FFH that due to the plasma fluctuation, fully relaxational dynamics, model A (z=2), should be appropriate.     

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.     

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.     

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...     

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.     

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.     

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.     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

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.     

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.     

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.     

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.     

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.     

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.     

Bi2Sr2CaCu2O8＋x单晶的热导率测量

介绍了有关高温超导材料的热导率性质、测试方法和测试结果。结合我们的测试经验，讨论了稳态热流法测量高温超导体热导率应注意的问题。给出了Ｂｉ２Ｓｒ２ＣａＣｕ２Ｏ８＋ｘ单晶沿ａｂ面热导率的测试结果。     

The Peculiarities of Magnetic Flux Dynamics at Magnetothermal Instability in Textured Bi2Sr2CaCu2O8+δ

The magnetic flux dynamics was investigated in a textured Bi₂Sr₂CaCu₂O₈₊ sample using a miniature Hall probe placed on the surface of the superconductor. The process of magnetic flux penetration into the superconductor was found to have a complex structure and consist of two processes with different time scales. The first one has a length of tens of microseconds and is considered as locally adiabatic. The second one has a length comparable with the time of thermal diffusion. The importance of the above two processes in the development of magnetic instability is analyzed.