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.     

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).     

Field Dependent Microwave Resistivity in YBa2Cu3O7−δ

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa₂Cu₃O_7– films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.     

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.     

Comparison of 2-D Quantum and Thermal Critical Fluctuations of Underdoped Bi2Sr2CaCu2O8+δ with Ultrathin YBa2Cu3O7−δ

This paper addresses two issues. Underdoped Bi-2212 has been shown to exhibit two-dimensional quantum fluctuations, with T _c vs. N _s (0) scaling qualitatively similar to ultrathin YBCO films that are 2-D by construction. The present paper shows that the quantitative difference in scaling is resolved by growing Bi-2212 films that are much smoother than previously used. Also, the absence of 2-D thermal critical behavior from the superfluid density of Bi-2212 films that exhibit 2-D quantum scaling is not likely due to an inhomogeneous T _c smearing out the 2-D thermal transition. Finally, we emphasize that there is as yet no consensus on the physics that drives the T-dependence of N _s at low to intermediate temperatures.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

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.     

Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

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.     

Thermal fluctuation effects on the magnetization above and below the superconducting transition in Bi2Sr2CaCu2O8 and YBa2Cu3O7−δ crystals in the weak magnetic field limit

We present detailed experimental data of the magnetization, M_ab(T,H), of Bi₂Sr₂CaCu₂O₈ and YBa₂Cu₃O_7– crystals on both sides of the superconducting transition, for magnetic fields, H, applied perpendicularly to the ab (CuO₂) planes. The data are analyzed in terms of thermal fluctuations in the weak H limit: In the reversible mixed state below the transition, by taking into account the vortex fluctuations, as first proposed by Bulaevskii, Ledvij and Kogan, which are much more important in the Bi-based compounds. Above the transition, by taking into account the order parameter amplitude fluctuations (OPF), through a generalization to multilayered superconductors of the Schmidt-like approach.     

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...     

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.     

The Low Temperature Microwave Properties of GdBa2Cu3O7?δ and Sr2RuO4

Measurements of the microwave surface resistance R_s and reactance X_s of GdBa₂Cu₃O_7– and Sr₂RuO₄ single crystals are reported in the temperature range 0.4 to 25 K at frequencies from 10 to 20 GHz, using a hollow dielectric resonator technique. The GdBa₂Cu₃O_7– measurements are interpreted in terms of antiferromagnetic alignment of the Gd spins, resulting in a Nèel transition at 2.25 K, with a strongly temperature spin relaxation time both above and below the transition. Above T_c = 1.45 K, the normal state surface impedance of the suspected p-wave superconductor Sr₂RuO₄ agrees rather well with that expected from dc measurements; however, below T_c, we observe an unusual increase in the surface reactance, which we attribute to a long electron relaxation time. Using a two-fluid model, we obtain a normal fluid component that decreases almost linearly with temperature and extrapolates to a finite low-temperature value.     

Oxygen mobility and charge transport in YBa2Cu3O7-δ

The out-diffusion of oxygen during heat treatment of fully oxygenated c-axis oriented epitaxial films of YBa₂Cu₃O_7- leads to large electric fields in the films. During heat treatment under oxygen pressure a transient electric resistance peak has been observed also.     

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.     

High-Frequency Fluctuational Conductivity in YBa2Cu3O7−δ

We present measurements of the fluctuational excess conductivity at 48 GHz and 24 GHz in YBa₂Cu₃O_7– films above the transition temperature. The measurements depart from the gaussian prediction for finite-frequency fluctuational conductivity. We focus on the region not too close to T _c, where the real part of the excess conductivity drops much faster than the prediction of the gaussian model. We calculate the dynamic excess conductivity within a Ginzburg–Landau approach. In the calculation we insert a short-wavelength cutoff of the order of the inverse coherence length, in order to suppress high-momentum modes. The excess conductivity of all samples measured can be described very well by the modified model.     

Enhancement of Phase Coherence Between Superconducting Grains in Strongly Underdoped Bi2Sr2CaCu2O8+δ

We report a new finding on the phase coherence between strongly underdoped superconducting Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) grains. Contrary to the expectation based on the linear suppression of the superfluid density at T=0 K with the decreasing hole concentration in the underdoped region, the phase coherence between superconducting grains was remarkably enhanced at a hole concentration (p≃0.08) in the underdoped regime. The likely candidate giving rise to our finding is the phase-disordered superconducting quasiparticles around the antinode of the Fermi surface.