Magnetoresistance hysteresis of granular YBa2Cu3O7−δ high-temperature superconductor in weak magnetic fields

The variation of the transverse magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconducting ceramic with external magnetic field intensity H _ext first increasing from zero to ～500 Oe (Δρ⁺ / ρ_{273 K}) and then decreasing from about 500 Oe to zero (Δρ^? / ρ_{273 K}) is studied for transport current densities varying from j/j _c ≈ 0.01 to ≈0.99 (where j _c is the critical current density in the absence of the magnetic field) at 77.3 K. It is found that the field dependence of the magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconductor is characterized by pronounced hysteresis, the difference Δρ⁺ / ρ_{273 K} ? Δρ^? / ρ_{273 K} increasing with j/j _c. As j/j _c grows, the effective critical fields of Josephson weak links, H _c2J , and the lower critical fields of superconducting grains, H _c1A , decline. When field H _ext rises, the critical fields are lower than when H _ext diminishes: H _c2J ⁺ < H _c2J ^? and H _c1A ⁺ < H _c1A ^? .     

Vortex structure and magnetoresistance of granular YBa2Cu3O7 − δ high-temperature superconductors

The transverse magnetoresistance $ \frac{{\Delta \rho }} {\rho } $ (H _ext) of several series of YBa₂Cu₃O_6.95 high-temperature superconductor samples has been measured in external magnetic fields with a strength of up to H _ext = H _ext ^max ～ 500 Oe over a wide range of transport current strengths (～10 ≤ I ≤ ～1000 mA) at the temperature T = 77.3 K. It has been revealed that there are three different types of field dependences of the magnetoresistance at H _ext ⊥ I with the following typical features: (1) low critical fields of the Josephson weak links H _c2J , a high level of the magnetoresistance, and the appearance of a remanent magnetoresistance Δρ_rem/ρ with a decrease in H _ext to zero; (2) high fields H _c2J and a low level of the magnetoresistance (Δρ_rem/ρ ≡ 0); and (3) relatively high fields H _c2J and the presence of jumps in the dependences $ \frac{{\Delta \rho }} {\rho } $ (H _ext). A comparative analysis of the dependences $ \frac{{\Delta \rho }} {\rho } $ (H _ext) has made it possible to establish the nature of the revealed differences: the first type of behavior corresponds to the existence of a relatively ordered vortex structure of the Bragg-glass type over the entire range of fields H _ext, the second type corresponds to the existence of a disordered vortex structure of the vortex-glass type over the entire range of fields H _ext, and the third type corresponds to the occurrence of Bragg-glass-vortex-glass first-order phase transition in the magnetic field in the range 0 < H _BG-VG < H _ext ^max .     

Internal magnetic fields in the high-temperature superconductor YBa2Cu3O7−δ from muon spin rotation experiments

Muon spin rotation (SR) measurements of the internal magnetic fields in sintered YBa₂Cu₃O_7– samples are reported. Pronounced differences between field-cooled (FC) and zero-field-cooled (ZFC) SR signals, hysteresis effects and flux trapping are observed. The SR results are compared with the results of macroscopic experiments (susceptibility, magnetization, resistivity) and with predictions obtained from models of weakly-coupled superconducting regions.     

Superconducting cluster in YBa2Cu3O7−x ceramic

The method of point laser heating of a sample is used to perform experimental investigations of the electrical conductivity of YBa₂Cu₃O_7−x ceramic in the vicinity of its critical state. It is found that, in the vicinity of transition to the superconducting state, the electrical conductivity of the ceramic exhibits a clusterlike behavior substantially nonuniform over the sample cross section. The topology of a superconducting cluster is investigated, as well as its spatial localization in the sample. A model of the formation of a superconducting cluster in a ceramic superconductor is suggested.     

Dynamics of atoms in the high-temperature superconductor YBa2Cu3O7−δ

In order to obtain insight into the role played by the phonons in the superconducting phase transition, we have performed infrared reflectivity and inelastic neutron scattering experiments on single phase YBa₂Cu₃O_7–(0.1,T _c =94 K, T _10–90=1 K). These experimental results, together with published Raman scattering and specific heat data are used to determine the parameters of a rigid-ion model; the model is able to reproduce qualitatively the experimental data. On the basis of our model we derive and discuss the phonon density of states and the pattern of atomic displacements of some important vibrational modes. In particular, the model reveals the existence of high-frequency oxygen breathing modes and of low-frequency copper dimerization modes in both the CuO₃-chains and the CuO₂-planes. Furthermore, Jahn-Teller like oxygen modes exist in the chains. The possible role of these modes with respect to the pairing mechanism is discussed. In addition there exist low-frequency chain-degrading modes which favour jumps of oxygen atoms out of the CuO₃-chains into neighbouring vacant sites in the basal plane.     

Process of magnetic field penetration into the high-temperature superconductor YBa2Cu3O7−δ

The processes of magnetic field penetration into the ceramic samples of the HTSC YB₂Cu₃O_～6.95 at T<T _c are studied by the methods of internal friction and magnetization measurements. A clearly manifested correlation is observed between the field dependences of the internal friction spectrum parameters (the logarithmic damping decrement Q ^?1 and the resonance frequency f) and the trapped magnetic flux ΔM. The magneto-mechanical approach we used reveals a significant difference in the field dependences of the densities of pinned (N _p) and free (N _f) Abrikosov vortices for H>H _c1.     

Superconducting weak links in YBa2Cu3O7−σ an AC magnetic susceptibility study

AC magnetic susceptibility, χ' and χ″, and ac resistivity, ϱ, of two different samples of superconducting YBa₂Cu₃O_7-σ have been measured. The results show a strong non-linear behaviour and are very sensitive to the exciting amplitudes. From the analysis of the data and comparison with appropriate models and other superconducting compounds, evidence for the existence of weak superconducting links is inferred. Finally, the utility of ac χ measurements for detection of different superconducting phases is made evident.     

Positron annihilation anomaly in the high-temperature superconductor YBa2Cu3O7−x

We have measured the ac susceptibility of a wire with a Nb core (1.27 mm diam.) and a Cu cladding (0.37 mm thickness) atT50 K andB0.1 mG. Due to its proximity to Nb, the Cu becomes fully superconducting. From the data we find a breakdown fieldH _b =1.2 (mG) and a coherence length =2.2T ^–1/2 (m) for the Cu, as well as a field penetration depth -34T ^1/2 (m) at the Cu/Nb interface.     

Effect of temperature and magnetic field on the evolution of a vortex structure of the granular YBa2Cu3O7 − δ high-temperature superconductor

The temperature dependences of the electrical resistivity ρ(T) _{H = const} have been measured in external magnetic fields H _ext (0 ≤ H _ext ≤ 1420 Oe) at temperatures ranging from 70 to 273 K for samples of the granular YBa₂Cu₃O_{7 ? δ} high-temperature superconductor (HTSC). Cooling of the samples to the minimum temperature T _min (70 K) has been performed in external magnetic fields (FC mode) and in the absence of a magnetic field (ZFC mode). Moreover, the dependences ρ(T) _{H = 0} for samples cooled in the FC mode have been measured in a zero field. The curves ρ(T)H = const have been converted into isotherms of the magnetore-sistance ρ(H _ext) _{T = const}. A comparative analysis of the specific features in the behavior of the curves ρ(H) _{T = const} for samples with different “magnetic prehistories” has made it possible to elucidate the nature and mechanisms of the influence of the particular scenario of the magnetic treatment of granular HTSCs on the behavior of their galvanomagnetic properties. The temperature dependences of the critical magnetic fields of superconducting grains (H _c1g , H _c2g ) and Josephson weak links (H _c2J ) have been determined, and the H-T phase diagrams of granular YBa₂Cu₃O_{7 ? δ} HTSCs have been recovered.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

Effect of deoxygenation on the weak-link behavior of YBa2Cu3O7−δ superconductors

A systematic study of the weak-link behavior for YBa₂Cu₃O_7−δ polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques. The experiments were performed with two samples of similar grain, a sample of well-coupled grains, and a deoxygenated sample in such a way that the oxygen mostly comes from the intergrain region. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor normal–metal superconductor (SNS) for well-coupled samples to superconductor insulator normal–metal–superconductor (SINS) type of junctions for the deoxygenated sample. These results are interpreted in terms of oxygen depletion from grain boundaries, which in turn decreases the intergranular Josephson coupling energy with a concomitant decrease of pinning of the intergranular vortices.     

Onset of superconductivity at 107K in YBa2Cu3O7 − δ at high pressure

Electrical resistivity measurements under pressure have been carried out on the high-temperature superconductor YBa₂Cu₃O_{7 –} as a function of temperature T between 1 and 300 K at various pressures between 8 kbar and 149 kbar. The superconducting transition temperature T_c increases almost linearly with pressure at the rate dT_c/dP - 0.13 K/kbar. The onset of T_c, defined as the temperature at which(T) drops to 90% of its extrapolated normal state value, increases from 95 K at 8 kbar to 107 K at 149 kbar. These results suggest that higher pressures will yield yet higher values of T_c.     

The Hubbard model and pressure effects of YBa2Cu3O7−δ superconductors

We apply a mean field approach to the extended Hubbard model on a square lattice to the YBa₂Cu₃O_7−δ family of superconductors under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated by the zero pressure phase diagram (T_c×n_h). This scheme yields the non-trivial dependence of the superconductor critical temperature T_c as a function of the hole concentration n_h in the CuO₂ plane. With the assumption that the pressure P modifies the potential V and the on-plane hole content n_h, we can distinguish the charge transfer and the intrinsic contribution to T_c(P). We show that the changes on T_c(P) for the YBa₂Cu₃O₇ optimally doped compound at low pressures are almost entirely due to the intrinsic term.     

Electric field effect on superconducting YBa2Cu3O7−δ films

Experimental evidence for a significant electric field effect in thin superconducting films of YBa₂Cu₃O_7– is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×10⁶ V/cm across insulating SrTiO₃ barriers into thin cpitaxial YBa₂Cu₃O_7– channel layers. The results demonstrate that with these structures the electrical resistivity aboveT _c (R=0) and the density of free carriers in the YBa₂Cu₃O_7– films can be modified by 1–2% with gate voltages smaller than 50 V.     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

Resistivity and magnetic susceptibility of superconducting and non-superconducting ErBa2Cu3O7−y

The oxide ErBa₂Cu₃O_7−y has been obtained in the orthorhombic and the tetragonal forms by suitable heat treatments. The orthorhombic form is metallic and superconducting with T_c=88 K while the tetragonal form has high resistance and is non-superconducting. Magnetic studies on both types of compounds reveal that the rare earth ion moment is very close to its free ion value independent of the structure. In the tetragonal compound, deviations in the susceptibility from the Curie-Weiss behavior are noted at low temperatures which may be due to the effect of crystalline electric fields.     

Phonon transport in superconducting EuBa2Cu3O7−x

We have measured the thermal conductivity, Young modulus sound velocity and internal friction of a polycrystalline sample of the ceramic superconductor EuBa₂Cu₃O_7-x. The low temperature data can be quantitavely understood assuming the interaction of phonons with tunneling systems.     

Effect of oxygen deficiency on the magnetic field-dependent entropy in YBa2Cu3O7−δ

Roulin et al (1988), in one of their experimental papers, have presented a study of field-dependent entropy of high-purity YBa₂Cu₃O_7???δ (YBCO) as a function of oxygen deficiency. In order to explain their experimental results, we have used phenomenological GL-theory of anisotropic HTSCs in the London limit in line with of our earlier paper (Pattanaik et al, Physica B405, 3234 (2010)). Moreover, to account for the applicability of the theory at high field, we have incorporated the effect of vortex overlapping in the London theory done by Nanda (1995). Here, we have presented the variation of change in entropy (ΔS) with magnetic field for different oxygen deficiencies δ?=?0, 0.04, and 0.06. On comparison, we found that our results are in good agreement with the experimental data of Roulin et al (1988). The variation of penetration depth (λ) and anisotropic ratio of effective masses (γ) with concentration is also presented.     

Far infrared absorption studies of high Tc superconducting material YBa2Cu3O7−x

The high Tc superconducting material YBa₂Cu₃O_7−x has been studied for its absorption in the FIR region. Measurements have been carried out using a Fourier Transform Infrared (FTIR) spectrophotometer at different temperatures above and below Tc. Observed bands are examined in the light of theoretical calculations based on different models. Certain IR absorption bands show screening effects in the measurements below Tc. Experimental values for observed IR bands are compared with the work of other authors and the bands are assigned to different IR active modes of vibration of the material.     

Magnetic properties of superconducting EuBa2Cu3O7−x

The magnetic properties of superconducting EuBa₂Cu₃O_7–x with resistiveT _c 96.5 K are measured. The normal state magnetic susceptibility is analyzed within the framework of the Van Vleck-Frank theory, leading to the conclusion that the strong moments of the Eu³⁺ ions are uncorrelated, and do not affect the superconducting state.