Submillimeter surface impedance and relaxation: A case study of quasi-two-dimensional YBa2Cu3Ox

Surface impedance measurements in the normal and superconducting state are an excellent method to study the conduction electron dynamics in metals. This holds especially in the relaxation range, i.e., for distances traveled in one r.f. periods=υ _F/ω(υ_f is the Fermi velocity) being smaller or of the order of the penetration depth λ and mean free pathl. For materials withυ _F<-10⁷ cm/sec the relaxation range is easily accessible forf>0.1 THz. Then, in the normal state, relaxation defines the surface impedance with a penetration depth approaching the London penetration depth λ_L, andR≈μ ₀λ_l/2τ as surface resistance allowing a measure of λ_L and relaxation time τ(T, ω). In the superconducting state the photon interaction scales withξ _F/λ_L=l/γ (ξ _f is the dimension of Cooper pairs for l→∞) and causes at low frequencies an absorption rate growing withγ, which is decreasing withξ _F/l. The rate increase proportional toγ turns to a decrease above 0.1 THz, being accompanied by a decrease ofγ with frequency which is stronger for largeγ and smallξ _F/l. These characteristic dependences allow a measurement of material parameters, anisotropy, and dynamics of electrons. To evaluate the consequences of theâ, b, and?-direction anisotropy, the integral kernels for normal and superconducting surface impedances in its nonintegrated, angle-dependent form are presented, analyzed, and compared with impedance measurements above 0.1 THz of YBa₂Cu₃O_x.     

Phonon Dispersion Measurements of YBa2Cu3O6.15 and YBa2Cu3O6.95 by Time-of-Flight Neutron Spectroscopy

We measured the phonon dispersions of YBa₂Cu₃O_6.15 and YBa₂Cu₃O_6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.     

Fabrication of the New Y3Ba5Cu8O y Superconductor Using Melt–Powder–Melt–Growth Method and Comparison with YBa2Cu3O7?x

YBaCuO superconducting sample with nominal composition in the stoichiometric ratio of 3:5:8 was fabricated by using the Melt?CPowder?CMelt?CGrowth (MPMG) process. Microstructural property of the sample was examined by X-rays diffraction (XRD) pattern and also observed at micrographs taken by both a polarized optical microscope and a scanning electron microscope (SEM) with energy dispersion analysis of X-rays (EDAX). Identification of chemical composition of the sample was investigated by the SEM-EDAX. Electric and magnetic properties were performed with a standard four-probe dc technique and magnetic levitation force measurements, respectively. In addition, all of these properties were compared with a standard MPMG YBa₂Cu₃O_7?x ?(Y123) superconductor. According to the obtained results, it was determined that empirical resulting formula of the sample was Y₃Ba_4.89Cu_8.17O _y ?(Y358) and also the magnetic levitation force of the Y358 was 169.8 mN under zero-field-cooled (ZFC) regime at 77 K while that of the Y123 was 99.3 mN.     

Fabrication and properties of YBa2Cu3O7-δAg composite superconducting wires by plastic extrusion technique

YBa₂Cu₃O_7-δ (Y123)-Ag composite superconducting wires were fabricated by the plastic extrusion method which involves plastic paste making, die extrusion, binder burn-out and the firing process. The as-extruded Y123-Ag wires were so flexible that they can be easily fabricated into a desirable shape. The current-carrying properties of the wire are dependent on sample size, sintering temperature and silver content. The critical current density, J _c, of the Y123 wire with a large cross-section was lower than that of the wires with a small cross-section, probably due to the large self-induced magnetic field. J _c of the Y123-Ag wires increased with increasing sintering temperature but abruptly decreased above 910 °C, which is close to the eutectic temperature of the Y–Ba–Cu–O system. A silver addition of 10–20 wt% slightly increased J _c of the Y123 (at 77 K and 0 T, it was 140 and 250 A cm ^-2 for the undoped Y123 wire and the Y123 wire with 20 wt% Ag addition, respectively), but further silver addition had a deleterious effect on J _c (180 A cm ^-2 for 30 wt% Ag addition). The small increment in J _c in the Y123 wire with 10–20 wt% Ag addition appears to be due to the enhanced densification and the associated microstructural variation. The decreased J _c of the Y123 wire with 30 wt% Ag addition is considered to be due to the formation of non-superconducting phase, Y₂BaCuO₅ (Y211), BaCuO₂ and CuO phases via the decomposition of the Y123 phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Band Structure and Quantum Oscillations in YBa2Cu3O7: A Local Spin Density Approximation with the On-Site Coulomb Interaction Study

First-principles calculations have been performed on the Bravais lattice, the density of states (DOS), the band structure (BS) and the Fermi Surface (FS) topology for high-T _c superconductor YBa₂Cu₃O₇. We used the method of the Full-Potential non-orthogonal Local-Orbital Minimum-Basis Band-structure Scheme (FPLO) [in the local spin density approximation with the on-site Coulomb interaction (LSDA+U) and the coherent potential approximation (CPA)] coupled with the XFSF program. Our FPLO Fermi surface consists of four large quasi-2D cylinders centered on the Brillouin zone corners with mostly CuO₂ plane character and two quasi-1D sheets with mostly Cu–O apex (or apical O) chain character. The main feature of this Fermi surface is that each CuO chain-sheet shows up in its center a small tubular pocket with a funnel-like shape (Fermi pockets) with mainly Ba–O apex character which could give rise to the quantum oscillations observed by Doiron-Leyraud et al. (Nature 447: 565, 2007). Our FPLO band structure (BS) shows that three flat bands cross the Fermi level (FL) which suggests that the two Fermi pockets could be produced by the conjunction near the Fermi level of two flat bands arising from the apex oxygen (BaO layer). This is consistent with the density of states (DOS) where the Fermi level is dominated by the two O apex (BaO) peaks (～90 %). Moreover, the strong narrow peak exhibited in the DOS just below the Fermi level (FL) corresponds to the van Hove singularity (VHS) and originates from the hybridization of Cu 3d and apex O2p (BaO) orbitals. In other hand, the calculated value of the bare band-structure Sommerfeld constant γ _0band=10.60 mJ/(mol?K²) corresponds to a very high Fermi level density of states N(E _F )=4.72 states/(eV×cell) and the electron–phonon coupling constant λ is found to be about 1.64 which implies that YBa₂Cu₃O₇ compound is a strong-coupling superconductor and the cyclotron effective mass m ^? is enhanced in comparison with the band mass m _b [m ^?=(1+λ)m _b ]. Also, the discrepancy between our FPLO calculated Debye temperature (274.15 K) and that obtained experimentally (437 K) may originate from a phonon anisotropy likely generated by the antiferromagnetic spin fluctuations. Our results therefore provide evidence of changes in the topology of the Fermi surface materialized by the Ba–O apex small pockets formed in the center of the Cu–O apex Fermi surface sheets.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7−δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7?δ crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2?x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Effect of columnar defects on theJ c anisotropy of YBa2Cu3O7−δ thin films and YBa2Cu3O7−δ /PrBa2Cu3O7−δ multilayers

Epitaxial YBa₂Cu₃O_7?δ thin films (YBCO) and YBa₂Cu₃O_7?δ /PrBa₂Cu₃O_7?δ multilayers (Y/Pr) were irradiated with high-energy heavy ions (770 Mev²⁰⁸Pb) under various directionsφ relative to thec-axis. The irradiation resulted in columnar defects tilted byφ from thec-axis. The angular dependence of their pinning activity was studied by measuring the anisotropy of the critical current density. TheJ _c (B, T,?) behavior of the irradiated YBCO thin films showed an additional peak, which exceeds the intrinsic pinning peak, exactly at the irradiation direction. The Y/Pr multilayers, however, showed an isotropicT _c -enhancement by a factor of 5, without any additional structure in theJ _c(B, T,?) curve.     

Microstructures of high-J c melt-textured YBa2Cu3O7−x /Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7?x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7?x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

Electronic Raman scattering of superconducting YBa2Cu3O y single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^?1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^?1 fory～7.0 to 550 cm^?1 fory～6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

Specific heat of YBa2Cu3O7

Specific heat measurements, including measurements in magnetic fields and at both low temperatures and nearT _c , on a number of YBa₂Cu₃O₇ samples have revealed several correlations among strongly sample-dependent parameters. These correlations suggest that the sample dependence of the parameters reflects a sample dependence of the volume fraction of superconductivity, which is in turn correlated with a low concentration of Cu²⁺ moments. The correlations give a criterion for recognizing the values of the parameters characteristic of the fully superconducting material. Preliminary results on the effects of sample heat treatment are reported. New data on the “linear term” is presented and discussed.     

In plane anisotropy in optimally doped YBa2Cu3O6.95

Single crystals untwinned optimally doped YBa₂Cu₃O_9.65 show a large degree of anisotropy in a-b plane properties such as resistivity, infrared conductivity and value of zero temperature penetration depth. This source of orthorhombicity must ultimately reside in the CuO chains. Using a three plane model; 2 CuO₂ and one CuO (chain plane), we study solutions of three coupled BCS equations which include inter- as well as intra-plane coupling. The gaps are found to contain s- as well as d-wave symmetries. From our solutions, we calculate the penetration depth and compare with experiments.     

Clues Obtained from the Oxygen Isotope Effect on NMR/NQR Parameters Observed in YBa2Cu4O8

Nuclear magnetic and nuclear quadrupole resonance (NMR, NQR) techniques have a precision allowing one to determine rather small isotope effects. Well-defined oxygen stoichiometry and negligible oxygen diffusion makes YBa₂Cu₄O₈ an ideal compound for studies of small isotope effects that require experimental results not hampered by reproducibility problems. We report on high-precision measurements of the temperature dependence of plane-^63,65Cu NMR/NQR parameters such as Knight shift, spin–lattice relaxation rate R = 1/T ₁, NQR line frequency ν_Q and NQR linewidth δν_Q, as well as ⁸⁹Y Knight shift, performed in normal and superconducting ¹⁶O and ¹⁸O exchanged YBa₂Cu₄O₈.     

Anisotropic superconducting state parameters of Nd-Ce-CuO and La-Sr-CuO systems

Major superconducting state parameter of high-T _c electron-doped Nd-Ce-CuO and hole-doped La-Sr-CuO systems have been estimated. The analysis is based on the electronic energy band structure (EEBS) within the local density approximation. Anisotropy of the layered structure is well reflected in the shape of open Fermi surface. Determination of the effective mass of the charge carrier from the Fermi velocity using EEBS and estimates of the anisotropic superconducting state parameters, particularly the magnetic penetration depth ( _L), lead to smaller values than those observed by muon spin relaxation (SR) measurements on polycrystalline samples. The coherence length ( and ) is evaluated and appears to be higher. The temperature dependence of the Ginzburg-Landau parameter (K) and along the plane magnetic penetration depth ( _L ) shows the same nature as those revealed from experiments. Furthermore, results on lower and upper critical magnetic field are also presented. The results based on EEBS studies are consistent but do not agree well with experiments. On the other hand, if the Fermi Liquid approach (FLA) parameters are used for this analysis, the results are in better agreement with those reported earlier. The implications of the above investigations are discussed.     

Measurement of the surface resistance of YBa2Cu3O7−x thin films using stripline resonators

We review methods of measuring surface resistance (R _s ) of thin films using stripline resonators, and present our measurements of theR _s of YBa₂Cu₃O_7?x films as a function of frequency, temperature, and r.f. magnetic field. The films were deposited on LaAlO₃ substrates by two methods: (1) electron-beam coevaporation of Y, BaF, and Cu followed by annealing in O₂, and (2) single-targetin situ sputtering. The measurements were obtained at frequencies from 0.4 to 10 GHz, temperatures from 4 to 90 K, and an r.f. magnetic field range from 0 to 30 Oe. At low temperature and low r.f. field at 0.4 GHz, theR _s values obtained for the two deposition methods are approximately 7×10^?6 and 4×10^?6 Ω, respectively.     

Charge Transport in Spin-Textured YBa2Cu3O6.25

The electric transport of the charged particles in a spin texture was investigated in a strongly underdoped YBa₂Cu₃O_6.25 single crystal in order to identify the characteristic electrical transport mechanism. The in-plane resistivity revealed three different regimes of charge transport: a chiral 2D VRH regime up to 55 K with a characteristic temperature T _d ≈ 12,400 K, an impurity band conduction regime above 55 K, and a metallic-like regime beyond 170 K. The out-of-plane resistivity has only one crossover at 115 K, but the conduction mechanisms controlling the two regimes are not clear.     

Significant Change in Mechanical Properties of YBa2Cu3O7−x Bulk Superconductors Diffused with Sn Nanoparticles

Mechanical behavior of YBa₂Cu₃O_7?x (Y123) superconductors exposed to Sn nanoparticles diffusion is determined by the way of Vickers microhardness (H _v ) conducted at different applied loads (0.245N≤F≤2.940 N). Load dependent microhardness, load independent microhardness, elastic modulus, and yield strength values are estimated from the microhardness curves. Unpredictably, the findings of the H _v values reveal that the undiffused sample and Sn diffused sample prepared at 500 ^°C exhibit reverse indentation size effect (RISE) behavior while the other samples obey indentation size effect (ISE) nature. Further, we extract the load independent (true) microhardness using the Meyer’s law, proportional specimen resistance (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model, and compare the true hardness with the apparent hardness.     

Preparation and properties of YBa2Cu3O7 thin films on sapphire with Yttria-stabilized zirconia buffer layer

YBa₂Cu₃O₇ superconducting thin films were deposited onR-plane sapphire substrates with Yttria-stabilized zirconia buffer layer by laser ablation. The structure of the films was characterized by X-ray diffraction. Superconducting transitional temperatures as high as ≥90 K and critical current densities up to 2.2×10⁶ A/cm² (77 K) were achieved. The results of Auger depth profile showed that no obvious diffusion occurred between the substrate and the YBa₂Cu₃O₇ thin film.     

Influence of Ba content and Ce doping on the structural features of YBa2Cu4O8 superconductor—a neutron study

The structures of A: YBa₂Cu₄O₈ (T _c=80 K), B: Y_0.9Ce_0.1Ba₂Cu₄O₈ (T _c=5K), and C: YBa_2.2Cu₄O₈ (T _c=80 K) have been investigated using the powder neutron technique with a view to assessing the influence of the bridging Cu(2)-O(4) and Cu(1)-O(4) bonds on the superconducting transition temperature. The Cu(2)-O(4) bond for samples A and C (withT _c=80 K) appears to be compressed as compared to the reported data. On the other hand, the substitution of Ce at 10% Y sites elongates the Cu(2)-O(4) bond substantially and also brings theT _cdown to 55 K. The role of Cu(2)-O(4) bond length as well as other parameters expected to influence theT _cbehavior of Y-124 samples are discussed in the light of the present data. The study also shows that the ambient oxygen pressure route, if tried with the starting composition YBa_2.2Cu₄O _y , leads to a quite homogeneous 124-phase.     

Improved Flux Pinning in Melt-Textured YBa2Cu3O7 − δ and ErBa2Cu3O7 − δ Through Chemical Additions

The effects of Ce-based additions (CeO₂ and BaCeO₃) in combination with MgO additions on the magnetic properties of melt-textured YBa₂Cu₃O_{7 ? δ} and ErBa₂Cu₃O_{7 ? δ} have been investigated. The additions lead to improvements in the magnetic properties of both RE-123 compounds compared to samples with either addition alone or with no additions. The Ce–Mg addition combination produces a “peak effect” in the magnetic hysteresis loop without significant T _c degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the RE-Ba₂Cu₃O_{7 ? δ} lattice, creating defects that produce a “peak effect” in the magnetic hysteresis loop. Mg additions alone lead to a reduced T _c, while Ce additions restore the T _c and enhance the magnitude of the peak.     

BCS temperature-dependent superconducting energy gap in domain boundaries of melt-textured YBa2Cu3O7

Attenuation and velocity measurements in low magnetic fields in the superconducting state of a melt-textured sample of YBa₂Cu₃O₇ are reported. Changes in both attenuationα and relative velocitydv/v are observed at the penetration fieldH′ _c1 of its domain boundaries. The change indv/v atH′ _c1 appears to be proportional to the repulsive force between vortices. The change inα atH′ _c1 may be produced by a relaxation process involving superconducting quasiparticles in the vicinity of the normal core of the vortices. Analysis of the relaxation time associated with this process yields a temperature-dependent energy gap which follows the BCS dependence. The relaxation time associated with the process is about 10^?11 sec.