Vladimir M. Pan

Formation of edge dislocations in thin epitaxial YBCO films

Transmission electron microscopy (TEM) of thin YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films on different single-crystalline substrates revealed (by Moire patterns) edge dislocations having non-superconducting cores normal to the substrate. The dislocations are in small-angle boundaries with the average density as high as 10/sup 11/ cm/sup -2/. An extremely high density of dislocations is thought to be the cause of the high critical current density in YBCO epitaxial films. The mechanism for dislocation formation is considered in the framework of a computer model. Computer modeling provided the details of the dislocation arrangement either in domain boundaries or in twist boundaries, depending on the angle of the in-plane misorientation between film and substrate lattices. The model is found to be in goad agreement with experimental data on dislocations in YBCO superconducting films.

Linear defects in epitaxial Y-Ba-Cu-O films: their role in anisotropic vortex pinning and microwave surface resistance

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films exhibit two orders of magnitude higher critical current density, J/sub c/, and a few times higher microwave surface resistance, R/sub s/, than single crystals. This evidences a different nature of crystal defects, which are responsible for the pinning and HF losses. Several types of dislocation arrays are identified in YBCO films by TEM/HREM and XRD. An analysis of J/sub c/(H,T,/spl theta/) and R/sub s/(T)-dependencies along with X-ray and HREM data for films grown via various modes allows to differentiate contributions to J/sub c/ and R/sub s/ from out-of-plane and in-plane edge dislocations. The higher the density of edge dislocations within YBCO film the higher are J/sub c/ and R/sub s/. Stress-strain field calculations allow one to comprehend the difference between out-of-plane and in-plane dislocations effects.

Microwave impedance of YBa2Cu3O7−δ high-temperature superconductor films in a magnetic field

The temperature, field, and intensity (amplitude) dependences of the surface impedance of magnetron-sputtered YBa2Cu3O7−δ quasi-single-crystal films on a sapphire substrate with a CeO2 buffer layer were measured. The measurements were performed with a coplanar resonator at 5.25 GHz in a weak constant magnetic field 0<B<12 mT in the temperature range 13 K <T<80 K. They made it possible to obtain the surface resistance and penetration depth versus the temperature and the magnetic field strength and to determine the contribution of Abrikosov vortices to the impedance. The cases with a frozen magnetic field and zero-field cooling with the field subsequently switched on at temperatures below the critical value were investigated separately. A substantial difference in the behavior of the high-frequency response of the vortices in these two cases was found. The Coffey-Clem and Brandt theories were used to study the linear response and the critical state model the nonlinear response of the vortices.

Vortex transfer mechanisms in C-oriented YBCO films with small-angle-boundaries

The mechanism of vortex depinning and motion along the row of parallel equidistant linear pins in an anisotropic type-II superconductor is considered, The essential role of the surface effects for these processes is emphasized, The activation energy of vortex motion and the critical current value are calculated for the current flow perpendicular to this row. We suppose this model to be applicable for description of resistive properties and the critical current dependence on the azimuthal misorientation angle between neighboring grains for YBCO bicrystals and rather thick c-oriented films with low-angle tilt grain boundaries, containing edge dislocations, which are parallel to the c-axis.

Vortex pinning/dynamics in YBCO films with J{sub c} (77 K) {gt} 3{center{underscore}dot}10{sup 6} A/cm{sup 2} studied by direct transport measurements

Critical current density, Jc, is studied for highly biaxially-oriented YBCO thin films with Jc(77 K) > 3 MA/cm2in a range of magnetic fields, temperatures, angles between magnetic field vector and film c-axis. Jc(H, θ) is shown to be dominated by vortex interaction with high density (> 1011lines/cm2) of linear pins — edge dislocations. A model is developed to describe different vortex arrays behavior in the presence of two-dimensional square network of linear pins.

Abrikosov Vortices Behavior in Different Pinning Potential for Moderately Anisotropic High-Tc Superconductors

Abrikosov vortex line interaction with different type of crystal defects is considered for moderately anisotropic high-Tc superconductor YBa2Cu3O7–δ. Two types of a quenched disorder are shown to give the major contributions into the anisotropic and nonmonotonous field dependencies of the critical current density, jc(H,θ), the irreversible magnetization Mirr(H,θ) and volume pinning force, Fp,(H,θ) for the perfect single crystals: 1. point-like defects - oxygen vacancies 2. planar defects - twin domain boundaries.

Nonlinear optical waveguides produced by MeV metal ion-beam implantation of lithium niobate

We report linear and nonlinear optical properties of planar waveguides produced by implantation of MeV Ag ions into LiBnO3. The linear optical properties include spectrum of propagation modes and optical extinction spectrum. The nonlinear properties include optical spectrum of nonlinear refractive index and nonlinear absorption. Light guiding in the implanted crystal is associated with the modification of the linear refractive index. The modification is a result of two concurrent processes such as crystal is damage in the nuclear stopping region producing a low index optical barrier between the top light guiding layer and the bulk crystal and the diffusion of the implanted ions into the implanted ions into the top layer causing its index increase. Relatively high nonlinear refractive index is a result of enhancement of intrinsic third order nonlinearity of the implant by the mechanism of surface plasmon resonance. The intrinsic nonlinearity is associated primarily with intraband and interband electron transitions. The spectrum of the nonlinear index correlates with the spectrum of optical extinction featuring prominent peak due to surface plasmon resonance. Possible applications of the waveguides include ultrafast photonic switches and modulators.

Superconducting Transport Properties, Mechanisms of Growth, and Microstructure of Deposited HTSC Materials

It is well known now the high T c superconductors layered cuprates can be described as stacking up of the metallic superconducting CuO 2 layers separated from each other with layers of different atoms constituting an insulator. The strong-coupled supercurrent flow in this anisotropic structure has been shown to be confined within CuO 2 (ab) planes.

Influence of planar defects on mixed state dynamics of high-Tc epitaxial films

Abstract The resistive state behaviour of YBa 2 Cu 3 O 7 films deposited by laser ablation has been studied. The analysis of the creep and viscous flux flow regimes allowed to conclude that the one dimensional “easy slip channels”, within which vortices preferably move, play an important role. Such channels arise at planar defects (twin boundaries). Easy slip regime has been observed at temperatures above t ⩾ T/T c ⋍ 0.9 , where drastic decrease of elementary pinning force occured.

Nonlinear Microwave Properties of High-Temperature Superconducting YBa 2 Cu 3 O 7-δ Single Crystal Thin Films

This work establishes the possible sources of nonlinear microwave response in perfect epitaxially-grown single-crystal HTS YBa2Cu3O7-δ (YBCO) thin films, their origin and conditions at which they become essential, as well as possibilities for their mitigation. Microwave measurements of YBCO thin film coplanar and strip-line resonators are performed at different experimental conditions. The nonlinear surface resistance of these resonators are studied as a function of the rf current amplitude at different temperatures and dc magnetic field for field-cooled experiments.