Alexey Semenov

Mechanisms of limitation and nature of field dependence of critical current in HTS epitaxial YBaCuO films

Magnetic field and temperature dependencies of the critical current density, J/sub c/(H/spl par/c, T) were measured by SQUID-magnetometry, ac magnetic susceptibility, and dc transport current techniques in the single-crystalline epitaxially-grown by off-axis dc magnetron sputtering YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films with J/sub c/(H/spl par/c, 77 K) /spl ges/ 2 /spl middot/ 10/sup 6/ A/cm/sup 2/. The mechanism of vortex depinning from growth-induced linear defects, i.e., out-of-plane edge dislocations in low-angle tilt domain boundaries, is shown to describe quantitatively measured J/sub c/(H/spl par/c, T). The developed model takes into account a statistical distribution of the dislocation domain boundaries ordered in a network as well as the interdislocation spacing within boundaries. Actual structural features of YBCO film known from HREM data turn out to be extracted from J/sub c/(H/spl par/c, T)-curves by a fitting procedure within the proposed model.

Magnetic field dependence of the critical current density in thin epitaxial HTS YBa2Cu3O7−δ films with dislocation low-angle domain boundaries

Highly biaxially oriented YBa 2 Cu 3 O 7-δ (YBCO) films exhibit critical current densities, J c , two orders of magnitude higher than any sort of bulk samples. Out-of-plane edge dislocations (mean 2D density is 10 11 lines/cm 2 in the film area) in low-angle tilt domain boundaries are shown to play a crucial role in J c (H||c, T)-behavior. Measurements of the magnetic field dependence of J c for the pulse-laser deposited YBCO films are carried out by AC magnetic susceptibility technique. Dislocation cores are shown to provide an extremely strong pinning force leading to J c (77 K) ≅ 2 × 10 6 A/ cm 2 . Peculiarities of J c (H||c) are found in the low-field range. A simple model of pinning by dislocation domain boundaries is developed. The model consistently describes the J c (H||c, T)/J c (0, T)-dependencies, accounting for the fraction of vortices pinned by dislocations.

Thickness dependence mechanisms of the critical current density in high-Tc cuprate superconductor films

The obtained electron backscattering diffraction data and high-resolution transmission electron microscopy have shown that the nanostructure of epitaxial HTS YBCO films evolves essentially with the film thickness. This happens due to a high deposition temperature and a high dislocation mobility that induce polygonization, dislocation rearrangement and a remarkable reduction of the out-of-plane dislocation density. The analysis of experimental thickness dependences of the critical current in the framework of existing pinning models leads to the conclusion that the c-axis-correlated pinning by out-of-plane edge dislocations plays a dominant role. Thus, the evolution of the dislocation nanostructure is responsible for the critical current density reduction. Embedding of nanoparticles into HTS films should be useful only if they are coherently coupled with the matrix and extra dislocations are being formed during the film growth. The nanoparticles just preserve a high density of dislocations. Another effect is self-assembling of nanoparticles within dislocation cores forming a bamboo structure. This phenomenon may result in an enlargement of the dislocation normal core and an essential increase of the elementary pinning force.

Critical current density of HTS single crystal YBCO thin films in applied dc field

Magnetic field and angle dependencies of critical current density J/sub c/(H,/spl theta/) are measured for single-crystal c-oriented epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films with high J/sub c//spl ap/2 MA/cm/sup 2/ at 77 K. Films are deposited by off-axis dc magnetron sputtering onto r-cut sapphire substrates buffered with CeO/sub 2/. Experimental evidences of the dominant contribution of extended linear defects (growth-induced out-of-plane edge dislocations) to pinning mechanism and critical current behavior are presented. A consistent model of vortex lattice depinning from a linear defect system is developed. Detailed J/sub c/(H) measurements start from very low fields (<0.001 T). Qualitatively different angle dependencies J/sub c/(/spl theta/) are obtained in different field ranges. Their evolution is comprehended on the base of depinning model. The peak-effect observed in J/sub c/(H)-dependencies at parallel magnetic field is discussed as a contribution of electromagnetic pinning.

Characteristics of superconducting tungsten silicide WxSi1-x for single photon detection

Superconducting properties of three series of amorphous WxSi1−x films with different thickness and nstoichiometry were investigated by dc transport measurements in a magnetic field up to 9 T. These amorphous nWxSi1−x films were deposited by magnetron cosputtering of the elemental source targets onto silicon substrates nat room temperature and patterned in the form of bridges by optical lithography and reactive ion etching. nAnalysis of the data on magnetoconductivity allowed us to extract the critical temperatures, superconducting ncoherence lengths, magnetic penetration depths, and diffusion constants of electrons in the normal state as nfunctions of film thickness for each stoichiometry. Two basic time constants were derived from transport and ntime-resolving measurements. A dynamic process of the formation of a hotspot was analyzed in the framework nof a diffusion-based vortex-entry model. We used a two-stage diffusion approach and defined a hotspot size by nassuming that the quasiparticles and normal-state electrons have the same diffusion constant.With this definition nand these measured material parameters, the hotspot in the 5-nm-thickW0.85Si0.15 film had a diameter of 107 nm nat the peak of the number of nonequilibrium quasiparticles.

On the mechanism of thickness dependence of the critical current density in HTS cuprate epitaxial films

The critical current density, Jc, in epitaxial HTS YBCO films strongly depends on the film thickness, degrading from 7-8 MA/cm2 in very thin films (50-70 nm) to less than 1 MA/cm2 in much thicker ones. Electron backscattering diffraction and high-resolution transmission electron microscopy reveal that the nanostructure of pulsed laser deposited epitaxial HTS YBCO films is evolving during growth due to high dislocation mobility. An analysis of existing pinning models leads to a conclusion that any other pinning centres but dislocations can not provide observed Jc. Extended linear defects provide a maximum elementary pinning force, if vortices are parallel to dislocation lines, while randomly distributed nanoparticles produce much lower pinning. Thus, the thickness dependence of Jc is determined mainly by the evolution of dislocation structure. Embedding of nanoparticles into HTS cuprates films results in Jc enhancement if extra dislocations are being formed during growth, preserving a high density of dislocations. Nanoparticles reduce the dislocation mobility, fix the dislocation cellular nanostructure and prevent its polygonization.

On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperat...

Current dependence of the hot-spot response spectrum of superconducting single-photon detectors with different layouts

We show that avoiding bends in a current-carrying superconducting nanowire enhances the probability for low energy photons to be detected and that this enhancement is entirely due to the increase in the experimentally achievable critical current. We studied nanowires shaped as either meander or spiral. The spirals had different layouts, a double-spiral layout with an S-turn in the middle and a single-spiral layout without such turn. Nanowires were prepared from films of niobium nitride with a thickness of 5 nm. For specimens with each layout we measured the spectra of the single-photon response in the wavelength range from 400 nm to 1600 nm and defined the cut-off wavelength

Improved energy resolution of a superconducting single-photon detector

{lambda}c

Field transients of coherent terahertz synchrotron radiation accessed via time-resolving and correlation techniques

beyond which the response rolls off. The largest and the smallest