L. Ya. Vinnikov

Direct observation of the lattice of Abrikosov vortices in high-Tc superconductor YBa2Cu3Ox single crystals

Abstract The magnetic structure of the YBa2Cu3Ox single crystal has been investigated using the technique of decorating the sample with small ferromagnetic particles. The lattice of the Abrikosov vortices and the effects caused by pinning on the twin boundaries have been observed in both the electron and optical microscopes. The twin structure of the single crystals has been investigated using the polarized light in the optical microscope. The penetration depth for the magnetic field parallel to the c axis has been evaluated as ≲ 0.3 μm at 4.2 K for the YBa2Cu3Ox single crystal.

Observation of the flux line lattice in high Tc-superconductor Bi2.2Sr2CaCu2OX and Ti2Ba2CaCu2OX single crystals

Abstract The flux-line lattice (FLL) has been observed on the (001) face of high T c Bi 2.2 Sr 2 CaCu 2 O X and Ti 2 Ba 2 CaCu 2 O X single cristals using the technique of decorating the sample with small ferromagnetic particles. Strong pinning of the Abrikosov vortices by plane defects along (100) and (010) planes has been found out on Bi 2.2 Sr 2 CaCu 2 O X . The triangular FLL with a long-range order has been observed on the perfect Ti 2 Ba 2 CaCu 2 O X single crystal. The penetration depth for a magnetic field parallel to (001) axis has been evaluated as ≤0.2μm at 4.2K for both materials.

Anisotropy of the resistivity and trapped magnetic flux in single crystals YBa2Cu3O7−x

Abstract In the best crystals of YBa 2 Cu 3 O 7−x the temperature behaviour of the c-axis resistivity is metallic. The structure of Abrikosovs vortexes in ab-plane does not reveal the anisotropy of the penetration depth, possibly, due to the presence of a dense net of twins.

Direct observation of the vortex structure in high-Tc superconductors in tilted magnetic fields: II. Vortex pinning by twins in YBaCuO single crystals

The high resolution Bitter technique was used for observation of the vortex structure in twinned YBaCuO single crystal in the magnetic field tilted with respect both to the c-axis and to the twin planes. The dominating effect in vortex distribution was strong pinning by twin planes which was seen as an alignment of vortices along twins with approximately two times density higher density than in the bulk. This strong pinning remained unchange for a surprisingly broad interval of tilt angles 0<25°. For larger tilt angles, pinning did not disappear abrupity but the signs of a decrease of the effective pinning potential were observed-the difference of vortex density between twin boundaries and the bulk was gradually decreasing and finally disappeared at a critical angle as large as 42°. The observed behaviour is discussed in terms of the formation of zig-zag vortices partly pinned at twin planes.

Observation of the magnetic domain structure in Cu0.47Ni0.53 thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu0.47Ni0.53 alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the surface of the films well below the Curie temperature TCurie (∼60 K). In contrast to magnetic force microscopy, this technique allowed visualization of the domain structure without its disturbance while the larger areas of the sample were probed. Maze-like domain patterns, typical for perpendicular magnetic anisotropy, were observed. The average domain width was found to be about 100 nm.

Visualization of the magnetic flux structure in phosphorus-doped EuFe2As2 single crystals

Magnetic flux structure on the surface of EuFe2(As1-xPx)2 single crystals with nearly optimal phosphorus doping levels x = 0.20 and x = 0.21 is studied by low-temperature magnetic force microscopy and decoration with ferromagnetic nanoparticles. The studies are performed in a broad temperature range. It is shown that the single crystal with x = 0.21 in the temperature range between the critical temperatures TSC= 22 K and TC = (18 ± 0.3) K of the superconducting and ferromagnetic phase transitions, respectively, has the vortex structure of a frozen magnetic flux, typical for type-II superconductors. The magnetic domain structure is observed in the superconducting state below TC. The nature of this structure is discussed.

Anisotropy of the vortex structure and resistivity in the basal plane of YBa2Cu4O8 single crystals

The vortex lattice of the YBa2Cu4O8 high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρa/ρb measured at temperatures from Tc to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.

Vortex arrays in the BSCCO (2212) single crystals in the vicinity of steps on the sample surface

Abstract With using the decoration technique the vortex distribution near steps on the surface of BSCCO (2212) single crystals has been investigated. The vortex structure visualization was carried out by optical and scanning electronic microscopy (SEM) as well as by atomic force microscopy (AFM). The Meissner rim was revealed on the top side of the steps which height h was more than 0.1 a 0 —vortex lattice parameter. The width of the band is proportional to the height of the step h and does not depend on the magnetic field. The vortex lattice behavior in the vicinity of the sample steps is discussed.

Vortex structure in FeTeSe single crystals

Vortex structure in FeTe0.66Se0.44 and FeTe0.6Se0.4 single crystals with Tc ∼ 11.7 and 14.5 K, respectively, has been studied using the decoration technique. It has been found that in single crystals with the simplest crystalline structure of 11-family iron-containing superconductors (without interlayers), no regular vortex lattice is observed, similar to the case of the previously studied 122 and 1111 families. Using transmission electron microscopy, the dislocation structure with a density of ∼109 cm−2 has been observed. The problem of pinning in iron-containing superconductor single crystals is discussed.

Vortex lattice anisotropy in the conducting plane of organic superconductors

Abstract Vortex structure in organic superconductors κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2CuN(CN)2Br has been studied by use of the decoration technique at magnetic fields up to 23 Oe directed perpendicular to the two-dimensional superconducting bc-plane. A quantitative analysis of high quality vortex lattice (VL) images reveals an anisotropy of the VL which can be interpreted as a penetration depth anisotropy.