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Using the same extremely noised data, two correlation methods for finding the maxima of Brillouin spectra are compared. The first method is a well-known method of correlating the received signal with the ideal Lorentzian function. In the second method, developed by the authors earlier, instead of the Lorentzian function, the same spectrum under study, but inverted along the frequency axis, is used (Backward correlation method, BWC). In addition to evaluating the accuracy of both methods, they are compared with the classical method of Lorentzian curve fitting. The accuracy of the considered methods is estimated depending on the probability of artefacts appearing in the Brillouin scattering spectra. It is shown that when the 9% probability of the artifact occurrence is exceeded, the BWC method shows better results than the other methods considered.

A model has been constructed for determining the effect of instrumental and calibration errors on the accuracy of a method for strain−temperature discrimination using polarization-Brillouin reflectometry based on measuring the Brillouin frequency shift in two polarization axes of an optical fiber. It is shown that the errors caused by the discrepancy of the calibration coefficients are negligible. The optimal hardware requirements for the reflectometer are determined. The experimental results of spectral scanning at the maximum resolution attainable in this study are in agreement with the results of computer simulation.

This paper presents a mathematical model that describes the influence of defects in an anisotropic optical fibre on its polarised frequency domain reflectograms. The model relies on the Jones matrix formalism. Calculated reflectograms are presented for a uniform axial twist of an optical fibre. As input data for simulation, we use real distributed optical parameters of the fibre determined by polarised light Brillouin reflectometry.

Magneto-optical imaging was used to study a zero-field-cooled MgB2 film at 9.6K where in a slowly increasing field the flux penetrates by abrupt formation of large dendritic structures. Simultaneously, a gradual flux penetration takes place, eventually covering the dendrites, and a detailed analysis of this process is reported. We find an anomalously high gradient of the flux density across a dendrite branch, and a peak value that decreases as the applied field goes up. This unexpected behaviour is reproduced by flux creep simulations based on the non-local field-current relation in the perpendicular geometry. The simulations also provide indirect evidence that flux dendrites are formed at an elevated local temperature, consistent with a thermo-magnetic mechanism of the instability, 5 pages, 5 figures, submitted to Supercond. Sci. Technol

The magnetic structure of the doped perovskite manganites La0.7Sr0.3MnO3 and La0.8Ca0.2MnO3 at a temperature of 10 K has been investigated by using the decoration technique in a wide region of external magnetic fields (up to 1000 Oe). We observe the multidomain ferromagnetic state in La0.7Sr0.3MnO3 and the coexistence of the ferro- and nonmagnetic phases in La0.8Ca0.2MnO3. The dependence of the structure on an external magnetic field has been investigated in La0.8Ca0.2MnO3.

Decay of a zero-voltage state of a Josephson junction at low temperature occurs via quantum tunneling through an effective potential barrier. An extremely small probability of quantum tunneling may become not very small under the action of an ac component of the bias current. The tunneling rate has a peak as a function of a dc component of the bias current (Euclidean resonance). An analysis of this extremely multiphoton process is done on the basis of classical trajectories in imaginary time. The studied phenomenon does not involve transitions between energy levels and, thus, is distinctly different from the well-known process of photon-assisted tunneling.

We report experimental and numerical study of microwave-induced flow of vortices in long Josephson junctions at zero dc magnetic field. Our intriguing observation is that applying an ac-bias of a small frequency $f \ll f_p $ and sufficiently large amplitude changes the current-voltage characteristics ($I$-$V$ curve) of the junction in a way similar to the effect of dc magnetic field, well known as the flux-flow behavior. The characteristic voltage $V$ of this low voltage branch increases with the power $P$ of microwave radiation as $V_{s}\propto P^{\alpha}$ with the index $\alpha \simeq 0.5 $. Experiments using a low-temperature laser scanning microscope unambiguously indicate the motion of Josephson vortices driven by microwaves. Numerical simulations agree with the experimental data and show strongly {\it irregular} vortex motion. We explain our results by exploiting an analogy between the microwave-induced vortex flow in long Josephson junctions and incoherent multi-photon absorption in small Josephson junctions in the presence of large thermal fluctuations. In the case of long Josephson junctions the spatially-temporal chaos in the vortex motion mimics the thermal fluctuations. In accordance with this analogy, a control of the intensity of chaos in a long junction by changing its damping constant leads to a pronounced change in the shape of the $I$-$V$ curve. Our results provide a possible explanation to previously measured but not yet understood microwave-driven properties of intrinsic Josephson junctions in high-temperature superconductors., Comment: 8 pages, 13 figures

Magneto-optical imaging was used to study flux penetration into MgB2 films in a slowly increasing perpendicular applied field. A variety of flux jumps and avalanches have been observed at temperatures below 10 K. At small fields, jumps with typical size of 20 μm and regular shape occur at random locations along the flux front. Above some threshold field of 2–10 mT, big dendritic jumps with dimensions comparable to the sample size (mm scale) take place. The jumps are developing extremely fast, result in highly-branching irreproducible flux patterns, and effectively suppress the apparent critical current. Both types of jumps are believed to result from thermo-magnetic instability.

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10^7 A/cm^2. The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors., 6 pages, 6 figures, submitted to Phys. Rev. B