Development of the Reiss theory for binary homogeneous nucleation of aerosols

The Reiss theory for binary homogeneous nucleation in binary gas mixtures is critically analysed and further developed. Based on the analysis of phase space trajectories in the supercritical zone of the phase transition, carried out within the framework of the theory of two-dimensional dynamical systems and supplemented by the flux matching condition at the boundary of the critical zone, it is shown how the theory should be modified. The proposed modification is equivalent to the earlier modifications by Langer and Stauffer, based on additional trial assumptions (ansatz) for solving the steady state equation for the non-equilibrium size distribution function, but reveals and substantiates the approximation underlying their approach. The extension of the Reiss theory to binary vapours in inert carrier (atmospheric) gases is justified.Comment: 14 pages, 1 figur

Dislocation lines as the precursor of the melting of crystalline solids observed in Monte Carlo simulations

The microscopic mechanism of the melting of a crystal is analyzed by the constant pressure Monte Carlo simulation of a Lennard-Jones fcc system. Beyond a temperature of the order of 0.8 of the melting temperature, we found that the relevant excitations are lines of defects. Each of these lines has the structure of a random walk of various lengths on an fcc defect lattice. We identify these lines with the dislocation ones proposed in recent phenomenological theories of melting. Near melting we find the appearance of long lines that cross the whole system. We suggest that these long lines are the precursor of the melting process.Comment: 5 pages, 5 figures, accepted in Physical Review Letter

Effect of controlled artificial disorder on the magnetic properties of EuFe2(As1−xPx)2 ferromagnetic superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2 (As1−xPx )2 (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, Tc (H), shows an extraordinarily large decrease. It starts at Tc (H = 0) ≈ 24 K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only Tc moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in Tc to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe2 (As1−xPx )2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu2+ lattice act as efficient pairbreakers leading to a significant Tc reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of Tm (less than 1 K) after the irradiation drives Tc to below Tm . Our results suggest that FM and SC phases coexist microscopically in the same volume

High-Resolution Bitter Decoration Technique for Low Temperatures

We present the high-resolution technique to visualize magnetic flux structure of super-conductors and magnets at low temperatures. The method is based on magnetic nanoparticles preparation in situ low-temperature experiment by magnetic matter evaporation over sample surface in helium atmosphere

Vortex studies in superconducting Ba(Fe0.93Co0.07)2As2

We present small-angle neutron scattering (SANS) and Bitter decoration studies of the superconducting vortices in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$}. A highly disordered vortex configuration is observed at all measured fields, and is attributed to strong pinning. This conclusion is supported by the absence of a Meissner rim in decoration images obtained close to the sample edge. The field dependence of the magnitude of the SANS scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry, consistent with the decoration images which show primarily 6-fold coordinated vortex domains. An analysis of the scattered intensity shows that this decreases much more rapidly than expected from estimates of the upper critical field, consistent with the large degree of disorder.Comment: 5 pages, 4 figure

Observation of the magnetic domain structures in Cu0,47_{0,47}Ni0,53_{0,53} thin films at low temperatures

We report on the first experimental visualization of domain structure in films of weakly ferromagnetic Cu$_{0,47}$Ni$_{0,53}$ alloy with different thickness at liquid helium temperatures. Improved high-resolution Bitter decoration technique was used to map the magnetic contrast on the top of the films well below the Curie temperature T$_{Curie}$ ($\sim$ 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.Comment: 4 pages, 5 figures, will be published in JETP Let

Observation of the Triplet Spin-Valve Effect in a Superconductor-Ferromagnet Heterostructure

The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation.Comment: 7 pages, 4 fgures, Submitted to Physical Review Letter