37 research outputs found
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
Microwave analysis of the interplay between magnetism and superconductivity in EuFe 2 (As 1 -x P x ) 2 single crystals
This paper presents a microwave analysis of the interplay between magnetism and superconductivity in an iron-based ferromagnetic superconductor. By comparing the complex rf susceptibility with magnetic force images, the authors discuss the nature of the observed phase transitions and the possible presence of a quantum critical point
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(FeCo)As}. 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 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
Observation of the magnetic domain structures in CuNi thin films at low temperatures
We report on the first experimental visualization of domain structure in
films of weakly ferromagnetic CuNi 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 ( 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
Evidence of the Bean-Livingston barrier in type-II superconductors
A magneto-optical imaging (MOI) system capable to resolve single vortices is combined with a focused laser beam to reorganize vortex matter in dense vortex clusters. The local heating of the superconductor with the laser produces a temperature profile which induces an attraction of the vortices towards the center of the laser spot. We analyze the collective vortex dynamics under high-power laser irradiation. The formation of vortex clusters is described with a model very similar to the one describing the first vortex entry into a type-II superconductor.Des nanosondes ultra-sensibles pour explorer les distributions de charges électriques dans les supraconducteur
Effects of proton irradiation on the magnetic superconductor EuFe2(As1-xPx)2
We report on the effects of 3.5 MeV proton irradiation on single crystals of the magnetic superconductor EuFe2(As1- x Px)2, investigated at microwave frequencies by a coplanar waveguide resonator technique. We studied the relative strength of the two collective phenomena by analyzing the dependence of their onset temperatures on different perturbations: magnetic fields and structural disorder. Results suggest that superconductivity and magnetism in this material are two competing orders: as the former is suppressed by irradiation or by excess P doping, the latter reinforces and manifests itself at higher temperatures. The ability to modulate superconductivity and magnetism in EuFe2(As1-xPx)2 by disorder and in-plane magnetic field, respectively, offers the chance to tailor, even on a local scale, their relative weights. This could be useful for the aim of engineering this material for specific applications