Filamentary Phase Research Articles

Non-nematicity of the filamentary phase in systems of hard minor circular arcs

Non-nematicity of the filamentary phase in systems of hard minor circular arcs

Streamer-to-filament transition in pulsed nanosecond atmospheric pressure discharge: 2D numerical modeling

The streamer-to-filament transition in air at atmospheric pressure, in a nanosecond pin-to-pin discharge, is studied by a 2D model. The main aim is to implement a kinetic scheme providing a sharp electron density increase to the 2D PASSKEy code, to validate the results on the available experimental data, and to investigate the mechanisms responsible for the transition. Results show that after the formation of a conductive channel across the discharge gap, two discharge modes appear during a few nanoseconds: a glow phase, with a relatively homogeneous distribution with the electron density of cm−3, and a filamentary phase, with the electron density of cm−3. Two filaments appear at the cathode and anode respectively, and propagate towards the middle of the gap with a velocity of about cm s−1, forming a narrow channel. Simultaneously, the gas temperature increases from 350 to 2800 K. The diameter of the channel at the middle gap decreases from 210 to 90 µm. Dissociation and ionization of the electronically excited states of molecules N2(A, B, a, C), and ionization of ground and electronically excited states of O and N atoms are the most important processes for the transition. Numerical results also reveal the influence of the memory effect (pre-heating, pre-ionization, and pre-dissociation) from previous pulses on the streamer-to-filament transition.

Phase behavior of hard circular arcs

By using Monte Carlo numerical simulation, this work investigates the phase behavior of systems of hard infinitesimally thin circular arcs, from an aperture angle θ→0 to an aperture angle θ→2π, in the two-dimensional Euclidean space. Except in the isotropic phase at lower density and in the (quasi)nematic phase, in the other phases that form, including the isotropic phase at higher density, hard infinitesimally thin circular arcs autoassemble to form clusters. These clusters are either filamentous, for smaller values of θ, or roundish, for larger values of θ. Provided the density is sufficiently high, the filaments lengthen, merge, and straighten to finally produce a filamentary phase while the roundels compact and dispose themselves with their centers of mass at the sites of a triangular lattice to finally produce a cluster hexagonal phase.

Protected superconductivity at the boundaries of charge-density-wave domains

Solid 4He may acquire superfluid characteristics due to the frustration of the solid phase at grain boundaries. Here, introducing a negative-U generalized Hubbard model and a coarse-grained semiclassical pseudospin model, we show that an analogous effect occurs in systems with competition among charge-density-waves (CDW) and superconductivity in the presence of disorder, as cuprate or dichalcogenide superconductors. The CDW breaks apart in domains with topologically protected filamentary superconductivity at the interfaces. Our transport measurements, carried out in underdoped La2−xSrxCuO4, with the magnetic field acting as a control parameter, are shown to be in excellent agreement with our theoretical prediction. Assuming superconductivity and CDW phases have similar energies, at intermediate temperatures, the magnetic field drives the system from a fluctuating superconductor to a CDW as expected in the clean limit. Lowering the temperature, the expected clean quantum critical point is avoided and a filamentary phase appears, analogous to ‘glassy’ supersolid phenomena in 4He. The transition line ends at a second quantum critical point at high-fields. Within our scenario, the filamentary superconducting phase is parasitic with CDW and bulk superconducting phases playing the role of primary competing order parameters.

Spin-orbital polarons in electron doped copper oxides

Present work demonstrates the formation of spin-orbital polarons in electron doped copper oxides, that arise due to doping-induced polarisation of the oxygen orbitals in the CuO2 planes. The concept of such polarons is fundamentally different from previous interpretations. The novel aspect of spin–orbit polarons is best described by electrons becoming self-trapped in one-dimensional channels created by polarisation of the oxygen orbitals. The one-dimensional channels form elongated filaments with two possible orientations, along the diagonals of the elementary CuO2 square plaquette. As the density of doped electrons increases multiple filaments are formed. These may condense into a single percolating filamentary phase. Alternatively, the filaments may cross perpendicularly to create an interconnected conducting quasi-one-dimensional web. At low electron doping the antiferromagnetic (AFM) state and the polaron web coexist. As the doping is increased the web of filaments modifies and transforms the AFM correlations leading to a series of quantum phase transitions - which affect the normal and superconducting state properties.

Signatures of filamentary superconductivity in antiferromagnetic BaFe2As2 single crystals

In this paper, we present ac susceptibility and magnetotransport measurements on aged single crystals of the ferropnictide parent compound, BaFe2As2 with a paramagnetic-to-antiferromagnetic transition temperature of 134 K. The ac susceptibility shows the clear onset of a partial diamagnetic response with an onset temperature, commensurate with a subtle downturn in resistivity at approximately 20 K. Below 20 K the magnetotransport shows in-plane anisotropy, magnetic-field history dependence and a hysteretic signature. Above 20 K the crystals show the widely reported high-field linear magnetoresistance. An enhanced noise signature in ac susceptibility is observed above 20 K, which varies in character with amplitude and frequency of the ac signal. The hysteresis in magnetoresistance and the observed sensitivity of the superconducting phase to the amplitude of the ac signal are indicative characteristics of granular or weakly linked filamentary superconductivity. These features taken together with the observed noise signature above suggests a link between the formation of the superconducting filamentary phase and the freezing of antiphase domain walls, known to exist in these materials.

Direct observation of nanoscale interface phase in the superconducting chalcogenideKxFe2−ySe2with intrinsic phase separation

We have used scanning micro x-ray diffraction to characterize different phases in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ as a function of temperature, unveiling the thermal evolution across the superconducting transition temperature (T$_c\sim$32 K), phase separation temperature (T$_{ps}\sim$520 K) and iron-vacancy order temperature (T$_{vo}\sim$580 K). In addition to the iron-vacancy ordered tetragonal magnetic phase and orthorhombic metallic minority filamentary phase, we have found a clear evidence of the interface phase with tetragonal symmetry. The metallic phase is surrounded by this interface phase below $\sim$300 K, and is embedded in the insulating texture. The spatial distribution of coexisting phases as a function of temperature provides a clear evidence of the formation of protected metallic percolative paths in the majority texture with large magnetic moment, required for the electronic coherence for the superconductivity. Furthermore, a clear reorganization of iron-vacancy order around the T$_{ps}$ and T$_c$ is found with the interface phase being mostly associated with a different iron-vacancy configuration, that may be important for protecting the percolative superconductivity in K$_{x}$Fe$_{2-y}$Se$_2$.

Spectromicroscopy of electronic phase separation in KxFe2-ySe2 superconductor.

Structural phase separation in AxFe2−ySe2 system has been studied by different experimental techniques, however, it should be important to know how the electronic uniformity is influenced, on which length scale the electronic phases coexist, and what is their spatial distribution. Here, we have used novel scanning photoelectron microscopy (SPEM) to study the electronic phase separation in KxFe2−ySe2, providing a direct measurement of the topological spatial distribution of the different electronic phases. The SPEM results reveal a peculiar interconnected conducting filamentary phase that is embedded in the insulating texture. The filamentary structure with a particular topological geometry could be important for the high Tc superconductivity in the presence of a phase with a large magnetic moment in AxFe2−ySe2 materials.

Radiation feedback on dusty clouds during Seyfert activity

We investigate the evolution of dusty gas clouds falling into the centre of an active Seyfert nucleus. Two-dimensional high-resolution radiation hydrodynamics simulations are performed to study the fate of single clouds and the interaction between two clouds approaching the Active Galactic Nucleus. We find three distinct phases of the evolution of the cloud: (i) formation of a lenticular shape with dense inner rim caused by the interaction of gravity and radiation pressure (the lense phase), (ii) formation of a clumpy sickle-shaped structure as the result of a converging flow (the clumpy sickle phase) and (iii) a filamentary phase caused by a rapidly varying optical depth along the sickle. Depending on the column density of the cloud, it will either be pushed outwards or its central (highest column density) parts move inwards, while there is always some material pushed outwards by radiation pressure effects. The general dynamical evolution of the cloud can approximately be described by a simple analytical model.

Magnetic Filaments in Resistive Manganites

The magnetic phase separation in single crystals of the Pr0.67Ca0.33MnO3 manganites is studied using polarized small angle neutron scattering. The measured spectra give a fractal dimension consistent with a configuration in ferromagnetic filaments of nanometric diameter. We argue here that localized charge carriers hop in a random walk fashion mediating a ferromagnetic "hopping exchange" which coexists with superexchange to create the filamentary phase separation. The arguments for this physical picture are validated by Monte Carlo simulations, where magnetism and transport are treated in a self-consistent manner.

Fast vortex motion and filamentary phase separation in high-Tcthin films

We investigated the temperature dependence of the normalized logarithmic relaxation rate $S(T)$ and the corresponding temperature dependence of the critical current ${I}_{c}(T)$ in high-${T}_{c}$ thin films (YBCO-123, TlBCCO-2212, -2223, and -2201, and Bi-2212). Experiments have been performed using persistent critical currents flowing in the a-b planes of ring-shaped samples. The magnitude of ${I}_{c}$ and the relaxation rate have been extracted from the measurement of the self-field of the current. The results revealed a relationship between ${I}_{c}(T)$ and $S(T).$ ${I}_{c}(T)$ in YBCO is a superposition of two universal components: an underdoped Ginzburg-Landau- (GL-) like one with ${I}_{c}(T)\ensuremath{\propto}{(T}_{c}\ensuremath{-}{T)}^{3/2}$ and ${T}_{c}$ between 40 and 60 K, and an Ambegaokar-Baratoff-like one close to an optimum doping. The results revealed that when the amount of the GL-like phase increases above a certain threshold value, a peak appears in $S(T)$ at temperatures of 20--30 K, and its height gradually increases with the magnitude of ${I}_{c}$ at 10 K for this phase. We discuss similarities between these results and those reported for YBCO crystals with columnar defects. The presence of two maxima in $S(T)$ have been observed for TlBCCO films that are composed of three different phases, at temperatures close to ${T}_{c}$ of two underdoped components. The studies imply that the filamentary phase separation on a nanometer scale level in the a-b planes is responsible for the changes in vortex dynamics.

Fast vortex motion and filamentary phase separation in YBCO and TlBCCO thin films

We found a correlation between the temperature dependence of the normalized magnetic relaxation rate S( T)=d ln J/d ln t, and the filamentary phase separation in the a– b planes of high T c thin films, i.e., the ratio of the amount of an underdoped phase to that of an optimally doped one. These phases are characterized by different temperature dependences of the critical current density J c( T). J c( T) of the underdoped component is that of the Ginzburg–Landau (GL) one [with J c( T)∝( T c− T) 3/2]. For the phase closer to the optimum doping, J c( T) is described by the Ambegaokar–Baratoff (AB) behaviour at low temperatures with a crossover to the GL one above 0.8 T c, which is indicative of the presence of the Josephson nanostructures in the a– b planes. The results revealed that when the amount of the underdoped GL-like phase increases above a certain threshold value, a peak appears in S( T) at a temperature of 20–30 K and its height gradually increases with the magnitude of the critical current flowing through this phase.

NANOSCOPIC PHASE SEPARATION AND THE UNIVERSAL TRANSPORT AND MAGNETIC PROPERTIES OF HIGH Tc SUPERCONDUCTORS

We have found the correlation between nanoscopic phase separation in the copper-oxygen planes of YBCO and TlBCCO and the transport and magnetic properties of these materials in the a-b planes such as: the temperature dependence of the critical current density Jc( T ), the temperature dependence of the superfluid density ns( T )∝1/λ2( T ) at low temperatures, the temperature dependence of the normalized logarithmic relaxation rate S(T), and the dependence of the effective energy barrier against vortex motion on the current density Ueff( J ). These properties are controlled by the ratio of the amount of an underdoped filamentary phase to that of an optimally doped one.

Effect of hydrogen on the mechanical properties of a deformation processed Cu-20% Nb composite

The influence of H on the mechanical properties of a deformation processed Cu-20% Nb composite was analysed and compared with the results on similarly processed pure Cu and pure Nb. In this composite the matrix phase (Cu) is relatively resistant to H while the filamentary phase (Nb) is highly susceptible to H embrittlement. The results show that H, in an amount equivalent to that which causes embrittlement of pure Nb, causes no significant deleterious influence on the mechanical properties of Cu-20% Nb. Apparently the ductile Cu matrix creates a favourable stress state for the hydrogenated Nb filaments so that they are constrained from fracturing and continue to deform in a ductile manner.

Galaxy clustering and the dark-matter problem

Recent observational and theoretical results on galaxy clustering are reviewed. A major difficulty in relating observations to theory is that the former refer to luminous material whereas the latter is most directly concerned with the gravitationally dominant but invisible dark matter. The simple assumption that the distribution of galaxies generally follows that of the mass appears to conflict with evidence suggesting that galaxies of different kinds are clustered in different ways. If galaxies are indeed biased tracers of the mass, then dynamical estimates of the mean cosmic density, which give Ω « 0.2 may underestimate the global value of Ω. There are now several specific models for the behaviour of density fluctuations from very early times to the present epoch. The late phases of this evolution need to be followed by N -body techniques; simulations of scale-free universes and of universes dominated by various types of elementary particles are discussed. In the former case, the models evolve in a self-similar way; the resulting correlations have a steeper slope than that oberved for the galaxy distribution unless the primordial power spectral index n « 2. Universes dominated by light neutrinos acquire a large coherence length at early times. As a result, an early filamentary phase develops into a present day distribution that is more strongly clustered than observed galaxies and is dominated by a few clumps with masses larger than those of any known object. If the dark matter consists of ‘cold’ particles such as photinos or axions, then structure builds up from subgalactic scales in a roughly hierarchical way. The observed pattern of galaxy clustering can be reproduced if either Ω « 0.2 and the galaxies are distributed as the mass, or if Ω — 1, H 0 = 50 km s -1 Mpc -1 and the galaxies form only at high peaks of the smoothed linear density field. The open model, however, is marginally ruled out by the observed small-scale isotropy of the microwave background, whereas the flat one is consistent with such observations. With no further free parameters a flat cold dark-matter universe produces the correct abundance of rich galaxy clusters and of galactic halos; the latter have flat rotation curves with amplitudes spanning the observed range. Preliminary calculations indicate that the properties of voids may be consistent with the data, but the correlations of rich clusters appear to be somewhat weaker than those reported for Abell clusters.

Remarks on the use of Boltzmann's equation for electrical conduction calculations in metal matrix and in situ composites

The Boltzmann equation and its solutions are cental to the development of microscopic models describing the longitudinal and transverse electrical conductivity of metal matrix and in situ composites. Such solutions are needed to describe electron and phonon scattering and transport phenomena in the matrix due to the presence of a second filamentary phase, and to describe electrical conductivity at cryogenic and higher temperatures. In this paper, we derive solutions to the Boltzmann equation in the relaxation time approximation in cylindrical coordinates. It is shown that one solution for the electric field parallel to the fibre direction leads to an expression for composite conductivity at cryogenic and higher temperatures. We also present a solution for the case in which the electric field is normal to the fibre direction.

Eddy currents and forces in a three phase gas insulated cable with steel enclosure

A three phase gas insulated power cable has been analysed. Phase currents are sinusoidal in a steady state and balanced. Three tubular, thin phase conductors are symmetrically arranged within a steel tube of any thickness. Field, currents and forces have been found by applying complex potentials in the air and solutions of the diffusion equation in the shell material. It may be of importance that forces calculated differ significantly from the forces calculated elsewhere where simplified models with filamentary phase conductors were used.

Analysis of forces in three phase gas insulated cables

A three phase gas insulated cable, like these used in electrical power transmission or SF6 substations, has been analysed for steady state symmetrical currents. All conductors have zero thickness and are non ferrous, i.e. μ=μ0. Magnetic fields and forces have been calculated by using two dimensional complex potentials. It may be of practical importance that the forces found are significantly different than these published elsewhere in which simplified models with filamentary phase conductors were used. It is also shown how effective is in such calculations the use of complex potential.