Supercritical Field Research Articles

Mechanical and order rigidity of nematic elastomers

We calculate the ratio of moduli for extension parallel and perpendicular to the director of nematic elastomers. Elastomers in practice are not ideal nematics with a jump from a finite to zero-order parameter with increasing temperature. Some classes behave as if under effective external fields. These are really internal in origin and leave the system subcritical (still with jumps) or supercritical. We give expressions for general non-ideality and memory of formation conditions, along with its translation into values of the “fields". For the case of supercritical fields, we find that the modulus ratio deviates more from unity, in accord with experiment, than in the subcritical case.

Compton Scattering in Ultrastrong Magnetic Fields: Numerical and Analytical Behavior in the Relativistic Regime

This paper explores the effects of strong magnetic fields on the Compton scattering of relativistic electrons. Recent studies of upscattering and energy loss by relativistic electrons that have used the nonrelativistic, magnetic Thomson cross section for resonant scattering or the Klein-Nishina cross section for nonresonant scattering do not account for the relativistic quantum effects of strong fields (>4 × 1012 G). We have derived a simplified expression for the exact QED scattering cross section for the broadly applicable case in which relativistic electrons move along the magnetic field. To facilitate applications to astrophysical models, we have also developed compact approximate expressions for both the differential and total polarization-dependent cross sections, with the latter representing well the exact total QED cross section even at the high fields believed to be present in environments near the stellar surfaces of soft gamma repeaters and anomalous X-ray pulsars. We find that strong magnetic fields significantly lower the Compton scattering cross section below and at the resonance when the incident photon energy exceeds mec2 in the electron rest frame. The cross section is strongly dependent on the polarization of the final scattered photon. Below the cyclotron fundamental, mostly photons of perpendicular polarization are produced in scatterings, a situation that also arises above this resonance for subcritical fields. However, an interesting discovery is that for supercritical fields, a preponderance of photons of parallel polarization results from scatterings above the cyclotron fundamental. This characteristic is both a relativistic and a magnetic effect not present in the Thomson or Klein-Nishina limits.

Magnetic photon splitting: TheS-matrix formulation in the Landau representation

Calculations of reaction rates for the third-order QED process of photon splitting in strong magnetic fields traditionally have employed either the effective Lagrangian method or variants of Schwinger's proper-time technique. Recently, Mentzel, Berg and Wunner (1994) presented an alternative derivation via an S-matrix formulation in the Landau representation. Advantages of such a formulation include the ability to compute rates near pair resonances above pair threshold. This paper presents new developments of the Landau representation formalism as applied to photon splitting, providing significant advances beyond the work of Mentzel et al. by summing over the spin quantum numbers of the electron propagators, and analytically integrating over the component of momentum of the intermediate states that is parallel to field. The ensuing tractable expressions for the scattering amplitudes are satisfyingly compact, and of an appearance familiar to S-matrix theory applications. Such developments can facilitate numerical computations of splitting considerably both below and above pair threshold. Specializations to two regimes of interest are obtained, namely the limit of highly supercritical fields and the domain where photon energies are far inferior to that for the threshold of single-photon pair creation. In particular, for the first time the low-frequency amplitudes are simply expressed in terms of the Gamma function, its integral and its derivatives. In addition, the equivalence of the asymptotic forms in these two domains to extant results from effective Lagrangian/proper-time formulations is demonstrated.

Effects of pulsed electric fields on cell membranes in real food systems

High frequency current and voltage measurements were used to determine passive electrical properties, such as the polarization effect at intact membrane interfaces and field-induced electropermeability changes in the cellular materials during direct current pulses. The time sequence of the electropermeabilization at the level of the cell membrane showed a similarity to the breakdown phenomena observed in cell systems (potato, apple and fish tissues, as well as plant cell suspension cultures) when a single pulse with critical or supercritical field amplitude is applied. A slight membrane breakdown phenomenon occurred in the first few microseconds after the initiation of the pulse at a critical electric field strength of 150–200 V/cm. Significant membrane breakdown was observed when the field strength of the electric pulses applied directly on the cell systems was in the range of 400–800 V/cm. At various field intensities, the electrical potential across a cell membrane reached a critical value of approximately 0.7–2.2 V. The initiation of conductive channels across the membrane occurred within nanoseconds during the charging process of the membrane, whereas the formation of a high-conductance membrane due to pore expansion took place within a few microseconds. The application of a single pulse, even with supercritical field amplitude, does not necessarily cause an irreversible membrane rupture. The insulating properties of the cell membrane can be completely recovered within several seconds after the termination of the pulse. The biological and engineering aspects of the membrane permeabilization are discussed in this paper. These data are utilized as the basis for the design and optimization of high-intensity pulsed electric field applications in the areas of food science and biotechnology.

The supercritical fields of metastable Type I superconductors

A precise knowledge of the superheated and supercooled transition fields of Type I superconductors is of importance to the ongoing research and development of superconducting radiation detectors, since determining whether or not the material exhibits a full degree of metastability is necessary to knowing whether the measurements are performed in metastable or intermediate state regimes. We show that, in fact, much of the information regarding device performances has been obtained from devices not fully metastable. We further examine the relationship between the supercritical fields and the Ginzburg–Landau parameter, and review the available data in tin and indium in its regard. Empirical relations are developed from which the supercritical fields can be generally determined.

Theory of magnetars

The discovery of long periods and high period derivatives in soft gamma-ray repeaters (SGRs) over the past year has raised the possibility that magnetars, neutron stars having magnetic fields above 1014 G, may be real astrophysical objects. Magnetars are possibly the first astrophysical sources to derive their primary radiation power from magnetic energy, which substantially exceeds their rotational energy. In addition, the physics of supercritical magnetic fields operates in a more exotic realm than even that of normal pulsars. I will discuss acceleration and radiation models for the SGR bursts and quiescent emission from SGRs and anomalous X-ray pulsars.

Theory of magnetars

The discovery of long periods and high period derivatives in soft gamma-ray repeaters (SGRs) over the past year has raised the possibility that magnetars, neutron stars having magnetic fields above 1014 G, may be real astrophysical objects. Magnetars are possibly the first astrophysical sources to derive their primary radiation power from magnetic energy, which substantially exceeds their rotational energy. In addition, the physics of supercritical magnetic fields operates in a more exotic realm than even that of normal pulsars. I will discuss acceleration and radiation models for the SGR bursts and quiescent emission from SGRs and anomalous X-ray pulsars.

Pair production of charged vector bosons in supercritical magnetic fields at finite temperatures

The thermodynamic properties of an ideal gas of charged vector bosons (with mass m and charge e) is studied in a strong external homogeneous magnetic field no greater than the critical value B_{cr}=m^2/e. The thermodynamic potential, after appropriate analytic continuation, is then used in the study of the spontaneous production of charged spin-one boson pairs from vacuum in the presence of a supercritical homogeneous magnetic field at finite temperature.

Electric field pulses can induce apoptosis.

Injection of electric field pulses of high intensity (kV/cm) and short duration (microsecond range) into a cell suspension results in a temporary increase of the membrane permeability due to a reversible electric breakdown of the cell membrane. Here we demonstrate that application of supercritical field pulses between 4. 5 and 8.1 kV/cm strength and 40 microsec duration induce typical features of apoptosis in Jurkat T-lymphoblasts and in HL-60 cells including DNA fragmentation and cleavage of the poly(ADP ribose) polymerase. Apoptosis induction did not depend on the presence of any particular electrolyte in the extracellular medium. However, no apoptosis was observed in solutions without a minimum amount of salt. Apoptotic DNA fragmentation was prevented by the caspase inhibitor zVAD.

Critical currents in quantum Hall conductors with antidot arrays

We have investigated the abrupt onset of dissipation in the quantum Hall effect (QHE) in antidot arrays patterned on a two- dimensional electron system. Different lateral configurations (periodic or aperiodic antidot arrays, and single lines of antidots, with diameters between 40 and 100 nm and periods or average spacings between 300 and 1500 nm) show remarkable differences in their non-Ohmic transport properties. In periodic arrays with large antidot diameter (lithographic diameter 100 nm), the breakdown current is systematically reduced with increasing antidot density and determined by the peak value of the local current density. From these measurements, we determined the depletion width around the antidots. In aperiodic arrays, the breakdown current is markedly lower than in periodic arrays of the same antidot size and density due to higher local values of the current density at the same total current. This was experimentally confirmed by measurements of the current dependence of the electron temperature in periodic and aperiodic arrays. Single lines of antidots, placed across the direction of current flow, cause only a small reduction of the breakdown current in comparison with unpatterned reference areas. This is in accordance with the picture of avalanche electron heating for the breakdown of the QHE, where the electrons reach a quasistationary, elevated temperature only after travel distances of several 10 im in a supercritical electric Hall field. In antidot lattices with very small antidot diameter (40 nm) and small lattice period (300 nm), the antidots provide additional inelastic scattering, which effectively suppresses the electron heating. This effect overcompensates the geometrical effect of the antidots and was experimentally verified by the observation of higher breakdown currents compared to the reference area. A complete absence of a hot- electron-induced hysteresis in the current- voltage characteristics was observed for this type of array.

Magnetic Photon Splitting: Computations of Proper‐Time Rates and Spectra

The splitting of photons in the presence of an intense magnetic field has recently found astrophysical applications in polar cap models of gamma-ray pulsars and in magnetar scenarios for soft gamma repeaters. Numerical computation of the polarization-dependent rates of this third order QED process for arbitrary field strengths and energies below pair creation threshold is difficult: thus early analyses focused on analytic developments and simpler asymptotic forms. The recent astrophysical interest spurred the use of the S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In this paper, we present numerical computations of a full proper-time expression for the rate of splitting that was obtained by Stoneham, and is exact up to the pair creation threshold. While the numerical results derived here are in accord with the earlier asymptotic forms due to Adler, our computed rates still differ by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner, reflecting the extreme difficulty of generating accurate S-matrix numerics for fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time rates appear very accurate, and exceed Adler's asymptotic specializations significantly only for photon energies just below pair threshold and for supercritical fields, but always by less than a factor of around 2.6. We also provide a useful analytic series expansion for the scattering amplitude valid at low energies.

Modeling of the penetration of a supercritical magnetic field into a type II superconductor and the electromagnetic radiation generated thereby

Modeling of the penetration of a supercritical magnetic field into a type II superconductor and the electromagnetic radiation generated thereby

Fluid evolution during tectonic exhumation of oceanic crust at a slow-spreading paleoridge axis: evidence from the Lizard ophiolite U.K.

Zoned quatz crystals within late syntectonic veins from brittle-ductile shear zones situated within the lowermost plutonic rocks of the Lizard ophiolite contain three groups of fluid inclusions. Type I are a phase separated primary and pseudosecondary group. Liquid-rich inclusions contain 3.2–8.0wt% NaCl equivalent with a mode at 5.7wt% NaCl equiv. Vapour-rich inclusions contain 0.0–6.1wt% NaCl equiv. with a mode at 2.6wt% NaCl equiv. Inclusions homogenize either by disappearance of the liquid or vapour phase, with a mode at 380°C. Type II inclusions contain 2.1–5.7wt% NaCl equiv. with a mode at 4.1 wt% NaCl equiv. Inclusions homogenize by disappearance of the vapour phase (L + V ⇄ L) with a mode at 230°C. Type III inclusions are a pseudosecondary and secondary group, with textures which are suggestive of underpressure-induced re-equilibration. They contain methane and a low salinity aqueous solution (0.3–2.2wt% NaCl equiv.). Liquid-rich satellite inclusions homogenize at 310-240°C (L + V ⇄ L). Vapour-rich inclusions homogenize by disappearance of the liquid phase (L + V ⇄ V) at 370-320°C. By assuming that the range in salinity is exclusively the product of phase separation of a 3.2wt% NaCl seawater solution, and by applying microthermometric data to the NaClH 2O system, analysis of the data suggests two-phase boundary constrained conditions of crystal growth, initiated within the super-critical field at around ∼410°C, with declining temperature passing into subcritical portions of the system, beyond which the fluid entered single-phase liquid conditions of crystal growth. Microthermometric and petrographic data, in comparison with present day ocean-ridge hydrothermal systems, is consistent with a reaction zone situated within an inferred decollement at or near the petrological moho, from which fluids were derived from and expelled along intersecting listric faults, at temperatures broadly comparable to shallow subsurface levels within present-day systems. This interpretation suggests that high vent temperatures were maintained as the heat flux waned, and that the hydrothermal system migrated downward through the uplifting gabbros, culminating in the reaction zone intersecting the upper mantle during the terminal pulses of tectonic exhumation.

Stability of confined swirling flow with and without vortex breakdown

A numerical investigation of steady states, their stability, onset of oscillatory instability, and slightly supercritical unsteady regimes of an axisymmetric swirling flow of a Newtonian incompressible fluid in a closed circular cylinder with a rotating lid is presented for aspect ratio (height/radius) 1 [les ] γ [les ] 3.5. Various criteria for the appearance of vortex breakdown are discussed. It is shown that vortex breakdown takes place in this system not as a result of instability but as a continuous evolution of the stationary meridional flow with increasing Reynolds number. The dependence of the critical Reynolds number Recr and frequency of oscillations ωcr on the aspect ratio of the cylinder γ is obtained. It is found that the neutral curve Recr(γ) and the curve ωcr(γ) consist of three successive continuous segments corresponding to different modes of the dominant perturbation. The calculated critical parameters are in good agreement with the available experimental and numerical data for γ < 3. It is shown that the onset of the oscillatory instability does not depend on the existence of a separation bubble in the subcritical steady state. By means of a weakly nonlinear analysis it is shown that the axisymmetric oscillatory instability sets in as a result of a supercritical Hopf bifurcation for each segment of the neutral curve. A weakly nonlinear asymptotic approximation of slightly supercritical flows is carried out. The results of the weakly nonlinear analysis are verified by direct numerical solution of the unsteady Navier-Stokes equation using the finite volume method. The analysis of the supercritical flow field for aspect ratio less than 1.75, for which no steady vortex breakdown is found, shows the existence of an oscillatory vortex breakdown which develops as a result of the oscillatory instability.

Numerical solutions to dilaton gravity models and the semiclassical singularity.

A general homogeneous two dimensional dilaton gravity model considered recently by Lemos and S\` a, is given quantum matter Polyakov corrections and is solved numerically for several static, equilibrium scenarii. Classically the dilaton field ranges the whole real line, whereas in the semi-classical theory, with the usual definition, it is always below a certain critical value at which a singularity appears. We give solutions for both sub- and super-critical dilaton field. The pasting together of the spacetime on both sides of a singularity in semi-classical planar general relativity is discussed.

Charge screening in supercritical Coulomb fields and a lower bound for electron energy levels

We show that discontinuities in the vacuum polarisation give rise to charge screening effects which are large and result in a lower bound Δ with − m < Δ < m for the energy levels of an electron bound in a supercritical field.

Ground state instability in a gauge theory model of a doped planar antiferromagnet

We study the spectrum of bound states in the (2 + 1)-dimensional C P 1 model coupled to N flavours of Dirac fermions. The case N=2 corresponds to the long-wavelength limit of a microscopic model of the dynamics of holes in a planar antiferromagnet. Fermions of opposite charge with respect to the C P 1 gauge field are found to be subject to attractive forces corresponding to a 1 R two-body potential in the large- N limit. For N<N c= 8 π the corresponding quantum mechanical model is closely related to the problem of an electron moving in a supercritical Coulomb field. At zero temperature the formation of fermion bound states is energetically favoured and this is interpreted as leading to hole-pair condensation and a stable superconducting ground state. The zero temperature mass-gap and coherence length are calculated in this approach. Finite temperature corrections to the two-body potential are derived and the existence of a critical temperature at which superconductivity is extinguished is demonstrated. The model is generalised to include additional four-fermion operators which arise naturally in the underlying statistical model. These are found to have a large effect on the coherence length, reducing it by a factor of about 50. The possible relation of this model to high- T c superconductivity is discussed.

Interpretation of e+e− peaks in heavy-ion collisions as a Lee-Wick phase transition

We suggest that the e+e− lines seen in heavy-ion collisions are a manifestation of an inhomogeneous Lee-Wick phase transition to abnormal matter, initiated by the presence of the supercritical electric field generated by the heavy-ion collision.

Pair production in the collision of heavy nuclei: summing perturbative terms

The cross section for the production of an e+d- pair in the peripheral collision of two heavy fast nuclei is evaluated in a way which corresponds to a sum of leading perturbative contributions. The result, with particular attention to the role of the total charge of the nuclei, is compared when possible with the Bethe-Maximon result and with the experimental evidence. The sharp difference between the theoretical results and the experimental situation for high nuclear charge is interpreted as indirect but clear evidence of the formation, in a supercritical time-dependent Coulomb field, of configurations very far from the perturbative ones.

Limit theorems for supercritical branching random fields with immigration

A measure-valued process which carries genealogical information is defined for a supercritical branching random field with immigration. This process counts the particles present at a final time whose ancestors had specified locations at given times in the past. A law of large numbers and a fluctuation limit theorem are proved for this process under a space-time scaling. The fluctuation limit is a nonstationary generalized Ornstein-Uhlenbeck process. An example of interest in transport theory and polymer chemistry is given.