On the extreme luminosity of hot spots in Z-pinches

A simple universal upper estimate has been proposed for the intensity of X rays from hot spots in pinches, which is similar to the Eddington limit of luminosity of stars. The closeness of the experimental results at various setups to the theoretical limit has been discussed.     

Time-resolved X-ray spectroscopy of hot spots in an X-pinch

The plasma parameters in hot spots of an X-pinch are determined by using time-resolved data from X-ray spectroscopy in experiments on the implosion of crossed Ti wires in the XP device with a current of 480 kA and pulse duration of 100 ns. The electron densities and temperatures calculated from these data are in the ranges (0.8–3)×10²³cm^–3 and 1–2.5 keV, respectively. An analysis performed shows that the plasma processes are highly nonequilibrium.     

A polarization-insensitive plasmonic SECARS substrate with multiple hot spots

A plasmonic substrate providing high, reproducible and stable Raman signals should be highly desirable for the development of surface enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In this work, we theoretically present a design of SECARS substrate consisting of five different-sized gold nanodisks and investigate its enhancement properties under different excitation polarizations by using finite element method. The numerical results reveal that the pentamer supports a polarization-independent Fano-resonant scattering spectrum due to its symmetric geometrical arrangement. Multiple electromagnetic hot spots produced by the Fano resonance are overlapped spatially at three characteristic frequencies involved in SECARS process. Consequently, the theoretically estimated overall enhancement factor (EF) of SECARS nearly keeps the same order of magnitude up to ～10¹⁴ for any horizontally excitation polarizations, and the relative root mean square error of the logarithm of the overall EF (Log₁₀EF) is less than 2%. Giant and polarization-insensitive SECARS enhancements enable the pentamer structure to be promising for plasmonic substrates in SECARS as well as other enhanced nonlinear optical process.     

Delayed hot spots in a low energy plasma focus

In a low energy Mather-type plasma focus device, hot spots having temperature in the range of few KeV have been observed even 1 μs after the pinch disintegration and in regions away from the pinch area. These hot spots are perhaps created by the thermal runway due to temperature fluctuations in the background gas.     

Analysis of hot spots in deuterium plasma focus with SSNTD

Analysis of hot spots was conducted by using two kinds of pin-hole cameras, with LR-115 detectors. Hot spots emitting positive particles from nuclear processes were located. Their locations were compared with locations emitting soft X-ray. In this experiments neutron yield was ≈10⁸ n.     

Evaluation of hot spots parameters by simple soft x-ray diagnostics

Properties of the hot spots in Ar and Ne plasma of a gas-puff z-pinch were investigated. Pinhole cameras with entrance diameter 13–250 m, XRD, semiconductor detectors and flat crystal spectrographs with Si and KAP crystals were used for spatially, temporally and spectrally resolved soft x-ray diagnostics. The diameters of Ar (25–30 m) and Ne (40 um) hot spots were found and K-shell x-ray spectra were registered. Temporal evolution of x-rays within 600 ns was observed; the upper estimate of hot spots lifetime did not exceed 50 ns. The described processing and curve fitting of He-like Ar XVII lines provided the values the of electron temperature and density in Ar hot spotsT _e=1·0–1·1 keV,n _e=(1·8–4·0)×10²⁷ m^–3, which support the idea of the Bennett equilibrium validity for the spots.Reprinted with permission of American Institute of Physics from Dense Z-pinches, AIP Conf. Proc. 195 (Eds. N. R. Pereira, J. Davis, N. Rostoker), AIP, New York, 1989.     

Acoustic timescale characterisation of symmetric and asymmetric multidimensional hot spots

In this work, two-dimensional hot spots are modelled by combining a linear temperature gradient with a constant temperature plateau. This approach retains the simplicity of a linear temperature gradient, but captures the effects of a local temperature maximum of finite size. Symmetric and asymmetric plateau regions are modelled using both rectangular and elliptical geometries. A one-step Arrhenius reaction for H₂–air is used to model the reactive mixture. Plateaus with different ratios of excitation to acoustic timescales, spanning two orders of magnitude, are simulated. Even with clear differences in behaviour between one and two dimensions, the a priori prescribed hot spot timescale ratio is shown to characterise the 2-D gasdynamic response. The relationship between one and two dimensions is explored using asymmetric plateau regions. It is shown that 1-D behaviour is recovered over a finite time. Furthermore, the duration of this 1-D behaviour is directly related to the asymmetry of the plateau.     

Improving SERS uniformity by isolating hot spots in gold rod-in-shell nanoparticles

Surface-enhanced Raman scattering (SERS) tags show ultrasensitivity and multiplexing abilities due to strong and characteristic Raman signals and therefore can be utilized as optical labeling agents similar to fluorescent dyes and quantum dots for biosensing and bioimaging. However, SERS tags have the difficulty to realize quantitative analysis due to the uniformity and reproducibility issue. In this work, we have reported on a new type of SERS tag called Au rod-in-shell (RIS) gap-enhanced Raman tag (GERT). With the high-resolution transmission electron microscopy (TEM) and optical absorbance measurements, we have demonstrated the subnanometer sized gap junctions inside the RIS GERTs. SERS measurements and FDTD calculations show that the core–shell subnanometer gap geometry in the RIS GERTs not only generates strong SERS hot spots but also isolates SERS hot spots by Au shells to avoid the influence when the particle aggregates form, therefore showing better SERS uniformity and stronger SERS intensity than normal Au nanorods. Those RIS NPs exhibit great potential as the labeling agents for SERS-based bioimaging and biosensing applications.     

Solitons bound to heavy mesons

We improve the bound state approach of the Skyrme model applied to the heavy baryons by adopting a static heavy meson picture where the soliton moves around the fixed heavy meson. This allows to take into account the center of mass corrections in a more consistent way. The bound state masses so obtained are comparable to the experimentally observed Λ _c and Λ _c ^* masses. A loosely bound state of a soliton with an antiflavored heavy meson is found, which leaves a possibility of the nonstrange pentaquark baryon(s).     

Nonlinear dynamics of vortices pinned to unidirectional twins

The nonlinear resistive properties of superconductors in the mixed state in the presence of a system of unidirectional planar defects (twins) have been investigated theoretically within the framework of the two-dimensional stochastic model of anisotropic pinning based on the Fokker-Planck equations with a concrete form of the pinning potential. These equations allow one to obtain an exact analytical solution of the problem. Formulas are obtained for experimentally observable even and odd (relative to reversal of the direction of the external magnetic field) nonlinear longitudinal and transverse magnetoresistivities ρ _‖,⊥ ^± ( j,t,α,ε) as functions of the transport current density j, temperature t, the angle α between the directions of the current and the twins, and the relative volume fraction ε occupied by the twins. In light of the great variety of types of nonlinear resistive dependences contained in these expressions for ρ _‖,⊥ ^± the most characteristic of them are presented in the form of graphs with commentary. The desired nonlinear dependences ρ _‖,⊥ ^± are linear combinations of the even and odd parts of the function v(j,t, α,ε), which has the sense of the probability of overcoming the potential barrier of the twins; this makes it possible to give a simple physical treatment of the nonlinear regimes. New scaling relations for the Hall conductivity are obtained and investigated which differ from the previously known relations for isotropic pinning. The interaction of vortex motion directed along the twins and the Hall effect is considered for Hall constants which are arbitrary in magnitude and sign, and it is shown that in the case of small Hall viscosity vortex motion directed along the twins has an effect on the odd magnetoresistivities ρ _‖ ⁻ and ρ _⊥ ⁻ , whereas the reverse effect can be neglected. It is shown that pinning anisotropy is sufficient to manifest the new nonlinear (in the current) magnetoresistivities ρ _⊥ ⁺ and ρ _‖ ⁻ . Zh. éksp. Teor. Fiz. 116, 2103–2129 (December 1999)     

Superconductivity and lattice instability in compressed lithium from Fermi surface hot spots

The highest superconducting temperature Tc observed in any elemental metal (Li with Tc approximately 18-20 K at pressure 35-48 GPa) is shown to arise from increasingly strong electron-phonon coupling concentrated along intersections of Kohn anomaly surfaces with the evolving Fermi surface. First-principles linear response calculations of the phonon spectrum and spectral function alpha2F(omega) reveal very strong Q- and phonon-polarization dependence of coupling strength, resulting in values of in the observed range. The sharp momentum dependence of the coupling even for the simple Li Fermi surface indicates more generally that a fine Q mesh is required for precise evaluation of lamda.     

Measurement of hot spots inside the proton at HERA and LEP/LHC

In this paper we discuss the measurement of regions in the proton where the density of small-x partons is large, called hot spots, by means of an associated jet analysis. An analytical estimate of the cross section is presented and the jet kinematics is discussed in the HERA and LEP-LHC frame. A Monte Carlo estimate shows that the number of jets produced in deep inelastic scattering events at HERA, suitable for this analysis, amounts to a few 1000 jcts for a data sample with an integrated luminosity of about 10 pb^–1.supported in part by FONDECYT 0367/90     

Exchange coupling of bilayers and synthetic antiferromagnets pinned to MnPt

Exchange bias and blocking temperature were studied in MnPt based bottom-pinned bilayers and synthetic antiferromagnets (SAF) prepared by magnetron sputtering. The structure and magnetic properties were determined as a function of the MnPt layer thickness. Exchange coupling was found to be (Jex = 0.4 erg/cm2) for a MnPt (t ≤20 nm)/CoFe (5 nm) bilayer. The distribution of the blocking temperature TB was analyzed and its width ΔTB and center point TB,center determined. TB is about 280 ○C for thinner MnPt films, and increases to 330 °C for thick films. ΔT B is constant for thick MnPt but steadily increases as the thickness decreases. SAF structures show higher exchange bias and higher TB,center at thin layer thickness (tMnPt = 8.5 nm) compared to bilayers.     

Surface-enhanced Raman scattering substrates of high-density and high-homogeneity hot spots by magneto-metal nanoprobe assembling

Binary naoparticles composed of a superparamagnetic Fe(3)O(4) core and an Au nanoshell were prepared via a high-temperature hydrolysis reaction followed by seed-mediated growth. The nanoprobes render simultaneous dual functions of both fast magnetic response and local surface plasmon resonance. Using these nanoprobes, analyte molecules can be easily biologically captured, magnetically concentrated, and analyzed by surface-enhanced Raman scattering (SERS). Particularly, the complex particles were assembled under magnetic force direction into a SERS substrate. It was found to possess both a high enhancement factor (10(6)) and high homogeneity of "hot spot" distribution (fluctuation less than 20% for a 1 μm(2) area) with 4-aminothiophenol as the analyte.     

Hysteresis from dynamically pinned sliding states

We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F_ext. A critical threshold value F_c of the adiabatically applied force F_ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of “dynamic” stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F_ext. Hysteretic behavior is also observed as a function of the external driving velocity.     

Self-Induced Transparency Solitons and Dissipative Solitons in Microwave Electronic Systems

Radiophysics and Quantum Electronics - Solitons are a well-studied subject in many domains of nonlinear physics, including hydrodynamics, plasma physics, optics, etc. However, it has been...     

Transportation of pinned charge density waves

We investigated the transport of pinned charge density waves (CDWs) that is observed in low dimensional materials. We treated pinned CDWs as moving CDWs that were confined within a typical quantum well amongst the many different types where pinning occurs at the barrier. We calculated the current flowing out of the quantum well by confined CDWs. The calculated conductivity is in good correspondence with experimental data in TTF–TCNQ, where the measured Fröhlich–Peierls temperature is 60 K much higher than the theoretical value of 20 K. The voltage dependence of the conductivity was calculated, where this is easily transformed into the dependence of electric field. The magnetic susceptibility was also calculated with a similar trend of experimental data. The susceptibility is a diamagnetic contribution by CDWs in addition to the constant background Pauli paramagnetic part.