Revised Results for Non-thermal Recombination Flare Hard X-Ray Emission

Brown and Mallik (BM) recently showed that, for hot sources, recombination of non-thermal electrons (NTR) onto highly ionised heavy ions is not negligible compared to non-thermal bremsstrahlung (NTB) as a source of flare hard X-rays (HXRs) and so should be included in modelling non-thermal HXR flare emission. In view of major discrepancies between BM results for the THERMAL continua and those of the Chianti code and of RHESSI solar data, we critically re-examine and correct the BM analysis and modify the conclusions concerning the importance of NTR. Although the analytic Kramers expression used by BM is correct for the purely hydrogenic recombination cross section, the heuristic expressions used by BM to extend the Kramers expression beyond the `bare nucleus' case to which it applies had serious errors. BM results have therefore been recalculated using corrected expressions, which have been validated against the results of detailed calculations. At T ~ 10-30 MK the dominant ions are Fe 22+, 23+, 24+ for which BM erroneously overestimated NTR emission by around an order of magnitude. Contrary to the BM claim, NTR in hot flare plasmas does NOT dominate over NTB, although in some cases it can be comparable and so still very important in inversions of photon spectra to derive electron spectra, especially as NTR includes sharp edge features. The BM claim of dominance of NTR over NTB in deka-keV emission is incorrect due to a serious error in their analysis. However, the NTR contribution can still be large enough to demand inclusion in spectral fitting, the spectral edges having potentially serious effects on inversion of HXR spectra to infer fast electron spectra.Comment: 6 pages, 8 figures, 1 tabl

Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above Curie temperature

We report here the results of magnetic susceptibility, electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an AlB_2-derived hexagonal structure. The results establish that Eu ions are divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An interesting observation is that the sign of MR is negative even at temperatures close to 3T_C, with increasing magnitude with decreasing temperature exhibiting a peak at T_C. This observation, being made for a Cu containing magnetic rare-earth compound for the first time, is of relevance to the field of collosal magnetoresistance.Comment: To appear in PRB, RevTex, 4 pages text + 6 psFigs. Related to our earlier work on Gd systems (see cond-mat/9811382, cond-mat/9811387, cond-mat/9812069, cond-mat/9812365

Magnetic anisotropy, first-order-like metamagnetic transitions and large negative magnetoresistance in the single crystal of Gd2_{2}PdSi3_3

Electrical resistivity ($\rho$), magnetoresistance (MR), magnetization, thermopower and Hall effect measurements on the single crystal Gd$_{2}$PdSi$_3$, crystallizing in an AlB$_2$-derived hexagonal structure are reported. The well-defined minimum in $\rho$ at a temperature above N\'eel temperature (T$_N$= 21 K) and large negative MR below $\sim$ 3T$_N$, reported earlier for the polycrystals, are reproducible even in single crystals. Such features are generally uncharacteristic of Gd alloys. In addition, we also found interesting features in other data, e.g., two-step first-order-like metamagnetic transitions for the magnetic field along [0001] direction. The alloy exhibits anisotropy in all these properties, though Gd is a S-state ion.Comment: RevTeX, 5 pages, 6 encapsulated postscript figures; scheduled to be published in Phy. Rev. B (01 November 1999, B1

Complexified PSUSY and SSUSY interpretations of some PT-symmetric Hamiltonians possessing two series of real energy eigenvalues

We analyze a set of three PT-symmetric complex potentials, namely harmonic oscillator, generalized Poschl-Teller and Scarf II, all of which reveal a double series of energy levels along with the corresponding superpotential. Inspired by the fact that two superpotentials reside naturally in order-two parasupersymmetry (PSUSY) and second-derivative supersymmetry (SSUSY) schemes, we complexify their frameworks to successfully account for the three potentials.Comment: LaTeX2e, 28 pages, no figure

Solar physics at the Kodaikanal Observatory: A Historical Perspective

This article traces the birth and growth of solar physics at the Kodaikanal Observatory of the Indian Institute of Astrophysics, Bangalore, India. A major discovery took place here in 1909 by John Evershed who detected radial outflow of matter in the penumbra of sunspots. Major developments at the Observatory since its inception in 1899 as well as the scientific results are highlighted.Comment: 26 pages, 7 figures To appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200

Self-aligned silicidation of surround gate vertical MOSFETs for low cost RF applications

We report for the first time a CMOS-compatible silicidation technology for surround-gate vertical MOSFETs. The technology uses a double spacer comprising a polysilicon spacer for the surround gate and a nitride spacer for silicidation and is successfully integrated with a Fillet Local OXidation (FILOX) process, which thereby delivers low overlap capacitance and high drive-current vertical devices. Silicided 80-nm vertical n-channel devices fabricated using 0.5-?m lithography are compared with nonsilicided devices. A source–drain (S/D) activation anneal of 30 s at 1100 ?C is shown to deliver a channel length of 80 nm, and the silicidation gives a 60% improvement in drive current in comparison with nonsilicided devices. The silicided devices exhibit a subthreshold slope (S) of 87 mV/dec and a drain-induced barrier lowering (DIBL) of 80 mV/V, compared with 86 mV/dec and 60 mV/V for nonsilicided devices. S-parameter measurements on the 80-nm vertical nMOS devices give an fT of 20 GHz, which is approximately two times higher than expected for comparable lateral MOSFETs fabricated using the same 0.5-?m lithography. Issues associated with silicidation down the pillar sidewall are investigated by reducing the activation anneal time to bring the silicided region closer to the p-n junction at the top of the pillar. In this situation, nonlinear transistor turn-on is observed in drain-on-top operation and dramatically degraded drive current in source-on-top operation. This behavior is interpreted using mixed-mode simulations, which show that a Schottky contact is formed around the perimeter of the pillar when the silicided contact penetrates too close to the top S/D junction down the side of the pillar

Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic

Context. Flare Hard X-Rays (HXRs) from non-thermal electrons are commonly treated as solely bremsstrahlung (f-f), recombination (f-b) being neglected. This assumption is shown to be substantially in error, especially in hot sources, mainly due to recombination onto Fe ions. Aims. We analyse the effects on HXR spectra and electron diagnostics by including non-thermal recombination onto heavy elements in our model. Methods. Using Kramers hydrogenic cross sections with effective Z, we calculate f-f and f-b spectra for power-law electron spectra, in both thin and thick target limits, and for Maxwellians, with summation over all important ions. Results. We find that non-thermal electron recombination, especially onto Fe, must, in general, be included together with f-f, for reliable spectral interpretation, when the HXR source is hot. f-b contribution is greatest when the electron spectral index is large, and any low energy cut-off small. f-b spectra recombination edges mean a cut-off in F(E) appears as a HXR feature at Photon energy = Ec + Vz, offering an Ec diagnostic. Including f-b lowers, greatly in some cases, the F(E) needed for prescribed HXR fluxes and, even when small, seriously distorts F(E) as inferred by inversion or forward fitting based on f-f alone. Conclusions. f-b recombination from non-thermal electrons can be an important contributor to HXR spectra and should be included in spectral analyses, especially for hot sources. Accurate results will require use of better cross sections than ours and consideration of source ionisation structure.Comment: 13 pages, 2 tables, 9 figures, Accepted for publication in A&

Inverse Compton X-rays from relativistic flare electrons and positrons

<p><b>Context:</b> In solar flares, inverse Compton scattering (ICS) of photospheric photons might give rise to detectable hard X-ray photon fluxes from the corona where ambient densities are too low for significant bremsstrahlung or recombination. γ-ray lines and continuum in some large flares imply the presence of the necessary ~100 MeV electrons and positrons, the latter as by-products of GeV energy ions. Recent observations of coronal hard X-ray sources in particular prompt us to reconsider here the possible contribution of ICS.</p> <p><b>Aims:</b> We aim to evaluate the ICS X-ray fluxes to be expected from prescribed populations of relativistic electrons and positrons in the solar corona. The ultimate aim is to determine if ICS coronal X-ray sources might offer a new diagnostic window on relativistic electrons and ions in flares.</p> <p><b>Methods:</b> We use the complete formalism of ICS to calculate X-ray fluxes from possible populations of flare primary electrons and secondary positrons, paying attention to the incident photon angular distribution near the solar surface and thus improving on the assumption of isotropy made in previous solar discussions.</p> <p><b>Results:</b> Both primary electrons and secondary positrons produce very hard ICS X-ray spectra. The anisotropic primary radiation field results in pronounced centre-to-limb variation in predicted fluxes and spectra, with the most intense spectra, extending to the highest photon energies, expected from limb flares. Acceptable numbers of electrons or positrons could account for RHESSI coronal X/γ-ray sources.</p> <p><b>Conclusions:</b> Some coronal X-ray sources at least might be interpreted in terms of ICS by relativistic electrons or positrons, particularly when sources appear at such low ambient densities that bremsstrahlung appears implausible.</p&gt

Analytic structure of rho meson propagator at finite temperature

We analyse the structure of one-loop self-energy graphs for the rho meson in real time formulation of finite temperature field theory. We find the discontinuities of these graphs across the unitary and the Landau cuts. These contributions are identified with different sources of medium modification discussed in the literature. We also calculate numerically the imaginary and the real parts of the self-energies and construct the spectral function of the rho meson, which are compared with an earlier determination. A significant contribution arises from the unitary cut of the pi-omega loop, that was ignored so far in the literature

Pion propagation in real time field theory at finite temperature

We describe how the thermal counterpart of a vacuum two-point function may be obtained in the real time formalism in a simple way by using directly the $2\times 2$ matrices that different elements acquire in this formalism. Using this procedure we calculate the analytic (single component) thermal amplitude for the pion pole term in the ensemble average of two axial-vector currents to two loops in chiral perturbation theory. The general expressions obtained for the effective mass and decay constants of the pion are evaluated in the chiral and the nonrelativistic limits. We also investigate the effect of massive states on these effective parameters.Comment: 17 pages TeX and 9 eps figure